Minsheng Huang

The size effect and plastic deformation mechanism transition in the nanolayered polycrystalline metallic multilayers

The strength and deformation mechanisms of the nanolayered polycrystalline metallic multilayers (NPMMs) are investigated via molecular dynamics simulation, with special attentions to the coupling effect of grain size and layer thickness. The results indicate that the strength of multilayers does not always increases sensitively with the decrease of layer thickness or grain size, and the smaller one of them governs substantially the size effect on the strength. Due to the constraint of GBs and phase interface to gilding dislocations, there are several possible deformation mechanisms, which can govern the strength of NPMMs, including the confined partial dislocation slip, confined extended dislocation slip, and confined grain boundary slip. With the increase or decrease of the characteristic size of multilayers (i.e., layer thickness or grain size), the dominant deformation mechanism changes from one to another, resulting in very intricate size effect on the strength of multilayers. The underlying reason of...

Modeling of abnormal mechanical properties of nickel-based single crystal superalloy by three-dimensional discrete dislocation dynamics

Unlike common single crystals, the nickel-based single crystal superalloy shows surprisingly anomalous flow strength (i.e. with the increase of temperature, the yield strength first increases to a peak value and then decreases) and tension–compression (TC) asymmetry. A comprehensive three-dimensional discrete dislocation dynamics (3D-DDD) procedure was developed to model these abnormal mechanical properties. For this purpose, a series of complicated dynamic evolution details of Kear–Wilsdorf (KW) locks, which are closely related to the flow strength anomaly and TC asymmetry, were incorporated into this 3D-DDD framework. Moreover, the activation of the cubic slip system, which is the origin of the decrease in yield strength with increasing temperature at relatively high temperatures, was especially taken into account by introducing a competition criterion between the unlocking of the KW locks and the activation of the cubic slip system. To test our framework, a series of 3D-DDD simulations were performed on a representative volume cell model with a cuboidal Ni3Al precipitate phase embedded in a nickel matrix. Results show that the present 3D-DDD procedure can successfully capture the dynamic evolution of KW locks, the flow strength anomaly and TC asymmetry. Then, the underlying dislocation mechanisms leading to these abnormal mechanical responses were investigated and discussed in detail. Finally, a cyclic deformation of the nickel-based single crystal superalloy was modeled by using the present DDD model, with a special focus on the influence of KW locks on the Bauschinger effect and cyclic softening.

Cyclic Hardening Behavior of Polycrystals with Penetrable Grain Boundaries: Two-Dimensional Discrete Dislocation Dynamics Simulation

A two-dimensional discrete dislocation dynamics (DDD) technology by Giessen and Needleman (1995), which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocation-penetrable grain boundaries (GBs) on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.

Brittle to ductile transition of metallic glasses induced by embedding spherical nanovoids

The lack of global plasticity at low temperature seriously limits the application of metallic glasses (MGs) as structural materials. An approach to enhance the MG-ductility by dispersed spherical nanovoids is suggested and validated by molecular dynamics in the present paper. By introducing these nanovoids, a deformation mode transition from localized shear banding to homogeneous flow occurs. The ratio of void-surface area to MG volume λ is revealed to be the dominant factor controlling this brittle-to-ductile transition. Generally, for a given void volume fraction, smaller nanovoids with larger λ have better toughening effects. It is also discovered that the ductile responses of porous MGs with embedded nanovoids remain unchanged, even after several cycles of tensile-compressive loads. The intrinsic mechanism may be the transition of energetic void-surface atoms into internal atoms with lower potential energy. This process induces many uniformly distributed potential nucleation sites for shear transforma...