The Effects of Macroscopic Residual Stress on Ring Collapse and Crack Growth in an in-situ Al-Al3Ni Functionally Gradient Material.

Abstract

The effect of macroscopic residual stress on the fracture of an Al-Al3Ni functionally gradient material (FGM) has been studied. The material has a graded volume fraction of Al3Ni intermetallic particles in a continuous Al phase. Specimens were machined from a thick-walled tube of in-situ Al-Al3Ni FGM that was fabricated by centrifugal casting. The level of macroscopic residual stress present in the hoop direction can be modelled accurately using an uniform temperature change of 160K from an assumed stress free temperature of half the melting point (of the Al matrix). The residual stress distribution changes through-thickness from tension (inner surface) to compression (outer surface). By a superposition of applied and residual stresses a failure (crushing) stress of 130 MPa is obtained. Radial cracks are initiated symmetrically on inner surfaces at positions of maximum applied tensile stress. These cracks do not propagate catastrophically under a constant applied stress, but rather they arrest. Crack opening displacement measurements confirm that as cracks propagate they close under the action of residual stresses. Fractographs show that Al3Ni intermetallic particles fail by transgranular cleavage, and they are surrounded by ductile failure of the continuous Al matrix. The crack path preferentially follows cracked Al3Ni particles.