Effect of Excess Atomic Volume on Crack Evolution in a Deformed Iron Single Crystal

This paper presents a molecular dynamics study of how the localization and transfer of excess atomic volume by structural defects affects the evolution and self-healing of nanosized cracks in bcc iron single crystals under different mechanical loading conditions at room temperature. It is shown that deformation is initially accompanied by a local growth of the atomic volume at the crack tips. The crack growth behavior depends on whether the excess atomic volume can be transferred by structural defects from the crack tips to the free surface or other interfaces. If an edge crack is oriented with respect to the loading direction so that dislocations are not emitted from its tip or only twins are emitted, then the sample undergoes a brittle-ductile fracture. The transfer of the excess atomic volume by dislocations from the crack tips prevents the opening of edge cracks and is an effective healing mechanism for nanocracks in a mechanically loaded material.

Energy Activation Spectrum of Low-Temperature Acoustic Relaxation in High-Purity Iron Single Crystal. Solution of the Inverse Problem of Mechanical Spectroscopy by the Tikhonov Regularization Method

When studying the temperature dependences of the acoustic absorption and the modulus of elasticity, absorption peaks are often observed, which correspond to the characteristic step on the temperature dependence of the modulus of elasticity. Such features are called relaxation resonances. It is believed that the occurrence of such relaxation resonances is due to the presence in the structure of the material of elementary microscopic relaxors that interact with the studied vibrational mode of mechanical vibrations of the sample. In a sufficiently perfect material, such a process is characterized by a relaxation time τ, and in a real defective material by a relaxation time spectrum P(τ). Most often such relaxation processes have a thermally activated character and the relaxation time τ(T) is determined by the Arrhenius ratio τ(T)=τ0exp(U0/kT), and the characteristics of the process will be U0 - activation energy, τ0 - period of attempts, Δ0 - characteristic elementary contribution of a single relaxator to the dynamic response of the material and their spectra. In the low temperatures region the statistical distribution of parameters τ0 and Δ0 can be neglected with exponential accuracy, and the relaxation contribution to the temperature dependences of absorption and the dynamic elasticity modulus of the material will be determined only by the activation energy spectrum P(U) of microscopic relaxors. The main task of mechanical spectroscopy in the analysis of such relaxation resonances is to determine U0, τ0, Δ0 and P(U). It is shown, that the problem of recovering of spectral function P(U) of acoustic relaxation of a real crystal can be reduced to the solving of the Fredholm integral equation of the first kind with an approximately known right part and concerns to a class of ill-posed problems. The method based on Tikhonov regularizing algorithm for recovering P(U) from experimental temperature dependences of absorption or elasticity module is offered. It is established, that acoustic relaxation in high-purity iron single crystal in the temperature range 5-100 K is characterized by two-modes spectral function P(U) with maxima at 0.037 eV and 0.015 eV, which correspond to the a-peak and its a' satellite.

Molecular Dynamics Simulation of Porous Layer-Induced Stress in α-Iron Single Crystal and Twin Crystal

Molecular dynamics (MD) simulation was carried out to investigate the stress distribution and the macro tensile stress for the α-iron single crystal and twin crystal. The results show that there was a maximum tensile stress located at the matrix near the interface between the porous layer and the matrix for the two crystals. It has been found that a steep drop of stress generated at the twin boundary of the twin crystal. The deflection and the macro tensile stress of the single crystal and the twin crystal increased with an increase of the relative depth of the porous layer. The value of the deflection and the macro tensile stress of the single crystal were larger than that for the twin crystal, because there is a steep drop of stress generated at the twin boundary due to the effect of twin interface.

First Principles Calculation of Hydrogen Embrittlement in Iron

Hydrogen embrittlement in iron and steels has been studied for long years. Although a few mechanisms of it have been proposed, it has not been clearly understood yet. Especially, the decohesion theory and the hydrogen enhanced localized plasticity theory have been discussed in the center of attention. Hydrogen is considered to induce a cleavage fracture in the former, but enhance slips in the latter. It is essential problem which hydrogen induces cleavage or slip in hydrogen embrittlement. The purpose of the present paper is to clarify the effects of hydrogen in iron single crystal on the fracture or deformation using first principles calculation. In the cleavage model, the changes of total energy were estimated with a distance of a pair of {001} planes in a unit cell composed of 12 iron atoms and a hydrogen atom. In the slip model, the changes of total energy were estimated with a displacement of a few {110} layers toward <111> direction. Hydrogen reduces the total energy in the cleavage model, but does not change that in the slip model.