High temperature deformation of nanocrystalline metallic materials

High temperature deformation at the nanoscale was investigated using two different one-phase materials: Ni3Al produced via High-Pressure Torsion, and alpha-Fe produced through devitrification of Vitroperm (Fe 73.5Cu1Nb3Si15.5B7), a commercially available melt-spun metallic glass. Nanocrystalline Ni3Al specimens were studied using in-situ high temperature straining in the TEM at 750°C and resulted in the direct observation of Cooperative Grain Boundary sliding. This process was observed to take place in two distinct steps: (1) Re-orientation of individual grain boundaries through local grain boundary sliding to form long-range surfaces oriented favorably for sliding, and (2) Long-range sliding along these surfaces, resulting in the macroscopic elongation in excess of 200% usually seen in superplastic deformation. A nanocrystalline single-phase alpha-Fe matrix with a grain size of approximately 15 nm was produced by devitrifying Vitroperm metallic glass using a 600°C anneal for 1 hour. All alloying elements were found to be in solid solution after annealing. This microstructure is stable during tensile testing at 600°C at strain rates between 10-5 s -1 and 10-4 s-1 and shows a strain rate exponent of 0.5 corresponding to grain boundary sliding as a dominant deformation mechanism, but shows great strength and little corresponding ductility. Further testing at temperatures up to 725°C show breakdown of the initial single-phase structure to a multiphase microstructure with grains of approximately 150 nm diameter, with a strain rate-exponent of 0.2. It was found that while some similarities exist between superplasticity in microcrystalline and nanocrystalline materials, such as the occurrence of cooperative grain boundary sliding, nanomaterials may not deform to the large elongations seen in their microcrystalline counterparts. This is believed to be due to restricted dislocation accommodation as grain sizes drop below ∼20 nm.