Shuyong Jiang

Dynamic recovery and dynamic recrystallization of NiTi shape memory alloy under hot compression deformation

Abstract Mechanical behavior of nickel–titanium shape memory alloy (NiTi SMA) under hot deformation was investigated according to the true stress—strain curves of NiTi samples under compression at the strain rates of 0.001–1 s −1 and at the temperatures of 600–1000 °C. Dynamic recovery and dynamic recrystallization of NiTi SMA were systematically investigated by microstructural evolution. The influence of the strain rates, the deformation temperatures and the deformation degree on the dynamic recovery and dynamic recrystallization of NiTi SMA was obtained as well. NiTi SMA was characterized by the combination of dynamic recovery and dynamic recrystallization at 600 °C and 700 °C, but the complete dynamic recrystallization occurred at other deformation temperatures. Increasing the deformation temperatures or decreasing the stain rates leads to larger equiaxed grains. The deformation degree has an important influence on the dynamic recrystallization of NiTi SMA. There exists the critical deformation degree during the dynamic recrystallization of NiTi SMA, beyond which the larger deformation degree contributes to obtaining the finer equiaxed grains.

Multi-pass spinning of thin-walled tubular part with longitudinal inner ribs

Based on the process experiments, micrography analysis was dedicated to advancing the understanding of plastic flow of the metal in backward ball spinning of thin-walled tubular part with longitudinal inner ribs. Micrography analysis reveals that severe plastic deformation leads to grain refinement, grain orientation and grain flow line of the spun part. Based on rigid-plastic finite element method, DEFORME3D finite element code was used to simulate and analyze multi-pass backward ball spinning of thin-walled tubular part with longitudinal inner ribs. Finite element simulation results involve the distributions of the strain, the shape variation of the inner ribs as well as the prediction of the spinning loading.

Effect of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi shape memory alloy

Abstract As-received nickel–titanium (NiTi) shape memory alloy with a nominal composition of Ni50.9Ti49.1 (mole fraction, %) was subjected to solution treatment at 1123 K for 2 h and subsequent aging for 2 h at 573 K, 723 K and 873 K, respectively. The influence of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi alloy was systematically investigated by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and compression test. Solution treatment contributes to eliminating the Ti2Ni phase in the as-received NiTi sample, in which the TiC phase is unable to be removed. Solution treatment leads to ordered domain of atomic arrangement in NiTi alloy. In all the aged NiTi samples, the Ni4Ti3 precipitates, the R phase and the B2 austenite coexist in the NiTi matrix at room temperature, while the martensitic twins can be observed in the NiTi samples aged at 873 K. In the NiTi samples aged at 573 and 723 K, the fine and dense Ni4Ti3 precipitates distribute uniformly in the NiTi matrix, and thus they are coherent with the B2 matrix. However, in the NiTi sample aged at 873 K, the Ni4Ti3 precipitates exhibit the very inhomogeneous size, and they are coherent, semi-coherent and incoherent with the B2 matrix. In the case of aging at 723 K, the NiTi sample exhibits the maximum yield strength, where the fine and homogeneous Ni4Ti3 precipitates act as the effective obstacles against the dislocation motion, which results in the maximum critical resolved shear stress for dislocation slip.

Influence of cooling rate on phase transformation and microstructure of Ti-50.9%Ni shape memory alloy

Heat treatment of Ti-50.9%Ni (mole fraction) alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence of cooling rate on transformation behavior and microstructures of NiTi shape memory alloy. The experimental results show that three-stage phase transformation can be induced at a very low cooling rate such as cooling in furnace. The cooling rate also has a great influence on the phase transformation temperatures. Both martensitic start transformation temperature (Ms) and martensitic finish transformation temperature (Mf) decrease with the decrease of the cooling rate, and decreasing the cooling rate contributes to enhancing the M→A austenite transformation temperature. The phase transformation hysteresis (Af-Mf) increases with the decrease of the cooling rate. Heat treatment is unable to eliminate the textures formed in hot working of NiTi sample, but can weaken the intensity of them. The cooling rate has little influence on the grain size.

Microstructure evolution and deformation behavior of as-cast NiTi shape memory alloy under compression

Abstract The as-cast ingot of equiatomic nickel-titanium shape memory alloy (SMA) was prepared via vacuum consumable arc melting. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) analysis, energy dispersive X-ray (EDX) measurement and microanalysis were adopted in order to understand the microstructure evolution and deformation behavior of the as-cast NiTi SMA under compression at various strain rates and temperatures. The microstructures of as-cast NiTi SMA composed of dendritic grains and equiaxed grains are inhomogeneous and show segregation. The as-cast NiTi SMA consists of B19′ martensite, B2 austenite and Ti2Ni phase simultaneously at room temperature. The as-cast NiTi SMA is sensitive to strain rate under compression at high temperature, at which NiTi SMA is characterized by dynamic recrystallization at strain rates of 0.1 and 0.01 s−1, but by dynamic recovery at strain rate of 0.001s−1. The strain rates have little influence on the true stress—strain curves as well as microstructure of NiTi SMA at room temperature and −100°C.

Crystallization of amorphous NiTi shape memory alloy fabricated by severe plastic deformation

Abstract Based on the local canning compression, severe plastic deformation (SPD) is able to lead to the almost complete amorphous nickel-titanium shape memory alloy (NiTi SMA), in which a small amount of retained nanocrystalline phase is embedded in the amorphous matrix. Crystallization of amorphous NiTi alloy annealed at 573, 723 and 873 K was investigated, respectively. The crystallization kinetics of the amorphous NiTi alloy can be mathematically described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. NiTi SMA with a complete nanocrystalline phase is obtained in the case of annealing at 573 K and 723 K, where martensite phase transformation is suppressed due to the constraint of the grain boundaries. Crystallization of amorphous NiTi alloy at 873 K leads to the coarse-grained NiTi sample, where (001) martensite compound twin is observed at room temperature. It can be found that the martensitic twins preferentially nucleate at the grain boundary and they grow up towards the two different grains. SPD based on the local canning compression and subsequent annealing provides a new approach to obtain the nanocrystalline NiTi SMA.

Equal channel angular extrusion of NiTi shape memory alloy tube

Abstract As a new attempt, equal channel angular extrusion (ECAE) of nickel–titanium shape memory alloy (NiTi SMA) tube was investigated by means of process experiment, finite element method (FEM) and microscopy. NiTi SMA tube with the steel core in it was inserted into the steel can during ECAE of NiTi SMA tube. Based on rigid-viscoplastic FEM, multiple coupled boundary conditions and multiple constitutive models were used for finite element simulation of ECAE of NiTi SMA tube, where the effective stress field, the effective strain field and the velocity field were obtained. Finite element simulation results are in good accordance with the experimental ones. Finite element simulation results reveal that the velocity field shows the minimum value in the corner of NiTi SMA tube, where severe shear deformation occurs. Microstructural observation results reveal that severe plastic deformation leads to a certain grain orientation as well as occurrence of substructures in the grain interior and dynamic recovery occurs during ECAE of NiTi SMA tube. ECAE of NiTi SMA tube provides a new approach to manufacturing ultrafine-grained NiTi SMA tube.

Fracture behavior and microstructure of as-cast NiTi shape memory alloy

Abstract The as-cast ingot of equiatomic nickel–titanium shape memory alloy (NiTi SMA) was prepared by vacuum consumable arc melting. The tensile tests and the compressive tests with respect to as-cast NiTi SMA were performed to study its mechanical properties of fracture. The microanalysis of as-cast NiTi SMA as well as its fractured samples was performed so as to better understand microstructure evolution and fracture behavior of NiTi SMA. Under tensile loading, the as-cast NiTi SMA shows higher plasticity and is characterized by ductile fracture at 750 °C, but it demonstrates poorer plasticity and is characterized by cleavage fracture as well as transcrystalline fracture at room temperature and −100 °C. Under compressive loading at −100 °C, the as-cast NiTi SMA is characterized by shear fracture where the normal to the shearing fracture surface inclines about 45° to the compressive axis, and belongs to cleavage fracture where the cracks expand via transcrystalline fracture.

Constitutive equation and processing map of equiatomic NiTi shape memory alloy under hot plastic deformation

Abstract In order to describe the deformation behavior and the hot workability of equiatomic NiTi shape memory alloy (SMA) during hot deformation, Arrhenius-type constitutive equation and hot processing map of the alloy were developed by hot compression tests at temperatures ranging from 500 to 1100 °C and strain rates ranging from 0.0005 to 0.5 s −1 . The results show that the instability region of the hot processing map increases with the increase of deformation extent. The instability occurs in the low and high temperature regions. The instability region presents the adiabatic shear bands at low temperatures, but it exhibits the abnormal growth of the grains at high temperatures. Consequently, it is necessary to avoid processing the equiatomic NiTi SMA in these regions. It is preferable to process the NiTi SMA at the temperatures ranging from 750 to 900 °C.

Finite element simulation of ball spinning of NiTi shape memory alloy tube based on variable temperature field

Abstract As a new attempt, ball spinning was used to manufacture the nickel-titanium shape memory alloy (NiTi SMA) tube at elevated temperature. The NiTi bar with a nominal composition of Ni 50.9 Ti 49.1 (mole fraction, %) was solution treated and was used as the original tube blank for ball spinning. Based on the variable temperature field and the constitutive equation, rigid-viscoplastic finite element method (FEM) was applied in order to simulate the ball spinning of NiTi SMA tube. The temperature field, the stress field, the strain field and the load prediction were obtained by means of FEM. FEM results reveal that there is a temperature increase of about 160 °C in the principal deformation zone of the spun part. It can be found from the stress fields and the strain fields that the outer wall of NiTi SMA tube is easier to meet the plastic yield criterion than the inner wall, and the plastic deformation zone is caused to be in a three-dimensional compressive stress state. The radial strain and the tangential strain are characterized by the compressive strain, while the axial strain belongs to the tensile strain. The variation of spinning loads with the progression of the ball is of great importance in predicting the stable flow of the spun part.