Atomistic simulation of martensitic phase transformation at the crack tip in B2 NiAl

Mechanisms of low-temperature deformation at the crack tip in B2 NiAl are studied by molecular dynamics simulations. The stress-induced martensitic transformation is found to occur at the crack tip when a sufficiently high stress concentration exists. For cracks with 〈1 0 0〉 crack fronts, the layered structures of martensites are formed at the crack tip, which is caused by the atoms’ relative displacement on a basal plane due to the shear stress at the crack tip. The mechanism of the martensitic transformation from the B2 to the L1₀ structures occurs along the Bain path. For cracks with 〈1 1 0〉 crack fronts, the martensitic transformation occurs without any layered structures existing. The phase transformation is caused by the atoms’ relative displacements at different atoms layers in the entire martensite formed region.     

Atomistic simulation of dislocation nucleation and motion from a crack tip

A recently developed fully atomistic technique for fracture simulations is applied to the study of dislocation emission from a crack tip in an elastically anisotropic f.c.c. crystal. The detailed atomicscale mechanisms of dislocation nucleation and motion are investigated as a function of the external load. Analysis of the atomic configurations around the crack tip demonstrates an intimate coupling of the nucleating dislocation with a step formed at the crack surface. Displacement and stress fields around both nucleating and moving dislocations are compared to the predictions of the Peierls-Nabarro continuum-elastic model by Rice. The size of a nucleating (‘incipient”) dislocation is found to be larger than that of a fully-formed dislocation. Also, we elucidate the reasons why the value of the unstable-stacking energy estimated by means of the rigid-block sliding concept, a feature common to several continuum-elastic models, overestimates the activation energy for dislocation nucleation. We conclude that the concept of unstable-stacking energy should be replaced by the true energy barrier for dislocation nucleation, incorporating the full inhomogeneity of the displacement field.     

Effect of stress-induced martensitic transformation on the crack tip stress-intensity factor in Ni–Mn–Ga shape memory alloy

The Ni–Mn–Ga shape memory ferromagnetic shape memory alloys (FSMAs) are prone to fracture during thermal cycling. The present research shows that the primary reason for the thermally induced fracture of Ni–Mn–Ga FSMAs is the increase in the crack tip stress-intensity factor (SIF) due to stress redistribution around the crack tip as a result of stress-induced martensitic (SIM) transformation. On lowering the temperature to M_s, the crack tip SIF of Ni–Mn–Ga FSMA increases significantly, being very different from that of Ni–Ti and Cu–Al–Ni SMAs. The sensitive temperature dependence of crack tip SIF in Ni–Mn–Ga is responsible for its brittleness under thermal cycling. The temperature dependence of crack tip SIF is strongly related to the yield stress, the temperature dependence of the critical stress for SIM transformation and the transformation interval. Temperature rate also plays an important role in the fatigue behavior of Ni–Mn–Ga FSMAs under thermal cycling.     

Atomistic simulation of kink-pairs of screw dislocations in body-centred cubic iron

The nudged elastic band method is used to calculate the activation paths connecting jumps between the two degenerate states A and B of the screw dislocation core in body-centred cubic (bcc) iron. Kink-pairs are found to be involved in all such jumps. By comparing the activation energies, it is shown that the jump from state A to state B on the {101} plane is not the same as that from B to A, and whichever is easier depends on the direction of the applied stress. This asymmetry results in a characteristic zigzag pattern of slip, and forms the basis of an explanation for pencil glide at elevated temperatures. The activation energy of the rate-determining jump is found to decrease with increasing stress, but the predicted flow stress is about three times greater than the experimental value at all temperatures.     

Multiscale simulation of crack tip shielding by a dislocation

Crack tip shielding by dislocations is an essential feature of the modeling of semi-brittle crack propagation and the brittle to ductile transition. The stress field, in the elementary configuration where a single dislocation interacts with a crack, is obtained by two independent methods, at two different scales. Analytical formulas, by Lin and Thomson, are used in the case of a semi-infinite crack in an isotropic linear elastic medium. Atomistic simulations are used to simulate the emission of a perfect dislocation from the tip of a nano-scale flaw. Constrained molecular dynamics enables the dislocation to be pinned during the relaxation of the system. With this method, it is possible to obtain the static stress field which can be compared to the elastic solution. Semi-infinite and nano-scale cracks produce the same stress field in the vicinity of the tip, as predicted by continuum fracture mechanics. This property is the basis of the multiscale approach proposed here, where critical stress intensity factors are computed at the atomic scale, on the nano-scale crack, and transfered to the meso-scale simulation, based on the elastic theory of discrete dislocations.     

Dislocation shielding and crack tip decohesion at the atomic scale

Anti-shielding of a crack tip by a dislocation is examined at the atomistic level for a simple geometry to test classical singular-crack and recent cohesive-crack models of crack/dislocation interactions. The atomistic model shows that, as an anti-shielding dislocation approaches the crack tip, it causes less anti-shielding than predicted by the singular-crack model. The trend is qualitatively consistent with predictions of a cohesive-crack model, but the atomistic effect is even larger. The cohesive-crack model is consistent with the atomistic results if a reduced cohesive strength of ～3.5 GPa is used instead of the actual value of 13 GPa. The difference is shown to be due to the non-linear deformation of material around the crack tip, which cannot be fully represented by a cohesive zone law along the fracture surface. It is then shown that, at the point of fracture, there is a unique traction–displacement cohesive law acting behind the crack tip, independent of the position of the anti-shielding dislocation. The maximum traction of 12.8 GPa and fracture energy of 1.9 J m^?2 are both in excellent agreement with the values obtained from independent atomistic calculations on this material. Both the shielding and cohesive results have implications for the accurate modeling of fracture processes in metallic materials.     

脉冲电流处理过程中的相变和再结晶细化晶粒(英文)

在室温下对铸态TiAl合金和冷轧TA15合金进行高密度脉冲电流处理。应用光学金相显微镜和透射电子显微镜研究脉冲电流处理前、后试样的显微组织。实验结果表明:通过脉冲电流处理可以细化铸态TiAl基合金的晶粒,从约1000μm的原始粗大层片组织,经脉冲电流处理后可以得到尺寸为3050μm细小、均匀的晶粒。对于冷轧TA15合金,脉冲电流处理后发生了完全的再结晶,晶粒组织由原始的α板条晶粒转变为细小的α等轴晶粒。研究结果表明,脉冲电流处理是一种有效的细化晶粒方法;由于不需要热机处理所要求的挤压等变形工序和高温加热、真空保护等条件,简化了工艺过程。     

应力诱导相变对氧化锆陶瓷断裂韧性的影响

利用放电等离子烧结技术制备了氧化锆陶瓷,研究了应力诱导相变对氧化锆陶瓷断裂韧性的影响。结果表明,Zr O2陶瓷在应力诱导下可发生应力诱导相变,且应力诱导相变量随烧结温度和晶粒尺寸增加而增大。应力诱导相变可提高Zr O2陶瓷材料的断裂韧性,在1300℃时,晶粒尺寸、应力诱导相变量、断裂韧性分别为0.36μm、4.85%、3.05 MPa·m-0.5;在1500℃时,晶粒尺寸、应力诱导相变量、断裂韧性分别为1.29μm、17.49%、4.80 MPa·m-0.5,其中断裂韧性相比前者提高了57.4%。     

Atomistic simulation of hydrogen diffusion at tilt grain boundaries in vanadium

Molecular dynamics simulations of hydrogen diffusion at Σ3 and Σ5 tilt grain boundaries in bcc vanadium (V) have been performed based on modified embedded-atom method interatomic potentials. The calculated diffusivity at the grain boundaries is lower than the calculated bulk diffusivity in a temperature range between 473 and 1473 K, although the difference between the grain boundary and bulk diffusivities decreases with increasing temperature. Compared with that of the other directions, the mean square displacement of an interstitial hydrogen atom at the Σ3 boundary is relatively small in the direction normal to the boundary, leading to two dimensional motion. Molecular statics simulations show that there is strong attraction between the hydrogen atom and these grain boundaries in V, which implies that the role of grain boundaries is to act as trap sites rather than to provide fast diffusion paths of hydrogen atoms in V.     

Texture of primary recrystallization on nonoriented electrical steel sheet with phase transformation

The magnetic properties of nonoriented (NO) electrical steel sheet are commonly improved if the texture of their products possesses “cube texture” (e.g., {100 〈0vw〉, “goss texture”) (i.e., 110〈011〉, and less 222 texture). Industrially “cube type” has not been obtained, but “goss texture” has been. In a greater or lesser degree, {222} texture exists. To improve “goss texture” and reduce 222 texture, the grain size of the material prior to cold rolling should be larger. When the grain size before cold rolling is larger, during primary recrystallization, “goss texture” is enriched, 222 texture is decreased, and the grain grows so easily that higher induction and lower core loss can be obtained. This does not depend on the presence of phase transformation. In case of NO steel with phase transformation, heat treatment before cold rolling has been done below the austenite transition temperature (Acin1) in order to prevent the fine grain size caused by α → γ(+α) → α transformation. By using material that was heated over Acin1 and cooled with changing cooling rates, this study describes (a) the relationship between textures before cold rolling and the texture of the final product, and (b) the development of the magnetic properties.     

Interactions between carbon solutes and dislocations in bcc iron

Carbon solute–dislocation interactions and solute atmospheres for both edge and screw dislocations in body-centered cubic (bcc) iron are computed from first principles using two approaches. First, the distortion tensor and elastic constants entering Eshelby’s model for the segregation of C atoms to a dislocation core in Fe are computed directly using an electronic-structure-based the total energy method. Second, the segregation energy is computed directly via first-principles methods. Comparison of the two methods suggests that the effects of chemistry and magnetism beyond those already reflected in the elastic constants do not make a major contribution to the segregation energy. The resulting predicted solute atmospheres are in good agreement with atom probe measurements.     

High-temperature chlorination of gold with transformation of iron phase

Gold in cyanide tailings from Shandong Province is mainly encapsulated by hematite and magnetite at distribution rates of 76.49 % and 10.88 %, respectively.Chlorination–reduction one-step roasting of cyanide tailings was conducted under the following conditions: calcium chloride dosage of 6 %, bituminous coal dosage of30 %, calcium oxide dosage of 10 %(all dosages are vs.the mass of cyanide tailings) at 1000 °C of roasting temperature. X-ray diffraction(XRD), scanning electron microscopy(SEM), and chemical-phase analysis were performed to investigate the effects of iron phase transformation on the high-temperature chlorination of gold.Results indicate that the lattice structure of hematite undergoes expansion, pulverization, and reorganization when hematite is reduced to magnetite, which leads to42.03 % gold exposure, and the high-temperature chlorination rate of gold is 41.17 % at the same time. The structure of wustite formed by the reduction in magnetite is porous and loose, and thus 44.02 % of gold is exposed. The high-temperature chlorination rate of gold is increased by41.98 percentage points. When wustite is reduced to metallic iron, 4.42 % of gold is exposed, and the hightemperature chlorination rate of gold is increased by3.38 percentage points. Accordingly, the high-temperature chlorination of gold mainly occurs in two stages, in which Fe_2O_3 is reduced to Fe_3O_4, and Fe_3O_4 is reduced to Fe_xO finally.     

Formulating stress corrosion cracking growth rates by combination of crack tip mechanics and crack tip oxidation kinetics

A theoretical equation for stress corrosion crack growth rate of austenitic alloys in high temperature water is reformulated based on crack tip asymptotic fields and crack tip transient oxidation kinetics. A general oxidation kinetic law is introduced, emphasizing the role of mass transport through solid oxide film at the crack tip. The effects of several parameters on crack growth rate are evaluated. The results are compared with available experimental data and other equations. A good prediction of the effect of K on stress corrosion cracking growth rate of typical austenitic alloys in simulated light water reactor environments has been achieved.