Thermal behavior of short fiber reinforced AlSi12CuMgNi piston alloys

The mismatch in thermal expansion coefficients could introduce large thermal stresses during thermal processing and correspondingly increase the dislocation density due to the addition of ceramic reinforcement to the metal or alloy. Compared with the unreinforced metals or alloys, the aging kinetics and thermal cycling response in metal matrix composites are modified as a result of microstructural changes. This work investigated both the aging and thermal cycling behaviors of the short fiber reinforced AlSi12CuMgNi piston alloys using X-ray diffraction, microhardness and thermal dilatometry tests. Compared with the unreinforced alloy, the aging is accelerated in the composites containing the inorganic Saffil((R)), Maftech((R)) and Supertech((R)) fibers. The precipitates formed during aging are related to the reinforcement composition. The magnesium is essential for the formation of strengthening precipitates, but it could deplete due to its reaction with the SiO2 in the fiber or in the binder, and therefore the age hardening could be suppressed. The coefficient of thermal expansion (CTE) is affected by both the microstructure and internal thermal stresses generated from the both heating and cooling. The thermal stresses can produce a positive or negative additional strain to the dilatometric strain, causing the CTE to change with the temperature in a different way from the other single-phase metals or alloys. The aging treatment does not influence the thermal expansion largely, but indeed increases the critical temperature above, which the CTE divergence occurs. The thermal strain response during thermal cycling is affected by the material composition, fiber orientation and aging heat treatment. The fluctuation of residual strain has been observed that is caused by the evolution of thermal stress. The experimental results show that during thermal cycling up to 47 times the stress relaxation proceeds through the plastic deformation and matrix recovery. The effect of creep on stress relaxation is small. Both the fiber fracture and interface debonding responsible for the stress relaxation could be excluded. The reduction in hardness after thermal cycling is related to the overaging and matrix recovery. (C) 2003 Elsevier Ltd. All rights reserved.