SANS investigation of microstructure evolution in single crystal Ti‑15Mo metastable β-Ti alloy at elevated temperatures

P. Strunz1, V. Ryukhtin1, P. Zháňal2,3, P. Harcuba2, J. Šmilauerová2, U. Keiderling4

1Nuclear Physics Institute v.v.i. of the CAS, Department of Neutron Physics, 25068 Řež, Czech Republic

2Charles University, Mathematics and Physics Faculty, Department of Physics of Materials, Ke Karlovu 5, 12116 Prague, Czech Republic

2Research Centre Řež, Hlavní 130, 250 68 Řež, Czech Republic

4Helmholtz Zentrum Berlin for Materials and Energy, D-14109 Berlin, Germany

strunz@ujf.cas.cz


Titanium alloys have a plenty of applications in industry and medicine [1] due to unique combination of high strength, low density and excellent biocompatibility. Ti-15Mo alloy is a metastable β-Ti alloy containing ω (hexagonal) and α (hcp) precipitates in β-phase matrix. Particular microstructure resulting from the heat treatment has a large impact on mechanical properties and thermal stability of the alloy. One of the techniques able to deliver bulk information on the precipitate evolution directly at elevated temperatures is Small-Angle Neutron Scattering (SANS). V4 SANS facility of HZB Berlin [2] was used for investigation of Ti-15Mo (wt.%) alloy. SANS data were acquired in-situ up to 600°C at three orientations of the single crystal sample – with <111>, <110> and <100> directions of β-phase parallel to the neutron beam. The rate of 1 K/min was used for the in-situ heating during SANS measurement. The orientation of the crystal in this case was <110> direction of the β-phase matrix parallel to the neutron beam.

Strongly anisotropic scattering pattern, moreover evolving with temperature increase, was detected. Observed 2D intensity distribution at temperatures 400-530°C originated from isothermal prolate-spheroidal ω precipitates arranged on a simple cubic-like grid. When heating above 400°C, the mean size and interparticle distance of ω particles gradually increased. Initially, volume fraction times scattering contrast of ω-phase increases up to 443°C, and it then gradually decreases up to 530°C.

In the temperature range 531-538°C, both ω-phase ordered spheroids and a population of α plates is needed to fit the observed SANS patterns well. Fig. 1 (left side) shows an example with already well visible α-phase streaks (near the edge of the detector), but with still present major part of the scattering (including interparticle-interference maxima near the detector centre) originating from the ω spheroids.

Above 538°C, no more scattering from ω particles exists; nevertheless, the scattering coming from the population of α plates remains. Moreover, a second type of streaks appears, which has orientation clearly distinguished from the first ones caused by α plates. The two types of streaks in the scattering pattern can be seen in Fig. 1, right side. The cause of the second type of streaks can be a formation of a second population of α plates which has orientation clearly distinguished from the first population of α plates. The data were evaluated and interpreted using this hypothesis.

In the initial stage after their appearance, the scattering intensity from both α-phase populations gradually increases on temperature increase. Both α phase populations remain present up to the highest measured temperature of 608°C.

 

 

It was also found that the observed orientation of both populations of α plates do not fit the standard orientation [3]. The streaks in SANS patterns are oriented parallel to  or  crystallographic directions for the first population of α plates, and to  or  for the second population of α plates, or in an orientation very similar to these crystallographic directions. Microstructure of α precipitates was not changed during cooling down from 608°C to the room temperature.

Morphology (size and distance) of ω particles and their evolution were also deduced from the SANS data using 3D modelling and fitting of the 2D SANS data by NOC software [4].

Figure 1. Left: SANS pattern measured in ω-α phases transition region (T=533°C). Right: SANS pattern in α-phase region (T=559°C). For both cases,  is perpendicular to the figure plane. Color or gray scale depicts measured data, white contour lines show the fit to the data.

 

1. J. Disegi, Implant Materials. Wrought Titanium –15% Molybdenum (Synthes, 2009).

2. U. Keiderling, C. Jafta, Journal of large-scale research facilities 2, 97 (2016), 97.

3. G. Lütjering and J.C. Williams. Titanium (Engineering Materials and Processes). Berlin, Heidelberg: Springer-Verlag, 2007, p.31.

4. P. Strunz, J.Šmilauerová, M. Janeček, J. Stráský, P. Harcuba, J. Pospíšil, J. Veselý, P. Lindner, L. Karge, Philosophical Magazine 98 (2018) 3086-3108, https://doi.org/10.1080/14786435.2018.1520403.

V. Ryukhtin and P. Strunz acknowledge partial support from the long-term conceptual development project RVO 61389005 of the Nuclear Physics Institute of the Czech Academy of Sciences and from the Czech Academy of Sciences in the frame of the program “Strategie AV21, No. 23”.