Grain Boundary Engineering for Control of Fatigue Fracture in 316L Austenitic Stainless Steel

Roles of grain boundaries in fatigue crack nucleation and propagation in 316L austenitic stainless steel were investigated to obtain a clue to the grain boundary engineering for control of high-cycle fatigue fracture. The fatigue crack nucleation preferentially occurred at grain boundaries at the low-stress amplitude conditions less than about 160 MPa. In particular, the 82% of cracked grain boundaries were random boundaries. The fatigue crack nucleation at the random boundaries occurred irrespective of the geometrical configuration of grain boundary plane to the stress axis and the persistent slip bands (PSBs) in the neighboring grains. Although the fatigue cracks nucleated even at the annealing twin boundaries, namely the {111}/Sigma 3 coincidence site lattice (CSL) boundaries the crack nucleation occurred only when the surface trace of the Sigma 3 CSL boundaries was parallel to the PSBs in the neighboring grains. Moreover, in-situ observations of the fatigue crack propagation revealed that the grain boundaries played important roles as crack path, crack deflection sites and barrier of crack propagation, depending their character. In particular, although the Sigma 3 CSL boundaries became crack propagation path, the crack propagation rate locally decreased when the crack propagated along the Sigma 3 CSL boundaries. On the other hand, the crack propagation rate considerably increased when the crack propagated along random boundaries. The usefulness of grain boundary engineering for control of high-cycle fatigue fracture was demonstrated. The higher fraction of CSL boundaries achieved higher fatigue strength and longer fatigue life in 316L stainless steel.