EFFECT OF THERMAL RESIDUAL-STRESS ON AGE- HARDENING BEHAVIOR OF SIC WHISKER AL-CU COMPOSITES/

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 transmissio n 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 i s water quenching from solution temperature to R.T. (W.Q. composite) d irectly, the other is heating of the W.Q. composite to R.T. after hold ing for 3.6 ks at liquid nitrogen temperature (L.N. composite). Tensil e residual stresses in the matrix of the whisker axis (sigma(11)) and the perpendicular direction (sigma(22), sigma(33)) of the L.N. composi te are markedly reduced compared with those of the W.Q. composite. In the SiCW/Al-Cu system, both the W.Q. and L.N. composites show the supp ression of the G.P. zone formation and the acceleration of the theta' (Al2Cu) precipitation compared with the unreinforced matrix alloy. Als o, the precipitation of the G.P. 1 zone and theta' (Al2Cu) in the L.N. composite is more accelerated than in the W.Q. composite. In SiCW/Al- Cu-Mg system, both the W.Q. and L.N. composites show more remarkable a ge hardening due to the precipitation of the G.P.B. 1 zone and S' (Al2 CuMg) phase than the unreinforced matrix alloy. Compared with the W.Q. composite, especially the precipitation of metastable S' phase is muc h more accelerated in the L.N. composite.