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The Effects of Alloying Elements on the Microstructure of Al-Zn alloy

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The effects of alloying elements and rare earth elements such as Zr, Sc, Er, etc., are presented. Structural analysis shows that these rare earth elements all form an intermetallic phase with Al, with the same formula Al₃RE. This intermetallic phase plays a role in preventing the formation of harmful phases in the alloy. These intermetallic phases can be located within the grain, acting as heterogeneous nucleation centers or at the grain boundary, blocking positions to prevent grain growth. The research presented the roles of the intermetallic phase as the crystallization seed center and the intermetallic phase as the inhibitory phase for grain growth. XRD analysis results have determined the structure of these intermetallic phases, showing their conformity with the background phase. Through theoretical and experimental studies, the deformation and uniform annealing process for the Al-Zn-Mg-Cu alloy was determined in the presence of La and Ce grain refiners. The uniform annealing process removed the dendritic structures after casting and determined the intermetallic phases' role in preventing the matrix phase's growth. XRD analysis results confirmed the structure of the intermetallic phases, demonstrating their conformity with the matrix phase. Research about the influence of rare earth elements in the grain reduction process will help establish a grain reduction protocol for studying alloys during both the melting and homogenization annealing stages. This is particularly crucial for creating high mechanical strength and superplastic alloys within this alloy group.
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Vol.14 (2024) No. 4
ISSN: 2088
-
5334
The Effects of Alloying Elements on the Microstructure of Al-Zn alloy
Thi Ngoc Mai Bui a, Minh Thai Duong b, Van Phuc Nguyen c, Duc Chuan Nguyen c, Thanh Nam Dang d,*
a School of Mechanical Engineering, Vietnam Maritime University, Haiphong, Vietnam
b Institute of Mechanical Engineering, Ho Chi Minh City University of Transport, Ho Chi Minh, Vietnam
c Institute of Maritime, Ho Chi Minh City University of Transport, Ho Chi Minh, Vietnam
d Center of Human Resources & Maritime Training, Ho Chi Minh City University of Transport, Ho Chi Minh, Vietnam
Corresponding author: *nam.dang@ut.edu.vn
AbstractThe effects of alloying elements and rare earth elements such as Zr, Sc, Er, etc., are presented. Structural analysis shows that
these rare earth elements all form an intermetallic phase with Al, with the same formula Al₃RE. This intermetallic phase plays a role
in preventing the formation of harmful phases in the alloy. These intermetallic phases can be located within the grain, acting as
heterogeneous nucleation centers or at the grain boundary, blocking positions to prevent grain growth. The research presented the
roles of the intermetallic phase as the crystallization seed center and the intermetallic phase as the inhibitory phase for grain growth.
XRD analysis results have determined the structure of these intermetallic phases, showing their conformity with the background phase.
Through theoretical and experimental studies, the deformation and uniform annealing process for the Al-Zn-Mg-Cu alloy was
determined in the presence of La and Ce grain refiners. The uniform annealing process removed the dendritic structures after casting
and determined the intermetallic phases' role in preventing the matrix phase's growth. XRD analysis results confirmed the structure
of the intermetallic phases, demonstrating their conformity with the matrix phase. Research about the influence of rare earth elements
in the grain reduction process will help establish a grain reduction protocol for studying alloys during both the melting and
homogenization annealing stages. This is particularly crucial for creating high mechanical strength and superplastic alloys within this
alloy group.
KeywordsIntermetallic phase; rare earth elements; crystallization seed; stable phase; heterogeneous nucleation; Al-Zn alloy.
Manuscript received 12 Mar. 2024; revised 29 Jun. 2024; accepted 17 Jul. 2024. Date of publication 31 Aug. 2024.
IJASEIT is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
I. INTRODUCTION
Industry development has led to the development of
technology and materials engineering. Among the materials,
metal materials such as steel, aluminum alloys, and copper
alloys are commonly used in the fields of industrial
production, construction, and transportation [1]–[7]. For steel,
there are many studies conducted to improve its mechanical
properties, such as alloying solutions, heat treatment, or using
modifiers [8]–[17]. However, the solutions for non-ferrous
alloys have not been analyzed comprehensively. Being non-
ferrous alloys, the Al-Zn alloy is a base alloy with aluminum
(Al) as the primary element and zinc (Zn) as the main alloying
element, while other alloying elements include magnesium
(Mg) and copper (Cu) [18]–[20]. With characteristics such as
high castability, good strength, and lower melting
temperature, the Al-Zn alloy was developed to compete with
copper, cast iron, and aluminum [21]–[25]. This aluminum
alloy has significant potential and is utilized in aerospace,
weapon manufacturing, and sports equipment production.
Studying fine-grain Al-Zn alloys using rare earths, combined
with technological methods suited to local conditions and
equipment, requires thorough research to produce high-
ductility alloys. The goal is to develop new materials for
manufacturing thin parts in industries such as automotive,
motorcycle, computer, and phone manufacturing [26]–[30]. This
will help increase the localization rate and promote the
application of high-ductility alloys in these industries [31]–[33].
Aluminum alloys exhibit vastly different crystallization
properties depending on their composition. As a result, the use
of grain refining agents varies for each alloy. Without grain
refining agents, pure aluminum requires a supercooling of 3–
4°C for nucleation to occur. However, once the first crystals
reach an acceptable size, the thermite reaction causes the
liquid metal temperature to rise again, preventing the
formation of new crystallization nuclei [34], [35]. This leads
to large, coarse columnar grain growth during solidification.
Modifying aluminum alloys serves two main purposes:
1248
refining the primary aluminum grains and refining the
secondary grains. Under normal solidification conditions,
most aluminum alloys develop a three-zone structure: a
delicate outer shell, a layer of columnar dendritic crystals, and
coarse, regular dendritic crystals at the center of the casting.
The grain coarseness and the length of cylindrical crystals
depend on the pouring temperature, the temperature gradient
in the mold, and the number of nucleation centers formed
during crystallization.
Adding alloying elements to liquid aluminum typically
reduces the grain size [36], [37]. Alloys containing elements
with good solubility, such as Cu, Mg, and Zn, are easier to
modify regarding grain size than those containing only
silicon. For example, modifying Al-Si-Mg-Cu alloys can
result in small and refined grains. However, in Al-Si alloys
with high silicon content, grain refinement is more
challenging due to silicon's high growth inhibition coefficient
[36], [38]–[41]. Thermodynamically, the crystallization
process of metals involves two stages: nucleation and
nucleation growth. The nucleation and growth rates of the
nuclei are critical parameters that determine the structure and
properties of the alloy. The main goal of modification is to
influence these two parameters [36], [37], [42].
A solid solution β based on zinc dissolves aluminum in
small quantities. At 382°C, the solubility of aluminum in β is
approximately 1.1%. At this temperature, with an aluminum
content of 5%, a reaction occurs alongside the solid solution
[31], [43]:
L → (β + α) (1)
In the α phase, there is 79% zinc. Rapid cooling can
maintain this supersaturated concentration in α at room
temperature. Upon slow cooling, a favorable reaction will
occur [31], [43]:
α→ [α’ + β] (2)
The Al-Zn alloy has excellent castability and high
mechanical properties in the cast state. At low temperatures,
the α phase will precipitate the β phase with the same
composition as pure Zn because Al dissolves in Zn very
minimally, only about 2%. However, at 380°C, Zn can
dissolve up to 84%. The α and β phases form a coarse
microstructure during normal solidification without
intervention. To overcome the coarse structure of the α and β
phases, measures must be taken to refine the granularity and
improve the alloy's microstructure, according to the Hall-
Petch equation [44]–[49]:
σ =
.
(3)
In this context:
σ 0 represents the stress needed to initiate deviation.
d stands for particle size.
k denotes lattice constant.
The alloy's strength improves with smaller grain sizes. To
achieve finer grain structure, refining methods are employed
during casting to achieve the desired structure. This article
explores the impact of critical elements and rare earth
elements on the microstructure of Al-Zn alloy.
II. MATERIALS AND METHOD
A. Materials
The alloy studying belongs to the Al-Zn-Mg-Cu system,
closely resembling the 7475 (AA) alloy grade, classified
among high-strength deformed aluminum alloys. Table 1
details the composition of this alloy.
TABLE I
CHEMICAL COMPOSITION OF THE STUDIED ALLOY [50]
Elements
Zn
Mg
Cu
Si
Fe
Mn
La
Ce
Al
Sample 1
8.26
2.3
1.11
0.7
0.15
0.12
x
x
remaining
Sample 2
7.6
1.9
1
0.6
0.19
0.17
0.21
0.155
remaining
1) Ingredients for preparing aluminum ingots: To prepare
aluminum ingots for the experiment, follow these steps: First,
ensure each ingot weighs approximately 7-8 kg. Next, cut the
aluminum billets into small pieces to fit into the cooking pot,
aiming for batches weighing around 1-1.2 kg each. After
cutting, thoroughly clean the workpieces to remove dust, then
dry them at 200°C to eliminate any remaining surface
moisture. Additionally, pure metals should be included to
achieve the desired composition for each batch. Pure zinc (Zn)
and magnesium (Mg) should be chopped and dried with
aluminum ingots before smelting. [50].
2) Grain refiner preparation: The grain refiner utilized
consists of rare earth elements, primarily La (lanthanum) and
Ce (cerium). The rare earth pieces are first sawed into small
segments to form intermediate alloy components. Due to the
tendency of rare earths to ignite during fine sawing,
continuous cooling with water is necessary. After sawing, the
rare earth segments are dried on the furnace surface to ensure
they are moisture-free before use. [50]–[53]. Commercial
cryolite slag salt, constituting 3% of the mass of the processed
aluminum in each batch, is utilized. Before addition to the
batch, this salt undergoes drying at 200°C to ensure it is free
from moisture [54], [55].
3) Laboratory instruments: Medium-frequency furnace:
This type of induction electric furnace is versatile, used for
quenching and tempering mechanical parts, melting metals,
and heating billets for forging and pressing. Medium-
frequency furnaces operate by inducing currents that generate
heat to melt metals efficiently.
4) Cooking pot: The cooking pot used in the process is a
graphite pot housed within an iron frame, designed for easy
handling and manipulation. The graphite pot serves to reduce
the iron content that may enter the liquid aluminum,
minimizing harmful effects as previously discussed.
5) Stirring, slag removal, and aeration equipment: All
equipment used for stirring, slag removal, and aeration is
coated with graphite and dried using a torch to eliminate
moisture. This precaution is crucial because liquid aluminum
is highly reactive to hydrogen gas.
6) Mold paint: For enhancing gloss and heat resistance,
mold surfaces are painted with materials such as graphite
powder, charcoal powder, or glass water. This coating aids in
releasing the cast aluminum from the mold surface
effectively.
1249
7) Pouring mold: Two types of molds are prepared for
research and comparison purposes: a large cast (~9kg) and a
small cast (~2kg). The mold cavities are coated with graphite
to easily remove solidified aluminum samples. Graphite's low
wettability with aluminum ensures the sample does not adhere
to the mold walls after solidification. The design of metal
molds for large and small batches is depicted in Fig. 1 [50].
Fig. 1 Metal molds are designed for large and small batches [50]
8) Gas: Industrial Argon (Ar) gas is used. Aeration with Ar
gas breaks down dendritic structures and facilitates de-
aeration. Combined with gentle stirring and oscillation of the
liquid-slag interface, degassing helps achieve uniform
composition throughout the batch.
9) Pouring tube and pouring bowl: During pouring, the
method involves bottom-up pouring through a tube. This
approach enhances productivity and improves the surface
quality of ingots by allowing the steel water surface to rise
steadily without splashing, which helps gases, impurities, and
slag float to the top more effectively. It also facilitates the
placement of an anti-reoxidation barrier.
10) Slag filter: A slag filter blocks light non-metallic
impurities that float on the surface during pouring. This
allows clean metal to flow through the gaps into the mold,
ensuring high-quality castings.
B. Melting and Denaturation Process
Here are the steps for arranging and drying ingredients in
the process:
1) Preparation of Utensils: Ensure the furnace, pot, and
other utensils are prepared and ready for use.
Placing Aluminum Billet: Place the chopped aluminum
billet into the pot. Add the cryolite so that when the metal
melts, a slag layer forms.
2) Drying Intermediate Ingredients: Place zinc,
magnesium, intermediate alloys, and cryolite salt on the
furnace lid for drying.
3) Melting Aluminum Billet: Once the aluminum billet has
melted completely, gradually add the required pure metals by
pressing them into the molten aluminum.
4) Flow of Intermediate Metals: Allow the intermediate
metals and alloys to flow entirely into the melt.
5) Aeration Process: Initiate aeration with low flow
(approximately 1 liter/min) for 2-3 minutes. Gentle stirring
should be applied to avoid strong surface fluctuations and to
ensure separation of liquid and slag phases, preventing
outside gases from entering.
6) Slag Removal: Remove the slag from the surface after
completing the aeration process.
7) Raising Heat: Increase the heat to reach the denaturation
temperature, compensating for any heat loss during operation.
These steps ensure the proper preparation, melting, and
treatment of ingredients to achieve consistent and high-
quality results in the aluminum processing procedure [36],
[50], [56].
8) Pouring Liquid Metal: Pour the metal at a temperature
of 750°C quickly to minimize heat loss and ensure a stable
flow of liquid metal.
9) Solidification: Allow the poured liquid metal to solidify
within the mold.
10) Mold Removal: Remove the mold from the solidified
metal after solidification.
11) Sample Preparation: Cut the solidified metal into
small samples suitable for microscopic examination and
durability testing.
12) Sample Grouping: Divide the samples into groups
based on whether they are unmodified or modified, and
further categorize them by whether they were cast in large or
small molds. This process ensures the samples are prepared
and categorized effectively for subsequent analysis and
testing to evaluate their microscopic structure and durability
characteristics.
C. Heat Treatment Process
To achieve uniform incubation, follow these steps:
1) Incubation Temperature: Set the temperature to 480°C
in the furnace.
2) Heat Retention: Maintain this temperature for 16 hours
to ensure consistent incubation.
3) Cooling Process: After incubating, allow the sample to
cool gradually in the same furnace. Ensure the sample
temperature stabilizes with the ambient temperature before
removing it from the furnace.
These steps ensure that the sample undergoes a controlled
incubation process, followed by a gradual cooling phase,
crucial for achieving uniform and reliable experimental
conditions.
III. RESULTS AND DISCUSSION
A. Influence of Alloying Elements
Adding copper to an Al-Zn-Mg alloy can significantly
enhance its strength. Here are the specifications with the alloy
element ratios considered in the annealed state:
1250
Zinc (Zn): 4.0-10.0%
Magnesium (Mg): Less than 6.5%
Copper (Cu): Approximately 3.0%
In the annealed state, the alloy typically exhibits the
following mechanical properties:
Ultimate Tensile Strength (durability limit): 200-290
MPa
Yield Strength (limit flow): 100-180 MPa
Elongation: Greater than 10%
The addition of copper within these compositional ranges
enhances the alloy's mechanical properties, particularly its
strength and durability while maintaining good elongation
characteristics suitable for various applications [57]–[59].
In heat-treatable Al-Zn-Mg-Cu alloys, the composition of
zinc (Zn) and magnesium (Mg) plays a critical role in
determining mechanical properties, especially in the heat
treatment process involving quenching and aging [60]–[62].
Zn and Mg are highly soluble in aluminum at high
temperatures and precipitate sharply during cooling,
significantly strengthening the alloy when quenched and
aged. Copper enhances the quenching effect but has minimal
impact on aging performance [63]–[65]. The highest
durability limit is typically achieved with around 2% copper,
increasing relative elongation. For alloys containing 6-7% Zn
and 2% Mg, increasing copper content to 3% further boosts
durability and significantly enhances fatigue resistance.
Higher levels (6-8%) increase durability but decrease alloy
ductility and impact toughness. Zinc also reduces resistance
to stress corrosion cracking more than magnesium. Content
ranging from 1.5% to 3% varies based on desired mechanical
properties. Higher magnesium levels above 2.5% notably
reduce alloy ductility. Industry guidelines set upper limits for
Zn and Mg to balance achieving the required strength against
potential reductions in ductility, impact toughness, corrosion
resistance, and fatigue resistance.
Understanding these relationships allows for optimizing
Al-Zn-Mg-Cu alloy compositions to meet specific strength
and performance requirements while managing potential
trade-offs in other mechanical properties [57], [66]–[73].The
Al-Zn-Mg-Cu alloy is a high-strength aluminum alloy with
versatile properties suitable for various industrial
applications. Comprising zinc (Zn), magnesium (Mg), and
copper (Cu), this alloy achieves remarkable strength. Its yield
limit is only 20-30 MPa lower than its durability limit,
surpassing common aluminum alloys like Duyra D16 in aging
conditions by 40-50%. It exhibits superior durability limit and
abrasion resistance compared to forged aluminum alloys such
as AK6 and AK8. This makes it ideal for applications
requiring high deformation capability while maintaining
durability. Zn and Mg are key alloying elements with high
solubility in aluminum. Their maximum solubility
concentrations in aluminum are 82% for Zn and 17.4% for
Mg, enabling effective strengthening through solid solution
strengthening and precipitation hardening mechanisms. The
alloy is suitable for deformation processing, surface heat
treatment, and cutting machining, enhancing its versatility
and applicability across various manufacturing processes.
Overall, the Al-Zn-Mg-Cu alloy represents a robust choice for
industries demanding high mechanical performance,
durability, and processing flexibility in aluminum
applications [74]–[80]. You seem to refer to the intermetallic
phases that form in Al-Zn-Mg-Cu alloys. These phases
determine the alloy's properties and heat treatment stability.
Here are some typical intermetallic phases found in these
alloys. The η Phase (MgZn2) phase forms primarily with
magnesium and zinc. It contributes to strengthening the alloy
through precipitation hardening mechanisms. The T-phase
(Al2Mg3Zn3) phase involves specific ratios of aluminum,
magnesium, and zinc. It also enhances the alloy's mechanical
properties, particularly in heat-treated conditions. The S
Phase (Al2CuMg) is formed with aluminum, copper, and
magnesium. This phase contributes to strengthening and
stability during heat treatment. The θ Phase (Al2Cu) phase
involves aluminum and copper, providing additional
strengthening through precipitation hardening. These
intermetallic phases strengthen the alloy and contribute to its
heat treatment stability by forming stable structures that resist
changes at high temperatures. Their presence and distribution
in the alloy matrix are crucial in achieving desired mechanical
properties such as strength, toughness, and corrosion
resistance in Al-Zn-Mg-Cu alloys.
TABLE II
PHASES IN AL-ZN-MG AND AL-CU-MG ALLOYS [13], [14], [16], [23]
Phase Al-Zn-Mg Alloy Al-Cu-Mg Alloy
θ
Al
2
Cu
Z
Mg
2
Zn
11
Cu
6
Mg
2
Al
5
S
-
Al
2
CuMg
T (τ) AlMgZn (Al2Mg3Zn3;
Al
6
Mg
11
Zn
11
; (Al, Zn)
49
Mg
32) CuMg4Al6
η (M, σ)
MgZn
2
Al
6
Cu
4
Mg
2
According to the research results of Isadare et al. [82], in
Al-Zn-Mg-Cu alloys, the morphology of the MgZn2 (η) phase
can vary significantly depending on the cooling rate: Slow
Cooling (a) and Fast Cooling (b). These differences in phase
morphology between slow and fast cooling rates impact the
mechanical properties and heat treatment response of the Al-
Zn-Mg-Cu alloy. Microstructure of Al-Zn-Mg-Cu alloy is
depicted in Fig. 2. At the same time, controlled cooling rates
can be used to manipulate the distribution and size of these
intermetallic phases, thereby optimizing the alloy's
performance characteristics for specific applications.
a) Slow cooling
b)
F
ast cooling
Fig. 2 Microstructure of Al-Zn-Mg-Cu alloy [82]
According to Fan et al. [73], various intermetallic phases
can form during solidification in Al-Zn-Mg-Cu alloys and
influence the alloy's properties. SEM image of Al-Zn-Mg-Cu
alloy in the cast state is illustrated in Fig. 3. Here are some
typical intermetallic phases: MgZn2 (η) Phase; Al2Mg3Zn3 (T)
Phase; AlCuMg (S) Phase; Al2Cu (θ) Phase; Al7Cu2Fe and
Al13Fe4 Phases; Mg2Si Phase. Research by Fan and others
highlights the complex microstructure of Al-Zn-Mg-Cu
1251
alloys, emphasizing the presence of these intermetallic phases
and their distribution within the alloy matrix. Understanding
their formation and morphology is essential for tailoring alloy
compositions and processing conditions to achieve desired
mechanical and performance characteristics.
Fig. 3 SEM image of Al-Zn-Mg-Cu alloy in the cast state [73]
In Fig.4, the ratio of alloying elements, particularly the
Mg/Zn ratio, significantly influences the formation of the
MgZn2 phase, which acts as the primary chemical stabilizer in
the Al-Zn-Mg-Cu aluminum alloy system [81]. Here are key
points about the alloy's composition and properties. Zn and
Mg are highly soluble in aluminum at high temperatures but
precipitate sharply upon cooling, enhancing the alloy's
strength significantly through quenching and aging processes.
Alloys with Zn content ranging from 5.0% to 9.0% show
optimal durability when quenched. Copper enhances the
quenching effect but has minimal impact on aging
performance. Higher Zn content (6-8%) increases the
durability limit of the alloy but reduces its ductility.
Magnesium content typically ranges from 1.5% to 3%,
depending on the desired mechanical properties. Above 2.5%,
magnesium notably decreases the alloy's ductility. These
insights underline the importance of precise control over alloy
composition to tailor mechanical properties to specific
application requirements in various industrial sectors.
Fig. 4 Al-Cu-Mg alloy ternary phase diagram; Al-Cu-Zn and Al-Mg-Zn at 400
o
K [81]
In the Al-Zn-Mg-Cu alloy system, the addition of copper
(Cu) significantly impacts the alloy's mechanical properties.
Here are key points about the role of Cu and the resulting
properties of the alloy. As Cu content increases in the Al-Zn-
Mg-Cu system, some Zn in the T phase (Al2Mg3Zn3) is
replaced by Cu. All three intermetallic phases (including the
S phase Al2CuMg and θ phase Al2Cu presented in Fig. 4)
remain stable during aging. Adding Cu to the Al-Zn-Mg alloy
increases the strength by approximately 20-50 MPa. With
alloy compositions (Zn: 4.0-10.0%, Mg < 6.5%, Cu: 3.0%) in
the annealed state, the durability limit ranges from 200-290
MPa, with a flow limit of 100-180 MPa and elongation more
significant than 10%. The durability limit reaches its peak
when the Cu content is about 2%, enhancing relative
elongation. For alloys containing 6-7% Zn and 2% Mg,
increasing Cu content to 3% further boosts the durability limit
and significantly improves the fatigue limit.
B. Influence of other Alloying Elements
The Al-Zn-Mg-Cu system is often alloyed with additional
elements such as Mn, Cr, Zr, and Ti. The elements Cr, Mn,
and Zr accelerate the decomposition process of solid
solutions, make particles more minor, and raise the
recrystallization temperature. Therefore, adding Cr, Mn, and
Zr stabilizes and causes a structural stabilizing effect shown
in the Al-Zn-Mg-Cu-Sc-Zr alloy phase diagram (Fig. 5) [81].
In addition, transition metal elements improve granularity
upon crystallization and enhance corrosion resistance under
stress. When present in alloys, transition metals such as Cr,
Mn, and Zr enormously change the organization in terms of
phase distribution and the shape of grain boundaries. On the
1252
one hand, transition metal elements promote the secretion of
η, T, and S phases from the α solid solution in a fine, dispersed
state; on the other hand, they create a jagged and elongated
shape on the boundary. the total boundary of the α particle.
With this characteristic organization, the intensity of
corrosion cracking is reduced, and the rate of development of
corrosion cracks along grain boundaries is also hindered,
preventing corrosion cracking. Manganese increases the
quenching effect to about 60 MPa and the aging effect to
about 50 MPa. The maximum processing heat effect is nearly
100 MPa with 0.6% Mn. With 0.3% Cr, the alloy structure
will not recrystallize, and fatigue strength will also increase
significantly.
In the Al-Zn-Mg-Cu alloy system, additional alloying
elements such as Mn, Cr, and Zr significantly enhance the
alloy's properties. Here's how these elements affect the alloy.
Manganese increases the quenching effect by about 60 MPa
and the aging effect by about 50 MPa. The maximum
processing heat effect is nearly 100 MPa with 0.6% Mn. With
0.3% Cr, the alloy structure resists recrystallization,
significantly increasing fatigue strength. Along with Cr and
Mn, Zr accelerates the decomposition process of solid
solutions, making particles smaller and raising the
recrystallization temperature. This has a stabilizing effect and
enhances structural stability.
When adding Cr, Mn, and Zr, the alloy improves
granularity upon crystallization and enhances stress corrosion
resistance. Promote the secretion of η (MgZn2), T
(Al2Mg3Zn3), and S (Al2CuMg) phases from the α solid
solution in a fine, dispersed state. In Fig.6, the microstructure
of the alloy at annealing showed the S phase [81]Creating
jagged and elongated grain boundary shapes reduces the
intensity of corrosion cracking and hinders the development
of corrosion cracks along grain boundaries. Including Mn, Cr,
and Zr in the Al-Zn-Mg-Cu alloy system demonstrates a
strategic approach to optimizing the alloy's performance for
demanding industrial applications, where enhanced strength,
stability, and corrosion resistance are critical.
Fig. 5 Al-Zn-Mg-Cu-Sc-Zr alloy phase diagram [81]
Iron and silicon impurities in aluminum alloys lead to the
formation of insoluble residual intermetallic compounds such
as Al6FeCuZn, Al7Cu2FeZn, Al6MnFeCuCr, and Mg2Si.
These compounds precipitate from the liquid phase during
crystallization, forming coarse particles. These particles act as
stress concentrators, which can initiate microscopic cracks,
reducing the alloy's ductility, impact toughness, and fatigue
resistance. Hence, controlling impurities is crucial.
Fig. 6 Microstructure of alloy when annealed: alloy with Cu content < 2% (c; e) and with Cu content > 2% (d and f) [81]
C. Research on Modification with Rare Earths
The research group of Nie et al. [32] studied the influence
of the rare earth element erbium (Er) on some aluminum
alloys' microstructure and mechanical properties, including
Al and Al-Mg alloys. In Fig. 7, increasing Er increases the
hardness and strength due to forming the Al3Er phase. As
shown in Fig. 8, in Al – Cu alloy, the alloy's branch structure
1253
is finely chopped when Er is added to the alloy. The
recrystallization temperature increases, although the strength
remains unchanged. In Fig.7 and Fig.8, the formation of the
Al8Cu4Er phase has a low melting temperature, which reduces
the formation of the CuAl2 phase - the main stable chemical
element.
a)
b)
Fig. 7 Microstructure of A l-Mg - Er alloy after casting. Al – 5Mg (a), Al –
5Mg – 0.5Er (b) [32]
a)
b)
Fig. 8 Microstructure of Al – Cu alloy after casting. Al – 4Cu (a), Al – 4Cu
– 0.2Er (b) [32]
Al – Zn – Mg and Al – Zn – Mg – Cu: The tensile strength
σb and yield stress σ0.2 of the Al – Zn Mg alloy increase
rapidly when adding Er, while the elongation decreases
slightly, the strength increases the most when adding 0.1% Er.
Increasing the amount of Er will continue to increase strength,
but the increase will decrease. The microstructure becomes
much smoother, and the branch structure almost disappears.
This can be explained by the formation of a small, fine Al3Er
phase, which plays a role as a crystallization nucleation zone.
Similar results for Al - Zn - Mg - Cu alloy.
According to He et al. [83], the result presented the
influence of Sc and Zr on Al-Zn-Mg-Cu alloy. By OM, which
is shown in Fig. 9, the microstructure of this alloy is
presented. The alloy is completely melted and then poured
into a cold mold. After being thermally homogenized at 400oC
for 5 hours, 450oC for 20 hours, and 470oC for 12 hours, the
billets were quenched in water and kept at 420oC for 2 hours,
then rolled into 4 mm thick sheets. The tensile test specimen
was heat treated at 470oC for 2 hours, kept at 480oC for 1 hour,
and finally quenched in water. The study results are
summarized as follows: Adding 0.18% Zr to Al - Zn - Mg -
Cu alloy castings will reduce grain fineness to a certain extent,
with an average grain size of 80 - 130 μm. The grain structure
is a mixture of fibers and a small number of coaxial grains.
Partial recrystallization occurs during hot rolling. Strength
increased by 99 MPa, and elongation increased by 3.6%.
Adding 0.18%Sc to Al - Zn - Mg - Cu composite castings
reduces grain size. The microstructure consists of coaxial
grains, and recrystallization occurs during hot rolling. Tensile
strength increased by 166 MPa, and elongation increased by
9.2%. The main stabilization mechanism is the excreted Al3Sc
phase, dislocation blocking elements, and dislocation cutting
of dispersed particles. Given both Sc and Zr, strong grain
reduction and small equiaxed grains are obtained. The grain
structure includes complete fiber organization; strength
increases to 148.83 MPa and elongation up to 7.67%. Strength
and elongation increase with the addition of Sc. The main
stabilization mechanisms are particle smoothing, stabilization
of the matrix structure, and shear deflection of dispersed
particles. If both Zr and Sc are added simultaneously, the
grain size will be finer many times due to the Al3(ScxZr1-x)
phase secretion mechanism on the Al-alpha base, shown in
Fig. 10.
Fig. 9 Microscopic image (a) Al-Zn-Mg-Cu; (b) Al-Zn-Mg-Cu-0.18Zr; (c) Al-Zn-Mg-Cu-0.18Sc; (d) Al-Zn- Mg-Cu-0.30Sc-0.18Zr [83]
Fig. 10 SEM image and EDX analysis [83]
Zr occupies most of the center of the particles, while Sc
surrounds the outside of the particles. The chemical formula
of the known compound Al3(ScxZr1-x). The Al3Zr phase is
secreted first. Then, the Al3Sc phase is secreted on the Al3Zr
surface; then the Al3Zr phase is secreted again on the Al3Sc
substrate. Just like that, many layers will eventually overlap
with each other. The second phase is the seed center of α –Al
during solidification. When adding both Zr and Sc to the
above alloy at the same time, smooth, round particles are
formed, causing both the strength and elongation of the Al
alloy to increase [84]–[87].
1254
TEM analysis results show that Fig. 11a structure contains
Zr. Many dispersed phases are clearly visible in both Fig. 11a
and Fig. 11b. When adding Zr and Sc simultaneously, the
secretion of the phase increases and reduces the size of the
secreted phase. Fig. 11c showed coffee bean-like deformation
in contrast to the phase and EDP secretion in Fig. 11d [83].
Fig. 11 TEM of Al3Zr, Al3Sc và Al3(Sc, Zr) in studying alloy: (a) Al-Zn-
Mg-Cu-0.18%Zr; (b) Al-Zn-Mg-Cu-0.18%Sc; (c) Al-Zn-Mg-Cu-0.2%Sc-
0.18%Zr; (d) EDP of (c) [83]
In Fig. 12, For Zr-containing alloys, recrystallization
occurs where the density of the Al3Zr dispersed phase is low
during deformation and heat treatment [83]. This
phenomenon is related to Zr in grains or grain boundaries;
where Zr reduces the amount of solute available for phase
secretion, the Al3Zr dispersed phase will be depleted.
Therefore, the weak recrystallization nucleation inhibits
phase secretion during the hot rolling and aging process.
Recrystallization occurs first in the Al3Zr-poor region. The
grain structure with the addition of 0.30% Sc and 0.18% Zr
will form a complete fiber, and there will be no
recrystallization during the hot rolling process [50], [83], [85],
[86]. Studies on the effects of Sc and Zr to modify Al - Zn -
Mg alloy, creating the Al3(Sc1-xZrx) phase on the Al(α) base
[35,43,45,46]. As observed in the bright area of the TEM
image (Fig. 13(a-c)), in the early deformation stage (ε = 0.69
or 1.10), secondary circular Al3(Sc1-xZrx) nanoparticles are
located at the boundary. The grain boundaries and the
contrasting Ashby – Brown deformation zone (indicated by
the blue arrow) show that these grains maintain a binding
relationship with the Al(α) matrix. Increasing the strain level
to 2.4, the contrast of the irregular deformation region of
Al3(Sc1-xZrx) nanograins in the highlight image suggests that
as their size increases, the connectivity gradually decreases.
Fig. 12 Microstructure of alloy after hot rolling: (a) Al-Zn-Mg-Cu-0.18%Sc; (b) Al-Zn-Mg-Cu-0.18%Zr; (c) Al-Zn-Mg-Cu-0.1%Sc-0.18%Zr [83]
In addition, combination of the results from the bright
region and dark central superlattice in the TEM images of sub-
nano Al3(Sc1-xZrx) particles by high ductility reflection (110)
(Fig. 13d and Fig. 13g), it is possible that the Al3(Sc1-xZrx)
particles strongly hinder dislocation movement (indicated by
yellow arrows) and grain/subgrain boundary displacement
(indicated by white arrows). Furthermore, due to the strong
influence of dislocations or grain boundaries in the
deformation zone, the Ashby Brown contrast of Al3(Sc1-
xZrx) elements along with the precipitation process occurs and
thus, the dislocation density decreases sharply (Fig. 13(e&f)
and Fig. 13 (h&i)) [88]. It can be concluded that during
deformation, Al3(Sc1-xZrx) grains play an important role in
hindering dynamic recrystallization and grain enlargement
[89].
During high ductility deformation, the grain size in the Al
Zn Mg alloy increased rapidly to 15.4 μm, and small
subgrains were formed within the initial recrystallized grains.
Compared with the fine grain surface in the Al –Zn – Mg alloy
(Fig. 14(a&b)), grain boundary sliding can be observed in the
Al Zn – Mg Sc Zr alloy. At the peak stress stage of
deformation, the deformed grains elongate with the rolled
texture having recovered and partially recrystallized (Fig.
14(c&d)).
Fig. 13 TEM image of microstructure of Al3(Sc1-xZrx) nanoparticles during
high plasticity deformation at 500oC and 0.01s-1, ε=0.69; (a) and (d) bright
area image, (g) dark area image with superlattice in the center, ε=1.10; (b)
and (e) bright area images, (h) (110) dark area images with super gill in the
center; ε=2.40: (c) and (f) bright area images, (i) (110) dark area images with
superlattice in the center [88].
1255
Furthermore, the fine particles with rolled texture
accumulate a lot of energy, especially the S and Copper faces,
which gradually escape and begin to form surrounding parent
particles for nucleation (Fig. 14d&e), due to the accumulated
energy. higher deformation storage. When straining to a true
strain of 1.10, it is clear that large regions of recrystallized
grains with S and Brass faces have formed and the grain size
continues to increase (Fig. 14e). At the softening stage
(ε=2.40), the normal organization consists of extremely fine
and uniform micro-sized recrystallized particles (particle size
<5μm) with random arrangement (Fig. 14f&g). The fine grain
size can be attributed to the Zener stopper from the secondary
nano Al3(Sc1-xZrx) grains at the demigrain or grain
boundaries, and the uniform grains from the generation of the
selective limit of the nucleation center and growing by nano
Al3(Sc1-xZrx) particles in high ductility deformation.
Therefore, secondary nano Al3(Sc1-xZrx) particles are
important in accelerating grain boundary deformation and
achieving high ductility at high strain rates. The authors
concluded that secondary Al3(Sc1-xZrx) nanoparticles only
affect the dynamic softening deformation mechanism of the
Al - Zn - Mg alloy. During the deformation hardening stage,
the studied alloy is mainly controlled by the shear-driven
dislocation creep mechanism. During the dynamic softening
stage, the grain boundary sliding mechanism is dominant in
the Al - Zn - Mg alloy with secondary Al3(Sc1-xZrx)
nanoparticles [50], [88].
Fig. 14 EBSD, TEM, and SEM microstructure images of the two alloys under
different real deformations at 500oC and 0.01s-1, the Al-Zn-Mg alloy
deformed to destruction (ε~1.10) : (a) EBSD mapping, (b) SEM fracture
surface; Al-Zn-Mg-Sc-Zr alloy: (c) SEM fracture image, EBSD arrangement
map (d) ε = 0.69, (e) ε=2.40, (g) color equivalent to arrangement different in
Figure 2.5 (d)-(f), the strength of the color reflects the deviation from the ideal
arrangement [88]
According to Zhang et al. [88], Al-0.3Mg alloy was cast
into 0.1 to 0.5 RE-modified specimens according to the
conventional method. The study results are shown in Fig.15:
The α-Al phase is the dominant phase with a fractional peak
in all the alloys studied. The Al8Mg5 and FeAl3 phases are
found in Al-3.0%Mg alloys with 0 and 0.1% RE additions.
The intensity of the Al4Ce/Al4La phases increased while the
Al8Mg5 and FeAl3 phases disappeared when the RE content
increased to 0.2%. Thus, Al-Mg alloy can effectively remove
Fe impurities and limit the formation of Al8Mg5. The addition
of lanthanum and cerium can improve the solid solubility of
magnesium into aluminum and reduce the formation of
compounds Al8Mg5.
Fig. 15 Sample of XRD [88]
Through the EDS imaging results in Fig. 16, with the Al-
0.3Mg alloy without RE added, the oxides make up the
majority of the alloy weight, but when RE is added, the oxides
are no longer found, proving that rare earth elements are
capable of Deoxidation on molten aluminum base. The
addition of rare earths refines the particle size. The second
phases are concentrated at the grain boundaries and dispersed
within the grains during solidification. La and Ce promote
branch growth. Al4Ce and Al4La will form in molten
aluminum and play a role in increasing the heterogeneous
particles. In addition, increasing RE reduces porosity,
possibly because La and Ce have reacted with hydrogen in
molten aluminum to create stable CeH2 and LaH2 to eliminate
air bubbles and porosity in the alloy. Adding Lanthanum and
Cerium improves the tensile strength of Al- 0.3Mg alloy. With
the addition of rare earths, the tensile strength reaches its
highest value at 0.3%RE. At that time, the tensile strength
increased from 177MPa to 220MPa. However, the elongation
is significantly reduced compared to 0.2 %RE.
From the studies conducted, it has been observed that Al-
Zn alloys are commonly modified with Zr or a combination
of Zr and Sc. Chinese researchers have also explored using
the rare earth element Er to modify Al-Zn alloys. Adding
small amounts of rare earth or transition elements helps refine
the alloy's grain structure. This is achieved through
intermetallic phases that prevent the formation of detrimental
phases such as CuAl₂. These intermetallic phases tend to
concentrate at the center or surround the grains, inhibiting
their growth. As a result, the alloy exhibits a finer grain
structure, and the dendritic structure diminishes, leading to
improved ductility and elongation.
1256
Fig. 16 EDS (a): 0%RE and (b) 0.2%RE [88]
D. Analysis of Structure Changes in Al-Zn-Mg-Cu Alloy
after Casting
Upon examining Sample 1, it is evident that it does not
exhibit the coarse grains typically associated with La or Ce,
nor does it display large or uneven grains. The sample's
microstructure, characteristic of the post-molding state, shows
a tree-branch shape. However, after introducing La and Ce as
grain-refining agents, the grain size significantly decreased
compared to the unmodified sample. The grains became more
uniform and finer, with sizes comparable to samples that do
not contain La or Ce. Specifically, the average grain size was
reduced by 88% compared to the sample without La and Ce,
suggesting that these two elements significantly influence the
casting process. This finding aligns with the theoretical
framework proposed by author Zhang Xin [5].
In Fig. 17, the alloy's microstructure is depicted, where the
brown background phases represent the aluminum matrix.
The black phases observed at the grain boundaries are likely
the intermetallic phases, which could be either MgZn₂ or
Mg₃Zn₃Al₂. These intermetallic phases play a crucial role in
the mechanical properties and overall behavior of the alloy
[3]. Additionally, there is a contribution from intermetallic
phases formed between Al, La, and Ce. This contribution will
be analyzed in the subsequent sections through the results of
XRD diffraction analysis and SEM images of the sample.
These analyses will provide detailed insights into the presence
and distribution of these intermetallic phases and their impact
on the alloy's microstructure and properties.
Without La and Ce: The distance between the branches of
the alloy after casting without La and Ce is approximately 65
µm. With La and Ce: This distance is reduced to 40-50 µm for
samples containing La and Ce. This reduction in branch size
can be attributed to rare earth elements (RE) 's role in refining
the alloy's microstructure. RE elements influence the grain
structure, leading to finer branches. However, with the
addition of RE elements in the studied alloy, intermetallic
phases involving Al, and these elements may form. This
hypothesis will be further validated through additional
analyses such as XRD and SEM [50], SEM and EDS analyses
of post-cast samples, as shown in Fig. 18 and Fig. 19, clearly
evidence the formation of intermetallic phases between rare
earth elements (RE) and aluminum.
The analysis of SEM and EDS, images at grain boundaries,
confirms the presence of La and Ce within the composition at
these boundaries. The role of these rare earth elements is
significant, as they combine with other elements to form
intermetallic phases that inhibit grain growth, thereby refining
the alloy's microstructure.
1257
Sample 1
Sample 2
Fig. 17 Microscopic image results of the sample in the cast state, x200 [50]
In Fig. 18, it is crucial to conduct a more detailed SEM
analysis to further understand the distribution of rare earth
elements. This will determine whether the rare earth
elements are primarily concentrated at the grain boundaries
or if they are also present in the grain centers. Such detailed
analysis will provide insights into the effectiveness of rare
earth elements in modifying the alloy’s microstructure and
enhancing its properties [50]. According to [50], [90]–[92],
the detailed SEM and EDS analyses reveal that rare earth
elements are primarily concentrated at the grain boundaries.
Consequently, intermetallic phases involving rare earth
elements, specifically Alx (La, Ce), are predominantly
found at these grain boundaries. These intermetallic phases
play a crucial role as blockers, effectively preventing the
grain enlargement process and contributing to the
refinement of the alloy’s microstructure (Fig. 19). By
concentrating on the grain boundaries, these Alx (La, Ce)
phases hinder the movement and growth of grains, thereby
enhancing the mechanical properties of the alloy. This grain
boundary pinning mechanism ensures that the grains remain
fine and uniformly distributed, leading to improved
ductility and strength [50].
Sample 1
Sample 2
Fig. 18 Analysis of the microstructure of the sample after casting (SEM
image) [50]
Analyzing the sample results after adding denaturants, it is
evident that rare earth elements are distributed across the
entire surface of the phases, including both the background
and the phase boundaries. The rare earth elements used as
denaturants, such as La and Ce, do not dissolve in the liquid
metal due to their higher melting temperatures than the
denaturation temperature. Instead, they contribute to reducing
energy required to form crystallization nucleation and
crystals. Introducing rare earth elements into the Al-Zn-Cu-
Mg alloy acts as foreign crystallization seed centers, resulting
in a finer and smoother grain structure. The rare earth
elements effectively inhibit the growth of primary crystals by
providing numerous nucleation sites, leading to a more
refined microstructure. This process enhances the mechanical
properties of the alloy by creating a more uniform and fine-
grained structure [50], [90], [93]–[96].
1258
Fig. 19 SEM and EDS analysis of sample 2 [50]
Fig. 20 analyzes the sample's microstructure after uniform
incubation at 480oC for 16 hours. It shows that the
characteristic structure in the molded state (tree branch
structure) of both sample 1 and sample 2 is no longer there.
The black phases in the grain boundary region in the cast state
of both samples have also gradually exuded into the aluminum
matrix. This shows that this annealing regime is effective for
7475 alloys as demonstrated by the results of author
Guillaume Fribourg [12]. In Fig. 20, the analysis of sample 1
without La and Ce reveals the following characteristics: These
observations suggest that adding La and Ce is crucial in
limiting grain growth during the heat treatment process. The
formation of intermetallic phases involving rare earth
elements effectively inhibits the growth of coarse grains,
resulting in a finer and more uniform microstructure. This
phenomenon is consistent with the documented findings by
author Guillaume Fribourg, highlighting the effectiveness of
rare earth elements in enhancing the microstructural stability
and mechanical properties of the alloy after heat treatment
[12]. According to [9], SEM results will provide a more
detailed analysis due to their higher magnification capabilities
than optical microscopes to clarify the presence of sub-
particles in the boundary area with sizes ranging from 1-3 µm.
These sub-particles are crucial for understanding the
microstructural changes induced by rare earth elements (La,
Ce). Regarding the black phases identified as MgZn₂ or
Mg₃Zn₃Al₂, which initially exist in the grain boundary region
of the sample in the cast state and have subsequently diffused
into the aluminum matrix to form intermetallic phases with
elements like Al, La, and Ce, this transformation will be
thoroughly elucidated through XRD analysis results.
Sample 1
Sample 2
Fig. 20 Microstructure after annealing, x200 [50]
From the X-ray diffraction (XRD) pattern analysis in Fig.
21, it could be seen in Sample 1 (without La or Ce): In the
post-cast state, no phases are observed that connect the
elements Al, Zn, Mg, and Cu. The surface of the alloy
primarily consists of aluminum. This indicates that without
rare earth elements, the alloy does not form significant
intermetallic phases involving these elements. In sample 2
(containing La and Ce), in the post-cast state, a small amount
of AlCe phases is detected in sample 2 [50]. This suggests that
adding an intermediate alloy between Al and rare earth
elements (La, Ce) effectively modifies the Al-Zn alloy. The
XRD results also show the presence of phases involving Mg-
Al-Cu elements, such as Mg₂Al₃ and S-(Al₂CuMg), indicating
that rare earths influence the phase formation of other
elements in the Al-Zn alloy. After annealing, additional AlCe₃
phases appear in sample 2. This indicates that the annealing
regime at 480°C for 16 hours effectively promotes the
dispersion and interaction of rare earth elements with the
aluminum substrate.
1259
Fig. 21 XRD diffraction results, (2.1) Sample 1 in the post-cast state (1.1) Sample 2 in the post-cast state; (1.2) Sample 2 is in the state after uniform annealing [50]
This aligns with findings from Guillaume Fribourg,
suggesting that this annealing process optimally distributes
rare earth elements on the aluminum surface. The XRD
analysis confirms that incorporating rare earth elements (La,
Ce) into the Al-Zn alloy leads to beneficial intermetallic
phases, such as AlCe and AlCe₃. These phases enhance the
alloy's properties by influencing phase formation and
distribution, particularly after annealing treatments. The
results underscore the suitability of using intermediate alloys
and specific annealing conditions to effectively modify and
improve the microstructure and performance of Al-Zn alloys
[40], [81], [97].
IV. CONCLUSION
The article compiles research demonstrating the significant
role of alloying elements, particularly La and Ce, in shaping
the microstructure of Al-Zn alloys. Intermetallic phases
formed with La and Ce act as heterogeneous nucleation
centers during crystallization or inhibit particle growth in the
alloy. For instance, the Al11Ce3 phase, present in intermediate
alloys, resembles the α-Al base phase and promotes the
formation of fine grains. This results in a refined
microstructure compared to alloys without La and Ce directly
added. Al11Ce3 phase acts as a seed center for crystallization,
facilitating the formation of refined grains. While the Al3La
phase prevents the development of α-Al matrix phases,
contributing to microstructural stability and refinement. The
article highlights the effectiveness of uniform annealing
processes for Al-Zn-Mg-Cu alloys containing La and Ce grain
refiners. This annealing method eliminates the casting-
induced microstructure, creating a more uniform
organization. This uniformity is advantageous for subsequent
deformation processes, enhancing the alloy's mechanical
properties and structural integrity. Overall, the research
underscores how the strategic addition of La and Ce, both
directly and as intermediate alloys, influences the
microstructural evolution of Al-Zn alloys. These elements
play pivotal roles in grain refinement and phase control,
ultimately improving the alloy's performance in industrial
applications.
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