Influence of training time and temperature on shape memory effect in Cu-Zn-AI alloys
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I.
INTRODUCTION
T H E application of the Cu-Zn-AI shape memory alloy (SMA) was associated with the two-way memory effect (TWME) in many cases and, thus, had been thoroughly studied by many researchers,v-n However, there were some serious problems in applying Cu-Zn-A1 SMAs, one of which was the shift of transformation temperatures in thermal cycling and service process. [3~It was generally believed that the formation of preferential thermoelastic martensite is the origin of the TWME in Cu-Zn-A1 alloys.t1,21 In order to obtain stable forming paths for preferential martensite, samples should be subjected to thermomechanical training. Presently, the TWME could be achieved by cyclings of SME training[4] or stress induced martensite (SIM) trainins ~Sj as well as a series of combined SIM and SME training. t61 The combined SIM and SME training was the most effective procedure for inducing the TWME in Cu-Zn-A1 SMAs.[7] Enhancement of the TWME had an attraction for many researchers. Their conclusions indicated that the preferential dislocations and retained martensite generated in training were favorable to the formation of preferential martensite variants, tl,21 However, overtraining, i.e., excesses of applied stress or number of training cyclings may result in large residual deformations in the parent phase and eventually the loss of the TWME. t81 The optimum constraint stress was the minimum stress to induce the stress-assisted transformation effect (SATE) near to the saturation condition, while the optimum number of training cycles corresponded to the state where the TWME reached the maximum. The sufficient number of training cycles was considered to be approximately 10 to 15.m However, conceming the effect of training temperature on the TWME, few references were found. Usually, training for the TWME was conducted in a range between the upper temperature Tp (above As) and the lower temperature T m (below Ms),tl,8]
but effects of the training temperature interval on the TWME were scarcely studied. Consequently, one may consider that training time may also be an important factor influencing shape memory behaviors. In the present investigation, it has been found by the authors that there exists an optimum upper training temperature resulting in the best SME for Cu-Zn-A1 SMA as well as a certain training time without any shift of A~ temperature at that temperature after 1000 thermal cyclings.
II.
EXPERIMENTAL
A polycrystalline SMA with composition Cu-25.66Zn4.02A1 (wt pct) was melted and then homogenized at 1123 K for 8 hours. Ingots were then rolled into sheets with interrupted annealings at 923 K and cut into strips of 100 • 4 • 0.5 ram. A step quenching of beta solution treating at 1123 K for 15 minutes following by a holding at 413 K for 10 minutes was adopted. With this treatment, the consequent transformation temperatures are determined to be M, = 316K, M l = 308K, As = 315K, andA I = 324K. Samples are finally subjected to the combined SIM and SME training by bending up strips into " U " samples of 15-mm radius o
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