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 crystals containing a dense distribution of small (almost-equal-to 10 nm) gamma-phase precipitates have been studied. The sigma-epsilon curves show a reduction of hysteresis after the first transformation cycles caused by a shift of the direct transformation to progressively lower stresses. More prolongued cycling produces a tilting of the sigma-epsilon curves. Microscopic observations show the formation of dislocation bands in the beta matrix, but in lower amounts in relation to precipitate-free specimens and after prolongued cycling. Moreover, dislocation loops encircling the precipitates have been observed since the first cycles, which improve the accommodation of the precipitates and martensite and, then, are responsible of the diminution of sigma(beta-m) and the 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 to deliver small amounts of work on cooling.