A comparative study of elastic constants of Ti-Ni-based alloys prior to martensitic transformation

Single crystal elastic constants of Ti-Ni alloys without (quenched) and with (aged) Ti3Ni4 precipitates were measured systematically by rectangular parallelepiped resonance method as a function of composition and temperature, and compared with Ti-Ni-Cu and Ti-Ni-Fe alloys, in an attempt to answer some long-standing questions as to the origin of the unique monoclinic B19' martensite, and why composition and thermomechanical treatment greatly changes the path of martensitic transformation. The results showed that softening in c(44) in additional to c', is a common feature for all Ti-Ni binary (both quenched or aged) and ternary alloys. This general feature just corresponds to the fact that all these alloys ultimately transform into B19', suggesting that softening in c(44) is responsible for the unique B19' martensite, which found no analogy in other beta phase alloys. We also found an interesting correspondence between the temperature dependence of anisotropy factor and transformation path. Prior to B2-B19' transformation anisotropy shows a decrease with lowering temperature; prior to B2-B19 an anisotropy increase, while prior to B2-R transformation a constant anisotropy. We further showed that three possible martensite candidates (R, B19, B19') are rooted in anomalies in specific phonon modes and elastic softening. We showed that the multi-stage transformations are restricted by a general rule: multi-stage transformation occurs in the sequence of increasing transformation strain. With this rule we can explain all known transformation paths by considering the effect of alloying addition and fine precipitates/dislocation network on relative stability of different martensites. We further predict that there may exist a new transformation path in Ti-Ni-based alloys: B2-R-B19-B19'. (C) 2001 Elsevier Science B.V. All rights reserved.