Effect of strain of cryorolling on structure and strength of nickel

Abstract

The effect of isothermal rolling at liquid nitrogen temperature with reductions up to 90 % on the structural-mechanical behavior of coarse-grained nickel was evaluated by TEM, SEM-EBSD and X-ray analysis methods, and microhardness measurements. The highest hardening rate of the metal was found at the initial stage of rolling (up to about 50 %), after which it decreased slightly with further reductions. Detailed analyses of the evolution of dislocation density, grain size and grain boundary spectra have shown that the behavior found was attributed to an extremely low deformation temperature, resulted in a decreased rate of accumulation of crystalline defects, as well as the formation and rearrangement of dislocation structure. Due to the strong suppression of dynamic recovery and recrystallization, even when rolling to a reduction of 90 %, a relatively homogeneous coarse-fibered structure with a developed nanocellular substructure and less than 5 % fraction of nanoscale dynamically recrystallized grains was formed. It was concluded that the main structural strengthening factors in cryogenically rolled nickel was the formation of low-energy dislocation structures and their transformations into a well-developed substructure with nanosized crystallites separated predominantly by low-angle boundaries.