Effect of Thermal Residual Stress on Age Hardening Behavior of SiC Whisker/Al-Cu Composites

Abstract

The effect of thermal residual stress on the age hardening behavior of the extruded SiC whisker reinforced Al-4 mass%Cu and Al-4 mass%Cu-1.5 mass%Mg alloys composites is investigated with micro-Vickers hardness, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. To control the thermal residual stress induced during heat treatment before isothermal ageing at 373, 423 and 473 K, two kinds of heat treatments are carried out for the composites. One is water quenching from solution temperature to R.T. (W.Q. composite) directly, the other is heating of the W.Q. composite to R.T. after holding for 3.6 ks at liquid nitrogen temperature (L.N. composite). Tensile residual stresses in the matrix of the whisker axis (σ₁₁) and the perpendicular direction (σ₂₂, σ₃₃) of the L.N. composite are markedly reduced compared with those of the W.Q. composite. In the SiC_w/Al-Cu system, both the W.Q. and L.N. composites show the suppression of the G.P. zone formation and the acceleration of the θ^′(Al₂Cu) precipitation compared with the unreinforced matrix alloy. Also, the precipitation of the G.P. 1 zone and θ^′(Al₂Cu) in the L.N. composite is more accelerated than in the W.Q. composite. In SiC_w/Al-Cu-Mg system, both the W.Q. and L.N. composites show more remarkable age hardening due to the precipitation of the G.P.B. 1 zone and S^′(Al₂CuMg) phase than the unreinforced matrix alloy. Compared with the W.Q. composite, especially the precipitation of metastable S^′ phase is much more accelerated in the L.N. composite.