Effect of bimodal grain structure and heterogeneous SiC p distribution on friction properties of ultrahigh-strength aluminum matrix composites

This study investigates the microstructural characteristics and wear mechanism of ultrahigh-strength aluminum matrix composites (AMCs) featuring a bimodal grain structure with silicon carbide particles (SiCp) linearly distributed along the extrusion direction (ED). Microstructural analysis and wear experiments reveal the coupled effects of the bimodal grain structure and particle distribution on hardness gradients and wear behaviour. In the transverse direction (TD), grains predominantly exhibit an equiaxed morphology, while ED grains form banded structures with pronounced anisotropy. Hardness is higher in ultrafine-grained (UFG) zone than in coarse-grained (CG) zone, revealing a bimodal grain size distribution and a corresponding hardness gradient. Under varying loads, the TD friction coefficient decreases from 0.35 to 0.30 and then stabilises, while that of ED declines continuously. At the same load, TD exhibits higher friction coefficient and wear rate than ED. As the load increases, the wear regions in both orientations broaden, the wear depth increases, and the surface roughness rises, which points to a coupled load-wear mechanism. ED exhibits superior wear resistance whereby the CG zones serve as load buffers, transferring stress to deeper areas. The bimodal Schmidt factor in ED activates two grain textures, which enhances wear resistance by balancing stiffness and ductility.