Relaxation of the residual stress in 6061Al-15 vol.% SiCw composites by isothermal annealing

The residual stress (RS) in extruded 6061Al-15 vol.% SiCw composites manufactured by a powder metallurgical route has been studied by neutron diffraction. Several precipitation states were investigated, going from a fully hardened state, T6, to an over-aged (OA) condition. For comparison, the unreinforced powder metallurgy 6061Al alloy was also studied. The end precipitation stages T6 and OA were obtained after appropriate heat treatments. The T6 condition involves a quenching step, which provokes a macro-RS state. Instead, the OA treatment (at 300 °C) does create relaxation at intermediate times and leads to total relaxation of the axial deviatoric component and to partial relaxation of the hydrostatic term of the stress in the composite. Total relaxation of both deviatoric and hydrostatic terms occurs in the unreinforced alloy. Furthermore, it is found that relaxation of the macro-RS occurs more rapidly in the unreinforced alloy than in the composite. These results are in agreement with the idea that discontinuously reinforced metal matrix composites (DRMMCs) have a more stable microstructure than the corresponding unreinforced alloys. On the other hand, no decrease of the deviatoric micro-RS developed in the composite and a small reduction of the hydrostatic one occurs when treating from the T6 to the OA condition. This implies that the geometry of the reinforcing particles plays an important role and the geometrically necessary dislocations do not disappear with heat treatment.