Article

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

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## Abstract

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.

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... 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. ...
Article
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.
... 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. ...
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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Evolution of $$\omega$$ phase during heating of metastable $$\beta$$ titanium alloy Ti–15Mo was investigated in situ during heating by electrical resistance measurements and accompanied by transmission electron microscopy. Different heating rates were employed aiming to determine kinetics of occurring phase transformations. Sharp change of temperature dependence of electrical resistance caused by complete dissolution of $$\omega$$ phase was observed at 560 $$^{\circ }\hbox {C}$$ independently of heating rate. Majority of $$\omega _{\mathrm{iso}}$$ particles revert back to $$\beta$$ phase at 560 $$^{\circ }\hbox {C}$$; therefore, they are not direct precursors of $$\alpha$$ precipitation during continuous heating.
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The process of transition of the cubic-body-centred modification into the hexagonal-close-packed modification of zirconium can be described by means of a combination of shearing- and dilatation-processes parallel to definite crystallographic directions. The transition has, therefore, “homogeneous” or “oriented” character. See for particulars the detailed summary at the end of the paper.
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Solid-solid displacive, structural phase transformations typically undergo a discrete structural change from a parent to a product phase. Coupling electron microscopy, three-dimensional atom probe, and first-principles computations, we present the first direct evidence of a novel mechanism for a coupled diffusional-displacive transformation in titanium-molybdenum alloys wherein the displacive component in the product phase changes continuously with changing composition. These results have implications for other transformations and cannot be explained by conventional theories.
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