SUCCESSIVE STRESS-INDUCED MARTENSITIC TRANSFORMATIONS AND ASSOCIATED TRANSFORMATION PSEUDOELASTICITY IN CU-AL-NI ALLOYS

Successive stress-induced martensitic transformations in Cu-Al-Ni single crystals have been extensively studied as a function of temperature and strain rate, using tensile tests, optical microscope observation and the back-reflection Laue method. As a result, a phase diagram relating various martensitic phases and the matrix has been determined in temperature and stress coordinates. Complicated stress-strain curves, which drastically change with temperature, have been explained in consistent terms by using the phase diagram. Strong orientation dependence and remarkable elongation (exceeding 15% in total) of the associated multistage pseudoelasticity have been rationalized by the calculations based on the crystallographic theory. The temperature and strain rate dependence of these pseudoelastic curves have been obtained and analyzed by the surface dislocation theory developed by Sumino. It is proposed that the mechanism of successive transformations between martensites with long-period stacking order structures is the successive nucleation of regularly spaced partial dislocations. This is a unique mechanism to account for the above pseudoelastic behavior. © 1979.