Study On Deformation Mechanism Of NiTi-based Shape Memory Alloys Under Extreme Conditions

As the important functional materials, NiTi-based shape memory alloys (SMAs) have the unique shape memory effect, superelastic and the excellent mechanical properties. They are very suitable for engineering applications, involving biomedicine, machine and electron, aerospace, and many other areas. At present, NiTi-based SMAs mostly focus on research under conventional experimental conditions. While, they are also under extreme conditions (e.g., high speed-rate, high pressure, high and low temperatures) during practical applications, such as armour defeat mechanisms, crashworthiness testing and satellite protection. According to the above reasons, the single gas gun, high-speed tensile machine and variable-temperature tensile machine were used to study the NiTi and Ni47Ti44Nb9 SMAs under extreme conditions. Their mechanical properties, phase transformation behaviors and microstructural evolution were systematic investigated. The main research results were obtained as follows:After shock loading on NiTi SMA, three endotherms are observed in the first heating cycle, showing the presence of three-step reverse phase transformation; whereas during the second heating only one endotherm is seen, because the other two endotherms attributed to stress-induced martensite have disappeared. A small shoulder is detected in exothermic peak, indicating that the intermediate phase (R-phase) results in two-step phase transformation. In shocked NiTi specimens, the main martensitic twin type are<011> type ? twin and (11-1) type ? twin. Moreover, (001) compound twin is also observed in detwinning area of <011> type ?.After shock loading on Ni47Ti44Nb9 SMA, the initial sheet textures of{111} <1-10> and {111}<0-11> gradually evolve into{111} plane texture. Meanwhile, a weak texture component{001} is also observed in all the shocked specimens, indicating that the recovery strains in shocked specimens decrease. During the shock loading, the dislocations first gathered around the ?-Nb particles phase. The ?-Nb particles first absorb the shock energy by their own deformation, only when the shock energy is more than the critical bearing capacity of ?-Nb particles, the deformation process would diffuse into NiTi matrix phase. As the shock velocity up to 958m/s, the deformation twins are formed in NiTi matrix of Ni47Ti44Nb9 SMA.During the tensile experiments of NiTi SMA under various strain-rates, it is found that the martensitic detwinning stress has the positive strain-rate dependence. A large number of detwinning regions were found in the NiTi specimen, which was deformed at the strain-rate of 10/s under tension. While, with the strain-rate further up to 100/s and 1200/s, no detwinning region was detected. It indicates that the detwinning rate of martensitic twins are in the range of 10/s-100/s. Meanwhile, thermal-induced austenite was detected in the NiTi specimens deformed at high strain-rates (?10/s). It is ascribed to that there is a change from the isothermal process to the adiabatic process when the tensile strain-rate is up to a critical value.The detwinning stress, tensile strength and max strain of NiTi alloy have the temperature dependence, when the NiTi alloy is in martensite. During the tensile test of NiTi alloy at 450 ?, with the stress is up to 50MPa, a stress-plateau of a length about 13% is observed, which is related to the process of dynamic recrystallization. The detwinning process of <011> type II twin and the formation of (001) compound twin could proceed simultaneously.Differ from the NiTi SMA, no detwinning stress-plateau was detected in the low temperature tensile curves of Ni47Ti44Nb9 SMA. It is due to that the ?-Nb particles phase becomes hard at low temperature, so the resistence of detwinning is increased. The DSC peaks only appear in the Ni47Ti44Nb9 specimen deformed at 450? and the DSC peaks in the others are all not observed. While, the DSC peaks are always presence in the NiTi specimens deformed at various temperatures. This difference is also caused by ?-Nb particles. During the tensile tests, the dislocations gather around the ?-Nb particles and their deformation stability could be enhanced, so the phase transformation was prevented. Owing to the 450? is high enough to eliminate some dislocations, the DSC peaks exist in this specimen.