THERMAL-STABILITY OF SUBMICROCRYSTALLINE COPPER STRENGTHENED WITH HFO2 NANOPARTICLES IN THE TEMPERATURE-RANGE 20-500-DEGREES-C

The thermal stability of the yield stress and Young's modulus was inve stigated in ultrafine-grained copper (99.98%) and a Cu-HfO2 composite obtained by intensive plastic deformation using the method of equichan nel angular pressing. It is shown that both the pure copper and the co mposite strengthened with HfO2 nanoparticles demonstrate in this state a high yield stress (sigma(0.2) approximate to 400 MPa). When the two -hour annealing temperature T-a is increased above 200 degrees C, the yield stress in pure copper decreases to 40 MPa at T-a = 400 degrees C , whereas in the Cu-HfO2 composite, high yield stresses are conserved up to T-a = 500 degrees C. A recovery stage of Young's modulus is foun d at around 200 degrees C both in pure copper and in the Cu-HfO2 compo site. It is concluded that this stage reflects the transition of the g rain boundaries from a nonequilibrium to an equilibrium state, and the high-strength properties of the materials are determined mainly by th e grain size and depend weakly on the grain-boundary structure. (C) 19 98 American Institute of Physics.