Determination of heat of transformation in a cold-rolled martensitic tini alloy
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INTRODUCTION
A M O N G many shape memory alloys, TiNi alloys are the most popular because they can be deformed in a ductile manner to more than 50 pct strain p r i o r to fracture and show the shape memory effect o r pseudoelasticity (PE) over a wide range o f strain up to 8 pctJ II The transformation behavior and mechanical properties in TiNi binary alloys [2-6J and TiNiX ternary alloys t7-1°j have been studied extensively. These studies have confirmed that transformation behaviors and mechanical properties can be affected by internal stresses induced by many thermalmechanical treatments, including thermal cycling, t~,12] aging treatment in Ni-rich alloys, t13-~61 and annealing immediately following cold working. [~7,~8] Many recent articles have reported deformation behavior and stress effects in TiNi alloys07,18,2°,2tj and stated that the stress in these alloys significantly affects the start and finish transformation temperatures and induces the R-phase transformation and that the microstructure with dislocations o r precipitates impedes the movement o f martensite interfaces and depresses the Ms point. All o f these reported studies, however, were conducted on TiNi specimens w h i c h had been deformed with subsequent annealing. TiNi specimens that have been deformed but not annealed are seldom studied, although some results have been implied in the r e p o r t s . [22,23,241 In our previous article, [wl we discussed the effects o f cold rolling on the martensitic transformation o f equiatomic TiNi alloy primarily through transmission electron microscope (TEM) observation and internal friction measurement. W e observed the phenomenon o f martensite stabilization in the cold-rolled TisoNis0 alloy at r o o m temperature. W e expected the deformed structures, such as distorted martensite and dislocations/vacancies, to impede the reverse H.C. LIN, f
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