Content uploaded by Amelia Almeida
Author content
All content in this area was uploaded by Amelia Almeida on Dec 19, 2017
Content may be subject to copyright.
page 1 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Research Article Open Access
Scifed Journal of metallurgical Science
Almeida A, SF J Met Sci, 2017, 1:1
Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying
*1Almeida A, 2Carvalho PA, 3Vilar R
*Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisbon, Portugal
Keywords
Al-Cr alloys; laser alloying; transmission electron
microscopy; solidification microstructure; intermetallics;
hardness
1. Introduction
Aluminium-transition metal alloys present
excellent stability when exposed to moderately high
temperatures, so they are promising materials for many
automotive and aerospace applications where high strength
light alloys with an extended working temperature range
are required [1]. Among the alloying elements capable
of generating dispersion-hardened Al alloys stable at
relatively high temperatures (alloying elements with low
solubility and low diffusivity in solid Al), Cr is one of
the most interesting [2, 3]. Previous work by the authors
[4, 5] demonstrated the possibility of producing surface
alloys with improved hardness and corrosion resistance
by laser alloying Cr into Al or 7175 Al-alloy substrates.
Luft et al. [6]. Studied the laser surface melting of an Al-6
wt.% Cr alloy prepared by casting. In the cast condition
the alloy’s microstructure consisted of blocky particles of
laser melting the material presented very fine particles
with a dendritic morphology, which were considered to be
Al7
et al. [7]. Investigated the microstructure of an Al-15 at.%
Cr alloy treated by laser surface melting and reported that
it consists of Al7Cr and Al11Cr2 intermetallic compound
*Corresponding author: Almeida A, Center of Physics and Engineering
of Advanced Materials, Instituto Superior Tecnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisbon, Portugal. E-mail: amelia.almeida@
tecnico.ulisboa.pt Tel No: 351-218419176
Received July 31, 2017; Accepted September 22, 2017; Published
September 30, 2017
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys
Synthesised by Laser Alloying. SF J Met Sci 1:1.
Copyright:© 2017 Almeida A. This is an open-access article distributed
under the terms of the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium,
provided the original author and source are credited.
Abstract
Al-Cr surface alloys were developed by laser surface alloying of an Al-Cr powder mixture into a commercially
pure Al substrate. The alloying treatment was followed by laser remelting aiming to homogenise the alloys and refine
their microstructure. The surface alloy contains 14.9±5.5 wt.% Cr and its microstructure consists of long faceted
particles of the hexagonal Al4
remelting a columnar microstructure of Al4
on approaching the surface a transition from columnar to equiaxed solidification occurs leading to a microstructure
consisting of equiaxed cells comprised of extremely fine Al7
microstructure forms because Al4Cr is only partially remelted and the surviving particles remaining in the liquid act as
heterogeneous nuclei for the equiaxed cells. This transition from a columnar to equiaxed structure is favoured by larger
constitutional undercooling. As the remelting speed increases, the equiaxed cells structure becomes finer and harder as
the volume fraction of intermetallic compounds increases. Yet, the alloys show considerable toughness demonstrating
page 2 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
Luft et al. [6], but no evidence for the identification of the
intermetallic compounds is provided.
The interpretation of the microstructures formed in
Al-Cr alloys is difficult due to the controversy that remains
in what concerns the configuration of the Al-rich region
of the Al-Cr equilibrium phase diagram [8-11]. In the
configuration proposed by Murray [10] the intermetallic
compound Al11 Cr2 forms by a peritectic reaction, while
Neto et al. [11] consider that this compound does not exist
as an equilibrium phase and Mahdouk et al. [8, 9] suggest
that it is a high temperature phase stable only in the range
reported to form by a peritectic reaction in the phase
diagram published by Murray [10], while Neto et al. [11]
and Mahdouk et al. [8] proposed an eutectic reaction instead.
The work of Due et al. [12] based on DSC experiments,
confirmed the eutectic nature of the invariant reaction, also
reported by the present authors in previous work [13]. More
alloys based on planar solidification experiments. The fact
that the invariant reaction temperature is extremely close to
the melting point of Al makes the experimental work with
the accuracy necessary to solve the ambiguity extremely
difficult and discrepancies in results occur. Nevertheless,
this alloy system continues to attract researcher’s attention
and theoretical and experimental studies on phase diagram
assessment and phase structure determinations have been
reported recently [15-17].
Only very few papers exist on the microstructure
and properties of Al-Cr alloys prepared by laser surface
treatment techniques. On the contrary, considerable
research has been carried out on materials produced by
rapid solidification methods [18-26]. These studies have
concentrated on one hand, on the Cr supersaturation
22] and, on the other, on the structure and stability of
several crystalline [20,24] and quasicrystalline phases
[24-26] formed during rapid solidification. Warlimont
et al. [18,19] observed an equiaxed structure with
radially distributed petal-like particles of an intermetallic
compound in Al-Cr alloys containing up to 7.1 at.% Cr,
which they designated spherulites. The authors suggested
that this structure forms in the solid state, by discontinuous
precipitation of a highly supersaturated solid solution that
result from the rapid solidification process. By contrast,
Benderski et al. [20] suggested that the spherulites formed
directly from the liquid and consist of a core of very fine
particles of several unidentified intermetallic compounds
on which larger crystals of Al7Cr have nucleated and
grown. The primary solidification origin of the spherulites
was corroborated by Sridhara et al. [27] but the authors
showed that spherulites are formed of a quasicrystalline
core and Al7Cr arms. Inoue et al. [25] considered that the
spherulites formed by rapid solidification consist initially
of a quasicrystalline phase, which decomposes into the
equilibrium intermetallic compounds Al7Cr and Al11Cr2
by heat treatment at temperatures between 625 and 644
rapid solidification methods and by casting. Cast alloys
containing up to 15 at.% Cr consist of Al7
while in more concentrated alloys Al4Cr and Al7Cr form,
though no microstructural study is presented. In rapidly
solidified alloys with up to 15 at.% Cr quasicrystalline
which were replaced by an Al4
more concentrated alloys. The authors also reported that
the quasicrystalline phase transforms to Al7Cr by heating
11Cr2 as proposed by Inoue et al.
[25], while Rosen et al. [26] confirmed the conclusions of
Swamy et al. [24] but considered that the decomposition
product is Al11Cr2 instead of Al7Cr.
From the above, it is clear that the microstructure
and properties of Al-rich Al-Cr alloys remain largely
unknown. It is well known that laser melting processes
lead to cooling and solidification rates that are
intermediate between conventional casting and rapid
solidification methods. Solidification hardly ever occurs
under equilibrium and, in particular, peritectic reactions
are often suppressed. Besides, due to the geometry of the
process and the shape of the melt pool, the temperature
gradient at the solid-liquid interface and the solidification
rate vary during solidification, often leading to transitions
in the morphology of the resulting microstructure. As
laser-based additive manufacturing methods are rapidly
gaining importance in industry and the final properties
and defects of manufactured parts depend critically on
the microstructure, the knowledge and control of the
microstructure formation is of utmost relevance.
The present paper reports results of a study of the
microstructure and mechanical properties of Al-Cr alloys
with compositions between 10 and 20 wt.% Cr synthesised
by laser surface alloying. Based on the results achieved
alloys extensive to other alloys with similar constitution is
devised.
page 3 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
Figure 1: a) Cross-section of Al-Cr alloy in the as-alloyed condition. b)
2. Experimental
The surface alloys were prepared either by direct
laser alloying or using a two-step process whereby an alloy
with suitable composition prepared by direct laser alloying
its structure and properties. The laser treatments were
carried out by the blown powder technique, using a CO2
laser at a laser beam power of 2 kW (leading to a power
density of 1.5x105 W/cm2), on a substrate of a commercial
purity Al, previously sand blasted, degreased and cleaned.
Alloying was carried out by injecting a mixture of 25 wt.%
Cr and 75 wt.% Al high purity powders into the melt pool
mass flow rate of 0.03 g.s-1. To reduce moisture, which
often results in the formation of pores due to hydrogen
release [28], the powder and the substrates were dried in
an oven at 80 °C for 12 hours just before the experiments.
Oxidation was prevented by blowing Ar gas over the melt
pool during the laser treatment. Due to the high melting
temperature of Cr a long interaction time and a high power
density are required in order to melt the Cr particles and
produce relatively homogeneous alloys. A scanning speed
(Vs
s, where d is the laser beam diameter), was
used. Surface coverage was accomplished by overlapping
consecutive laser alloyed tracks by 50% of their width.
Since the microstructure depends on the solidification rate,
remelting at different scanning speeds allowed producing
a wider variety of microstructures. Therefore, remelting of
the alloyed layers was carried out at scanning speeds of 5,
10, 20 and 40 mm/s in a direction perpendicular to the one
previously used for alloying.
by optical microscopy, scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and
X-ray diffraction (XRD). Metallographic samples were
intermetallic compound particles was studied by SEM.
radiation, TEM selected area electron diffraction (SAED)
and microdiffraction. Thin foils for TEM observation were
prepared by cutting 1 mm thick material slices parallel to
the surface of the sample, mechanical thinning to less than
100 µm and punching of 3 mm diameter discs. The discs
were then further thinned by single jet electropolishing
at 10 V using a 5% perchloric acid-ethanol solution.
Simulations of the electron diffraction patterns were
carried out in dynamical conditions using and CaRIne
Crystallography © [29] software. The chemical composition
of the alloys was determined by energy dispersive X-ray
spectrometry (EDS). Quantitative characterisation of the
microstructure was performed on SEM images using
standard stereological methods and a commercial image
analysis computer program. The values presented are the
average of measurements made on 10 images of different
regions of the sample.
3. Results
3.1 Chemical composition
(Table 1) The alloys prepared in the alloying step
are heterogeneous, with an average Cr content of 14.9 ±
5.5 wt.%. After laser remelting at speeds between 5 and 40
mm/s homogeneous alloys with the chemical compositions
presented in Table 1 were produced. The average Cr content
%, both in the as-alloyed and in the remelted material.
Vs (mm/s) 5 10 20 40
wt.% Cr 9.7 ± 1.1 11.2 ± 2.2 13.8 ± 1.1 19.6 ± 1.6
Table 1: Chemical composition of remelted Al-Cr alloys
page 4 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
3.2 Microstructure
A micrograph of the cross-section of a sample
in the as-alloyed condition is presented in fig. 1a. The
formed of faceted particles of an intermetallic compound
with a branched morphology, dispersed in a matrix of
consecutive melt tracks, which is between 30 and 50 µm
thick (T in figure. 2a), the intermetallic compound particles
are coarser than elsewhere (figure. 2a).
Moreover, some of these particles present a two-
phase structure, the core consisting of a phase richer in
Cr than the surrounding region (figure. 2b). This structure
suggests that an incomplete peritectic reaction between the
intermetallic compound in the particle core and the liquid
occurred, leading to the formation of a lower Cr-content
phase. (Figure 3)
Micrographs of the cross-section of samples
remelted at 10 and 40 mm/s are presented in fig. 3. Two
the fusion line (C in figure. 3) has a columnar structure
similar to that of the as-alloyed material, formed of faceted
particles of an intermetallic compound with a branched
Figure 3:
microstructures (E)
Figure 4:
Figure 2: a) Microstructure of the overlapping region between two
consecutive tracks (T). b) Particle showing surrounding (indicated by
arrow)
page 5 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
This microstructure develops by epitaxial
solidification, each phase growing on the crystals of
the same phase of the underlying material (the alloyed
the remelted layer (C in figure. 3) are finer than in the
as-alloyed material due to the higher solidification rates
prevailing in the remelting treatment. On the contrary,
formed of equiaxed cells consisting of very fine particles
of an intermetallic compound radially distributed within
decreases with increasing remelting speed, from 50 µm
when remelting was carried out at 5 mm/s to 5 µm for
40 mm/s (figure. 4). In the samples remelted at higher
scanning speeds some intermetallic compound particles
from the as-alloyed microstructure survived remelting
and remained in the microstructure (figure. 4d), acting as
heterogeneous nuclei for the cells (figure. 4e). Cells may
also nucleate on particles floating at the surface of the
liquid (figures. 4a to c). Comparison between figures. 3a
and b shows that the extension of the columnar region (C)
decreases with increasing remelting speed, the equiaxed
region (E) becoming predominant for the higher speeds
(figure. 3b). (Figure 5)
The X-ray diffraction patterns of the Al-Cr alloys
are presented in fig. 5. Despite the proximity of the high
intensity peaks of most Al-Cr intermetallic compounds, all
phases present in the microstructure can be unambiguously
identified. The diffraction pattern of the as-alloyed sample
7Cr, Al11Cr2 and
Al4Cr (A5 in figure. 5). The simultaneous existence of
all these intermetallic compounds is probably due to the
chemical heterogeneity of the material. After remelting,
the peaks corresponding to Al11 Cr2 disappear and only
those corresponding to Al7Cr, Al4
(figure. 5, R5 to R40). (Figure 6)
TEM analysis of the branched intermetallic
compound particles existing in the as-alloyed
microstructure (figure. 6) showed that faceted particles,
with a plate-like morphology observed by SEM present
an average chemical composition of the intermetallic
compounds, determined by EDS/TEM corresponding to
the Al4Cr stoichiometry. Selected area electron diffraction
analysis (figures. 7 and 8) confirmed that these particles
correspond to the µ-Al4Cr phase, with hexagonal crystal
structure proposed by Audier et al. [30]. The particle in fig.
8 presents an inner structure, but SAED analysis showed
that it consists of a single phase. This structure does not
result from an incomplete peritectic reaction as seen
in overlap region and is probably due to a composition
variation in the crystal (coring). (Figure 7,8)
Figure 6:TEM images of faceted particles of Al4Cr intermetallic
compound present in the as-alloyed microstructure
Figure 5: X-ray diffraction patterns of Al-Cr alloys as -alloyed (A5)
and remelted at speeds between 5 and 40 mm/s (R5 to R40)
page 6 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
Figure 7:a) Electron diffraction patterns of faceted Al4Cr particle indicated by arrow. b) Experimental pattern. c) Pattern simulated on the basis of
the hexagonal structure proposed by Audier et al. (30)
Figure 8: a)
and [111], respectively
page 7 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
The TEM analysis of the microstructure of the
remelted alloys shows that the equiaxed cells consist of
small, elongated particles distributed radially around
solid solution. SAED of these particles showed that they
consist of Al7Cr intermetallic compound (figure. 9). The
results obtained confirm also the absence of Al11Cr2 or
quasicrystalline phases. (Figure 9)
3.3 Quantitative phase analysis and microhardness
The volume fraction of intermetallic compounds
determined by quantitative image analysis is about 60
% for the as-alloyed material and in the range 70 - 85 %
after remelting (figure. 10a). This difference is due to the
fact that the predominant intermetallic compound in the
as-alloyed microstructure is Al4Cr while after remelting
it is Al7Cr, with a lower Cr content. The increase in the
volume fraction of Al7Cr with remelting speed reflects
the variations in chemical composition reported in Table
1. The increase in the volume fraction of intermetallic
compounds results in an increase in the hardness of the
alloys from 130 to 260 HV, as observed in figure. 10b.
However, the observation of the hardness
indentations (Figure. 11) shows that while there is
considerable cracking around indentations in the as-alloyed
material, the remelted alloys do not show any cracks but
extensive plastic deformation instead.
Figure 9: a)
of Al7Cr particles in a). c) Electron diffraction pattern of Al7Cr particle
in b). d) Simulated [123] pattern on the basis of the structure proposed
by Cooper (31)
Figure10: a) Volume fraction of intermetallic compounds in Al-
Cr alloys. A5 - as-alloyed; R5 to R40 - remelted at speeds from 5 to
40 mm/s. b) Variation of microhardness with the volume fraction of
intermetallic compounds
Figure 11: Hardness indentations in a) as-alloyed and b) remelted Al-
Cr alloys
page 8 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
4. Discussion
4.1 Chemical composition
Chemical heterogeneity has been a problem in
laser surface alloying of aluminium alloys [31-33]. This
difficulty is due, on one hand, to the very short contact
time between the alloying element particles and the liquid
and, on the other hand, to insufficient convective mass
transport in liquid aluminium [34, 35]. In the present
case, the interaction time used in the alloying treatment
(0.24 s) was not sufficiently long to ensure complete Cr
particle dissolution and liquid homogenisation for the
selected power density. To obtain homogeneous alloys it is
necessary to use high power densities and long interaction
times, in order to reach high maximum temperatures and
long melt pool durations. However, the present results
show that even for the parameters used in this work
from reached. Strong convection in the liquid is also
required. This is due to the fact that convective Marangoni
is limited in aluminium alloys due to the small value of the
alloys [36]. The remelting treatment allows achieving the
homogeneous surface composition required for consistent
surface properties. This second melting treatment dissolves
completely the alloying element particles and redistributes
the solute by convection, allowing obtaining homogeneous
material.
4.2 Microstructure
alloys produced in the present work (1.1 wt.%) is much
lower than that observed in similar materials produced
by rapid solidification methods (11 wt.% Cr) [19, 20] and
very close to the equilibrium value (0.71 wt.%). This low
supersaturation is due to the moderate solidification rate
prevalent in the present experiments. The solidification
rate (S) of a phase growing perpendicularly to the solid-
liquid interface is given by:
s
Where Vs is the laser beam scanning speed and
normal to the interface. Since the scanning speed is in the
in the present work are typically in the range 2.5 - 20 mm/s,
several orders of magnitude lower than the solidification
rates used in processes such as melt spinning and gas
atomisation. Similarly, the cooling rates are considerably
4-
1056-108
rates used also explain why no quasicrystalline phases
have been observed in the present study, contrarily to
the observations of previous authors in rapidly solidified
alloys [24-26]. Due to their low Cr supersaturation these
alloys are thus not expected to be age hardenable.
4.2.1 Alloying microstructure
The microstructure resulting from alloying at
a scanning speed of 5 mm/s is similar to that reported
previously for alloys of the same composition prepared by
[37], and laser surface melting [6]. The analysis of the
intermetallic compound particles by TEM showed that they
consist of the µ-Al4Cr phase, with the hexagonal crystal
structure proposed by Audier et al. [30] and Grushko et
al. [38]. Contrarily to what was previously reported by
Luft et al. [6] they are not dendrites but an arrangement of
particles that nucleated on each other. The fact that some
of the branches show the same angular relationship with
the main stem is due to the existing epitaxial relationship.
The microstructure of the Al-Cr surface alloys
can be interpreted on the basis of currently proposed
phase diagram configurations. Basically two different
configurations were proposed for the Al-rich region
of the Al-Cr phase diagram [8-10]. Both agree that the
solidification of Al-xCr alloys with 3<x<13 wt.% Cr
of Al11 Cr2 and Al4Cr, respectively, but according to the
diagram proposed by Murray [10], Al-Cr alloys with
Cr>13 wt.% should undergo the peritectic reactions
L+Al4CrAl11Cr2 at 940 °C, L+Al11Cr2Al7Cr at 790 °C
and L+Al7Cr
temperature, while for Cr<13 wt.% only the peritectic
reactions L+Al11Cr2Al7Cr and L+Al7Cr
occur. According to this diagram, the microstructure of
the alloys studied in the present work should consist of
7Cr. On the contrary, in the phase
diagram configuration suggested by Mahdouk et al. [8, 9],
result of the eutectic reaction L7Cr occurring at
656 °C and not by a peritectic reaction. The microstructure
of these alloys should consist of primary Al7Cr particles
7Cr eutectic matrix. Due to the
short interaction times used in laser materials processing,
peritectic reactions are usually suppressed because, once
page 9 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
a continuous layer of reaction product forms, it prevents
further contact between the solid and the liquid reactants
and, since solid-state diffusion across the reaction layer is
too slow to allow any significant transformation to occur
the peritectic reaction is inhibited [34]. As a result, in
laser alloying the above-mentioned peritectic reactions
would be suppressed and solidification should start with
the formation of primary Al4Cr, followed by the columnar
from the substrate. Assuming that this is the solidification
mechanism, the presence of small amounts of Al7Cr and
Al11Cr2 in the microstructure may be explained by the local
variations in chemical composition observed. Al11Cr2 may
also result from an incipient peritectic reaction between
the liquid and Al4Cr in the overlapping areas of remelting
tracks, as suggested by the presence of the two-phase
particles such as the one observed in fig. 2. In either case,
the volume fraction of Al7Cr and Al11 Cr2 would be low,
the predominant intermetallic compound being Al4Cr, in
agreement with the experimental results.
The fact that Al4Cr particles grow with a faceted
solid-liquid interface can be explained by the high value
of the melting entropy of this phase. According to Jackson
[39] the morphology of the solid-liquid interface at the
atomic scale may be predicted on the basis of a criterion
based on the value of the dimensionless entropy of melting
Equation. 2,
Where
Sf
∆
is the molar entropy of melting and
the solid-liquid interface
tends to be diffuse or atomically rough and the resulting
crystals present a non-faceted morphology, while values
corresponding to low index crystallographic planes, and
interfaces may exist in the same crystal and transitions
between the two types of interface may occur. The
intermetallic compounds Al7Cr, Al11Cr2 and Al4Cr present
4Cr presenting the
7Cr the lowest. The fact that the
Al4Cr particles observed in the present study are faceted
while Al7Cr particles present both faceted and non-faceted
interfaces is consistent with the theoretical predictions.
4.2.2 Remelting microstructure
During remelting solidification starts by the
epitaxial growth of faceted primary particles of Al4Cr
microstructure transition occurs to an equiaxed structure
as solidification proceeds. This equiaxed structure presents
a cellular morphology comprised of fine particles of Al7Cr
E, figure. 3). A particle of the Al4Cr intermetallic compound
is sometimes observed at the centre of the cells, suggesting
that the cells nucleated heterogeneously on particles of this
intermetallic compound that survived remelting (figure.
4e). Heterogeneous nucleation occurs also on oxide
particles floating at the melt surface, as observed in figs.
4a to c. In fact, Al4Cr is the main intermetallic compound
formed in the laser-alloying step. Some particles of Al4Cr
do not melt completely in the second laser treatment and
remain in suspension in the liquid, especially in the upper
region of the melt pool, where the temperature may be
m
insufficient to melt Al4Cr completely (Tm>1030-1130 °C),
in particular if the laser remelting treatment is performed
at high scanning speeds (figure. 4d).
In a previous paper it was shown that the two-phase
component forming this cellular structure results from an
eutectic reaction [13]. The existence of an eutectic reaction
on the Al-rich region of the Al-Cr phase diagram at 657
ºC was suggested by Neto et al. [11] on the basis of DTA
(differential thermal analysis) results and this hypothesis
was retained in the revised phase diagram configuration
published by Mahdouk et al. [8, 9], which includes the
7Cr at 658 °C.
The formation of a columnar microstructure at the
the top of the remelted layer indicates that a columnar to
equiaxed transition (CTE) occurred during solidification.
This is not very common in laser melting processes as
solidification usually occurs under constrained growth
conditions, i.e. with superheated liquid [40]. Yet, as
the laser beam moves away from a particular point of
Sf
R
α
∆
=
Table 2: Dimensionless melting entropy for intermetallic compounds
in the Al-Cr system
Phase αEstimated from ref.
Al7Cr 2.5 (21)
Al11Cr22.7 (21)
Al4Cr 2.8 (8)
page 10 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
the melt pool the temperature decreases and a region of
undercooled liquid may form ahead of the solid-liquid
interface, where equiaxed (unconstrained) growth is
possible. The relation between the solidification rate (S)
and the laser beam scanning speed (Vs) (Equation. 1) and
the solidification rate increases from 0 at the bottom of the
melt pool to a maximum value below Vs at the surface. On
the other hand, the temperature gradient across the solid-
liquid interface decreases as solidification progresses from
the bottom of the melt pool to the surface. As a result of
this evolution constitutional undercooling increases, and
if heterogeneous nuclei exist in the liquid a columnar to
equiaxed solidification transition may occur. This requires
that the liquid undercooling in front of the solid-liquid
interface is higher than the heterogeneous nucleation
undercooling.
The solidification path in these alloys during
Solidification starts at the bottom of the melt pool by
epitaxial growth of Al4
material. In this region solidification is columnar. The
growth of Al4Cr involves rejection of Al to the liquid
in front of the solid-liquid interface generating a strong
constitutional undercooling, which becomes more
significant as the solidification rate and the solute content
of the alloys increases. A large constitutional undercooling
is favoured by the fact that Al4Cr is a faceted phase that
grows with a large kinetic undercooling. The Al4Cr/liquid
interface will lag behind the melting temperature isotherm,
of the columnar interface. If the undercooling ahead
of the solid-liquid interface becomes sufficiently large
7Cr eutectic
phases will occur on particles of Al4Cr that survived
remelting or on oxide particles floating on the liquid
and the eutectic reaction will develop in the undercooled
liquid. The growth of the eutectic cells will block the slow
columnar growth of Al4
bottom of the melted region.
4.3 Quantitative analysis and microhardness
The variation of the volume fraction of intermetallic
compounds with the remelting scanning speed results
from the fact that this speed affects the depth of remelted
material. Since the as-alloyed layer is heterogeneous and
presents undissolved Cr particles, variations in the melt
pool depth result in the incorporation of different amounts
of Cr in the liquid. In particular when the remelting speed
is high, the volume of the melt pool is small and the
incorporation of undissolved alloying element particles in
the liquid originates surface alloys with higher Cr content.
On the other hand, the intermetallic compound formed
during remelting (Al7Cr) has a lower Cr content than
the one formed during alloying (Al4Cr). This fact alone
justifies the increased volume fraction of intermetallic
The hardness increases with the remelting speed
due to the refinement of the microstructure but mostly as a
result of the increased amount of intermetallic compounds
formed. The hardness of the as-alloyed material containing
the acicular intermetallics is comparable to that reported by
other authors in alloys of similar composition [6, 7, and 19].
By contrast, the remelted alloys presenting the equiaxed
cells morphology present higher hardness values precisely
due to the presence of finer and denser intermetallic
compound particles in the microstructure. Nevertheless,
while there is considerable cracking around indentations
in the as-alloyed material formed of the coarse needle-
type Al4Cr intermetallics, the remelted material with the
finer equiaxed morphology shows considerable plastic
deformation around indentations, demonstrating that,
besides improving hardness, the finer equiaxed morphology
induces considerable toughness to the material. This result
shows that alloying and microstructural refinement may
be a relevant strategy to obtain dispersion hardened Al
5. Conclusions
• Homogeneous Al-Cr surface alloys with compositions
between 9.7 and 19.6 wt.% Cr were produced by laser
alloying followed by remelting. Depending on their
chemical composition, the alloys contain from 60 to 85
vol.% of intermetallic compound particles dispersed in a
• The microstructure of the as-alloyed material consists in
long faceted primary particles of Al4Cr dispersed in a matrix
4Cr particles present a branched morphology and
are formed of individual plate-like particles nucleated on
each other. The results confirm that this phase corresponds
to the µ-Al4Cr with hexagonal crystal structure proposed
by Audier et al. [30].
• Remelting causes the formation of a layered
page 11 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
starts by epitaxial growth of Al4Cr particles on the alloyed
material microstructure and proceeds by columnar
(constrained) growth. At some distance from the bottom
of the melt pool a columnar to equiaxed growth transition
occurs. The equiaxed cells consist of eutectic Al7
nucleated heterogeneously in the undercooled liquid ahead
of the columnar solid-liquid interface on undissolved Al4Cr
particles and on oxides floating in the liquid.
• The hardness of the alloys increases with the remelting
speed reflecting the increase in the volume fraction of
intermetallic compounds observed with this parameter and
the refinement of the microstructure. Besides, the remelted
alloys present considerable toughness, showing that the
synthesis strategy used allows obtaining Al-Cr surface
alloys with improved mechanical properties.
References
Rapidly solidified Al-3Cr-X (Ni, Mo) ribbons: structure and
decomposition behaviour. Mater Sci Engn A 134: 1204-1207.
2. McConnell MC, Partridge PG The effect of microstructure
and composition on the properties of vapour quenched Al-Cr
alloys - I. Young’s modulus. Acta Metall 35: 1973-1980.
3. Partridge PG, McConnell MC (1987) The effect of
microstructure and composition on the properties of vapour
quenched Al-Cr alloys - II. Tensile properties. Acta Metall 35:
1981-1993.
4. Almeida A, Anjos M, Vilar R, et al. (1995) Laser alloying of
aluminium alloys with chromium. Surf Coat Technol 70: 221-
229.
5. Almeida A, Vilar R, Li R, et al. (1993) Laser Alloying of
Aluminium and 7175 Aluminium Alloy for Enhanced Corrosion
Resistance. In: Denney P, Miyamoto I, Mordike BL, editors.
ICALEO’93-12th International Congress on Applications of
Lasers and Electro-Optics; Orlando, Florida: Laser Institute of
America 902-912.
6. Luft U, Bergmann HW, Mordike BL (1987) Laser Surface
Melting of Aluminium Alloys. In: Mordike BL, editor.
Laser Treatment of Materials. Oberursel, Germany: DGM
Informationsgesellschaft mbH 147-162.
and alloying of Al-based alloys with CO2 laser radiation. In:
Conference on Laser Advanced Materials Processing - Science
and Applications; Nagaoka, Japan 787-793.
of the Aluminum-Chromium System. J Phase Equilib 21: 157-
166.
binary phase diagram. Arch Metall 46: 157-166.
10. Murray JL (1998) The Al-Cr (aluminum-chromium) system.
J Phase Equilib 19: 368-375.
11. Neto JGC, Gama S, Ribeiro CA (1992) Experimental study
of the Al-Cr equilibrium diagram. J Alloys Compd 182: 271-
280.
12. Du Y, Schuster JC, Chang YA (2005) Experimental
identification of the degenerated equilibrium in extreme Al end
of the Al-Cr system. J Mater Sci 40: 1023-1025.
13. Almeida A, Vilar R (2010) Al-Al7Cr eutectic in Al-Cr alloys
synthesised by laser alloying. Scripta Mater 63: 811-814.
on the Al-rich side of Al-Cr system. J Alloys Compd 621: 283-
286.
15. Cao BB (2017) On the structure of the hexagonal µ-Al4Cr
698: 605-610.
and thermodynamic study of the Al-Cr and Al-Cr-Mg systems. J
Alloys Compd 698: 1038-1057.
reassessment of the Al–Cr–Si system with the refined description
of the Al–Cr system. Thermoch Acta 561: 77-90.
Cooled Al-Cr Alloys. Mater Sci Engn 23: 101-105.
19. Furrer P, Warlimont H (1977) Crystalline and amorphous
structures of rapidly solidified Al-Cr alloys. Mater Sci Engn A
28: 127-137.
20. Bendersky L, Schaefer RJ, Biancaniello FS, et al. (1986)
Rapidly solidified Al-Cr alloys: structure and decomposition
behaviour. J Mater Sci 21: 1889-1896.
21. Adkins NJE, Saunders N, Tsakiropoulos P (1988) Rapid
Solidification of Peritectic Aluminium Alloys. Mater Sci Engn
98: 217-219.
22. Tsakiropoulos P, Pratt RC, Jones H, et al. (1988) Development
page 12 of 12ISSN:xxxx-xxxx SJOMS, an open access journal
Volume 1 · Issue 1 · 1000001SF J Met Sci
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys Synthesised by Laser Alloying. SF J Met Sci 1:1.
the Melt. Mater Sci Engn A 98: 143-147.
23. Shao G, Tsakiropoulos P (1994) Prediction of phase selection
in rapid solidification using time dependent nucleation theory.
Acta Metall Mater 42: 2937-2942.
Solidified Al-Cr Alloys: Crystalline and Quasicrystalline Phases.
J Mater Res 4: 539-551.
Stability and Electrical Resistivity of Quasicrystalline Phase in
Rapidly Quenched Al-Cr Alloys. J Mater Sci 22: 1758-1768.
26. Rosen GI, Shechtman D (1994) Formation of microstructure
of the icosahedral phase in rapidly solidified aluminium-
chromium alloys. Acta Metall Mater 42: 177-184.
27. Sridhara S, Muddle BC (1991) Dispersed intermetallic
phases in rapidly solidified Al-Cr-Fe and Al-Cr-Ce alloys. In:
temperature powder metallurgy alloys. Warrendale PA: TMS
169-181.
Wiley & Sons.
29. Boudias C, Monceau D (2001) CaRIne Crystallography.
Senlis France: Divergent SA.
30. Audier M, Durand-Charre M, Laclau E, et al. (1995) Phase
equilibria in the Al-Cr system. J Alloys Compd 220: 225-230.
31. Hegge HJ, Hosson JTMD (1991) The Influence of
Convection on the Homogeneity of Laser-Applied Coatings. J
Mater Sci 26: 711-714.
Microstructure in Laser Surface Alloying of Aluminium with
Nickel. Mater Sci Engn A 174: 75-84.
Workstation for Variable Composition Laser Cladding - Its Use
for Rapid Alloy Scanning. Surf Coat Technol 72: 62-70.
34. Almeida A, Petrov P, Nogueira I, et al. (2001) Structure and
Properties of Al-Nb Alloys Produced by Laser Surface Alloying.
Mater Sci Engn A 303: 273-280.
35. Almeida A, Vilar R (2008) Alloy Formation in Laser Surface
Alloying of Al with Transition Metals. Lasers Engn 18: 49-60.
pure metals. Int Mater Rev 38: 157-192.
37. Goodwin PS, Tsakiropoulos P, Saunders N (1988) The
microstructure of rapidly solidified Al-Cr-Fe powder. Mater Sci
Engn 28: 61-64.
constitution of the high-Al region of the Al-Cr alloy system. J
Alloys Compd 454: 214-220.
and Crystal Growth Morphology. In: ASM editor Solidification
Chapter 5 Ohio: American Society for Metals 121-154.
materials processing. Lasers Engn 1: 193-212.
Citation: Almeida A (2017) Microstructural Study of Al-Cr Alloys
Synthesised by Laser Alloying. SF J Met Sci 1:1.
