Critical Assessment of the Al-Ti-Zr System
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Critical Assessment of the Al-Ti-Zr System Z. Kahrobaee1 • M. Palm1
Submitted: 28 July 2020 / Accepted: 8 September 2020 The Author(s) 2020
Abstract Only within the last couple of years, complete isothermal sections for the Al-Ti-Zr system have been determined. Up to then there had been several investigations, but these were limited to the Ti-Zr and Ti-Al sides and the Ti- and Al corners of the system. Also recently the system has been evaluated by thermodynamic ab initio calculations and CALPHAD-type (CALculation of PHAse Diagrams) modelling. As Al-Ti-Zr alloys are of interest for a variety of applications and as the last assessment has been performed more than 15 years ago, this new critical evaluation of the existing literature about phase equilibria in the Al-Ti-Zr system has been performed. Keywords assessment crystal structures modelling phase equilibria
1 Introduction In view of applications, different parts of the Al-Ti-Zr system are of interest and investigation of phase equilibria over the decades reflects these interests. With the development of Ti-base alloys, the extension of the homogeneity ranges of b(Ti,Zr) and a(Ti,Zr) within the ternary system in dependence of temperature have been investigated.[1–3] In the Al corner of the system precipitation of (Ti,Zr)Al3 has & Z. Kahrobaee [email protected] M. Palm [email protected] 1
Max-Planck-Institut fu¨r Eisenforschung GmbH, Du¨sseldorf, Germany
been investigated for strengthening Al alloys at higher temperatures[4–7] and for grain refining.[8–10] As ductility may improve if instead of tetragonal TiAl3 (D022) or ZrAl3 (D023) metastable (Ti,Zr)Al3 with cubic L12 structure forms, quite some effort has been spent on establishing phase stability in the Al-rich part of the system.[11–17] With the industrialisation of c TiAl based alloys, phase equilibria among the Ti-Al based phases in this system came into focus.[18–23] While all these activities were limited to specific parts of the system, studies of full isothermal sections have been performed only very recently.[24,25] Though of some practical importance, experimental difficulties in investigating Ti-Al(-X) systems at high temperatures make the determination of phase equilibria difficult. High susceptibility to impurities, specifically oxygen, which can have a pronounced effect on phase equilibria, reactions of melts with crucibles or preferential evaporation of Al are some of these problems.[26,27] The addition of Zr does not improve things, as it is also highly susceptible to oxygen uptake and because of its high melting point and low diffusivity, quite long annealing times are necessary to attain equilibrium even at high temperatures.[24,25] And the fact that even ‘‘high-purity’’ Zr contains comparable high amounts of impurities,[18,28] specifically Hf, is most of the time neglected. The key for any aimed alloy development is a sound knowledge of phase equilibria in dependence of temperature and composition. In this regard, assessments play an important role as they collect all the available
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