Unusual annealing effects on hardness and strain rate sensitivity of nanocrystalline Nb

The dependence of microstructure, hardness (H) and strain rate sensitivity (m) of nanocrystalline (NC) body-centered cubic Nb thin films at various annealed temperature were experimentally evaluated. The annealed temperatures ranged from 100 to 400 degrees C, and kept for 30 min respectively. Despite nearly unchanged grain sizes upon annealing, the strengthening effect was observed in nanoindentation hardness testing as annealing temperature was increased above 200 degrees C. In particular, the strain rate sensitivity of NC Nb increased significantly after annealing at 100 degrees C and slightly decreased at higher annealing temperatures. Competitive mechanisms underlying the different variation trends of hardness and strain rate sensitivity of NC Nb annealed in the two different temperature regimes were proposed.