Effect of Thermal Cycling on Creep Behavior of Powder-Metallurgy-Processed and Hot-Rolled Al and Al-SiC Particulate Comp
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METAL matrix composites have earned significant interest worldwide, for their potential to combine the strength and high modulus of the ceramic reinforcement with the ductility and toughness of the metals. The discontinuously reinforced aluminum (DRA) composites possess a high strength-to-weight ratio and a high specific stiffness; and as a result, they are of interest for use in various automotive and aerospace components, particularly for weight reduction. Many of the automotive components are subjected to thermal cycles during use. The DRA composites are sensitive to thermal cycles, primarily because of the differences in the coefficients of thermal expansion (CTEs) of the Al matrix and the ceramic reinforcement phases. Significant residual stress is also generated through thermal cycling and this has been measured by diffraction techniques.[1,2] Relaxation of the thermal residual stresses during exposure to an elevated temperature in the course of a given thermal cycle leads to dimensional changes.[3] Dimensional changes and hysteresis loops SHARMILEE PAL, Research Scholar, and R. MITRA and K.K. RAY, Professors, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur–721 302, India. Contact e-mail: [email protected]. ernet.in V.V. BHANUPRASAD, Scientist ‘‘F,’’ is with the Ceramics and Composites Group, Defence Metallurgical Research Laboratory, Hyderabad–500 058, India. Manuscript submitted January 31, 2009. Article published online September 23, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
have been observed both in the case of pure Al and in the Al-based particulate composites. Furthermore, quench hardening has been observed in pure metals, such as in Al, due to the formation of vacancy loops and jogged dislocations.[4,5] The Young’s modulus (E), CTE, and yield strength (YS) of Al (E = 70 GPa, CTE = 4 9 106 K1, and YS = 35 MPa) and SiC (E = 450 GPa, CTE = 24 9 106 K1, and YS = 600 MPa) are significantly different. The strain due to mismatch in the CTEs of Al and SiC leads to the generation of considerable internal stress at the particle-matrix interfaces on cooling from the temperature of fabrication to the ambient temperatures.[6–8] Simple calculations have shown that the internal stress due to the CTE mismatch exceeds the yield stress of the matrix.[9] The plastic deformation inside the matrix near the interface has been confirmed by observations of dislocation generation through in-situ dynamic studies in the transmission electron microscope (TEM) and by correlating the dependence of the composite YS with the conditions of quenching from higher temperatures.[10–12] A recent study[13] has shown that the matrix close to the particle-matrix interfaces of the Al-TiC and Al-SiC composites shows an increase in microhardness with an increasing number of thermal cycles, indicating that strain hardening is caused by dislocation generation and movement. Aided by increased internal interfacial strains, thermal cycling of the Al alloys and composites during u
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