Phase transformations during consolidation treatments of an attrition‐milled amorphous yttria‐partially‐stabilized zirconia (Y‐PSZ: ZrO2–3 mol% Y2O3)–20 mol% Al2O3 powder and the resulting microstructures have been investigated. A metastable cubic phase (c‐ZrO2 solid solution) together with an α‐Al2O3 phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase (t‐ZrO2 solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α‐Al2O3 particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic‐to‐tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition‐milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.

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Nanocrystalline (nc) ceramic–metal composite bulk samples have been fabricated by consolidation of mixture of attrition‐milled (AM) amorphous base ceramic ((ZrO2–3 mol% Y2O3)–20 mol% Al2O3) and AM amorphous base metallic (Ti–48 mol% Al) powders using a pulse‐current pressure sintering system. Microstructural observations revealed that the ceramic and metallic colonies appear blocky in morphology in the composite bulk samples, and both the ceramic and the metallic colonies consist of a large number of equiaxed fine grains with the sizes of 78–82 and 81–86 nm, respectively. Mechanical mixing treatments by wet ball milling in ethanol before consolidation process are effective for refinement of the ceramic and metallic colonies. In all the obtained composite bulk samples, the ceramic colonies consist of the dominant phase of tetragonal (t) ZrO2 solid solution (ss) together with the minor phases of monoclinic (m) ZrO2ss and α‐Al2O3. On the other hand, the dominant phase in the metallic colonies changes from Ti3Al (α2) to Tiss (α) with an increase in the t‐ZrO2ss volume fraction by abrasion of 3 mol% yttria‐stabilized tetragonal polycrystalline zirconia balls during wet mechanical mixing treatments. Such a phase transformation from α2 to α is considered to be due to the decrease in the aluminum content in the metallic colonies by combination of aluminum with oxygen (i.e., the formation of α‐Al2O3), which is probably taken from ethanol (C2H5OH) into the powders during wet mechanical mixing treatments. The obtained nc composite bulk samples show good Vickers hardness values, which are considerably higher than those estimated from the rule of mixture.

The instrumented pulse electric discharge consolidation method is used to provide a way of in process nanocrystalline control densification of the amorphous ZrO 2 -20 mol%Al 2 O 3 powder as prepared by rotating-arm reaction ball milling. The cylindrical compact height (h f ) of the amorphous ZrO 2 -20 mol%Al 2 O 3 powder is found to be a dominant process variable; at 1 mm, it leads to the densification prior to major crystallization after a high relative density of roughly 0.86 at 800 K, and significant decreases down to 1284 K in temperature necessary to obtain the full densification under 100 MPa. The rapid densification for amorphous and nanocrystalline supersaturated cubic ZrO 2 -20 mol%Al 2 O 3 is fairly well expressed by an Arrhenius-type equation of Newtonian viscous flow: p ¼ po expðQ=kTÞ having greatly decreasing apparent activation energy Q from 300 to 72 kJ mol À1 and process viscosity p at 1200 K with decreasing h f from 14.6 to 1 mm. The Berkovich indentation testing permits us to derive a relatively low level of 4.4 GPa for the value of the yield stress at room temperature in the full-density nanocrystalline ZrO 2 -20 mol%Al 2 O 3 sample having the Vickers hardness number of approximately 800 DPN.