Growth and microstructure of Al2O3SiCSi(Al) composites prepared by reactive infiltration of silicon carbide preforms

Alumina matrix composites have been grown by directed melt oxidation of an AlSiZnMg alloy into sintered SiC preforms. Reaction between melt and the silica layer on the pre-oxidised SiC yields a matrix in which the residual alloy is principally silicon. However, there is no evidence of the formation of Al₄C₃. The coarsest particles display the least reaction while porosity increases as the particle size decreases .Thus, owing to the enrichment of silicon, growth rates are retarded compared to those in free space. The microstructure of the oxide is monocrystalline over distances of the order of the inter-particle spacing, however, a new type of growth fault that constitutes an inversion boundary in the Al₂O₃, has been found. Owing to the substantial reaction between the particle and the melt, the volume fraction of the alloy constituent in the final composite depends sensitively on particle size, in contrast to its relative invariance in directed melt oxidation into inert preforms.     

The coarsening of lamellae in Ti48Al2Mn2Nb

The thermal stability of as-cast Ti48Al2Mn2Nb alloys has been investigated as a function of time at temperatures of 1200, 1350 and 1420°C. The resultant structures have been characterised using optical microscopy, electron probe microanalysis and transmission electron microscopy. The as-cast structure consists mainly of lamellae of α₂ and γ. Annealing at 1350 and at 1420°C resulted in the continuous and discontinuous coarsening of the primary lamellae. Discontinuous coarsening, which often originates at the sample surface, results in branched irregular secondary lamellae. The average interlamellar spacing of the secondary lamellae is up to a thousand times larger than the primary interlamellar spacing. The extremum principles of maximum velocity and maximum rate of entropy production, used to predict the secondary interlamellar spacing, are inconsistent with the experimental observations. The theoretical predictions of the velocity of the discontinuous coarsening by Livingston and Cahn are used to estimate the value of grain boundary diffusivity at 1350°C. The temperature dependence of discontinuous coarsening is weak, in contrast to the strong effect of lamellar orientation on the rate of discontinuous coarsening.     

Solidification paths of TiTaAl alloys

Microstructure evolution during solidification and high temperature phase equilibria were investigated for TiTaAl alloys in the vicinity of the 50 at.%Al isoconcentrate. Examination of dendrite morphologies and segregation profiles were used to deduce the phase sequencing during solidification and the boundaries of the relevant liquidi surfaces. In situ high-temperature X-ray diffraction and isothermal annealing experiments were conducted to determine the phases present at elevated temperatures and coupled with extensive characterization by transmission electron microscopy to elucidate the solid state transformations during cooling of the solidification microstructure. For approximately equiatomic TiAl, the primary phase selected from the liquid was α for the leaner Ta concentrations (< 7% Ta), as in the binary alloys of equivalent Al content, but changed to β with increasing Ta levels (> 10 %). With increasing Al and Ta the α liquidus penetrates deeply into the ternary. The interdendritic segregate was always γ. Dendrites of the β-forming alloys were heavily segregated leading to different microconstituents in the core and bulk dendrite regions during post-solidification cooling. In alloys with < 15 % Ta, (α₂ + γ_t) lath formed in the dendrite bulk due to the decomposition of α, with σ precipitating in the core (> 10% Ta). With increasing Ta levels the lath is gradually replaced by polycrystalline γ which grows into the dendrite bulk, and the core decomposes into a lamellar (γ + σ) microstructure from the decomposition of σ. The γ segregate does not transform further.     

Al66Mn9Ti25金属间化合物的微合金化

研究了微量镧、铈、铌、钇对Ａｌ６６Ｍｎ９Ｔｉ２５金属间化合物显微组织、微裂纹生成及扩展倾向、压缩性能、微观断裂和形变行为的影响。研究表明，添加微量元素后，保持Ｌ１２结构，且晶格常数基本无变化，位错可动性得到不同程度的增强。镧、铈有促进晶粒枝晶化和第二相弥散化作用，从而使微裂纹生成和扩展以及沿晶开裂受到一定程度的抑制，因之，压缩强度提高，压缩塑性也有改善。加入微量铌和钇显微组织仍为等轴状晶粒和粗大第二相，压缩性能未能改善。     

Mechanical behavior of Al−Li/SiC composites: Part II. Cyclic deformation

The deformation and failure mechanisms under cyclic deformation in an 8090 Al−Li alloy reinforced with 15 vol pct SiC particles were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes in low-cycle fatigue life is discussed.