Coarse Silicon Particles Research Articles

Microstructural studies of aluminium-silicon alloy reinforced with alumina fibres

The microstructure of an alumina fibre reinforced Al-7wt% Si alloy has been investigated. It was shown that the Al-Si eutectic structure which characterized this alloy was markedly changed by the presence of the fibres, with coarsening of silicon particles and a reduction in primary aluminium grain size. The coarse silicon particles exhibited twinning but no orientation relationship with the aluminium. Fine silicon precipitates were also present and these had a cube-cube orientation relationship with the aluminium lattice. Lath-like intermetallics, FeSiAl5 and FeSi2Al4 with monoclinic and tetragonal structures, were identified which existed in equilibrium and had the epitaxial relationship (001)mono//(001)tet and [100]mono//[100]tet. The iron was a contaminant introduced in the course of composite fabrication. Dislocations were a common feature of the aluminium matrix, with a typical density of ∼4×107mm−2. Nevertheless, dislocation hardening of the metal matrix was not detected. No evidence of Mg2Si precipitates in the metal matrix was found, but the small addition (0.2wt%) of magnesium to the alloy was discovered to segregate at the fibre-aluminium interface. This segregation was believed to result in improved wettability of the two constituents, encouraging the formation of a strong fibre-matrix bond, and producing desirable properties of the composite in the transverse direction.

Microstructural studies of matrix and interface in ceramic fibre-reinforced aluminium alloys

This study is a part of an ongoing research programme aimed at optimising the properties of ceramic fibre reinforced aluminium alloys. It involves a detailed study of matrix and interface and an understanding of the overall mechanical behaviour of the composite. Here, we report an investigation of an Al-7wt%Si alloy reinforced with alumina fibres. A range of techniques including analytical electron microscopy are used to characterise the structure of the fibre matrix interface, the crystallography and morphology of matrix precipitates, and dislocation substructures.It is shown that the eutectic structure of the Al-Si alloy is markedly changed in the presence of fibres, with silicon showing a tendency to segregate on the fibres. Coarse silicon particles are characterised by twins having the twin the {111} silicon habit plane and related by a rotation of 180° about the [110] direction.The particles have a lamellar morphology with the longitudinal axis lying in the {111} habit plane, but show no evidence of any epitaxial relationship with the aluminium lattice.

Precipitation of silicon in aluminum-silicon: A calorimetric analysis of liquid-quenched and solid- quenched alloys

The precipitation of silicon in aluminum-silicon was investigated by calorimetric analysis of liquid-quenched (LQ; rapidly solidified) and solid-quenched (SQ: quenched after annealing the solid at elevated temperature) alloys. Nonisothermal annealing experiments (differential scanning calorimetry) were performed using specimens containing 1.3 to 19.1 gross at. pct Si. Initial and resulting microstructures were characterized by X-ray diffraction and transmission electron microscopy. Quantitative analysis showed that heat production in LQ alloys is due to precipitation of dissolved silicon and coarsening of silicon particles already present in the as-liquidquenched state, whereas heat production in SQ alloys can be ascribed solely to precipitation. The kinetics of silicon precipitation are affected by quenched-in excess vacancies in both LQ and SQ alloys. The grain-boundary area as a function of gross silicon content is discussed in terms of its effect on vacancy annihilation and the resulting effective activation energy.

粉末押出しした黒鉛粒子分散Al-Si合金の摩耗特性

Graphite-dispersed alloys with the Al-Si matrix phase consisting of various amounts and sizes of silicon particles were prepared by the powder extrusion. Effects of the dispersion of silicon and graphite particles on wear characteristic of the alloy were investigated at various sliding speeds. In the graphite-dispersed Al-Si(C) alloy with the matrix phase consisting of coarse silicon particles, the wear rate has a small value in the range of low sliding speed, but it is remarkably increased at high sliding speed. The increase of the wear rate is caused by plastic deformation of the aluminum phase and its breaking-away at silicon or graphite particles. In the case of the graphite-disparesed Al-Si(F) alloys with the matrix phase consisting of fine silicon particles, the sliding speed which causes a small wear rate is higher than that of the alloy with the matrix phase consisting of Al-Si(C) alloy. The wear rate increases with increasing sliding speed, but it decreased with the formation of hard layer containing aluminum oxide and fine silicon particles, accompanied by the lubricating effect of the graphite particles. This tendency becomes remarkable with increasing silicon and graphite contents. When the sliding surface with an angle θ=90° to the extrusion direction was rubbed, the increase of wear rate with the sliding speed is higher than that in the case of θ=0°. In the graphite-dispersed alloy with the matrix phase prepared from Al-12 wt%Si alloyed powder, the wear rate attains a maximum at a certain sliding speed, then decreases and reaches a minimum with increasing sliding speed. At higher sliding speed, the wear rate again increases rapidly.

Ａｌ‐Ｓｉ系合金鋳物の破壊じん性におよぼす凝固組織の影響

Fracture toughness of unidirectionally solidified Al-7%Si and Al-7%Si-0.3%Mg alloys is evaluated as a function of cooling rate. The toughness of the Al-7%Si alloy is slightly improved when the primary and secondary arm spacings and the particle size of silicon are reduced with making fast the cooling rate. Cracks are initiated at the matrix-silicon particle interface by cleavage of silicon particles. The initial cracks at coarse silicon particles are interconnected and propagated. If the silicon particles are refined at faster cooling rate, the plane strain fracture toughness is raised. Coarse silicon particles are easily fractured. Fracture toughness of the Al-7%Si-0.3 Mg alloy is not so susceptible to the cooling rate because the dissolution of magnesium into the matrix and solution hardening oecur.

Over-modification band in aluminum-silicon eutectic solidified structure modified with sodium

The nature and origin of so called "over modification band" which appears in the microscopic structure of eutectic Al-Si alloy treated with an excessive amount of sodium has been investigated in detail by the method of 1) the optical and scanning electron microscope observation of structures, 2) the X-ray microprobe analysis of the distribution of sodium in solidified structures and 3) the change in the thermal analysis curves depending on the added amount of sodium.The over modification band may be classified into two groupes. The first group of band is formed when the temperature gradient in the liquid in front of the solid-liquid interface is decreased abruptly, for example, when the melt is cast in an insulating mold. The increase in sodium concentration and the decrease in temperature gradient results in the increase in the constitutional supercooling of aluminum. The condition will be satisfied for the change from the co-operative Al-Si eutectic growth with a plane solid-liquid interface to the dendritic growth of aluminum. The dendrite of silicon is not formed due to the existence of excess amount of sodium. When the growth of aluminum dendrite reaches to certain extent, the supercooling will be reduced and the co-operative growth of the eutectic is resumed leaving behind the "band" of aluminum dendrites with coarse silicon particles in between.The second group of band is formed due to the local segregation of sodium when the sodium addition is highly excessive. The band formed in such case is composed of aluminum dendrites, silicon particles and a ternaly compound having chemical composition close to AlSiNa.The formation of over modification band, in the case of either groupes, is governed by the solidification rate, temperature gradient at the solidliquid interface and the sodium content of the liquid. Therefore, by the proper consideration of the solidification condition, the formation of the band may be avoided even when the sodium content is in excess.