E. W. Lui

Processing and characterization of Al-Al3Nb prepared by mechanical alloying and equal channel angular pressing

Mechanically alloyed Al with immiscible elements such as Nb can lead to a uniform distribution of nanoscaled precipitates which are highly stable compared to conventional alloying and with excellent interface, resulting in significant increase in strength without problems associated with nano ceramic particles in metal matrix composites. Although immiscible, Nb can be alloyed with Al through mechanical milling, forming trialuminide (Al3Nb), either directly or upon subsequent precipitation, which possesses high strength, stiffness and stability at elevated temperatures. In the present study, Al-5 at.% Nb supersaturated solid solution was achieved after prolonged ball milling and nano Al3Nb precipitates were formed during subsequent ageing at 530°C. The Al-Al3Nb powder was consolidated by equal channel angular pressing (ECAP) at 400°C, resulting in a fully dense material with a uniform distribution of nanoscaled Al3Nb precipitates in the Al matrix.

Multiscale composition modulated Ti–Al composite processed by severe plastic deformation

AbstractUsing severe plastic deformation processes to consolidate and co-deform powder mixtures to make ultrafine grain composites is a very attractive approach because it offers an almost non-limited room for combinations of phases and composite structures. The aim of this work was to investigate the mechanisms operating at different length scales and leading to multiscale structures, namely co-deformation, fragmentation and mechanical mixing. A Ti–Al composite was processed from a Ti–Al powder mixture prepared by ball milling and subsequently deformed by equal channel angular pressing. Microstructures were characterized at all length scales, down to the nanometre, using optical microscopy, scanning electron microscopy and transmission electron microscopy. It was found that the final structure exhibits unique features at various length scales. Chemical heterogeneities at the micron scale are the result of co-deformation, while at the sub-micron scale they result from the fragmentation and necking of the Ti hard phase. Then, at the nanometer scale, intermixing occurred and nanoscaled intermetallic particles were discovered. This work highlights the possibilities offered by all these mechanisms to design ultrafine grain composite structures for optimized properties.

Oxide Dissolution and Oxygen Diffusion in Solid-State Recycled Ti-6Al-4V: Numerical Modeling, Verification by Nanoindentation, and Effects on Grain Growth and Recrystallization

AbstractThe oxide dissolution and oxygen diffusion during annealing of Ti-6Al-4V solid-state recycled from machining chips by equal-channel angular pressing (ECAP) have been investigated using nanoindentation and numerical modeling. The hardness profile from nanoindentation was converted into the oxygen concentration distribution using the Fleisher and Friedel model. An iterative fitting method was then employed to revise the ideal model proposed previously, leading to correct predictions of the oxide dissolution times and oxygen concentration profiles and verifying nanoindentation as an effective method to measure local oxygen concentrations. Recrystallization started at the prior oxide boundaries where local strains were high from the severe plastic deformation incurred in the ECAP recycling process, forming a band of ultrafine grains whose growth was retarded by solute dragging thanks to high oxygen concentrations. The recrystallized fine-grained region would advance with time to eventually replace the lamellar structure formed during ECAP.

Nanostructured Dual Phase Ti-Al through Consolidation of Particles by Severe Plastic Deformation

A two-phase Ti-Al material was fabricated by severe plastic deformation. Particles of finely mixed elemental Ti and Al were mechanically milled and then consolidated by equal channel angular pressing. The bulk material has a unique interpenetrating structure of Ti and Al phases with multiple scales from micro to nano. Compared to its coarse structured counterpart, the multiscale structured material exhibited a significant increase in strength without compromising plasticity.