Iain Brooks

Thermal Conductivity and Electrical Resistivity in Polycrystalline and Nanocrystalline Nickel

The grain-size dependences of thermal conductivity and electrical resistivity of polycrystalline and nanocrystalline nickel were measured by the flash method and four-point probe method, respectively. Nanocrystalline nickel (grain size: 28 nm) was made by the pulsed-current electrodeposition process, while polycrystalline nickel (grain size: 57 μm) was the same material in fully annealed condition. Noticeable differences in thermal conductivity and electrical resistivity were observed for both materials. These results can be explained on the basis of the rapid increase in the intercrystalline grain boundary and triple junction volume fractions at very small grain sizes. The relationship between thermal conductivity and electrical resistivity of nanocrystalline nickel follows the classic Wiedemann-Franz law.

Strain Hardening in Polycrystalline and Nanocrystalline Nickel

The work hardening behavior of electrodeposited nanocrystalline (grain size: 100 nm) and fully annealed polycrystalline nickel (grain size: 160 µm) was examined by hardness indentation analysis. First, plastic strain was introduced into the materials through large Rockwell hardness indentations. A series of Vickers micro-hardness traces below and away from the Rockwell indentation then measured the change in hardness as a function of distance from the plastic zone. The results showed that polycrystalline nickel exhibited considerable strain hardening, with micro-hardness values closest to the Rockwell indentation averaging twice the hardness value of the bulk material. On the other hand, for the nanocrystalline nickel the Vickers micro-hardness values changed only by a few percent indicating a limited strain hardening capacity.

Temperature Changes during Deformation of Polycrystalline and Nanocrystalline Nickel

Commercially available polycrystalline nickel (grain size: 30 µm) and electrodeposited nanocrystalline nickel (grain size: 30 nm) were analyzed for the effect of stress-induced heat generation during plastic deformation at room temperature. Tensile coupons in conformance to ASTM E8 standard were tested at a strain rate of 10-1/s to record the amount heat dissipated using a high resolution infrared detector. The maximum temperature increases recorded for nanocrystalline and polycrystalline nickel close to sample fracture were 58°C and 70°C, respectively. Grain growth in nanocrystalline nickel due to stress-induced heat generation is unlikely since the maximum temperature during deformation is below the previously reported onset temperature for grain growth in nanocrystalline nickel.