Orientation Dependence of Functional Properties in Heterophase Single Crystals of the Ti36.5Ni51.0Hf12.5 and Ti48.5Ni51.5 Alloys

The features of orientation dependence of stress-induced thermoelastic B 2–( R )– B 19′-martensitic transformations in single crystals of the Ti 48.5 Ni 51.5 and Ni 51.0 Ti 36.5 Hf 12.5 (at.%) alloys, which contain disperse particles of the Ti 3 Ni 4 and H-phase, respectively, are revealed along with those of their shape-memory effects (SME) and superelasticity (SE). It is experimentally demonstrated that irrespective of the crystal structure of disperse particles measuring more than 100 nm, for their volume fraction f > 16% there is a weaker orientation dependence of the reversible strain in the cases of manifestation of SME and SE. In the orientations of Class I , wherein martensitic detwinning introduces a considerable contribution into transformation strain, the values of SME |ε SME | and SE |ε SE | decrease by over a factor of two compared to the theoretical lattice strain value |ε tr0 | for a B 2– B 19′-transformation and the experimental values of reversible strain for quenched TiNi crystals. In the orientations of Class 2 , wherein detwinning of the martensite is suppressed as is the case in quenched single-phase single crystals, the reversible strain is maintained close to its theoretical value |ε tr0 |. Micromechanical models of interaction between the martensite and the disperse particles are proposed, which account for the weaker orientation dependence of |ε SME | and |ε SE | due to suppression of detwinning of the B 19′-martensite crystals by the particles and a transition from a single-variant evolution of the stress-induced martensitic transformations to a multiple-variant evolution of transformations in the cases of increased size of the particles and their larger volume fractions.