Martensitic transformation, reverse martensitic transformation, twinning, and detwinning of equiatomic nickel–titanium shape-memory alloy (NiTi SMA) under the action of a shock wave are studied using a molecular-dynamics simulation. In the loading process of a shock wave, B2 austenite is transformed into B19′ martensite, whereas in the unloading process of the shock wave, B19′ martensite is transformed into B2 austenite. With repeated loading and unloading of the shock wave, martensitic transformation occurs along with twinning, but reverse martensitic transformation appears along with detwinning. The mechanisms for the twinning and detwinning of NiTi SMA subjected to a shock wave are revealed in order to lay the theoretical foundation to investigate the shape-memory effect and superelasticity.

Microstructural evolution of NiTi shape memory alloy (SMA) with a nominal composition of Ni_50.9Ti_49.1 (at %) is investigated on the basis of heat treatment and severe plastic deformation (SPD). As for as-rolled NiTi SMA samples subjected to aging, plenty of R phases appear in the austenite matrix. In terms of as-rolled NiTi SMA samples undergoing solution treatment and aging, Ni₄Ti₃ precipitates arise in the austenite matrix. In the case of as-rolled NiTi SMA samples subjected to SPD and aging, martensitic twins are observed in the matrix of NiTi SMA. With respect to as-rolled NiTi SMA samples subjected to solution treatment, SPD, and aging, neither R phases nor Ni₄Ti₃ precipitates are observed in the matrix of NiTi SMA. The dislocation networks play an important role in the formation of the R phase. SPD leads to amorphization of NiTi SMA, and in the case of annealing, amorphous NiTi SMA samples are subjected to crystallization. This contributes to suppressing the occurrence of R phase and Ni₄Ti₃ precipitate in NiTi SMA.

The processing map of Ni₄₇Ti₄₄Nb₉ (at %) shape memory alloy (SMA), which possesses B2 austenite phases and β-Nb phases at room temperature, is established in order to optimize the hot working parameters. Based on true stress-strain curves of NiTiNb SMA during uniaxial compression deformation at the temperatures ranging from 700 to 1000 °C and at the strain rates ranging from 0.0005 to 0.5 s⁻¹, according to dynamic material model (DMM) principle, the processing map of NiTiNb SMA is obtained on the basis of power dissipation map and instability map. The instability region of NiTiNb SMA increases with increasing the true strain and it mainly focuses on the region with high strain rate. The workability of NiTiNb SMA becomes worse and worse with increasing plastic strain, as well as decreasing deformation temperature. There exist two stability zones which are suitable for hot working of NiTiNb SMA. In one stability region, the deformation temperature ranges from 750 to 840 °C and the strain rate ranges from 0.0003 to 0.001 s⁻¹. In the other stability region, the deformation temperature ranges from 930 to 1000 °C and the strain rate ranges from 0.016 to 0.1 s⁻¹. The severe microstructure defects, such as coarsening grains, band microstructure, and intercrystalline overfiring appear in the microstructures of NiTiNb SMA which is subjected to plastic deformation in the instability zone.

Based on molecular dynamics (MD) simulation, the behaviors and mechanisms of diffusion welding between 304 stainless steel (304 SS) and pure Ni were investigated in the present study. The results show that surface roughness has a significant influence on the diffusion behaviors of atoms during diffusion welding between two different materials, and it is suggested that the rough surface should be set on the pure Ni rather than the 304 SS during the diffusion welding between them. Temperature plays an important role in the interface diffusion. With the increase of temperature, the number of atoms diffusing into the opposite side increases and the diffusion distances increase as well. As a consequence, the diffusion welding should be performed at a suitably elevated temperature. The influence of vertical pressure on the diffusion bonding between the two materials includes two aspects. One is to increase the contact area via deforming the asperities or grooves at the interface, which provides more opportunities for the diffusion between the two materials. The other is to reduce the mobility of atoms within a lattice. As a consequence, the pressure effect is smaller than temperature effect during diffusion welding between 304 SS and pure Ni.

Deformation behavior and microstructure evolution of NiTiCu shape memory alloy (SMA), which possesses martensite phase at room temperature, were investigated based on a uniaxial compression test at the temperatures of 700~1000 °C and at the strain rates of 0.0005~0.5 s⁻¹. The constitutive equation of NiTiCu SMA was established in order to describe the flow characteristic of NiTiCu SMA, which is dominated by dynamic recovery and dynamic recrystallization. Dislocations become the dominant substructures of martensite phase in NiTiCu SMA compressed at 700 °C. Martensite twins are dominant in NiTiCu SMA compressed at 800 and 900 °C. Martensite twins are not observed in NiTiCu SMA compressed at 1000 °C. The microstructures resulting from dynamic recovery or dynamic recrystallization significantly influences the substructures in the martensite phase of NiTiCu SMA at room temperature. Dislocation substructures formed during dynamic recovery, such as dislocation cells and subgrain boundaries, can suppress the formation of twins in the martensite laths of NiTiCu SMA. The size of dynamic recrystallized grains affects the formation of martensite twins. Martensite twins are not easily formed in the larger recrystallized grain, since the constraint of the grain boundaries plays a weak role. However, in the smaller recrystallized grain, martensite twins are induced to accommodate the transformation from austenite to martensite.

Based on the strain rates of 2 × 108 s−1 and 2 × 1010 s−1, molecular dynamics simulation was conducted so as to study mechanisms of crack propagation in nanoscale polycrystal nickel. The strain rate has an important effect on the mechanism of crack propagation in nanoscale polycrystal nickel. In the case of a higher strain rate, local non-3D-crystalline atoms are induced and Lomer-Cottrell locks are formed, which plays a critical role in crack initiation and propagation. Orientation difference between adjacent grains leads to the slipping of dislocations along the different directions, which results in the initiation of a void near the triple junction of grain boundaries and further contributes to accelerating the crack propagation.