Fatigue Crack Propagation in an In-Situ Al-Al3Ni Functionally Gradient Material.

Abstract

Micromechanisms of fatigue crack growth in an Al-Al3Ni functionally gradient material (FGM) have been investigated. The distribution of Al3Ni intermetallic particles is graded radially through the continuous Al phase. Ring testpieces and bend testpieces have been machined from a thick-walled tube of FGM, and they have been subjected to cyclic loading under diametrical compression and four point (pure) bending, respectively. Fatigue cracks initiate more easily on the inner surfaces of the ring testpieces and their growth is affected by the residual stress state which changes through the ring from tension (at the inner surfaces) to compression (at the outer surfaces). For such testpieces the crack growth resistance curve is extremely difficult to determine. In testpieces subjected to cyclic bending, a stress ratio dependence of crack growth rates is observed, and Paris exponents of 8-10 have been determined. Crack paths are deflected along chains of brittle Al3Ni particles. These particles appear to fail by transgranular cleavage, and this fraction of the fracture surface is then relatively smooth. Crack growth through the Al matrix produces rougher features, and striations are found occasionally. Of interest, periodic markings are also observed on Al3Ni particles in some cases. It is concluded, that the high Paris exponents can be attributed to "static mode" failure of the intermetallic particles.