In Part I of this study the benefits of using microwave radiation as a volumetric heating source for sintering tungsten were delineated. Grain growth was curtailed when submicron size particles were used as the starting material followed by sintering in a microwave furnace. Prior research has shown that grain growth in refractory metals and ceramics can also be reduced by the addition of suitable secondary phases. Examples of this effect are the addition of A1_2O_3 to ZrO_2,1 ZrO_2 (tetragonal or cubic) to beta"-Al_2O3,2 and Al_2O_3 to cubic ZrO_2.3 Kitsunai et al.4,5 demonstrated the effect of additives in the processing of fine-grain tungsten and molybdenum alloys utilizing TiC as a secondary phase. By means of hot isostatic pressing (HIPing), followed by forging, they were able to develop grain sizes in the range 50 nm to 1.6 mu m. However, the addition of the secondary phase to tungsten or molybdenum involved ball milling, and this resulted in a non-uniform dispersion of the inclusions, with an attendant wide distribution in grain size.
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