Elevated temperature tensile properties of squeeze-cast Al-Al₂O₃-MgO particulate MMCs up to 573 K

Abstract

Al-Al₂O₃-MgO squeeze-cast composites were prepared using a modified “MgO-coating” technique and their tensile behaviour up to 300°C (573 K) evaluated. In each case, 10 wt% total powder mixture (Al₂O₃ + MgO) containing 15% MgO was stirred into well-superheated Al melt and the stirred slurry was squeezed in the pressure range 80–140 MPa using a 60t semi-automatic hydraulic press and alloy-cast iron dies. The tensile behaviour at 100, 200 and 300 °C (373, 473 and 573 K) of the squeezed composites (70 mm diameter, 60 mm long) was then examined. It was found that the composite squeezed at 140 MPa and ambient die temperature displayed the best tensile properties up to 573 K, the ultimate tensile strength (UTS) values being 207.9, 197.8, 179 and 160.4 MN m⁻² at ambient, 373, 473 and 573 K, respectively. Compared to ordinary gravity chill-cast composite, the UTS of the above composite was higher by 52% at 373 K. This value rose to 161% and 162% at 473 and 573 K, respectively. The above composite retained about 77% of its ambient UTS value at 573 K, while the ordinary gravity chill-cast composite retained only 44% of its ambient UTS value. The performance of the squeezed composite with regard to 0.2% offset yield strength (YS) was distinctly superior to the gravity chill-cast composite. The YS values for the squeezed composite were 119.5, 117.3, 115 and 112.8 MN m⁻² at ambient, 373 K, 473 K and 573 K, respectively. The squeezed composite retained 94.4% of its ambient YS value at 573 K, while the gravity chillcast composite retained only 47.6% of its ambient YS value at the same test temperature. The YS of squeezed composite was higher by 105% compared with the YS value of gravity chillcast composite at 573 K. The performance of the squeezed composite progressively improved compared to gravity chill-cast composite as the test temperature was systematically raised to 573 K. Squeezed composites also exhibited fully ductile fracture features compared to semiductile to brittle fracture features of the gravity chill-cast composites; they also perform better because of the virtual absence of porosity and some degree of grain refinement obtained upon squeezing. Increasing the squeeze pressure in the range 160–240 MPa is likely to improve the properties of the composite further.