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In situ detection of stability limit of ω phase in Ti–15Mo alloy during heating

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Journal of Applied Crystallography
Authors:
Pavel Zhanal at Research Center Rez
Pavel Zhanal
Petr Harcuba
Petr Harcuba
Petr Harcuba
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Michal Hájek
Michal Hájek
Michal Hájek
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Josef Stráský at Charles University in Prague
Josef Stráský
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Abstract and Figures

Phase transitions in a single crystal of a metastable β-titanium alloy (Ti-15Mo) were investigated in situ during heating by synchrotron X-ray diffraction. The results were compared with previous measurements of electrical resistance. Single-crystalline samples allowed different crystallographic families of ω-Ti and α-Ti phases to be distinguished. The observed evolution of the intensity of the reflections of the ω phase during heating is consistent with the evolution of electrical resistance, which proves that the resistance is affected by the presence of ω-phase particles. Between approximately 673 and 833 K, both the resistance and the intensity of ω peaks sharply decrease. At 833 K, ω reflections disappear, indicating a complete dissolution of the ω phase due to achieving the solvus temperature of the ω phase in the Ti–15Mo alloy. The synchrotron X-ray diffraction experiment proved that the disappearance of the ω phase during heating of Ti–15Mo with a heating rate of 5 K min⁻¹ occurs by its dissolution back to the β phase and not by ω → α transformation.
Schematic representation of experimental setup.
Schematic representation of experimental setup.
… 
(a) Measured diffraction pattern at room temperature and (b) calculated pattern with indexed β reflections (black square symbols). The colored dots denote positions of ω diffraction spots. The three most intense crystallographically equivalent ω diffraction spots are larger and shown in green (), blue () and red (). Less intense ω diffraction maxima belonging to various crystallographically equivalent plane systems are indicated by small yellow dots.
(a) Measured diffraction pattern at room temperature and (b) calculated pattern with indexed β reflections (black square symbols). The colored dots denote positions of ω diffraction spots. The three most intense crystallographically equivalent ω diffraction spots are larger and shown in green (), blue () and red (). Less intense ω diffraction maxima belonging to various crystallographically equivalent plane systems are indicated by small yellow dots.
… 
Scheme illustrating the possibility of detection of diffraction maxima owing to the broadening effect caused by particle size. Black dots represent the exact positions of diffraction maxima, pink ellipses show the broadening effect, and the cyan line denotes the Ewald sphere. is the incident beam vector, and and are the diffracted and scattering vectors, respectively.
Scheme illustrating the possibility of detection of diffraction maxima owing to the broadening effect caused by particle size. Black dots represent the exact positions of diffraction maxima, pink ellipses show the broadening effect, and the cyan line denotes the Ewald sphere. is the incident beam vector, and and are the diffracted and scattering vectors, respectively.
… 
Calculated positions of β and ω diffraction spots in reciprocal space with respect to the Ewald sphere. Black squares represent the positions of diffraction maxima of the β phase. Positions of ω-phase diffraction spots are denoted by dots. The three equivalent types of the most intense ω diffraction spots are highlighted by larger symbols in green (), blue () and red (). Less intense maxima are shown by smaller symbols (β – black squares, ω – yellow dots). (a) The cyan-colored square defines the region of the Ewald sphere investigated in this study. ω1–4 together with gray ellipses identify families of the ω phase of the investigated diffraction maxima (zone axis ). (b) View perpendicular to the red part of the Ewald sphere from (a) (zone axis [010]β). (c) View perpendicular to the blue part of the Ewald sphere from (a) (zone axis [011]β).
Calculated positions of β and ω diffraction spots in reciprocal space with respect to the Ewald sphere. Black squares represent the positions of diffraction maxima of the β phase. Positions of ω-phase diffraction spots are denoted by dots. The three equivalent types of the most intense ω diffraction spots are highlighted by larger symbols in green (), blue () and red (). Less intense maxima are shown by smaller symbols (β – black squares, ω – yellow dots). (a) The cyan-colored square defines the region of the Ewald sphere investigated in this study. ω1–4 together with gray ellipses identify families of the ω phase of the investigated diffraction maxima (zone axis ). (b) View perpendicular to the red part of the Ewald sphere from (a) (zone axis [010]β). (c) View perpendicular to the blue part of the Ewald sphere from (a) (zone axis [011]β).
… 
Measured and computed diffraction patterns of Ti–15Mo during heating: (a), (b) in the initial condition at room temperature, (c), (d) at 699 K, (e), (f) at 833 K, (g), (h) at 935 K (maximum reached temperature), and (i), (j) after cooling to 333 K. Positions of β reflections are indicated by black squares. Colored dots denote positions of ω diffraction spots. The three most intense crystallographically equivalent ω diffraction spots are larger and shown in green (), blue () and red (). Less intense ω diffraction maxima belonging to various crystallographically equivalent plane systems are indicated by small yellow dots.
+4
Measured and computed diffraction patterns of Ti–15Mo during heating: (a), (b) in the initial condition at room temperature, (c), (d) at 699 K, (e), (f) at 833 K, (g), (h) at 935 K (maximum reached temperature), and (i), (j) after cooling to 333 K. Positions of β reflections are indicated by black squares. Colored dots denote positions of ω diffraction spots. The three most intense crystallographically equivalent ω diffraction spots are larger and shown in green (), blue () and red (). Less intense ω diffraction maxima belonging to various crystallographically equivalent plane systems are indicated by small yellow dots.
… 
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research papers
J. Appl. Cryst. (2019). 52, 1061–1071 https://doi.org/10.1107/S1600576719010537 1061
Received 3 January 2019
Accepted 24 July 2019
Edited by G. Kostorz, ETH Zurich, Switzerland
Keywords: Ti alloys; !phase; phase;
phase transitions; electrical resistance; X-ray
diffraction.
Supporting information:this article has
supporting information at journals.iucr.org/j
In situ detection of stability limit of xphase in
Ti–15Mo alloy during heating
Pavel Zha
´n
ˇal,
a,b
* Petr Harcuba,
a
Michal Ha
´jek,
a
Josef Stra
´sky
´,
a
Jana S
´milauerova
´,
a
Jozef Vesely
´,
a
Luka
´s
ˇHora
´k,
c
Milos
ˇJanec
ˇek
a
and Va
´clav Holy
´
c,d
a
Department of Physics of Materials, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic,
b
Material and
Mechanical Properties, Research Centre Rez Ltd, Hlavni 130, Husinec-Rez, Czech Republic,
c
Department of Condensed
Matter Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic, and
d
CEITEC at Masaryk University,
Kotla
´r
ˇska
´2, 61137 Brno, Czech Republic. *Correspondence e-mail: pavel.zh@karlov.mff.cuni.cz
Phase transitions in a single crystal of a metastable -titanium alloy (Ti-15Mo)
were investigated in situ during heating by synchrotron X-ray diffraction. The
results were compared with previous measurements of electrical resistance.
Single-crystalline samples allowed different crystallographic families of !-Ti and
-Ti phases to be distinguished. The observed evolution of the intensity of the
reflections of the !phase during heating is consistent with the evolution of
electrical resistance, which proves that the resistance is affected by the presence
of !-phase particles. Between approximately 673 and 833 K, both the resistance
and the intensity of !peaks sharply decrease. At 833 K, !reflections disappear,
indicating a complete dissolution of the !phase due to achieving the solvus
temperature of the !phase in the Ti–15Mo alloy. The synchrotron X-ray
diffraction experiment proved that the disappearance of the !phase during
heating of Ti–15Mo with a heating rate of 5 K min
1
occurs by its dissolution
back to the phase and not by !!transformation.
1. Introduction
At room temperature and pressure, pure titanium crystallizes
in a hexagonal close-packed (h.c.p.) structure, called the
phase. The stability limit of the phase in pure Ti is at 1155 K,
which is known as the -transus temperature. Above this
temperature, the structure of titanium allotropically trans-
forms into a body-centered cubic (b.c.c.) phase, which is
stable up to the melting point of titanium. The and phases
most commonly obey a Burgers orientation relationship
(Lu
¨tjering & Williams, 2007; Burgers, 1934),
f0001gkf110g;h1120ikh111i;ð1Þ
from which follow 12 possible orientations of with respect to
the phase.
According to the type and quantity of alloying elements and
the resulting phase composition, titanium alloys are commonly
classified as ,+, metastable and stable (Lu
¨tjering &
Williams, 2007). Metastable -Ti alloy is defined as an alloy
with a sufficient -stabilizer content to suppress !0or 00
martensitic transformation during quenching to room
temperature. In many metastable -titanium alloys, an !
phase occurs; its formation can be described as a collapse of
one pair of (111) b.c.c. planes of the phase to their inter-
mediate position, leaving the next plane unaltered, collapsing
the next pair and so on (Hickman, 1969). A complete plane
collapse will produce an ideal hexagonal !phase with
ISSN 1600-5767
#2019 International Union of Crystallography
... On the other hand, the employment of high energy X-ray diffraction (HEXRD) available at synchrotron facilities can overcome these drawbacks. Previous works have shown the potential of HEXRD for the investigation of phase transformations in Ti-alloys [18][19][20][21][22]. To the best of our knowledge, phase transformations in UFG β-Ti alloys have not been investigated yet by HEXRD. ...
... The evolution of phase fractions evaluated from the Rietveld fit for the non-deformed and the HPT-deformed condition is shown in Fig. 4(a) and (b), respectively. The evolution of phase content in Ti15Mo alloy during heating has been studied in several previous studies [22,26,27,40,41]. Initial decrease of the content of ω ath phase is followed by significant increase of content of ω isothermal (ω iso ) reaching a maximum at around 450 °C. ...
... Initial decrease of the content of ω ath phase is followed by significant increase of content of ω isothermal (ω iso ) reaching a maximum at around 450 °C. Upon further heating, the volume fraction of ω iso decreases and finally, ω iso phase dissolves at 560 °C [22] -the dissolution is represented by dashed line in Fig. 4 similarly to Fig. 1. α phase peaks could be resolved just before the dissolution of ω phase and the maximum α phase content of approx. 15% is observed at 700 °C. ...
In-situ investigation of phase transformations in ultra-fine grained Ti15Mo alloy
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A metastable β solution treated Ti15Mo alloy was deformed by high pressure torsion (HPT) resulting in a severely deformed microstructure with high density of lattice defects. In order to gain insight into the kinetics of phase transformations, both non-deformed and HPT-deformed materials were studied in-situ during linear heating; phase evolution was investigated using high energy synchrotron X-ray diffraction (HEXRD) complemented by the measurement of electrical resistance. It was shown that in the non-deformed material the dissolution of the ω phase is followed by precipitation of the α phase during linear heating. In contrast, in the HPT-deformed material the nucleation of the α phase is shifted to lower temperatures, resulting in the coexistence of all three β, ω, and α phases at ~ 550 °C. Moreover, in HPT-deformed samples, the growth of α particles is accelerated due to high density of dislocations and grain boundaries. Post-mortem observations of selected samples revealed that the microstructure of the HPT-deformed material after heating up to 650 °C remains ultra-fine grained with equiaxed grains of α phase.
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... It is widely accepted that these particles can influence the nucleation of the α phase during subsequent heat treatments, thereby providing a method for microstructural control [9]. However, the exact mechanism of α phase formation is still not well understood [10][11][12], and some of the most recent works have suggested that α phase nucleation may, in certain heating regimes, be only indirectly influenced by the ω phase or completely independent of it [6,13,14]. ...
... Let us now discuss in more detail the kinetics of phase transformations during the slower heating rate of 1.9 • C/min. ND patterns demonstrating the evolution of Ti-15Mo during heating and cooling are presented in Figure 2. The material at room temperature consists of β phase matrix and ω particles [13], although ω peaks are wide at this temperature (see the ω peaks in Figure 1a) due to the small size of the ω particles [14]. The presence of Nb peaks in Figure 2 is due to the Nb sample holder, as was explained in Section 2. The peaks of the ω phase begin to sharpen during heating around 300 • C, which is caused by coarsening of ω particles. ...
Transformation Pathway upon Heating of Metastable β Titanium Alloy Ti-15Mo Investigated by Neutron Diffraction
Article
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A transformation pathway during thermal treatment of metastable β Ti-15Mo alloy was investigated by in situ neutron diffraction. The evolution of individual phases α , β , and ω was investigated during linear heating with two heating rates of 1.9 ∘ C / min and 5 ∘ C / min and during aging at 450 ∘ C . The results showed that with a sufficient heating rate (5 ∘ C / min in this case), the ω phase dissolves before the α phase forms. On the other hand, for the slower heating rate of 1.9 ∘ C / min , a small temperature interval of the coexistence of the α and ω phases was detected. Volume fractions and lattice parameters of all phases were also determined.
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... Crucibles with salt baths were used to maintain a more stable temperature. In Timetal LCB, the solvus of the ω phase (ω solv ) is approximately 500 • C, as demonstrated in our previous research using electric resistance measurements [39]; the ω solvus manifests itself as a sharp change in the dependence of resistance on temperature, and was also confirmed by in situ x-ray diffraction [40]. The two lower ageing temperatures used in this study lie below ω solv ; and therefore, ω particles evolution may be expected during the early stages of ageing, especially at 460 • C and to a limited extent at 490 • C. ...
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Sequence of phase transformations in metastable β Zr-12Nb alloy studied in situ by HEXRD and complementary techniques
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Phase transformations in a metastable beta Zr–12Nb alloy were investigated by high-energy X-ray diffraction (HEXRD) measured simultaneously with thermal expansion in situ during linear heating from room temperature to 800 °C. Complementary in-situ methods of electrical resistance and differential scanning calorimetry, which were performed using the same heating conditions as in the HEXRD experiment, provided additional information on the transformation sequence occurring in the Zr–12Nb alloy. Two bcc phases with a different lattice parameter, βZr and βNb, were observed in the investigated temperature range and identified using the phase diagram of the Zr–Nb system. In the initial solution-treated condition, metastable βZr phase and athermal ω particles are present in the material. At about 300 °C, Nb-rich βNb phase starts to form in the material and the original βZr phase gradually disappears. Ex-situ observations of the microstructure using transmission electron microscopy revealed a cuboidal shape of the ω particles, which is related to a relatively large misfit between the ω and β phases. At 560 °C, ω solvus was observed, identified by an abrupt dissolution of ω particles which was followed by growth of the α phase.
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