Pinning-induced stabilization of martensite - Part II. Kinetic stabilization in Cu-Zn-Al alloy due to pinning of moving interfaces

Two concurrent mechanisms of thermal stabilization of martensite have been distinguished in a Cu-21.3wt%Zn-5.4wt%Al-0.05wt%B alloy: kinetic stabilization, occurring during the reverse martensitic transformation and associated with sweeping-up of quenched-in defects by moving interfaces; swept-up quenched-in defects inhibit the motion of interfaces similar to the dynamic strain ageing effect; static stabilization, associated with isothermal ageing in the martensitic state. These stabilization mechanisms demonstrate opposite dependences on the duration of martensite ageing: the kinetic stabilization is suppressed by martensite ageing, whereas the static component is promoted by martensite ageing. The kinetic stabilization can be strongly predominant over the static one so as to cause only partial reverse transformation for short martensite ageing periods. In that case, the reverse transformation becomes more complete with martensite ageing. The suppression of kinetic stabilization with martensite ageing reflects a dramatic modification of the system of quenched-in defects upon martensite ageing, which includes either vanishing of the defect-assisted diffusion of quenched-in defects or a redistribution of quenched-in defects in the bulk of the sample. The origin of such redistribution can be pinning of interfaces at their static positions, responsible for the static pinning-induced stabilization component. The kinetic stabilisation also exemplifies pure pinning mechanism of stabilization, related, in contrast to static pinning, to immobilisation of moving interfaces. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.