Cyclic Deformation Behavior and Related Micro-Mechanisms of Ni-Carbonyl Chemical Vapor Deposited (CVD) Material with Bimodal Grain Structures: Ultrafine (UF) Grains and Large Grains with UF/Nano Twins

Stress-controlled cyclic tests were conducted on bulk sheet Ni-carbonyl Chemical Vapor Deposited material (CVD Ni) with bimodal grain structures: ultrafine (UF) grains and large grains with UF/nano twins. The tests were run with the cyclic stress ratio R=0.05 and peak stress level of 0.9 to 1.5 times of the material's yield strength. Results show that within the applied peak stress ratio of 0.9-1.1, the material demonstrated cyclic hardening behavior first, followed by stress-strain saturation till fracture; upon increasing the ratio to 1.4-1.5, an additional softening stage was activated and continued till fracture. By transmission electron microscope (TEM) examination, it was found that such cyclic deformation responses were associated with the stability of the ultrafine- and nano-twin structures. Initial hardening was found mainly due to the increase in dislocation density and the activities of dislocations especially with their strong interactions with the dense Twin Boundaries (TBs). The saturation was contributed by the simultaneous operations of the softening effect due to massive detwinning and the hardening behavior due to dislocation interactions with existing TBs. Newly formed dislocation walls and cell structures were further found in samples with stress ratio of 1.4 at fracture, corresponding to the softening stage. Furthermore, such microstructural evolution, which were observed also through annealing and monotonic deformation of the same material, is identified as a consistent energy reduction path for the material. Thus, an energy criterion is further established to predict the massive detwinning events that cause the major softening phenomena under cyclic deformation.