PSEUDOELASTIC CYCLING AND 2-WAY SHAPE-MEMORY EFFECT IN BETA-CU-ZN-AL ALLOYS WITH GAMMA-PRECIPITATES

The effects of tensile pseudoelastic cycling of Cu-Zn-Al single crysta ls containing a dense distribution of small (almost-equal-to 10 nm) ga mma-phase precipitates have been studied. The sigma-epsilon curves sho w a reduction of hysteresis after the first transformation cycles caus ed by a shift of the direct transformation to progressively lower stre sses. More prolongued cycling produces a tilting of the sigma-epsilon curves. Microscopic observations show the formation of dislocation ban ds in the beta matrix, but in lower amounts in relation to precipitate -free specimens and after prolongued cycling. Moreover, dislocation lo ops encircling the precipitates have been observed since the first cyc les, which improve the accommodation of the precipitates and martensit e and, then, are responsible of the diminution of sigma(beta-m) and th e presence of the two-way shape memory effect. Once the two-way memory is present, which occurs after one order of magnitude lower number of cycles compared to precipitate-free specimens, the samples are able t o deliver small amounts of work on cooling.