EFFECT OF HEAT-TREATMENT ON THE MICROSTRUCTURE, TENSILE PROPERTIES, AND FRACTURE-BEHAVIOR OF PERMANENT-MOLD AL-10 WT PCT SI-0.6 WT PCT MG SIC/10(P), COMPOSITE CASTINGS/

The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 degrees C to 550 degrees C) a nd artificial aging (in the temperature range 140 degrees C to 180 deg rees C) on the variation in the microstructure, tensile properties, an d fracture mechanisms of Al-10 wt pet Si-0.6 wt pct Mg/SiC/10(p) compo site castings. In the as-cast condition, the SiC particles are observe d to act as nucleation sites for the eutectic Si particles. Increasing the solution temperature results in faster homogenization of the micr ostructure. Effect of solution temperature on tensile properties is ev ident only during the first 4 hours, after which hardly any difference is observed on increasing the solution temperature from 520 degrees C to 550 degrees C. The tensile properties vary significantly with agin g time and temperature, with typical yield strength (YS), ultimate ten sile strength (UTS), and percent elongation (EL) values of similar to 300 MPa, similar to 330 MPa, and similar to 1.4 pet in the underaged c ondition, similar to 330 MPa, similar to 360 MPa, and similar to 0.65 pet in the peak-aged condition, and similar to 323 MPa, similar to 330 MPa, and similar to 0.8 pet in the overaged condition. Prolonged solu tion treatment at 550 degrees C for 24 hours results in a slight impro vement in the ductility of the aged test bars. The fracture surfaces e xhibit a dimple morphology and cleavage of the SiC particles, the exte nt of SiC cracking increasing with increasing tensile strength and rea ching a maximum in the overaged condition. Microvoids act as nucleatio n sites for the formation of secondary cracks that promote severe crac king of the SiC particles. A detailed discussion of the fracture mecha nism is given.