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. ...
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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.
  • S Banerjee
  • P Mukhopadhyay
Banerjee, S. & Mukhopadhyay, P. (2007). Phase Transformations: Examples from Titanium and Zirconium Alloys, Pergamon Materials Series, No. 12. Oxford, Amsterdam: Elsevier/Pergamon.
  • P Barriobero-Vila
  • G Requena
  • F Warchomicka
  • A Stark
  • N Schell
  • T Buslaps
Barriobero-Vila, P., Requena, G., Warchomicka, F., Stark, A., Schell, N. & Buslaps, T. (2015). J. Mater. Sci. 50, 1412-1426.
  • P P Ewald
Ewald, P. P. (1969). Acta Cryst. A25, 103-108.
  • P D Frost
  • W M Parris
  • L L Hirsch
  • J R Doig
  • C M Schwartz
Frost, P. D., Parris, W. M., Hirsch, L. L., Doig, J. R. & Schwartz, C. M. (1954). Trans. Am. Soc. Met. 46, 231.
  • T Gloriant
  • G Texier
  • F Prima
  • D Laillé
  • D.-M Gordin
  • I Thibon
  • D Ansel
Gloriant, T., Texier, G., Prima, F., Laillé, D., Gordin, D.-M., Thibon, I. & Ansel, D. (2006). Adv. Eng. Mater. 8, 961-965.
  • T Gloriant
  • G Texier
  • F Sun
  • I Thibon
  • F Prima
  • J Soubeyroux
Gloriant, T., Texier, G., Sun, F., Thibon, I., Prima, F. & Soubeyroux, J. (2008). Scr. Mater. 58, 271-274.
  • M C Jon
  • H Fujimura
  • R J De Angelis
Jon, M. C., Fujimura, H. & De Angelis, R. J. (1972). Metallography, 5, 139-150.
  • T S Luhman
  • R Taggart
  • D H Polonis
Luhman, T. S., Taggart, R. & Polonis, D. H. (1968). Scr. Metall. 2, 169-172.
The Electrical Resistivity of Metals and Alloys, Cambridge Solid State Science Series
  • P L Rossiter
Rossiter, P. L. (1987). The Electrical Resistivity of Metals and Alloys, Cambridge Solid State Science Series. Cambridge University Press.
  • S Banerjee
  • P Mukhopadhyay
  • P Barriobero-Vila
  • G Requena
  • F Warchomicka
  • A Stark
  • N Schell
  • T Buslaps
Banerjee, S. & Mukhopadhyay, P. (2007). Phase Transformations: Examples from Titanium and Zirconium Alloys, Pergamon Materials Series, No. 12. Oxford, Amsterdam: Elsevier/Pergamon. Barriobero-Vila, P., Requena, G., Warchomicka, F., Stark, A., Schell, N. & Buslaps, T. (2015). J. Mater. Sci. 50, 1412-1426.
  • S Nag
  • A Devaraj
  • R Srinivasan
  • R E A Williams
  • N Gupta
  • G B Viswanathan
  • J S Tiley
  • S Banerjee
  • S G Srinivasan
  • H L Fraser
  • R Banerjee
  • J Nejezchlebová
  • M Janovská
  • H Seiner
  • P Sedlá K
  • M Landa
  • J Milauerová
  • J Strá Ský
  • P Harcuba
  • M Janeček
Nag, S., Devaraj, A., Srinivasan, R., Williams, R. E. A., Gupta, N., Viswanathan, G. B., Tiley, J. S., Banerjee, S., Srinivasan, S. G., Fraser, H. L. & Banerjee, R. (2011). Phys. Rev. Lett. 106, 245701. Nejezchlebová, J., Janovská, M., Seiner, H., Sedlá k, P., Landa, M., Š milauerová, J., Strá ský, J., Harcuba, P. & Janeček, M. (2016). Acta Mater. 110, 185-191.
  • P Zhá Ň Al
  • P Harcuba
  • M Há Jek
  • J Milauerová
  • J Veselý
  • M Janeček
Zhá ň al, P., Harcuba, P., Há jek, M., Š milauerová, J., Veselý, J., Janeček, M. (2017). Mater. Sci. Forum, 879, 2318-2323.
  • P Zhá Ň Al
  • P Harcuba
  • M Há Jek
  • B Smola
  • J Strá Ský
  • J Milauerová
  • J Veselý
  • M Janeček
Zhá ň al, P., Harcuba, P., Há jek, M., Smola, B., Strá ský, J., Š milauerová, J., Veselý, J. & Janeček, M. (2018). J. Mater. Sci. 53, 837-845.