Yuwei Xun

Correlations between the minimum grain size produced by milling and material parameters

Abstract In this paper, data reported for the minimum average grain size that was obtained by ball milling are analyzed. It is shown that when the normalized minimum average grain size, d min / b (where d min is the minimum average grain size and b is the Burger vector) is plotted as a function of melting temperature, T m , the activation energy for diffusion, Q , or the normalized hardness, H / G (where H is the hardness and G is the shear modulus), the resultant curves are similar in trend. The curves exhibit two regions: region I and region II. In region I, d min / b decreases rapidly with increasing T m , Q , or H / G , while in region II, d min / b decreases very slowly with each of these three parameters. The relationship between the normalized minimum average grain size and the normalized hardness is explained in terms of the limit of a dislocation pile-up in nanocrystalline materials.

Slip-accommodated superplastic flow in Zn-22 wt%Al

Experiments were conducted on the Zn-22 wt% Al eutectoid that contained nanometre-scale dispersion particles. These particles were introduced in the matrix of the alloy via powder metallurgy followed by cryomilling. Transmission electron microscopy observations made on specimens crept at a strain rate near the centre of the superplastic region (the intermediate-stress region or region II in the sigmoidal relationship between stress and strain rate) reveal clear evidence for lattice dislocation activities during superplastic flow. Such evidence is demonstrated in part by the presence of attractive particle-dislocation interactions that are only noted in some of the grains. It is suggested that each one of these grains serves as an obstacle for a group of grains sliding together as a unit. In addition, the configurations of the lattice dislocations in the interiors of the blocking grains are suggestive of viscous glide and single slip in the blocking grain. Combining the present findings with earlier observations reported for superplastic deformation leads to the conclusion that the generation and movement of lattice dislocations provide an accommodation process for grain-boundary sliding.

On the minimum grain size produced by milling Zn–22%Al

Abstract Nanocrystalline (nc) Zn–22%Al, which consists of an FCC Al phase and an HCP Zn phase, has been synthesized by cryomilling. The results show that during cryomilling, the grain sizes of the Al and the Zn phases decreased with milling time, and that after 16 h of milling, they approached minimum values of 33 and 41 nm, respectively. While the minimum grain sizes of the Al and the Zn phases agree well with the trends of very recent correlations between the minimum gain size and materials parameters such as melting point and hardening, they are higher than those reported for Al (22–26 nm) and Zn (20 nm). Possible reasons for this discrepancy are discussed.