Orientation Dependence of Functional Properties in Heterophase Single Crystals of the Ti_36.5Ni_51.0Hf_12.5 and Ti_48.5Ni_51.5 Alloys

The features of orientation dependence of stress-induced thermoelastic B2–(R)–B19′-martensitic transformations in single crystals of the Ti_48.5Ni_51.5 and Ni_51.0Ti_36.5Hf_12.5 (at.%) alloys, which contain disperse particles of the Ti₃Ni₄ 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 B2–B19′-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 B19′-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.