I.C. Stone

Isothermal grain coarsening of spray formed alloys in the semi-solid state

Grain coarsening of alloys in the semi-solid state is important in controlling grain sizes in spray forming and in other processes in which an alloy is formed or cast in the semi-solid state. Until now, this coarsening has been analysed in terms of classic LSW theory or in terms of the migration of grain boundary liquid films. These analyses suggest that the coarsening rate should increase with increasing solid fraction, fS, but this suggestion conflicts with previous experimental results which show that coarsening rate decreases with increasing fS for fS>0.7. This paper shows, for the first time, that coarsening rate does increase with fS for fS less than approximately 0.75 in agreement with the liquid film migration model, and then decreases again with further increasing fS for fS greater than approximately 0.75. A modified model of liquid film migration is proposed which takes into account the reducing area of the liquid film as fS increases for high fS. The formation of intragranular liquid droplets, and the pinning of grain boundary liquid films by dispersoids during coarsening are also discussed.

Metal and ceramic matrix composites

SECTION 1: INDUSTRIAL PERSPECTIVE Introduction Metal matrix composites for aeroengines Metal matrix composites in high performance internal combustion engines High modulus steel composites for automobiles Metal matrix composites for aerospace structures Ceramic matrix composites for industrial gas turbines Composite superconductors SECTION 2: MANUFACTURING AND PROCESSING Introduction Fabrication and recycling of aluminium metal matrix composites Aluminium metal matrix composites by reactive and semi-solid squeeze casting Deformation processing of particle reinforced metal matrix composites Processing of titanium-silicon carbide fibre composites Manufacture of ceramic fibre-metal matrix composites SECTION 3: MECHANICAL BEHAVIOUR Introduction Deformation and damage in metal matrix composites Fatigue of discontinuous metal matrix composites Mechanical behaviour of intermetallics and intermetallic matrix composites Fracture of titanium aluminide-silicon carbide fibre composites Structure-property relationships in ceramic matrix composites Microstructure and performance limits of ceramic matrix composites SECTION 4: NEW FIBRES AND COMPOSITES Introduction Silicon carbide based and oxide fibre reinforcements High-strength high-conductivity copper composites Porous particle composites Active composites Ceramic nanocomposites Oxide eutectic ceramic matrix composites

Effect of Nb on nanoquasicrystalline Al-based alloys

Nanoquasicrystalline Al-based alloys, containing icosahedral particles in an α-Al matrix, exhibit high strength at elevated temperature. The metastability of the quasicrystals can limit the use of these alloys. In the present work, the microstructural evolution of Al93(Fe3Cr2)7 and Al93Fe3Cr2Nb2 (at%) alloys was studied using heat treatments and structural characterization by XRD, TEM and STEM-EDX analysis. It was observed that the Nb is dissolved in the Al–Fe–Cr icosahedral phase. This provides higher thermal stability, retaining the fine nanoquasicrystalline microstructure for longer times at high temperature.

Effect of Si on the formation and stability of the icosahedral quasicrystalline phase in Al-Fe-Cr-Ti alloys

The formation and stability of the quasicrystalline icosahedral (i) phase in melt-spun Al93– x Fe3Cr2Ti2Si x (x = 0–5) ribbons are reported. Samples were characterized by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. Primitive (P-type) ordered i phase particles were found to be homogeneously distributed in an fcc α-Al matrix in the as-melt-spun ribbons. The size of the i phase particles decreased and their thermal stability increased with increasing substitution of Al by Si. The i phase had a decomposition temperature of approximately 480°C in an Al93Fe3Cr2Ti2 ribbon whereas that in an Al92Fe3Cr2Ti2Si1 ribbon was approximately 500°C. The i phase particles are resistant to coarsening prior to decomposition into crystalline phases. The presence of a small quantity of Si (up to 1.0 at.%) is beneficial to both the thermal stability and the hardness of nanoquasicrystalline Al–Fe–Cr–Ti alloys.

Spray Forming of Al-Fe-Cr-Ti and Al-Si-Li Alloys

This work presents an investigation of the spray forming and downstream processing of Al alloys that are difficult to produce in bulk by conventional solidification processing: Al-Fe-Cr-Ti alloys for intermediate temperature applications and Al-Si-Li alloys for high stiffness, low density applications in fast moving machinery. For the Al-Fe-Cr-Ti alloys, spray forming is being investigated to allow the scale-up of alloy compositions previously explored only as ribbons or powders in traditional rapid solidification routes. For Al-Si-Li alloys, spray forming is used to provide globular primary AlLiSi in a fully divorced AlLiSi/α-Al eutectic structure. For both alloys, the as spray formed and downstream processed microstructure of 20kg billets has been investigated by scanning electron microscopy, electron probe microanalysis, and X-ray diffractometry. Preliminary mechanical properties have also been investigated.