Processing and high temperature deformation of zinc-22%aluminum eutectoid containing nanoscale dispersion particles

Synthesis of Zn-22%Al eutectoid containing nanometer-scale dispersion particles through powder metallurgy (PM) was investigated in conjunction with a systematic microstructural examination. The two-phased powders produced by inert-gas atomization were milled in liquid nitrogen to introduce AlN and Al₂O₃ fine dispersions (4 ∼70 nm in size). The microstructures were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The principal processing factors and microstructural characteristics associated with the major processing steps, including spray-atomization, mechanical milling (MM), consolidation, and heat-treatment, were evaluated and discussed. The formation of the dispersions was attributed to the interaction between powder material (primarily Al phase) and environmental elements such as oxygen and nitrogen during milling. It was demonstrated that the processing route studied herein was efficient to incorporate nano-scale dispersions.; Tensile and creep tests were conducted on the resultant Zn-22%Al eutectoid that contained nanometer-scale dispersion particles. The primary objective of the investigation is to determine the effect of these nano-scale dispersion particles on creep and microstructure in region I (low-stress region) and region II (intermediate-stress region or superplastic region) of the sigmoidal plot between stress and strain rate that was previously reported for dispersion-free grades of the alloy. The experimental results show that while the particles have no significant effect on the sigmoidal plot, their presence influences several characteristics in region I including the value of the stress exponent, n, and the value of the activation energy, Q. An examination of the dislocation structures by means of TEM reveals two observations that are common to region II and region I. First, after deformation, only some grains contain dislocations, many of which are attached to the dispersion particles. Second, most of the dislocations are parallel, tend to be long and curved, and appear to sweep across the grain interior one by one. Consideration of these observations along with available information on superplastic flow leads to the conclusion that region I and region II are controlled by the same deformation process, in which the sliding of groups of grains is accommodated by dislocation motion in the blocking grains, and that the apparent differences in creep characteristics between region I and region II arise from the presence of a threshold stress. The origin of this threshold stress is most likely related to the interaction between moving dislocations and dispersion particles in the blocking grains. 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. A close comparison between dislocation activities observed and existing slip-accommodation models is made.