Influence of aging on transformation characteristics in shape memory CuZnAl alloys

Shape memory alloys exhibit superelasticity when they are deformed in a tem perature range where the thermoelastic martensite forms on application of a strain. The martensite persists upon removal of the applied strain, and th e alloy recovers the original shape on heating over the reverse-transformat ion temperature after removing the strain. The beta-phase CuZnAl alloys hav e beta-type superlattice in the parent case, and M9R or M18R martensites oc cur on quenching the alloys from the homogenization temperature. The basal plane of martensite is exposed to hexagonal distortion with martensitic tra nsformation as well as the monoclinic distortion in the crystal structure, and splittings are observed in some selected diffraction-peak pairs due to the differences in atom sizes in lattice points. These pairs have a great i mportance as ordering criteria and satisfy a special relation between Mille r indices. The present text reports the variation of the differences in int erplane spacings (Delta d) between some selected planes upon the further ag ing at room temperature at which alloys are fully martensitic. The decrease of Delta d during the aging implies that the monoclinic distortion decreas es. The mass increases are caused by the oxidation upon heating the alloys at high temperatures close to the betatizing temperature at free atmosphere .