DEFORMATION INDUCED STRONG AND STABLE NANOLAMINATED STRUCTURES IN NICKEL

摘要

Polycrystalline Ni of two different purities: 99.96 wt% and 99.88 wt% was subjected to dynamic plastic deformation (DPD) and surface mechanical grinding treatment (SMGT), respectively. Nanometer-scale flat and straight lamellae, i.e. 2-dimensional nanolaminated (NL) structures, were fabricated in both DPD and SMGT samples. Two types of NL structures with comparable spacing but different contents of high angle boundaries are observed and form alternatively distributed regions in the DPD sample. In the SMGT sample, a NL structure with a boundary spacing about 20 nm and mainly low angle boundaries is fabricated in the surface layer 10-80 μm thick. The hardness of the NL structures in DPD and SMGT samples reaches as high as -2.5 GPa and 6.4 GPa, respectively, which are inversely proportional to the square root of their respective boundary spacing. Such a correlation agrees well with that delineated by various deformation microstructures with different contents of high angle boundaries, structural morphologies and purity levels. During post annealing, a coarsening of the NL structure with low angle boundaries takes place at a relatively lower rate and higher onset temperature. The good combination of strength and stability breaks the traditional trade-off relationship between strength and thermal stability. A new route for fabricating high performance nanometals through a deformation-induced NL structure with low angle boundaries is proposed.