In situ fabrication and mechanical properties of Al-AlN composite by hot extrusion of partially nitrided AA6061 powder

Al-AlN composite powders containing 9-55 vol% AlN were fabricated in situ by nitriding a powder mixture of AA6061-2% Mg-1% Sn at 560 degrees C. Transmission electron microscopy (TEM) revealed that the in situ formed AlN reinforcements are present as nanoscale AlN whiskers on each powder particle. The composite powder was consolidated by hot extrusion at 450 degrees C with an extrusion ratio of 11:1. This produced an AlN-free and an AlN-containing lamellar structure along the extrusion direction. The nanoscale AlN is dispersed in the AlN-containing lamella and shows excellent bonding with the Al matrix, free of decohesion and voids. The lamellar composite containing 9 vol% AlN has an ultimate tensile strength of 332 MPa and tensile elongation of 3%. Composites containing >= 17.5 vol% AlN achieve much higher tensile strengths (538 MPa) but zero tensile elongation. However, they show a low coefficient of thermal expansion up to 450 degrees C and may therefore have potential for selected elevated temperature applications.