Experimental Investigation on Cyclic Deformation and Fatigue Behavior of Polycrystalline Cu–Al–Ni Shape Memory Alloys above Ms

Cyclic deformation and fatigue behavior of polycrystalline Cu–Al–Ni shape memory alloys have been investigated in detail at temperatures above Ms point. As a result, they have been found to be much influenced by ΔT(=T−Ms, where T is test temperature) and by cyclic modes. The ratio of the critical stress for induction of martensites (σc) to ΔT is 3.2 MPa/K, which is in good agreement with the value previously predicted. The slope of a flat region on stress-strain curves increases with increasing ΔT. When cyclic deformation has been performed in P mode in which stress-induced martensites (SIM) are transformed back to parent at every cycle, σc decreases with increasing number of cycles, and fatigue life (Nf) is very short and exhibits a maximum at a temperature of ΔT≅13 K. On the other hand, when cyclic deformation has been performed in M mode in which deformation cycles are continued without reversion of SIM to parent, Nf is longer than that in P mode. Microstructural observation during deformation shows that the γ′1 martensite tends to be stress-induced in a small ΔT region, whereas the β′1 martensite does in a large ΔT region, and that intergranular cracking occurs being associated with SIM formation.