W.W. Jian

Grain Size Effect on Deformation Mechanisms of Nanocrystalline bcc Metals

Nanocrystalline (NC) body-centered cubic (bcc) metals behave very differently from how NC metals with other crystal structures behave. Their strain rate sensitivity decreases with decreasing grain size, which is an observation that has not been well understood. Here, we report a significant effect of grain size on the deformation mechanism of NC bcc Mo. With decreasing grain size, the density of mixed and edge dislocations increases, while the density of screw dislocations decreases. When the grains become very small, the overall dislocation density decreases with decreasing grain size. These observations provide a logical explanation for the observed effect of grain size on strain rate sensitivity.

A new metastable precipitate phase in Mg–Gd–Y–Zr alloy

Mg–RE alloys are among the strongest Mg-based alloys due to their unique precipitation structures. A previously unobserved metastable phase (βT) is found to coexist with reported β″ and β′ metastable phases under peak ageing conditions in a Mg–Gd–Y–Zr alloy. The position of the RE elements within the βT phase is identified using atomic-resolution high-angle annular dark field scanning transmission electron microscopy imaging, and the βT phase is shown to have an orthorhombic structure with a stoichiometry of Mg5RE. Based on these observations, a new precipitation sequence is proposed.

Dynamic Void Growth and Shrinkage in Mg under Electron Irradiation

We report in situ atomic-scale investigation of late-stage void evolution, including growth, coalescence and shrinkage, under electron irradiation. With increasing irradiation dose, the total volume of voids increased linearly, while the nucleation rate of new voids decreased slightly and the total number of voids decreased. Some voids continued to grow while others shrank to disappear, depending on the nature of their interactions with nearby self-interstitial loops. For the first time, surface diffusion of adatoms was observed to be largely responsible for the void coalescence and thickening. These findings provide fundamental understanding to help with the design and modeling of irradiation-resistant materials.