Influence of martensite stabilization on the low-temperature non-linear anelasticity in Cu-Zn-Al shape memory alloys

The advanced acoustic technique has been used to investigate the mobility of partial dislocations/intervariant boundaries in the beta(1)' martensite of a Cu-Zn-Al alloy subjected to the martensite stabilization and to the beta-phase ageing, suppressing the stabilization effect. The non-linear anelasticity has been studied for frequencies of about 100 kHz and strain amplitudes 2x10(-7)-2x10(-4) over the temperature range 300-8 K. Measurements at low temperatures, below approximately 70 K, allowed us to eliminate anelastic effects associated with the motion of quenched-in defects, which are 'frozen' for these temperatures, and to assess the intrinsic mobility of partial dislocation/intervariant boundaries. The results obtained for stabilized samples are compared with those for beta-phase aged samples, and with the previously reported data for the Cu-Al-Ni alloy, which is not prone to the stabilization at ambient temperatures. We suggest distinguishing mechanisms of stabilization according to their localization: a homogeneous and a heterogeneous component. Namely, short-range reordering occurring in the bulk of the crystal is responsible for the homogeneous component of the stabilization. The local rearrangement of the martensite structure in the vicinity of lattice defects (pinning of partial dislocations/intervariant boundaries by quenched-in defects and more intense than in the bulk localized reordering) is assumed to be responsible for the heterogeneous component of the stabilization process. The acoustic technique is shown to be able to distinguish and to study details of various effects associated with the heterogeneous and homogeneous changes in the structure of martensite, induced by the stabilization and different heat treatments. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.