Ageing characteristics of aluminium alloy aluminosilicate discontinuous fibre reinforced composites

Development of continuous fibre reinforced metal matrix composites is aimed at providing high specific strength and stiffness needed for aerospace and some critical high temperature structural applications (1,2). Such materia ls, however, are generally expensive because of the high cost of continuous fibres and the difficult processing techniques used for their fabrications . Discontinuous fibres, on the other hand, are relatively inexpensive and c an be incorporated in metal matrices using conventional production processe s. Conventional secondary fabrication methods can also be used to produce w ide range of product forms making them relatively inexpensive compared to a dvanced composites with continuous reinforcements (3). Discontinuous reinfo rced composites may have slightly less specific strength than the continuou s reinforced ones but have more isotropic properties. The combination of pr operties and ease of fabrication make them attractive for many structural a pplications requiring high specific strength and stiffness. In addition to high strength and stiffness, discontinuous reinforced composites also posse ss other attractive properties like high damping capacity, machinability, f riction, wear and seizure resistance and low coefficient of thermal expansi on (4). In view of these properties and lower cost, the discontinuous reinf orced composites are expected to find wide application in automotive indust ries (4). Considerable efforts have been made, during the last decade, to improve the strength of age-hardening aluminium alloy matrix composites by suitable he at treatment (5-19). It has also been well established that age-hardenable aluminium alloy composites show accelerated ageing behaviour because of enh anced dislocation density at the fibre/matrix interface resulting from ther mal expansion mismatch between ceramic fibre and the metal matrix (7,8,9,10 -13,20). The accelerated ageing of aluminium alloy composites either from d islocation density or the residual stress, as a result of thermal expansion mismatch is dependent on the size of whisker (20) and particulate (21). In vestigations have also been made on the effect of volume fraction of partic ulate on the ageing behaviour of aluminium alloys (13,22). The present inve stigation is concerned with characterization of age-hardening behaviour of an Al-Si-Cu-Mg(AA 336) alloy alumino-silicate discontinuous fibre-reinforce d composites (referred to as aluminium MMCs in the present text) being deve loped for automotive pistons. An effort is made to study the effect of volu me fraction of the reinforcement on age-hardening behaviour of this composi te.