Microstructural characteristics of nickel processed to ultrahigh strains by high-pressure torsion

The microstructures of highly strained high-pressure torsion (HPT) nickel processed from bulk master metals pre-strained to different levels of strain and from electrodeposited (ED), ball milled (BM) samples and rapidly quenched (RQ) ribbons were studied by transmission electron microscopy and X-ray diffraction. High-resolution X-rays showed that the BM + HPT specimens had the smallest coherent domain size of ∼9 nm and the RQ + HPT samples possessed a high dislocation density of ∼6.7 × 1015 m−2. Gradually increasing the strain in bulk nickel led to an increasing dislocation density and a decreasing coherent domain size. Microhardness measurements revealed differences in the mechanical behavior in HPT-strained nickel specimens. There is a good correlation between the microhardness measurements and the dislocation density measured with high-resolution X-rays.