Microstructure stability, softening temperature and strengthening mechanism of pure copper, CuCrZr and Cu-Al2O3 up to 1000 ℃

To evaluate the microstructure and hardness stability at high temperatures, Cu-Al2O3 produced via internal oxidation method was subjected to annealing at 200, 300, 400, 500, 600, 700, 800, 900 and 1000 °C for 1 h. Pure copper (T2) and CuCrZr are selected for comparison. Then the grain size, particle characteristics and softening temperatures are characterized systematically. Finally, grain growth kinetics and strengthening mechanism were clarified by quantitative calculation. The results indicate that Cu-Al2O3 has the most stable grain size. After annealing at 1000 °C, the average grain size of T2, CuCrZr and Cu-Al2O3 increases from 16 to 185 μm, 5.63 to 83.13 μm and 0.18 to 0.31 μm, respectively. The second phase is Cr-rich particle in CuCrZr and Al2O3 in Cu-Al2O3. The former particle suffers partial dissolution and rapid Ostwald ripening while the latter one present stable feature during high-temperature annealing, which leads to the softening temperatures of 200–300 °C, 500–600 °C and 900–1000 °C for T2, CuCrZr and Cu-Al2O3, respectively. Besides, although the calculated activation energy of grain growth in CuCrZr and Cu-Al2O3 are 137 and 37.5 kJ·mol−1, respectively, stable Al2O3 and the greater difference of elastic modulus between Al2O3 and Cu matrix inhibit the grain growth in Cu-Al2O3. Moreover, for the CuCrZr and Cu-Al2O3 annealed at 800 and 1000 °C, the main strengthening contribution is from grain boundary. The stable nanometer grains induced by Al2O3 particles are chiefly responsible for the higher softening temperature of Cu-Al2O3 than that of CuCrZr.