Stress-Assisted Austenite Ageing Kinetics in 001-Oriented Ni_(50.6)Ti_(49.4) Single Crystals

摘要

The effect of ageing under high stress (666-1072 MPa) at 423 K on the stability of the shape memory effect and the superelasticity in single-phase and heterophase [001]-oriented Ni_(50.6)Ti_(49.4) single crystals was studied. It was shown that single crystals, containing dispersed Ti_3Ni_4 particles (after stress-free ageing at 823 K for 1 hour (h) and 573 K for 1.5 h particles size was 400 nm and < 10 nm, respectively), are more stable to stress-assisted ageing at 423 K than single-phase quenched crystals (annealed at 1253 K for 1 h followed by water quenching). In heterophase crystals, stress-assisted ageing practically does not affect the stress-induced martensitic transformations and the parameters of the shape memory effect and superelasticity, irrespective of the particle size. In single-phase quenched crystals, stress-assisted ageing leads to a decrease in the transformation temperatures by 20 K, an increase in thermal hysteresis, an increase in the critical stress for martensite formation by 110-150 MPa and an increase in the austenite yield strength by 70 MPa.