High temperature tensile properties of hypereutectic Al-25Si based alloy

The high temperature tensile deformation of a hypereutectic Al-25Si based alloy fabricated by spray forming and subsequent hot extrusion was investigated. Tensile tests were conducted at various temperatures and strain rates. It was revealed that the ductility of the alloy is sensitive to both the test temperature and strain rate. At a given strain rate, the peak value of elongation was obtained at 500 °C. At 490 °C and above, the peak value of elongation was observed at a strain rate of 1.0 × 10^-2 s^-1, although the elongation increased with decreasing strain rate at 460 °C and below. The high elongation was exhibited when a high strain rate sensitivity index (m value) was attained and a liquid phase existed during deformation. The liquid phase appeared as a filament-like structure which is aligned with the tensile direction on the fracture surface of the tensile deformed specimen. A higher elongation (>35%) was obtained when the volume fraction of the liquid phase was 0.7%-1.7%. The maximum elongation of 75% was achieved when the volume fraction of the liquid phase was about 1%. The transition of the activation energy was observed at 430 °C when incipient melting occurred.