Industrially fabricated in-situ Al-AlN metal matrix composites (part A): Processing, thermal stability, and microstructure

This study introduces in-situ aluminum (Al)-aluminum nitride (AlN) metal matrix composites (MMCs) manufactured by a powder metallurgy (PM) cost-effective approach realized at a large industrial scale. The Al-AlN MMCs were targeted for structural load-bearing applications with an expected service at elevated temperatures not normally associated with a use of conventional Al-based alloys and MMCs. Commercial Al, magnesium, and tin powders were processed by readily available PM techniques of blending, cold isostatic pressing (CIP), a solid-state nitridation in a static gaseous nitrogen, and a hot direct extrusion. Two sound voids free Al + 8.8 and 14.7 vol% AlN MMCs were reproducibly fabricated in a form of the long extruded bars with the cross-section of 80 × 15 mm. The microstructure of nitrided and extruded MMCs was presented in details. A typical yolk-shell-like microstructure of Al metallic core and nitrided layer was formed homogenously in a volume of the CIP bulky (~25 kg) billets upon nitridation. The microstructure of extruded Al-AlN MMCs consisted of Al grains elongated into the extrusion direction. The Al grain structure was embedded with the evenly distributed micrometric regions formed by a high density AlN nanocrystals in Al matrix. A stability of the tensile mechanical properties of as-extruded Al-AlN MMCs was pursued in transversal and longitudinal directions after the annealings performed at 300–600 °C for 24 h. Owing to an effective stabilization by the stable and fine AlN dispersoids by Zener pinning action no major changes to the mechanical properties took place after annealing up to 500 °C.