Structural and phase transformations, mechanical properties, and shape-memory effects in quasibinary Ni50Ti38Hf12 alloy obtained by quenching from the melt

Abstract

Methods of transmission and scanning electron microscopy and chemical microanalysis, electron diffraction, and X-ray diffraction were used to systematically study the structure and the chemical and phase composition of the Ni₅₀Ti₃₈Hf₁₂ alloy synthesized by rapid quenching from the melt and subjected to various heat treatments. The critical temperatures of the devitrification of the initially amorphous rapidly quenched alloy and the B2 ↔ B19′ thermoelastic martensitic transformations have been determined. The lattice parameters of the B2 austenite and thermoelastic B19′ martensite have been measured. The main features of the formation of an ultrafine-grained structure in the alloy and the subsequent phase transformations (martensitic transformation and the decomposition with the formation of an intermetallic phase of the (Ti,Hf)₂Ni type) have been studied depending on the regimes of heat treatment. Based on the results of measurements of mechanical properties upon tension (σ_M, σ_u, and δ) and the shape-memory effects (degree of shape recovery depending on the deformation by bending; and magnitude of the reversible strain ε_rev), regimes for obtaining high-strength and plastic states of the alloy with a shape-memory effect have been established.