Soutenance de thèse

Title : Study of phase stability and point defects in the medium entropy system Ti-Zr-Nb centered cubic at the atomic scale

This study aims to better understand the stability of refractory high entropy alloys (RHEAs) having a bcc structure. This class of alloys are of great current interest because of their excellent mechanical properties retained up to elevated temperatures. Some of them also show some transformation-induced plasticity, a phenomenon closely related to their stability. Selecting the Ti-Zr-Nb system as a model alloy for group IV and V RHEAs mixing both hcp and bcc elements, ab initio calculations are performed on dilute to high concentration equiatomic alloys of this system. Vacancy calculations versus alloy composition are also considered, as important defects existing in materials responsible for diffusion processes, and possibly influencing phase stability. In the dilute limit, any substitutional contraction center (vacancy, solute) introduced in the bcc Ti or Zr matrix induces a phase transition towards a more compact hcp or ω phase. This is understood using local analysis of both electronic and atomic structures; the latter agrees well with diffraction experiments. Such point defects will thus introduce important heterophase fluctuations at finite temperature, which might explain the appearance of ω phase in some concentration domains of binary Ti-Nb, Zr-Nb and Ti-Zr alloys. For high concentration cases, phase stability is studied by considering both ordered and disordered alloys (generated as special quasi random structures). Random alloys are always more stable, and depending on alloy composition, several instabilities involving Burgers and Cook’s mechanisms are observed. The bcc stability zone is identified as rather extended in alloy composition, due to very large local lattice distortions. Computation of the appropriate local order parameter shows that these bcc equiatomic alloys intrinsically comprise ω hetero-phase fluctuations, a behavior in close connection with the TRIP effect. Finally, vacancies are seen to enhance these phase fluctuations; an anomalous diffusion that is typical of bcc alloys is thus expected in such RHEAs.

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