Pct Si Alloy Research Articles

The tensile properties of a graphite-fiber-reinforced Al-Si alloy

Room temperature uniaxial tensile tests have shown that composites of an Al-13 wt pct Si alloy and Thornel 50 graphite fibers have strengths greater than a theoretical value that was calculated on the basis of the law of mixtures. At 28 vol pct fibers, the average uniaxial tensile strength was found to be 106,000 psi, and several values between 130,000 and 144,000 psi were obtained. The modes of deformation and failure in the composites have been studied by the microexamination of polished surfaces and fractures of tested specimens. The reasons for the high strengths and the unusual modes of fracture that were observed cannot be explained on the basis of the presented data. Specimens of the composite have been thermally cycled between −193° and +20°C twenty times and others between −193° and +500°C twenty times. Tensile tests and microexamination of these thermally cycled specimens show that thermal cycling does not degrade the tensile properties of the composites or change their microstructure.

The migrational characteristics of the cusp-oriented α/κ interface in the Cu-Si system

The mobility of the coherentα/κ boundaries in the copper-silicon system was studied in a Cu plus 5.16 wt pct Si alloy with a lamellar microstructure. A new resistometric technique was employed to study the isothermal kinetics ofα/κ interphase boundary migration as a function of driving force at two reaction temperatures, 702° and 568°C. The driving force was shown to be linearly related to the temperature change, δT, and the experimental results show that the functional relationship between mobility and δT exhibits an initial linear dependence followed by a sharp rise in mobility at higher values of δT. Upquench and down-quench results were found to be equivalent, implying that the mechanism for boundary motion is independent of whether the boundary is moving into theα or into theκ phase. The results demonstrate that theα/κ interface does not move normal to itself by the long range bulk diffusion of silicon at low values of δT during the initial periods of the reaction times. A pole mechanism with its associated dislocation node configuration, consisting ofa/2〈112〉 andc[00.1] pole dislocations anda/6〈112〉 transformation partial, is proposed as the mechanism for the migration of the coherent fcc/hcp boundary in the range of small δT. The results obtained at the different reaction temperatures are consistent with the mechanism proposed, and quantitative agreement exists between the experimental and predicted mobilities. The sharp rise in mobility at larger values of δT could not be explained in terms of the pole mechanism. Attempts to rationalize this behavior in terms of a two dimensional nucleation model were not successful since unrealistically low values of surface energy would be required.

Investigation of the grain coarsening behavior of some Aluminum alloys

Grain coarsening tests were carried out on Al-4.5 pct Cu and Al-4.5 pct Si alloys. The effects of three variables, melt composition, pour temperature, and mold temperature, were determined. It was found that the macrostructure generally coarsened with increased pour and mold temperatures. Coarsening was extreme in the unrefined alloys but was retarded by the active grain refiners like titanium and columbium. The effect of boron was spectacular in suppressing coarsening tendencies. The results of the investigation support the carbide theory of nucleation as opposed to the peritectic theory.