The Study Of Constrained Phase Transformation Of TiNi Shape Memory Alloy In Smart Composite

The concept of intelligent materials came into being in the late 1980's. Shape memory alloy smart composites have been one of the most absorbable things hi the diversity of intelligent materials. Phase transformation behaviors of shape memory alloy under constraint of matrix materials has been a crucial issue for application of the shape memory alloy smart composites. However, systemic research is absent in this field.Based on the practical feature of shape memory alloy smart composites, phase transformations and associated thermomechanical behaviors of prestrained near-equiatomic TiNi shape memory alloy under a constant strain constraint and TiNi/cement composite were explored using electric resistance measurement, DSC, resistance strain gauge, SEM, TEM etc., in this dissertation. And the major conclusions are listed as follows.Phase transformation and incomplete phase transformation behaviors of TiNi shape memory alloy within small prestrain level (<10%) were studied by DSC and recovery strain measurements. The results show that the reverse martensitic transformation start temperature increases with the increase of prestrain, which may be due to the elastic energy released by prestrain, and the energy dissipated increased by plastic deformation and the constrain effects of grains. The main reason that the start temperature of the second reverse transformation is lower than that of the first one is due to the plastic deformation of the grains and induced dislocations. After an incomplete reverse transformation, the prestrained TiNi alloy sample is composed of the self-accommodation martensite and the remained oriented martensite. In the following reverse transformation, the start temperature of the former is lowered and the start temperature of the latter is raised.The recovery stress and actuating characteristic were studied. To the TiNi alloy heat-treated at 973K, the recovery stress is a monotonous rise function of temperature and reaches its maximum at prestrain 8%. The parameter da/dT increases with increasing prestrain level. The route of the recovery stress upon the second heating is obviously different from that upon the first heating, but the amounts of the recovery stress is similar to each other at the first stop heating temperature. To the TiNi alloy heat-treated at 723 K, a decrease stage appears in the recovery-temperature curve, whichmay due to the particular oriented martensites formed during the wire drawing process.Phase transformation behaviors of TiNi shape memory alloy under constraint of a constant strain were explored by use of resistance measurements, DSC and TEM. The results indicated that the expansion of the temperature span of the reverse martensite transformation under constraint comparing with that in free state is due to two contributions, the effect of recovery stress and the oriented martensite re-deformation process induced by recovery stress. In the forward transformation, the stress-induced martensite is induced by the recovery stress and the appeared R phase transformation is associated with induced dislocations hi the oriented martensite re-deformation process. After an incomplete constrained transformation cycle, a two-stage recovery stress in constrained state or a two-stage recovery strain hi free state will emerge in the subsequent heating as the result of the reverse transformation of the stress-induced martensite and the remained oriented martensite to parent, respectively.A TiNi fiber/ cement composite with a perfect interfacial bonding was fabricated and its phase transformation behaviors were explored. The results show that if the prestrain level exceeds a certain amount, the composite will rupture due to the large recovery stress upon heating. The reverse transformation of the TiNi alloy fiber under the constraint of cement matrix is the oriented martensite re-deformation process. In the temperature span of the endothermic peak of the DSC heating curve, the average transformation rate is relatively large. The reverse transformation of oriented marte...