H. B. McShane

Reaction synthesis of titanium aluminides

The formation of titanium aluminides from the elemental powders has been investigated. A traditional powder metallurgy route of compaction (by cold isostatic pressing, hot pressing or hot extrusion) followed by heat treatment was compared with the novel technique of hot extrusion reaction synthesis (HERS). The products from these different production methods were characterised by x-ray diffraction and microscopy (light and scanning electron). The intermetallic compound formed under most processing conditions wasTiAl3. Only when there was a rapid increase in temperature to high temperatures, as found in induction heating of compacts or in HERS, were the compounds Ti3Al and TiAl formed.

Processing defects in hot extrusion reaction synthesis

Abstract Experiments were conducted on a miniature and a larger extrusion press to simultaneously form and extrude nickel aluminides (Ni3Al, NiAl) and nickel aluminide composites from elemental powders using a novel process called hot extrusion reaction synthesis. An overview of macroscopic and microscopic processing defects that can arise in this process is presented as well as strategies for overcoming these defects. Extrusion cracking was found to significantly increase with increased nickel content. Higher extrusion die exit temperatures promoted both a reaction converting elemental powders to the desired intermetallic or intermetallic composites and reduced cracking of NiAl extrusions. Processing defects in the form of matrix micro-cracking and reaction layers between intermetallic matrix and SiC reinforcements were also present in the composite material. The reaction always occurs seconds after the material had been extruded, thus bypassing the consolidation stage of extrusion resulting in the presence of reaction induced porosity. A novel high temperature transient window has been identified for the production of pore-free intermetallic and intermetallic composite rods and wires.

Preliminary evaluation of hot extrusion miniaturization

Experiments were conducted to examine the feasibility of hot extruding metals on a miniature scale thus providing a quick and cheap method of studying extrusion in general. Aluminium was successfully extruded using a new miniature hot extrusion rig, producing aluminium wires (maximum diameter 2.6 mm) as opposed to rods. A preliminary comparison of extrusion pressures on the miniature rig and on a larger extrusion press was conducted. The effects of temperature, extrusion speed and extrusion ratio on the extrusion pressure were examined for both miniature and larger scale extrusions. Extrusion speed had little effect on extrusion pressure, because the range of speeds examined was too small (due to speed limitations on the larger extrusion press). Both extrusion sizes generally displayed similar dependencies on temperature and extrusion ratio. However, the extrusion pressures for miniature extrusions were found to be always lower than for the larger scale extrusions. This may have been due the evaluation of parameters in the expression used for strain rate in the comparison. Finite element analysis may prove useful in gaining a fuller understanding of the miniaturization process.

Fabrication and properties of rapidly solidified powder-based high-temperature application light-alloy composites

Particulate-reinforced composites based on Al-Fe-Ce and SiC were fabricated by conventional powder metallurgy techniques, namely powder mixing, cold compaction and hot extrusion. Static mechanical properties at ambient temperature and at elevated temperature after prolonged exposure to the test temperature were measured and related to process parameters and the volume fraction of the reinforcement. The addition of SiC particles in considerable volume fractions help to retain the static properties at high temperature, even after prolonged exposure. It was also observed that the mechanical strength increases with decreasing temperature and extrusion ratio. Unavoidable non-uniform distribution of SiC particles and the associated porosity were observed to be responsible for wide variations in the properties within the same extrudate. Attempts to reduce this variation are discussed.

Coarsening and properties of extruded Al-8Fe-4Ni-1Mo alloys

Characterization of rapidly solidified Al-8Fe-4Ni-1Mo alloy shows that subsequent extrusion generates a non-uniform microstructure which may be attributed to the non-uniform deformation during extrusion. The mechanical properties were found to be closely related to the extrusion ratio. It is believed that the degradation of the microstructure is due to the severe deformation during processing, and hence a compromise is needed between optimum powder bonding and the lowest extent of deformation in the consolidation process.

Phase transformation reactions in rapidly solidified Al-6.5Fe-1.5V alloys

Phase transformation reactions, occurring during heating of as-atomised Al-6.5Fe-1.5V powders, extrusion of the powders, and heating of the as-extruded alloys produced from the powders, have been studied by DSC, XRD and TEM. The DSC studies of the as-atomised powders revealed several phase transformation reactions. The solid solution in zone A decomposed to form metastable phases at 360°C. These metastable phases further transformed to form equilibrium phases at 500°C. The microquasi-crystalline icosahedral (MI) phase particles present in zone A and zone B transformed to equilibrium phases at 500°C. The globular clusters of microquasi-crystalline icosahedral (GCMI) phase particles in zone C transformed polymorphously to icosahedral (I) phase particles at 450°C. These reactions were believed to occur during extrusion of the powders. During heating of the as-extruded alloys produced from coarse powder particles, I phase transformed polymorphously to hexagonal phase at 550°C. The hexagonal phase decomposed to monoclinic Al45(V, Fe)7 and Al13Fe4 phases upon heating for longer times.

Effect of extrusion parameters on the microstructure and properties of an Al-Li-Mg-Zr alloy

The effect of the extrusion temperature and the extrusion ratio on the microstructure and mechanical properties of an Al-Li-Mg-Zr alloy in the as-extruded, solution-treated and aged conditions were investigated. It was found that an increase in the extrusion temperature and ratio increased the degree of recrystallization in the extrudate. An increase in the extrusion temperature also increased the sub-grain size, whereas the extrusion ratio did not affect the sub-grain size. The strength of the extrudates decreased and the ductility increased in the asextruded condition with increases in the extrusion temperature and the extrusion ratio.However, in the solution-treated condition, although the effect of the extrusion ratio on the mechanical properties was found to be the same as in the as-extruded condition, the trend with the extrusion temperature changes and the extrusion temperature was not simple in its influence on the mechanical properties. The fracture toughness remained almost unaffected by variation either in the extrusion temperature or the extrusion ratio.