Chengzhi Zhao

Plastic deformation mechanisms of equiatomic Ni20Ti20Fe20Al20Cu20 high-entropy alloy at high temperatures

A novel equiatomic Ni20Ti20Fe20Al20Cu20 high-entropy alloy was designed and fabricated in order to investigate plastic deformation mechanisms at high temperatures. Plenty of nano-precipitates are able to be observed in the as-cast high-entropy alloy. The nano-precipitate belongs to a face-centered cubic (FCC) structure, while the matrix possesses a body-centered cubic (BCC) structure. The high-entropy alloy exhibits the poor plasticity at room temperature, but it exhibits the high plasticity at elevated temperatures. The nano-precipitates gradually grow with the increase in the deformation temperature at elevated temperatures, where there exists an orientation relationship of [011]FCC//[011]BCC between the precipitate and the matrix. The plastic flow stress of Ni20Ti20Fe20Al20Cu20 high-entropy alloy at elevated temperatures is characterized by a steady state, which can be attributed to a competition between the increase of dislocation density due to plastic strain and the decrease of dislocation density due to dynamic recrystallization. In the case of elevated temperatures, plastic deformation mechanism for dislocation slip occurs in the matrix of Ni20Ti20Fe20Al20Cu20 high-entropy alloy, while plastic deformation mechanism for deformation twinning is dominant in the precipitates. Therefore, dislocation slip and deformation twinning play a significant role in plastic deformation of Ni20Ti20Fe20Al20Cu20 high-entropy alloy at elevated temperatures.

Influence of Addition of Nb on Phase Transformation, Microstructure and Mechanical Properties of Equiatomic NiTi SMA

Three novel NiTiNb shape memory alloys, which possess a nominal chemical composition of Ni50−x/2-Ti50−x/2-Nbx (at.%) where x stands for 2, 4 and 6, respectively, were designed in order to investigate the influence of the addition of Nb on phase transformation, microstructure and mechanical properties of equiatomic NiTi shape memory alloy. All the three NiTiNb shape memory alloys contain B2 austenite phase, B19′ martensite phase and β-Nb precipitate phase. Martensite type II twin can be observed in the case of Ni49Ti49Nb2 alloy. In the case of Ni48Ti48Nb4 alloy, there exists a boundary between Ti2Ni precipitate phase and β-Nb precipitate phase. As for Ni47Ti47Nb6 alloy, it can be observed that there exists an orientation relationship of

Deformation Heterogeneity and Texture Evolution of NiTiFe Shape Memory Alloy Under Uniaxial Compression Based on Crystal Plasticity Finite Element Method

[01\bar{1}]_{{\upbeta{\text{ - Nb}}}} //[01\bar{1}]_{\text{B2}}

Microstructure and properties of 17-4PH steel plasma nitrocarburized with a carrier gas containing rare earth elements

[011¯]β- Nb//[011¯]B2 between β-Nb precipitate phase and B2 austenite matrix. The increase in Nb content contributes to enhancing the yield stress of NiTiNb shape memory alloy, but it leads to the decrease in compression fracture stress. The addition of Nb to equiatomic NiTi shape memory alloy does not have a significant influence on the transformation hysteresis of the alloy, which is attributed to the fact that NiTiNb shape memory alloy is not subjected to plastic deformation and hence β-Nb precipitate phase is unable to relax the elastic strain in the martensite interface.

Influence of Ni4Ti3 precipitates on phase transformation of NiTi shape memory alloy

The effect of plane strain compression and subsequent recrystallization annealing on microstructures and phase transformation of NiTiFe shape memory alloy (SMA) is investigated. Inhomogeneous plastic deformation at various deformation degrees occurs in NiTiFe SMA during plane strain compression. Nanocrystalline phase and amorphous phase increase as the deformation degree increases. B2 austenite, B19′ martensite, nanocrystalline and amorphous phases coexist in the NiTiFe samples subjected to large plastic strain. The static recrystallization mechanisms depend on the microstructures of the deformed NiTiFe samples. The static recrystallization mechanisms deal with nucleation and growth of the recrystallized grains, growth of nanocrystalline phase and crystallization of amorphous phase. Grain size, subgrain boundaries, geometrically necessary dislocation density and Schmid factor are captured on the basis of electron backscattered diffraction data. The process of recrystallization annealing cannot eliminate the deformation texture completely. The slip direction [110] is the most favorable slip direction in the recrystallized NiTiFe sample. Plane strain compression along with subsequent recrystallization annealing changes the phase transformation path of as-rolled NiTiFe SMA, and it results in the decreasing martensite transformation start temperature. The three annealed NiTiFe samples exhibit the similar phase transformation behavior since complete recrystallization annealing leads to the similar microstructures.

Physical mechanisms of nanocrystallization of a novel Ni-based alloy under uniaxial compression at cryogenic temperature

Crystal plastic finite element method (CPFEM) is used to simulate microstructural evolution, texture evolution and macroscopic stress-strain response of polycrystalline NiTiFe shape memory alloy (SMA) with B2 austenite phase during compression deformation. A novel two-dimensional polycrystalline finite element model based on electron back-scattered diffraction (EBSD) experiment data is developed to represent virtual grain structures of polycrystalline NiTiFe SMA. In the present study, CPFEM plays a significant role in predicting texture evolution and macroscopic stress-strain response of NiTiFe SMA during compression deformation. The simulated results are in good agreement with the experimental ones. It can be concluded that intragranular and intergranular strain heterogeneities are of great importance in guaranteeing plastic deformation compatibility of NiTiFe SMA. CPFEM is able to capture the evolution of grain boundaries with various misorientation angles for NiTiFe SMA subjected to the various compression deformation degrees. During uniaxial compression of NiTiFe SMA, the microstructure evolves into high-energy substructure and consequently the well-defined subgrains are formed. Furthermore, the grain boundaries and the subgrain boundaries are approximately aligned with the direction in which metal flows.