Kisaragi Yashiro

Molecular dynamics simulation of dislocation nucleation and motion at γ/γ′ interface in Ni-based superalloy

Abstract Molecular dynamics simulations are conducted on the dislocation behavior at the apices and edges of cuboidal Ni3Al precipitate in a pure Ni matrix, or the idealized γ/γ′ microstructure in a Ni-based superalloy. A tensile simulation of the [001] direction is implemented with a periodic cell that has eight cubic precipitates in order to investigate the nucleation site of dislocation in the idealized microstructure with no defects other than the γ/γ′ interfaces. The effect of residual internal stresses on the stability of the interfaces is also discussed. Other simulations are conducted on the behavior of edge dislocations nucleated from a free surface and proceeding in the γ matrix toward γ′ precipitates under shear force. Dislocation pinning at γ′ precipitates, bowing-out in the γ channel, pile-up and nucleation of superdislocation in the γ′ precipitate are simulated and inspected in detail. Discussions on the size of the γ/γ′ microstructure and the sharpness of the edge of the γ′ precipitate are also presented.

Deformation analysis of amorphous metals based on atomic elastic stiffness coefficients

The elastic limit of a crystal can be evaluated by the positiveness of elastic stiffness coefficients, Bijkl. We had demonstrated that the nucleation of lattice defects such as dislocation and cleavage cracking can be predicted by the atomic Bijkl at each atom point. Amorphous metals and bulk metallic glasses draw intense interest whether the criteria are applicable or not since they are regarded as the ultimate of lattice defects. In the present study, an amorphous Ni–Al binary alloy is made by a usual melt–quench simulation and subjected to tension by means of molecular dynamics simulation. During simulations, the positiveness of atomic Bijkl is discussed for all atoms. Contrary to an Ni–Al crystal, many atoms show negative value even in the initial equilibrium of the amorphous before loading. These unstable atoms turn out to be the non-clustered atom or the outer-shell of the local cluster such as 12(0, 0, 12, 0) icosahedron. On the other hand, the centre atoms of the local clusters show high stability resulting in the positive Bijkl of the whole system. It is also demonstrated that the change in the atomic Bijkl can reveal the collapse and re-configuration of local clusters during the deformation.

Molecular dynamics study on atomic elastic stiffness in Si under tension: homogenization by external loading and its limit

As a series of studies that discuss the onset of inelastic deformation based on atomic elastic stiffness (AES), we investigated the AES in silicon by the Tersoff interatomic potential. For a comprehensive discussion including the effect of structural inhomogeneity by surface and grain boundaries, we performed tensile simulations on bulk/nanowire of Si single crystal, laminate bulk/bamboo nanowire with ?5 twist grain boundary under a very low temperature (T?=?1?K). Not only the stress?strain response, but also the AESs at each atom point, , were evaluated numerically by (Voigt notation) against local strain perturbation. The deviation of vanishes/diminishes by tension both in the homogeneous case of bulk perfect lattice and inhomogeneous ones with surface and grain boundaries; however, there is a certain limit for the homogenization. That is, the subtle deviation of AES in the perfect bulk vanishes by tension but it increases again like an onset of resonance, showing precursor stress decrease just before the unstable stress drop. In the inhomogeneous cases, we demonstrated that the near-zero AESs at the initial structural defects, e.g. surface or grain boundary, do not change but the positive AESs of other stable atoms approach zero by tension. When these distributions overlap each other, the standard deviation of AES increases again as if it were the first homogenization limit. However, the real homogenization starts at that point; that is, the AES distribution changes its shape to have a single peak at the border, suggesting that the difference of initial defects and other stable part vanishes before the system instability.

Molecular Dynamics Simulation of Nanoindentation on Folded Chain Crystal of Polyethylene

Nanoindentation tests on a folded chain crystal of polyethylene are implemented with the molecular dynamics simulation. The orthorhombic crystal is made of the planar zig-zag chains and has the thickness of about 10nm. The ideal Berkovich indenter is plunged into upper surface of the crystal down to 2nm with the constant loading rate of 200m/s or 2000m/s. After the holding time of 1000fs at the maximum depth, the indenter is then pulled up with the same speed. The results are summarized as follows; a) The indentation of 2000m/s remains the residual depression while that of 200m/s recovers the hollow, b) No elastic component is found in the deformation under the both rate of 200m/s and 2000m/s, c) The crystal deforms statically under the indentation of 200m/s while that of 2000m/s shows delayed response.

Molecular Dynamics Simulations on Local Lattice Instability at Mode I Crack Tip in BCC Iron

Molecular dynamics simulations are performed on the [001](010), [001] (110) and [11 Open image in new window ](111) through cracks in bcc Fe under mode I loading, in order to discuss about the local lattice stability at the crack tip within the framework of FS potential function. The crack width is set to \(2c=0.1L_x, 0.2L_x\) and \(0.5L_x\), respectively, against the periodic cell length of \(L_x=20-30\) nm. The [001](010) crack shows ductile behavior of blunting by dislocation emission, resulting in the remarkable nonlinearity in the stress–strain curve. Both the [001](110) and [11 Open image in new window ](111) cracks propagates in a brittle manner, showing the abrupt stress drop by the rupture. Then the local stability is discussed by the positiveness of the determinant of \(6\times 6\) matrix of elastic stiffness coefficients, \(B_{ij}^\alpha \); that is, Wang’s B-criteria is applied to each atom. Negative atoms emerges at far smaller strain than the peak or rupture one, corresponding to the onset of local “plastic deformation”, i.e. dislocation emission or rearrangement at the crack surface. For the detail of the unstable mode, we have evaluated the eigenvalue of \(B_{ij}^\alpha \) of each atom. Since the negative 1st eigenvalue leads the negative \(\det B_{ij}^\alpha \), it is natural the tendencies of the former and the later are almost same. However, a few atoms at the [11 Open image in new window ](111) crack tip turn to negative in their 2nd eigenvalue just at or just after the stress–strain peak.

Effect of Silica Coupling Agents on Texture Formation and Strengthening for Silica-Filled Rubber

New finite element homogenization model with nonaffine constitutive equation of rubber is developed to study the deformation behavior of silica-filled rubber under monotonic and cyclic deformation. The obtained results clarified the effect of the volume fraction of the silica coupling agent and the networklike structure connecting the silica particles on essential physical enhancement mechanisms of deformation resistance and hysteresis loss for silica-filled rubber. The finding suggests that the material characteristics of silica-filled rubber are much more controllable than those of carbon-black-filled rubber.

Molecular Dynamics Study on the Characteristics of Edge and Screw Dislocations in Gamma/Gamma-Prime Microstructure of Ni-Based Superalloy

Fundamental characteristics of edge and screw dislocations in the y/y′ microstructure of Ni-based superalloys are investigated by using molecular dynamics simulations. Edge/screw dislocations are nucleated and glide in a slab cell of the Ni matrix involving an apex of a cuboidal Ni3Al precipitate, that mimics a part of the idealized y/y′ microstructure. The edge dislocation decreases its velocity at the y′ precipitate, showing dislocation pinning there, then penetrates it under the force from following dislocations. The screw dislocation runs through the precipitate without slowdown by shrinking the width between its Shockley partials. Detailed investigation of the stress distribution suggests that the constriction is due to interactions between the stress field around the precipitate and the partials: the stress causes a repulsive Peach-Koehler force on the leading partial and an attractive force on the trailing one since their edge components have opposite Burgers vectors.

Criteria for Nucleation of a Dislocation and a Cleavage Crack in a Nickel Single Crystal Based on Molecular Dynamics

The microscopic features of deformation and fracture in a nickel single crystal are investigated on the basis of the local strain of the crystal lattice. The process of dislocation nucleation from the surface without the constraint as well as the process of cleavage cracking under tension, with the constraint of transverse deformation, are simulated by molecular dynamics, and it is found that their nucleation criteria are successfully derived by the lattice instability.

Evaluation of Deformation Behavior of Silica-Filled Rubber under Monotonic and Cyclic Straining

To clarify the essential deformation characteristics of silica-filled rubber, we construct the finite element homogenization models of silica-filled rubber with newly proposed nonaffine molecular chain network model of rubber. These models can reflect the generation of complicated inter-fillers connecting phases where the characteristics of rubber are intricately changed depending on the volume fraction of silica coupling agent and relative size of particles and their location. The results obtained clarified the essential physical enhancement mechanisms of deformation resistance and hysteresis loss for rubber filled with silica with different distribution patterns under diffrent rate of deformation. The volume fraction of coupling agent essentially affects the deformation behavior of silica filled rubber which suggests the high controlability of the material characteristics of silica filled rubber as compared with carbon black filled rubber.

Molecular Dynamics Study on Characteristics of Misfit Dislocations in Ni-Based Superalloys

By using molecular dynamics simulation, misfit dislocation networks are made on semi-coherent interfaces in a laminate structure of Ni and Ni3Al single crystals. The core structure of the networks is discussed in detail, focusing on the different atomic configuration at the interfaces; e.g. with or without Al atoms on the Ni3Al side. It is revealed that the networks can be a source of partial dislocation loops under the external loading; however, the loops tend to form immobile wedge-like stacking faults, analogous to the stacking fault tetrahedron (SFT), near the interface with Al atoms. On the other hand, the loops propagate into both Ni and Ni3Al phases, from the network dislocations on the interface without Al atoms.