Mechanical properties of nanocrystalline copper and copper-iron based alloys

The mechanical properties of mechanically milled copper and copper-30 iron were studied in the as-milled and consolidated conditions. Hardness, elastic modulus, and strain-rate sensitivity were measured on mechanically milled powders using ultra-low load indentation techniques. The bulk nanocrystalline copper and copper-30 iron samples were prepared by consolidating mechanically milled powders using hot isostatic pressing to form compacts from which bars were machined for testing. Samples studied included unmilled copper, milled copper, milled copper-30 iron, and commercial grade 101 copper bar which had been recrystallized. The x-ray diffracting particle sizes of the nanocrystalline copper was 42 nm and the copper-30 iron was 19 nm. Low-frequency internal friction behavior of the consolidated specimens was measured from 100 to 600$\sp\circ$K at a frequency of approximately 3 Hz. The internal friction studies indicated the presence of a large grain boundary peak in the consolidated samples superimposed on a high temperature background. The grain boundary activation energy was determined to be 29.8 kcals/mole for the consolidated copper powder, 58.4 kcals/mole for the nanocrystalline copper, and 19.0 kcals/mole for the nanocrystalline copper-30 iron material. The 101 copper exhibited a low temperature dislocation damping peak at approximately 125$\sp\circ$K which was absent in the nanocrystalline copper, which is believed to be due to the lack of dislocation structures in heavily milled nanocrystalline materials. Compression tests were performed on the consolidated samples up to a total strain of 14%. The 0.2% offset yield strengths were measured to be 190 MPa for the consolidated copper powder, 254 MPa for the nanocrystalline copper, and 610 MPa for the copper-30 iron material. The work-hardening of copper-30 iron material was very low which is an indication that dislocations generated do not interact with each other and instead end up in the grain boundary of the material