Critical Assessment of the Al-Ti-Zr System

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.

Alloying Elements in Intermetallic γ‐TiAl Based Alloys – A Review on Their Influence on Phase Equilibria and Phase Transformations

“…Additionally, the eutectoid temperature increases upon alloying with Zr, as demonstrated in Figure 12, indicating the stabilization of the α 2 compared to the α phase similar to the cases with Mo and V. [ 228,229 ] At intermediate temperatures, i.e., the temperature range of the α 2 + γ phase field region, sufficient alloying with Zr may cause the formation of the (Zr,Ti) 5 Al 3 phase. [ 225 ] With respect to ternary intermetallic compounds, contradicting results can be found for this system. Some authors claim that no ternary phases are thermodynamically stable in the Ti–Al–Zr system.…”

“…Such a stabilization of the β phase at the expense of α phase is also evidenced by the isothermal sections presented in previous studies. [218,[225][226][227] The contraction of the α single-phase field region is further promoted by the increase of the γ-solvus temperature. In particular, Musi et al [228] reported that Zr stabilizes the γ phase at temperatures below this transition temperature, i.e., the amount of γ increases at the expense of α/α 2 .…”

“…[218,226] Other research work reported on the presence of the phases Ti 2 ZrAl and Zr 2 TiAl, which, however, could also be ordered substructures of the β phase as argued by some authors. [225,230,231]…”

Experimental Investigation of Phase Equilibria in the Ti—Al—Zr System at 1000–1300 °C

“…From this assessment, it is clear that discrepancies exist, especially between recent CALPHAD modellings [21,22] and experimentally determined phase equilibria [6,11,[23][24][25]. As pointed out in the assessment [20] as well as in the modellings [21,22], specifically at 800 • C, even the multi-phase equilibria are not settled, e.g., the experiments show TiAl in equilibrium with βTi at 800 • C, while the CALPHAD modellings show equilibria between TiAl and Zr 4 Al 3 [21] or between TiAl and ZrAl 2 [22], which "cut off" any possible phase equilibrium between TiAl and βTi. In consequence, adjacent phase equilibria differ completely from each other within a wide composition range at 800 • C in [20][21][22].…”

“…In a first step, a complete assessment of all available literature on phase equilibria in the Ti-Al-Zr system was performed [20]. From this assessment, it is clear that discrepancies exist, especially between recent CALPHAD modellings [21,22] and experimentally determined phase equilibria [6,11,[23][24][25].…”