Discrepancy in low cycle fatigue and creep-fatigue life of 720Li alloy tested at 720 °C: Role of crystallographic texture evolution

Low cycle fatigue (LCF) and creep-fatigue (CF) interaction behavior of 720Li alloy were studied for the first time at 720 degrees C with different values of total strain amplitudes (triangle epsilon(t)/2) and hold time (HT). At triangle epsilon(t)/2 = 0.6%, CF life is less than LCF life. However, at triangle epsilon(t)/2 = 0.44, 0.48 and 0.55% CF life increased with increase in HT. Texture analysis of specimen tested at triangle epsilon(t)/2 = 0.44%, revealed that initial loading direction. [1 (1) over bar1] found to be rotated: (i) [2 (1) over bar1] in LCF condition; and (ii) [1 (1) over bar0] and [0 (1) over bar1] in the CF condition predominantly. Notably, texture components developed in the CF specimens tested at triangle epsilon(t)/2 = 0.6% is quite similar to that in the LCF specimen tested at triangle epsilon(t)/2 = 0.44%. Development of [2 (1) over bar1] texture component is due to multi planner slip (i.e. octahedral and cubic plane slip) under the influence of compressive load which is also confirmed by viscoplastic-simulation. Evolution of [2 (1) over bar1] texture component leads to greater extent of inter crystalline cracks due to latent strain hardening and strain incompatibility between adjacent grains. Further, high static interfacial energy of annealing twin boundaries, confirmed through five-parameter stereological analyses, in LCF specimen tested at triangle epsilon(t)/2 = 0.44% and CF specimen tested at triangle epsilon(t)/2 = 0.6% resulted annealing twin boundary cracking. In contrast, evolution of low modulus texture components (i.e. [1 (1) over bar0] & [0 (1) over bar1]) has been inferred to be responsible for higher cyclic life in CF specimen tested at triangle epsilon(t)/2 = 0.44% and LCF specimen tested at triangle epsilon(t)/2 = 0.6%.