Mechanical and Microstructural Characterization of Al-PM Based Composites Prepared Via Hot Working Route

Project start:	01.01.2013
Project end:	31.12.2014
Programme:	Academic agreement
Institute position in the project:	Project co-lead partner
Project leader at the institute:	Martin Balog

The project aims to study microstructures and mechanical properties of two different aluminium (Al) based metal matrix composites (MMCs) prepared via powder metallurgical (PM) route.

A, In situ formed Al+Al₃Ti MMCs
An effective weight reduction of Al parts which are used in stiffness limited structural applications can be achieved only in the case when the specific stiffness (Young’s modulus-to-weight ratio) of Al is improved. The technologically feasible approach is to produce Al based lightweight composites reinforced with particles of high Young’s modulus. PM route based on hot working processes enables to prepare sound high quality Al MMCs where problems associated with a degradation of matrix-reinforcement interface are avoided. However, a conventional approach, where discrete particles such as Al₂O₃, SiC, BN, B4C are admixed into Al powder and followed by subsequent consolidation, features many technological obstacles linked with extreme wear of production and machining tool. This drawback can be overcome by in situ formation of small non-abrasive round-edged reinforcing intermetallic phases with high Young’s modulus within Al matric which does not lead to an extreme wear. Al₃Ti phase features one of the highest specific Young’s modulus from all aluminides. The idea is based on production of compacts of easy-to-deform Al+Ti powder blends followed by their transformation to Al+Al₃Ti MMCs by subsequent heat treatment.

B, Ultrafine grained Al+Al₂O₃ MMCs in situ formed during compaction of fine atomised Al powders
With the rising energy-saving concern, it is in need to build light-weight structural parts using high strength materials with expected service at elevated temperatures. Al alloys are critical to transportation due to their high specific strength, but they show poor performance at elevated temperatures. The high strength ultra-fine grained Al stabilized by small content of in situ formed nanoscale Al₂O₃ phase will be prepared for expected service at elevated temperatures. By compacting finely atomised Al powders, the nano-scale Al₂O₃ phase can be homogenously introduced into the material in-situ to produce bulk volume ultrafine-grained Al-Al₂O₃ composites of submicrometric grain size. This Al₂O₃ phase originates from the thin (~3nm) native oxide film present on the surface of the as-atomised powder. Such homogenous distribution of such fine ceramic reinforcing phase is virtually impossible to achieve by other technological approaches. As a result, superior mechanical properties can be attained at substantially lower Al₂O₃ dispersion contents (typically <2.5 vol.%) when compared to composites containing micro-metric size dispersoids. Al₂O₃ dispersoids present at the grain boundaries stabilized the microstructure against a grain growth up to very high temperatures not normally associated with service temperatures of Al based parts. Even the long term annealing up to the melting point does not affect the grain size of large angle grains. As a result, fine Al powder compacts maintain reasonably high strengths up to very high service temperatures (~500°C). Furthermore, Al₂O₃ dispersoids act as effective barriers against the grain boundary sliding which is the key creep mechanism of fine grained Al based materials.