STRUCTURE CHANGES DURING PSEUDOELASTIC DEFORMATION OF CUALMN SINGLE-CRYSTALS

Structure changes during pseudoelastic deformation of CuAlMn single cr ystals were investigated using in situ optical and high-voltage electr on microscopy (HVEM). Several crystal orientations were investigated, from an irrational to [100] and [110] tensile axis and plane orientati ons in the case of thin foils. The composition of the alloy was chosen such as to obtain superelastic behavior at room temperature. Optical microstructures allowed to identify parallel plates at the beginning o f the stress plateau, the number of which increased with strain. The s tress/temperature phase diagram was established within the range of ex istence of gamma'(1) and beta'(1). During in situ HVEM deformation in the [100] direction of gamma'(1) plates nucleated on pre-existing ones . At later deformation stages 18R martensite was formed in stacks of n arrow needles. The Following crystallographic relationship was observe d: [001]beta(1) parallel to[010]beta'(1), gamma'(1) and [110]beta(1) p arallel to[001]beta'(1), gamma'(1). A small permanent deformaticn obse rved after stress release was connected with the presence of residual martensite of a high random stacking fault density and consisting ofte n of alpha'(1), gamma'(1) and beta'(1) martensite layers. During defor mation in the (110) direction a larger permanent deformation and a den sity of dislocations was observed. Copyright (C) 1996 Acta Metallurgic a Inc.