Mechanical properties of bulk- and hybrid nanocrystalline materials

Paper i proceeding, 2014

The compressive strength at elevated temperatures of a Cu/nano-Ni-Fe periodic cellular material and of nanocrystalline Ni-Fe samples was investigated. An unconstrained compression test was performed on a Cu/nano-Ni-Fe periodic cellular material (PCM) at 150C. Finite element analysis and SEM characterization revealed that major stress concentrations occurred at the nodes of the struts where cracks were formed. Also plastic wrinkling at the hinges of the PCM was observed. Compressive deformation of the nanocrystalline Ni-Fe samples was performed between room temperature and 275C. Without any deformation, the microstructure with an initial grain size of 31 nm was found to be thermally stable up to 275C. However, compression at an elevated temperature after annealing the sample for 30 minutes at the respective temperature led to deformation-induced grain growth; grains with 50 +/- 30, and 100 +/-50 nm in diameter were found after compression at 200 and 275C, respectively. The material exhibited considerable plasticity of >30% fracture strain at higher temperatures compared to <15% at room temperature. Characterization by use of TEM and transmission EBSD suggests that the coarser grains enable strain hardening by intragranular dislocation accumulation, and this is responsible for the larger fracture strains at higher temperatures.