Dislocation Topological Evolution and Energy Analysis in Misfit Hardening of Spherical Precipitate by the Parametric Dislocation Dynamics Simulation

Interaction of a single dislocation line and a misfit spherical precipitate has been simulated by the Parametric Dislocation Dynamics (PDD) method in this research. The internal stress inside the precipitate is deduced from Eshelby’s inclusion theory, the stress of the dislocation line and outside the precipitate is calculated by Green’s function. The influence of different relative heights of the primary slip plane on dislocation evolution is investigated, while the cross-slip mechanism and annihilation reaction are considered. The simulation results show three kinds of dislocation topological evolution: loop-forming (Orowan loop or prismatic loop), helix-forming, and gradual unpinning. The dislocation nodal force and the velocity vectors are visualized to study dislocation motion tendency. According to the stress–strain curve and the energy curves associated with the dislocation motion, the pinning stress level is strongly influenced by the topological change of dislocation as well as the relative heights of the primary slip plane.

Internal Stress and Dislocation Interaction of Plate-Shaped Misfitting Precipitates in Aluminum Alloys

The fine misfit precipitates in age-hardenable aluminum alloys have important roles due to their excellent age-hardening ability, by their interaction with dislocations. The present study focused on the internal stress field of plate-shaped misfitting precipitates to evaluate their roles in dislocation overcoming the precipitates by means of micromechanics based on Green’s function method. The stress field of misfit precipitates on {001} and {111} habit planes were reproduced by homogeneous misfit strain (eigenstrain) of the precipitate (Eshelby inclusion method), and the dislocation motion vector on the primary slip plane was predicted by the force acted on the dislocation by the Peach–Koehler formula. According to simulation results, the dislocation interaction strongly depends on the stress field and geometry of misfit precipitates; repulsive and attractive forces are operated on the dislocations lying on the primary slip plane when the dislocation approaches the misfit precipitates. The hardening ability of different orientations of precipitation variants was discussed in terms of interaction force acted on the dislocation.

Change in Stress Axis with Creep Deformation in γ-Single Phase Ni-20mass%Cr Single Crystal within Standard Stereographic Triangle

The creep deformations of γ-single phase Ni-20mass%Cr single crystals with stress axes within standard stereographic triangle and at the three pole positions have been investigated. The most of the creep life is occupied by the transient stage, which consists of Stage I and Stage II. In Stage I, the creep rate just after loading remains constant. In Stage II, the creep rate decreases continuously. Except for the single crystals with stress axes of [001] and [1,–11] poles, the single crystals make the creep deformation using the primary slip plane of (111). As a result, the cross section of the specimens turns from circular to elliptical in shape. However, there are marked difference in deformation manner among single crystals with the stress axes within standard stereographic triangle. The single crystals whose angle between stress axis and primary slip plane of (111), θ. is more than 45° shows the heterogeneous deformation during creep. While, the homogeneous deformation will be expected in the single crystals with θ less than 45°. In this study, by using the four single crystals with θ less than 45°, the change in the stress axis with the creep deformation at 1173K-29.4MPa, is investigated and the deformation manner due to the primary slip plane of (111) is estimated by conducting the creep interrupting tests. In the two single crystals with stress axes in the standard stereographic triangle where the moving range of θ is narrow, comparing to the others, the spot of the stress axis in the inverse pole figure moves for <1,– 01> direction by using (111)<1,–01> slip system, and after arriving at the [001]-[1,–11] line, the spot turns to its direction for [1,–11] pole using (111)<1,–10> slip system. While, in the other two single crystals whose stress axes located in the area with wider moving range of θ, the spot of stress axis only move for <1,–01> direction. And, the widely spread spot of the stress axis is confirmed after subjecting the small strain.

Change in Stress Axis with Creep Deformation in Ni-20mass%Cr Single Crystal with Orientation of [011]

The features of the creep deformation of γ-single phase single crystals with the composition of Ni-20mass%Cr are characterized by the extended transient stage, which consists of Stage I and Stage II. In the Stage I, the creep rate just after loading remains unchanged, while the creep rate decreases continuously in Stage II. In the single crystals except for the single crystals with the stress axis of [001] and [1, – 11], the predominant creep deformation using the primary slip plane continues. By this deformation, the cross section of specimen turns to elliptical in shape. However, in the single crystals with the angle between stress axis and primary slip plane (111) is more than 45°, the deformation using the primary slip plane does not continue, as a result, the duration of Stage II turn to shorter one. The single crystal with the stress axis of [011] has the largest angle of 55°. In this study, the deformation manner during transient stage of single crystal with the stress axis of [011] orientation is investigated from the two viewpoints. The first one is to clarify the change in deformation manner with decreasing the stress. As a result, with decreasing the stress, the Stage I become clear and strain during Stage I and Stage II become small, furthermore, the decreasing ratio of creep rate with definite strain becomes larger. While, the second viewpoint is to investigate the change in crystallographic orientation of the [011] single crystals with creep deformation using the inverse pole figure obtained by the EBSD method. As a result, at the stress of 29.4 MPa, the spot of stress axis turns from the [011]-[1, – 11] line to the <1, – 01> direction. While, at the stress of 19.6 MPa, the stress axis moves for the [1, – 11] pole along the [011]-[1, – 11] line from the [011] pole. And, it is noteworthy that the spot widely spread from the [011] pole during transient stage. This indicates the large distortion in the primary slip plane and the evidence of heterogeneous deformation.

The Orientation Dependence of Work?Hardening in Crystals of Face-centred Cubic Metals

It is shown that the principal features of the observed orientation dependence of work-hardening can be accounted for in terms of the likelihood of formation. Of Lomer-Cottrell sessile dislocations in two directions in tb" primary slip plane. This is deduced from the known variation of resolved shear stress with orientation, for the possible secondary slip systems, and metallographic observations of slip and deformation bands.