Consolidation of Al-TiO₂ and Ti₃Al-Ai₂O₃ composite powders
Pressureless sintering, hot pressing and sinter-HIP of Ti₃Al-Al₂O₃ and Al-TiO₂ composite powders have been carried out under inert atmospheres. The Al-TiO₂ composite powders were produced by discus milling Al/TiO₂ powder mixtures followed by ball milling. The Ti₃Al-Al₂O₃ composite powders were produced by heating Al/TiO₂ powder mixtures. Densification, sintering behaviour, microstructural development and mechanical properties of the composites were investigated for the variables of pressure and temperature at different powder particle sizes. It was shown that higher bulk density and lower porosity were obtained by hot pressing compared to pressureless sintering. A temperature of 1600°C is required to achieve >95% theoretical density of the composites when fabricated by pressureless sintering Ti₃Al-Al₂O₃ composite powder. However, fully densified composites could be fabricated by hot pressing l.2μm Ti₃Al-Al₂O₃ composite powder at 1474°C under vacuum for a one hour holding period. The growth of Al₂O₃ particle size and the development of microcracks at the Ti₃Al-Al₂O₃ interfaces were evident with the increasing temperature above 1500°C for the composites produced by pressureless sintering and hot pressing. Near fully densified Ti₃Al-TiO-Al₂O₃ and Ti₃Al-Al₂O₃ composites (96% of theoretical density) with ≤4μm Al₂O₃ particle size in the composites could be achieved by pressureless sintering Al-TiO₂ composite powder at 1480°C in argon for 4-5 hours of holding period. Fully dense Ti₃Al-Al₂O₃ composite microstructures could be achieved by hot pressing four hours discus milled composite powder subjected to 24 hours of ball milling giving a starting median particle diameter of 7μm, at 1387°C under vacuum. The XRD traces show that the Ti₃Al phase in the Ti₃Al-Al₂O₃ composites is maintained above 50% XRD peak intensity up to 1500°C. The composite microstructure demonstrates that both the Al₂O₃ and Ti₃Al phases are continuous and exhibit an interpenetrating network. Near fully densified Ti₃Al-TiO-Al₂O₃ composites (≤98% of theoretical density) were fabricated by sinter-HIP of Al-TiO₂ composite powders with a starting median particle diameter of 7.7μm. The coarsening of the Al₂O₃ particles was suppressed after hot isostatic pressing at 1350°C in argon for two hours at 200MPa for the Ti₃Al-TiO-Al₂O₃ composites prepared by pressureless sintering Al-TiO₂ composite powder at 1480°C. Vickers hardness of the composites increased with increasing densification and increased Ah03 ceramic phase content. Vickers hardness of ≤17.5GPa was achieved for the fully densified Ti₃Al-TiO-Al₂O₃ composite whereas Vickers hardness of ≤15GPa was achieved for the fully densified Ti₃Al-Al₂O₃ composites with Ah03 particle size of ≤4μm. In the sinter-HIP experiments, the Vickers hardness increased by up to 2GPa due to the decreased total porosity after hot isostatic pressing of the pre-sintered Ti₃Al-TiO-Al₂O₃ composites with 96% theoretical density. Biaxial strength of ≤350MPa and ≤278MPa were achieved for Ti₃Al-TiO-Al₂O₃ and Ti₃Al-Al₂O₃ composites, respectively. In this study, fracture toughness as high as 7.5±0.8MPa.m½ was obtained when the phase structure exhibited fine (:'.S4μm) and homogeneous microstructure with 50% XRD peak intensity of Ti₃Al ductile phase in the matrix.
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