Work hardening behaviour at the worn surface of Al-Cu and Al-Si alloys

In order to predict the ability of a material to support the load applied at asperities, the flow stress of the surface regions must be known. However, there is little quantitative data on the work hardening behaviour resulting from plastic sliding contact. Both linear hardening and the achievement of a saturation flow stress have been predicted. Two binary Al alloys, Al-4wt.%Cu and Al-11.7wt.% Si, were worn against a cast iron disc in a tri-pin-on-disc machine, under dry sliding conditions at 1 m s(-1) over the load range 6-40 N. Detailed transmission electron microscopy was performed on cross-sections taken from worn surfaces of both materials. The subgrain size was found to decrease substantially as the surface was approached for both alloys. In the Al-Si alloy, the Si particle size was also found to decrease as the surface was approached. Taper sections were taken parallel to the sliding direction and the microhardness was measured as a function of depth below the worn surface. The equivalent strain was measured from the displacement of prominent microstructural features such as grain boundaries. This allowed stress-strain curves to be constructed for the worn surface of the two materials. The stress-strain data were interpreted in relation to established work hardening laws, and correlated with the subgrain size. The maximum flow stress observed correlated with a minimum subgrain width of approximately 0.2 mu m in the Al-Cu and approximately 0.4 mu m in the Al-Si alloy. (C) 1997 Elsevier Science S.A.