Recovery of Young's Modulus During Annealing of Submicrocrystalline Cu and Cu : ZrO₂ Composite

Abstract
Young's modulus of pure Cu (99.98%) and precipitate hardened Cu : ZrO₂ polycrystals with an ultrafine-grained (UFG) structure has been investigated at temperatures 20-600°C. Nanoparticles of ZrO₂ with a size of about 10nm have been formed by an internal oxidation of Cu-Zr solid solution. The polycrystals have been produced by a heavy plastic deformation using the equichannel angular pressing technique, which yields an average size of grains d of about 200nm. Young's modulus E has been measured as a function of temperature T and of the annealing temperature T_a. The dependence E(T_a) in Cu-ZrO₂ demonstrates a recovery stage near 200-250°C (4-6% increase in E). Structural investigations show that d practically does not change during 2 hours annealing up to 500°C. Such a structural stability was observed only in the composite material. Pure copper with the same type of initial structure (d=200nm) shows approximately the same recovery in E at T near 200-250°C, however, this recovery corresponds to a steep (an order of magnitude) increase in d. This fact is a direct experimental proof that the softening of the Young's modulus in UFG materials is due to not only the smallness of grains but also the structure of grain boundaries and, in particularly, the mobility of grain boundary dislocations.