DEGRADATION OF SHAPE MEMORY EFFECT

Abstract
An important parameter for deciding whether or not a SME alloy is suitable for practical applications is the magnitude of the strain reversa1 accompanying martensite reversion. This research is concerned with elucidating metallurgical factors that cause degradation of this heat-activated recovery strain, E_R. After explaining what is meant by degradation, two manifestations of degradation recently identified in near-monotectoid uranium-niobium alloys will be described. The first was associated with the onset of plastic deformation of the martensite beyond the reversible strain limit, E_L ; a reduction of E_R from 5.25% at 8% total strain, i.e. E_L, to 2.9% at 12% total strain was observed. A second type of degradation depended strongly on the heating rate during reversion ; the E_R for an imposed strain of 6.95% was reduced from a value of 5.25% to 1.3% when the heating rate was decreased from 40 deg/sec to 0.05 deg/sec. Degradation was attributed to a change in the transformation path and the interjection of time-dependent, low temperature aging reactions.