Evaluation of fracture behavior of iron aluminides

Comparative fracture tests of three Fe-28%Al iron aluminides showed that alloys with Zr and C addition (FA-187) or with B, Zr, and C addition (FA-189) are extrinsically more susceptible to environmental embrittlement than the base ternary alloy (FA-186) under constant tensile loading condition. This may be caused by the variations of grain boundary morphology (i.e. changes of grain size and grain boundary cohesive strength) caused by the alloy addition. The effect of grain boundary size and cohesive strength are further investigated with reference to the susceptibility of hydrogen embrittlement. Finite element simulation of initial intergranular fracture of two iron aluminides (FA-186 and FA-189) are made. The computational scheme involves coupling the stress and mass diffusion analyses to determine crack-tip stress state and the crack tip hydrogen diffusion. Maximum strain failure criteria was adopted to simulate intergranular fracture. The numerical modeling results correlated well with the experimental data. The result further confirmed that the grain boundary morphology is important as it appears to control the intrinsic and extrinsic fracture behavior of iron aluminides.