INFLUENCE OF #alpha#-UNIT SIZE AND MORPHOLOGY ON THE NEAR-THRESHOLD FATIGUE CRACK GROWTH BEHAVIOUR OF Zr-2.5Nb ALLOY

摘要

In this investigation, the influence of #alpha#-unit sizes and morphology on the room temperature near-threshold fatigue crack growth (NTFCG) behaviour of Zr-2.5Nb have been studied at two load ratios (0.1 and 0.6). Two a morphologies, namely lamellar and equiaxed, were produced by employing various rates of cooling from #beta#-phase field and annealing at different temperatures in the (#alpha#+#beta#) phase field respectively. NTFCG behaviour was found to be affected by the load ratio (R), a morphology, and #alpha# unit size. Best fatigue resistance is shown when microstructure contains lamellar-#alpha# morphology. Increasing load ratio shifts the NTFCG stage to lower stress intensity factor range (#DELTA#K) indicating a lower stress intensity factor range at threshold (#DELTA#K_(th)). Detailed crack path profiling and fractographic investigation have been carried Out to identity the controlling microstructural feature involved in FCG near threshold regime.