Modelling martensitic transformation in titanium alloys
F. Tancret, E. Bertrand
Controlling, via composition, martensitic transformation in titanium alloys would allow to design materials with a superelastic, “TRIP” or shape memory behaviour. The occurrence of this transformation, upon quenching or upon deformation, was not completely understood or would invoke controversial hypotheses. The design of alloys, on its side, was largely using empirical rules with no real physical basis. Within the frame of Madeleine Bignon’s PhD thesis (2020), two new theories have been proposed to predict, as a function of composition, the occurrence of martensitic transformation upon quenching or upon deformation in titanium alloys. The developed models notably allow to design TRIP alloys with optimised properties.
Titanium alloys can form martensite during quenching from the high temperature domain, β. However, beyond a certain content, βc, in beta-stabilising elements (Fe, Cr, Mo, V, W, Nb, Ta…), the β phase is retained at room temperature. In some cases, a subsequent strain can provoke the transformation, leading to a superelastic effect of to a TRIP effect. Beyond a certain concentration in beta-stabilising elements, βd, the martensitic transformation does not occur, even upon deformation.
 |
The thresholds βc and βd are specific to each alloying element. Most of the hypotheses of the literature to explain β phase retention would rely on the inhibition of the martensitic transformation by the formation of the athermal β phase during quenching, which would block martensite, or on a too low martensite start temperature, Ms. Nevertheless, these explanations only allow to explain a part of literature data; besides, they are contradicted by a set of experimental observations and by calculations in many cases. The proposed approach is based on the one hand on the concept of thermally activated nucleation of martensite, and on the other hand on the phenomenological theory of martensite crystallography (PTMC). The developed models, associating computational thermodynamics (Calphad), crystallography and micromechanics, allow to explain the whole results of the literature and to predict correctly the observed thresholds βc and βd.
 |
 |
References:
Keywords:
Martensite, modelling, modeling, PTMC, metastable
Collaborations:
Lancaster University