| The effect of aging on the microstructure, transformation behavior and mechanicalbehavior of Ti48.4Ni51Cr0.6and Ti48.3Ni51.1V0.6alloy were investigated in this study. Theeffect on microstructure was studied by means of optical metallographic microscope,scanning electron microscope, X-ray diffraction and transmission electron microscope. Theeffect on transformation behavior was studied using differential scanning calorimetry. Theeffect of aging treatment, deformation strain and testing temperature on mechanicalbehavior was investigated with tensile deformation and stress-strain cycling test. Thecorrelation between microstructure, transformation behavior, mechanical behavior andsuperelasticity was discussed.The solution treated TiNiCr and TiNiV alloys consist of parent phase and secondphase Ti2Ni at room temperature. Fine spheric Ti3Ni4precipitates formed in TiNiCr alloyafter aging treatment at400°C for30min. The Ti3Ni4precipitate became larger in sizewhen elevating the aging temperature. When aging temperature reached600°C, theprecipitate with lenticular morphology was observed. The precipitate showed a highdistribution density at matrix adjacent to grain boundary and second phase, while theprecipitate density at grain interior was low. TiNiCr alloy aged at500°C for differentduration consist of parent phase, Ti2Ni phase and Ti3Ni4precipitate.The solution treated TiNiCr and TiNiV alloy exhibited single-stage transformation（A M） upon cooling and heating. Aging treatment showed significant influence on thetransformation behavior of the alloys. To TiNiCr alloy, when aging temperature was below400°C, single-stage transformation A M occurred upon cooling and heating; when theaging temperature was between400-500°C, two-stage transformation （A R M） wasobserved；when ageing temperature reached550°C, the alloy again showed a single-stagetransformation （A M）. Sample aged at500°C for1h showed two-stage transformation（A R M） upon cooling and heating. Aging for over2h resulted in three-stagetransformation, the sequence of which is A R M1transformation at matrix nearprecipitate and A M2transformation at none-precipitate region. Aging treatment showssimilar effect on the transformation sequence of TiNiV alloy with TiNiCr alloy.The evolution of transformation temperature with aging is different between TiNiV and TiNiCr alloy. To TiNiCr alloy after aging at different temperature, the transformationtemperature firstly decreased with aging temperature and then increased. To TiNiV alloy,critical temperature for both forward and reverse transformation increased with agingtemperature.Solution treated alloy and alloy after aging treatment exhibited well-definedsuperelasticity at room temperature. The critical stress to induce martensitic transformation(σSIM) declined firstly and then rose with increasing aging temperature. The function ofaging temperature on recovery ratio was different with different deformation strain. Whenthe deformation strain was below8%, the recovery ratio first fell and then went up withincreasing aging temperature. When deformation strain reached10%, the recovery ratiobasically fell with aging temperature.For alloy aged at500°C for different duration, extending the aging duration resultedin the decrease in σSIMand the increase in εYM. TiNiCr alloy aged at500°C for differentduration performed well defined superelasticity at room temperature. TiNiV alloy afteraging for less than5h also pocessed well defined superelasticity. When aging durationexceeds5h, the superelasticity was depressed.When tested at40°C, alloy aged at400°C for30min and at500°C for differentduration showed favorable superelasiticity. When the testing temperature rose to60°C, nosuperelasticity can be observed. It is indicated that the superelasticity of TiNiCr and TiNiValloy is only applicable at temperature lower than60°C. |
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