Fatigue crack growth behavior of 2124/SiC/10p functionally graded materials

Powder metallurgy processing involving cold pressing and hot extrusion has been used to fabricate bulk functionally graded materials (FGMs) based on t he 2124/SiC/10p composite system. Two forms of single-core bulk FGMs with c ircular cross section were fabricated. One form (designated 10SiC-2124) had a central core of unreinforced Al-2124 alloy that was surrounded by a 2124 /SiC/10p reinforced surface layer: the other (designated 2124-10SiC) had a composite core and an alloy surface layer. These forms enabled the effect o f the radial graded core on fatigue to be investigated with fatigue crack p ropagation from either (1) a ductile core to a more brittle region or (2) a brittle core to a ductile region of the FGM. The fatigue crack growth rate was measured using a constant applied stress intensity factor range (Delta K = MPa rootm) technique. Two main fatigue crack growth rates were distingu ished corresponding to growth in the core and in the surface layer. The res ults show that FGMs may exhibit good fatigue crack propagation resistance. For example, when the crack propagated from the brittle core to the tough s urface layer, the average fatigue crack growth rate in the Al-2124 core (3. 9 x 10(-6) mm/cycle) was significantly lower than for the Al-2124 alloy (1. 5 x 10(-5) mm/cycle) at a similar DeltaK value (7 MPa rootm), due to the hi ghly tortuous crack path in the 2124/SiC/10p brittle layer. The 2124/SiC/10 p brittle layer had a lower fatigue crack growth rate (6.6 x 10(-6) mm/cycl e) than the 2124/SiC/10p conventional composite (7.5 x 10(-6) mm/cycle) bec ause of the compressive residual stresses in the surface layer. Thus, FGMs could be more acceptable for critical applications than their conventional composite counterparts.