Ti50Ni50及Ti49.3Ni50.7鈦鎳形狀記憶合金變態及機械性能之研究

Abstract

In this study, the property improvement of shape memory effect (SME), pseudoelasticity (PE) and stress-strain (σ-e) cycling of Ti49.3Ni50.7 and Ti50Ni50 shape memory alloys (SMAs) is investigated. Ti49.3Ni50.7 SMA aged at 300℃×100h and 400℃×8h can reach the maximal precipitation-hardening with the hardness of the former being higher than that of the latter. Tensile test indicates that the specimen aged at 300℃×100h has better SME/PE and σ-e cycling properties than that aged at 400℃×8h. Cold-rolling effect on the property improvement is studied on Ti50Ni50 SMA. Experimental results show that the degree of cold-rolling lower than 20% is insufficient to strengthen the SMAs to improve their properties, such as the σ-e cycling stability and the recoverable storage energy in σ-e curve. If the annealing of cold-rolled specimen is over, the SMAs’ properties can also be deteriorated. At the same time, the σ-e cycling test indicates that, after 20th cycles, both R-phase and B19’ martensitic transformations are depressed due to the dislocations pile-up during the cycling, and the B2→R transformation is more depressed than R→B19’ one. In this study, the maximal PE strain induced by stress-induced martensite (SIM) is found to be lower than ~7% and the plasticity deformation occurs if the strain is higher than 7% which will deteriorate the SMAs’ PE property. For the strain rate (e ) effect on the property improvement of Ti50Ni50 SMA, in the e range of 2.5×10-4s-1~1.0×10-2s-1, the σ-e cycling with higher e will be more beneficial to the forward SIM transformation, instead of the reverse SIM transformation during the cycling.