Elevated temperature deformation behavior of spark plasma sintered nanometric nickel with varied grain size distributions

The deformation behavior of spark plasma sintered Ni powders with unimodal and bimodal grain size distributions, made up of nanocrystalline (NC) and ultra-fine grained (UFG) was studied at homologous temperatures from 0.36 to 0.40. TEM and SEM analysis have determined that minimal grain growth occurs during testing, while the bimodal specimens show a propensity for twin formation and growth. Purely nanocrystalline and microcrystalline sample activation volumes and strain-rate sensitivities were found to be consistent with previous work on electrodeposited Ni of similar grain sizes. The microstructures, activation volumes/energies, and strain rate sensitivities are found to be discontinuously changing as a function of increased volume fractions of UFG grains. Current models of nc deformation do not fully explain the observed behavior. A new rate controlling process is suggested in which both competitive and cooperative mechanisms function to produce the observed, novel behavior.

Research highlights▶ EFAS sintered nickel powders with bimodal grain size distributions are possible. ▶ Bimodal grain size distributions exhibit cooperative grain boundary sliding. ▶ Dislocation mediated creep results in intragranular slip and/or twinning. ▶ Bimodal grain size distributions can show widely differing activation volumes. ▶ Relative volume fraction of large and small grains can change mechanisms.