C.K. Yao

Microstructure and properties of SiCw/6061 aluminium alloy composites after compression at temperatures around solidus of matrix

Abstract The effect of compressive deformation at temperatures around the solidus of the matrix on the microstructure and properties of SiCw/6061 aluminium alloy composites was investigated. It was found that the temperature, strain rate, and amount of deformation affect whisker distribution and breakage, densification and uniformity of composites, and SiCw/matrix alloy interfacial bonding. The microstructural evolution due to compression affects the properties of the composites, which is considered to be the most important aspect for evaluating high temperature plastic forming of the composites. The optimum parameters for compressive deformation were determined by analysing the microstructure and the properties of the composites.

Effect of annealing treatment on the stress corrosion cracking behavior of SiC whisker reinforced aluminum composite

Abstract Stress corrosion cracking (SCC) behavior of SiCw/pure Al or SiCw/2024Al composite in 3.5% NaCl solution was studied using double cantilever beam method. The effect of annealing treatment on SCC behavior of the composites was examined. The results indicated that the annealing treatment had an influence on the SCC properties of the composites. With the increase of the annealing temperature, the threshold stress intensity factor, K ISCC , increased and the steady propagation rate of SCC decreased, so that the SCC susceptibility of composites was improved.

Investigation of compression of SiCw/6061Al composites around the solidus of the matrix alloy

Abstract Compression test of SiCw/6061Al at the temperatures around the solidus of matrix alloy has been studied. Compression flow stress increases with increasing strain rate according to a power law σ=K ϵ m . At temperatures lower than the solidus of matrix, the m value is 0.32 and the activation energy of deformation is almost the same as that of aluminum self-diffusion, indicating a dislocation motion controlled by lattice diffusion deformation mechanism of the composite. At temperatures slightly higher than the solidus of the matrix, the m value is 0.5 and the activation energy of deformation is much higher than that of the self-diffusion of aluminum, indicating a grain boundary and interfacial sliding deformation mechanisms of the composite.

Microstructure and properties of SiCw/6061Al composite

Abstract In this paper the SiCw/6061Al composite fabricated by squeeze casting has been successfully extruded, the tensile tests for asfabricated and as-extruded composite carried out, and fracture surfaces and microstructure observed and analyzed by SEM, TEM and EDAX. It is demonstrated that the SiCw/6061Al composite has a good combination of room temperature specific strength and modulus, as well as excellent thermal properties. Extrusion improves the mechanical properties of the composite, and transforms an isotropic composite to an anisotropic one. The fracture surfaces consist of fine, equiaxed dimples of uniform size, indicative of localized ductility. It is observed that whiskers pulled out, which are covered with a coating of aluminum, are quite few. TEM observations indicate that there is a high dislocation density in the composite, and a stacking fault is a kind of common planar defect existing in a whisker. The EDAX results indicate that there is no penetration of the matrix aluminum into the whisker, and that interfacial bonding between the whisker and matrix is excellent.

Influence of hot-deformation on morphology and effective grain size of lath martensite in 18Ni maraging steel

Abstract The changes in the morphology of lath martensite with hot-deformation of austenite were studied by using an 18Ni maraging steel. In the case of conventional quenched (undeformed) or completely recrystallized specimens, an austenite grain contains several packets and each packet is almost completely partitioned by parallel blocks. However, this morphology of packets and blocks was markedly changed when martensite was formed from hot-deformed (not recrystallized) austenite. The packet size increases but the block width decreases with increasing amount of deformation. The relationship between packet size (or block width) of martensite and the grain size of the prior austenite was discussed in detail. The effective grain size of martensite with lath morphology which decides the mechanical proporties of the steel has been discussed on the basis of the Hall-Petch formula.

The morphology and substructure of SiC whisker in SiCw/Al composite

Abstract The morphology and substructure of SiC whisker in SiCw 6061Al composites were observed by a high voltage and high resolution electron microscope. The SiC whisker generally has a hexagonal or triangular cross section and a zigzag surface. Stacking faults parallel to the {111} planes were found in a doublebeam condition. It was considered that the stacking faults were connected with the matrix by the Frank partial dislocations whose Burgers vectors are a[111]/3. The stripes of the stacking faults were straight lines and were parallel to the [110] direction. There were a lot of dislocations in the aluminium matrix near the SiC whiskers. The high density dislocation zones were formed around the whiskers, and the dislocation density was as high as 10 /cm.

FRACTURE CHARACTERISTICS IN SICW/6061AL COMPOSITE

Tensile fracture processes in an extruded SiCw/6061Al composite were in-situ observed by means of SEM. The effects of off-axis angles on the strength of the composite were studied. It was found that both of the fracture characters and the strength of the composite were strongly depended on the off-axis angles.

Transformation to pearlite from hot-deformed and austenitized austenite

Abstract The influence of hot deformation and austenization temperature on the subsequent isothermal transformation to pearlite was studied for steels 40Cr and 50CrV. The deformation of austenite above or below its recrystallization temperature may accelerate the subsequent pearlite transformation. The acceleration in both cases, that is in recrystallized and work-hardened austenite, is considered to be mostly caused by the increase in the nucleation rate, in other words, by the increase in the nucleation sites.

The thermal conductivity of short-fiber composites

Abstract In this paper, the upper and lower bounds of thermal conductivity were derived for unidirectional short fiber composites and random short fiber composites. The micro-field of temperature was also calculated using the method developed in [1]. Lastly, the above theory was verified by the experimental results of a SiC whisker reinforced 6061 Al composite

Influence of flow stress of ausformed austenite on strength of ausformed martensite

Abstract The relationship between the flow stress of austenite at ausforming temperature and the hardness of the ausformed martensite (or austenite) at room temperature was studied using martensitic steel 18CrNiW and austenitic steel 18-8. Ausforming was performed with a torsion test machine at various temperatures between 723K and 923K and at strain rates between 2.5×10−3 and 2.6×10−1. After deformation, specimens were rapidly quenched by water spray. It was found that the increase in hardness of the ausformed austenite at room temperature (ΔHvγ) is uniquely determined only by the increase in the flow stress (i.e., the amount of work-hardening) of austenite (δσγ) by ausforming. On the other hand, the increase in hardness of ausformed martensite at room temperature (ΔHvγ) is determined b/y both Δσγ and Z (Zener-Hollomon parameter). When Δσgg is the same, ΔHvα is increased with an increase in ausforming temperature or decrease in strain rate (i.e., with a decrease in Z). Furthermore, when the ausforming temperature is fixed, the contribution of Δσgg on ΔHvγ is small at smaller Δσγ range and becomes large at larger Δσgg range. Present results suggest that the dislocation cell structure in austenite is very effective for the strengthening of ausformed martensite, and uniformly distributed (pile-up) dislocations in austenite have little effect on the strengthening of the ausformed martensite.