Papers by Author: Hong Bao Yu

Abstract: Bulk nanostructured (grain sizes in the range of 50-200nm) and ultrafine structured (grain sizes in the range of 100-500nm) -TiAl based alloys with compositions Ti-47Al (in at%) and Ti–45Al–2Cr–2Nb–1B–0.5Ta (in at%), respectively, have been produced using a combination of high energy mechanical milling of mixtures of elemental powders and hot isostatic pressing at 800 and 1000oC respectively, and the microstructures of the samples have been characterised. At room temperature, the HIPed samples fractured prematurely at tensile stresses in the range of 200-300MPa and showed no ductility, very likely due to the relative high oxygen content (0.6wt%) in the samples and very low tolerance of TiAl based alloys on dissolved oxygen. At 800oC, the HIPed samples showed a yield strength in the range of 55-70MPa, a tensile strength in the range of 60-80MPa, a large amount of elongation to fracturing around 100% and clear strain softening. Examination of the fractured tensile test specimens at room temperature and 800oC showed that the level of the consolidation was fairly high, but the HIPed samples do contain a small fraction of interparticle boundaries with weak atomic bonding. The fracture of the HIPed samples in tensile testing at room temperature and 800oC, respectively, is predominately intergranular, and the large amount of plastic deformation prior to fracture at 800oC is achieved mainly through grain boundary sliding in conjunction with dislocation gliding, in agreement with the deformation mechanisms of nanostructured and ultrafine structured alloys generally agreed by researchers.

Abstract: Ti-45Al-2Cr-2Nb-1B-0.5Ta (at.%) bulk alloys with dense and ultrafine grains were fabricated by Double Mechanical Milling (mechanical milling + heat treatment + mechanical milling) and spark plasma sintering method. The phase composition and microstructure of the milled powder and bulk alloy sintered by SPS at different temperature (900oC, 1000oC and 1100oC), and the relationship between microstructure and properties of bulk alloys were investigated. The results demonstrate that high-quality composite powders (low contaminant, size uniform distribution and elements homogeneous dispersion) can be obtained by double mechanical milling. The composite powders prepared by primary mechanical milling were uniform and partially solid solution. Ti3Al、Ti、Al3Ti and TiAl phases were found after heat treatment while Al phase disappeared. The fined grain size and particle size were achieved by subsequent mechanical milling. The whole mechanical milling leads to alloying and the refined grain and particle, which also cause lattice distortion and powders system energy increased. SPS results showed that the densified and ultrafine grained Ti-45Al-2Cr-2Nb-1B-0.5Ta alloy is mainly consisted of TiAl, Ti3Al phase and a small quantity of TiB2 phase. With the increasing of sintering temperature, grain size of TiAl based alloy increase. The mechanical properties depend on microstructure and grain size. The relationship between compression properties, bending properties and microstructure was discussed.