Segregation mediated heterogeneous structure in a metastable β titanium alloy with a superior combination of strength and ductility

In β titanium alloys, the β stabilizers segregate easily and considerable effort has been devoted to alleviate/eliminate the segregation. In this work, instead of addressing the segregation problems, the segregation was utilized to develop a novel microstructure consisting of a nanometre-grained duplex (α+β) structure and micrometre scale β phase with superior mechanical properties. An as-cast Ti-9Mo-6W alloy exhibited segregation of Mo and W at the tens of micrometre scale. This was subjected to cold rolling and flash annealing at 820 oC for 2 and 5 mins. The solidification segregation of Mo and W leads to a locally different microstructure after cold rolling (i.e., nanostructured β phase in the regions rich in Mo and W and plate-like martensite and β phase in regions relatively poor in Mo and W), which play a decisive role in the formation of the heterogeneous microstructure. Tensile tests showed that this alloy exhibited a superior combination of high yield strength (692 MPa), high tensile strength (1115 MPa), high work hardening rate and large uniform elongation (33.5%). More importantly, the new technique proposed in this work could be potentially applicable to other alloy systems with segregation problems

Characterisation of the wear mechanisms in retrieved alumina-on-alumina total hip replacements

Due to their superior wear performance and biocompatibility compared to alternative polymer/metal prostheses, alumina-on-alumina total hip replacements (THRs) are extensively used for young and more active patients. However, the understanding of the wear mechanisms of alumina in vivo remains relatively poor, and there remains little quantitative understanding of the structural and chemical changes at the articulating surface. In the current study, the surface and sub-surface microstructures of retrieved in vivo alumina THRs are presented. Severe wear, also called stripe wear, was observed in all cases. The transition between the stripe wear and the mild wear was very sharp. Site-specific cross-section TEM specimens were prepared by Focused Ion Beam (FIB) at the stripe boundary region. The results suggest predominantly intergranular fracture occurred that was restricted to the outer layer of grains below the surface, with transgranular fracture also occurring in the stripe wear region. Cracking was believed to be initiated by extensive dislocation slip. A thin layer of hydroxide was also observed at the extreme surface of the mild wear region by aberration-corrected high resolution transmission electron microscopy (HRTEM). The wear mechanisms are discussed