AGING EFFECTS IN COPPER-ZINC-ALUMINIUM SHAPE MEMORY ALLOYS (MARTENSITE, BAINITE)

Aging in both the beta phase and martensitic state of Cu-Zn-Al alloys causes deterioration and instability of shape memory properties, even at moderate temperatures. These aging effects have been studied by electrical resistivity measurements and bend tests. The corresponding changes in microstructure were investigated using optical microscopy, transmission and analytical electron microscopy, and X-ray diffraction.;Isothermal aging from 150 to 350 degC induces the formation of the alpha(,1) plates (bainite) which depress the martensite transformation temperatures and cause a widening transformation hysteresis and a strong cycling effect. The early stage alpha(,1) plates possess temperature dependent long range order and show the dual nature of long range substitutional diffusion and invariant plane strain (IPS) crystallographic features. It is suggested that a displacive lattice deformation is necessary to generate the IPS crystallography. The long range order and IPS geometry are sustained because of a contrained and correlated diffusion behavior in the interface.;Direct quenching and isothermal aging in the martensitic state result in stabilized martensite. The destabilization effect during step-quenching is related to reordering in the initial stage and primarily due to annihilation of quenched-in vacancies. The isothermal aging behavior has been shown to consist of two components, related to the differences in equilibrium vacancy concentration and equilibrium of the long range order state, respectively, between the parent beta(,3) phase and the martensite. Tempering eventually anneals out long range order and causes decomposition of the martensite.;In addition to the stability of the beta phase, stable long range order is a necessary factor to be considered for future alloy design of high temperature shape memory alloys.;The early stage aging effects in the parent beta phase state have been shown to be related to changes in its long range order. The beta phase possesses an L2(,1) superlattice structure rather than the previously interpreted DO(,3) structure. Post-flash-heating aging behavior is in good agreement with a long range order effect model previously proposed. Inconsistency was observed in the post-quenching behavior, which can only be accounted for if the nearest neighbor order effect makes a decisive contribution.