LOW-CYCLE FATIGUE OF NIOBIUM AND NIOBIUM-1 PCT ZIRCONIUM ALLOYS

Commercially pure niobium (CPNb) and a niobium-1 pct zirconium (Nb-1 Zr) alloy were tested under low-cycle fatigue conditions at plastic strain amplitudes in the range of 0.02 pct less-than-or-equal-to DELTA epsilon(pl)/2 less-than-or-equal-to 0.7 pct. At low temperatures, the cyclic deformation response of body-centered cubic (bcc) metals is strongly dependent on strain rate. Thus, it was necessary to test at slow (2 X 10(-4) s-1) and fast (2 X 10(-2) s-1) strain rates in order to fully characterize the cyclic deformation at ambient temperature. Only cyclic hardening was observed for both metals under all testing conditions. As expected, higher cyclic stresses were recorded at the fast strain rate compared to the slow strain rate. The Nb-1Zr alloy was always stronger than CPNb, although both metals had the same cyclic life at equal plastic strain amplitudes. Further, the strain rate had no effect on the cyclic life. At the fast strain rate, intergranular cracking occurred, and a microplastic plateau was observed in the cyclic stress-strain (CSS) curve for CPNb. At the slow strain rate, no definitely intergranular cracks were detected, and a microplastic plateau was not observed for CPNb. The results of these experiments are interpreted in terms of the influence of strain rate and solute content on the relative mobilities of edge and screw dislocations.