Z.J. Zhang

Competition between slip and twinning in face-centered cubic metals

A criterion is proposed to predict the transition from slip to twinning in the face-centered cubic (FCC) metals within the framework of the generalized stacking fault energy (GSFE) curves, which are calculated by using the full potential linearly augmented plane wave method incorporating local orbital based on density functional theory. The criterion is a function of the stacking fault energy, unstable stacking fault energy, and unstable twinning fault energy, which are obtained from the GSFE curves. Based on the competition between slip and twinning, a new parameter is proposed to rank the sequence of the twinnability for ten typical FCC metals. The ranking results are in good agreement with the experimental data available. Compared with the previous studies of heterogeneous nucleation of twin, it is found that the heterogeneous nucleation sites of twinning such as crack tip and grain boundary do not change the ranking sequence. It is suggested that the GSFE curve may be regarded as the dominant factor determining the twinnability of FCC metals.

Tensile Fracture Modes in Fe-22Mn-0.6C and Fe-30Mn-3Si-3Al Twinning-Induced Plasticity (TWIP) Steels

Tensile tests were carried out to investigate the differences in fracture mechanisms between Fe-22Mn-0.6C and Fe-30Mn-3Si-3Al (wt pct) twinning-induced plasticity steels. Although both steels possess a strong twinning capability during tensile deformation, they display different tensile fracture modes of shear and necking. The Portevin–le Chatelier band is proposed as the key factor influencing the different fracture mechanisms.

Forecasting Low-Cycle Fatigue Performance of Twinning-Induced Plasticity Steels: Difficulty and Attempt

We find the existing empirical relations based on monotonic tensile properties and/or hardness cannot satisfactorily predict the low-cycle fatigue (LCF) performance of materials, especially for twinning-induced plasticity (TWIP) steels. Given this, we first identified the different deformation mechanisms under monotonic and cyclic deformation after a comprehensive study of stress–strain behaviors and microstructure evolutions for Fe-Mn-C alloys during tension and LCF, respectively. It is found that the good tensile properties of TWIP steel mainly originate from the large activation of multiple twinning systems, which may be attributed to the grain rotation during tensile deformation; while its LCF performance depends more on the dislocation slip mode, in addition to its strength and plasticity. Based on this, we further investigate the essential relations between microscopic damage mechanism (dislocation–dislocation interaction) and cyclic stress response, and propose a hysteresis loop model based on dislocation annihilation theory, trying to quickly assess the LCF resistance of Fe-Mn-C steels as well as other engineering materials. It is suggested that the hysteresis loop and its evolution can provide significant information on cyclic deformation behavior, e.g., (point) defect multiplication and vacancy aggregation, which may help estimate the LCF properties.

Quantitative understanding of anomalous slip in Mo

Hexagonal dislocation networks (HDNs) formed by the reaction of <1 1 1>/2 screw dislocations are frequently observed in association with anomalous slip in body-centred cubic (bcc) metals. However, its role assigned in anomalous slip remains obscure due to the absence of quantitative description of its response to uniaxial loading. Here, systematic atomistic simulations are performed in molybdenum (Mo) to study the responses of a typical HDN to different applied loadings. The simulation results are used to develop a quantitative yield criterion for the HDN motion under uniaxial loading. Based on this criterion together with the yield equation that can account for the non-Schmid behaviours of an isolated <1 1 1>/2 screw dislocation, the transition from primary to anomalous slips with the loading direction is predicted to be consistent with the experimental observations in many bcc metals including Mo. This work also sheds light on other experimental results such as the lack of dead-band and the displacement accompanying anomalous slip. In addition, the reason for the absence of anomalous slip in bcc iron (Fe) is found by comparison of the reaction between <1 1 1>/2 screw dislocations in Mo and Fe.

Description of E4 transitions in A=192, 194, 196, 198 platinum isotopes in a microscopic sdgIBM-1

influence of the effective fermion hexadecapole force newly incorporated in a microscopic sdgIBM-1 on spectra, reduced E2 and E4 transition matrix elements (T(E2)s and T(E4)s) in the even-even platinum isotopes (A = 192, 194, 196, 198) is investigated in terms of numerical calculations. It is found that the introduced interaction causes only limited modification to the spectrum and T(E2)s, apart fi om a few exceptions. However, it plays an essential role in describing E4 transitions. Thus in the case that the interaction is incorporated with certain strength, a reasonable description of all the E4 transitions in the platinum isotopes is reached in the microscopic sdgIBM-1 in comparing both to experimental data and the results calculated in phenomenological bosom model.