Structural evolution of Cu(1−X)YX alloys prepared by mechanical alloying: Their thermal stability and mechanical properties

In the present study, an attempt has been made to synthesize copper based disordered solid solutions by mechanical alloying (MA) of non-equilibrium compositions. The blended compositions of Cu-1% Y, Cu-3% Y, Cu-5% Y and Cu-7.5% Y (at.%) (all the compositions will be addressed as % only hereafter until unless it is mentioned) were ball-milled for 8 h, and then annealed at different temperatures (200-800 °C) for different length of duration (1-5 h) under high purity argon + 2 vol.% H2 atmosphere. X-ray diffraction (XRD) analysis and Gibbs free energy change calculation confirm the formation of disordered solid solution (up to 7.5%) of Y in Cu after milling at a room temperature for 8 h. The XRD grain size was calculated to be as low as 7 nm for 7.5% Y and 22 nm for 1% Y alloy. The grain size was retained within 35 nm even after annealing for 1 h at 800 °C. Transmission electron microscopy (TEM) analysis substantiates the formation of ultra-fine grained nanostructures after milling. Microhardness value of the as-milled samples was quite high (3.0-4.75 GPa) compared to that of pure Cu. The hardness value increased with increasing annealing temperatures up to 400 °C for the alloys containing 3-7.5% Y, and thereafter it showed a decreasing trend. The increase in the hardness after annealing is attributed to the formation of uniformly distributed ultrafine intermetallic phases in the nanocrystalline grains. The stabilization effect is achieved due to segregation of Y to reduce the grain boundary energy to zero and hindrance of dislocation movement due to precipitation of intermetallic phases. The tensile yield strength (σy) of the HPT consolidated nc Cu-1% Y and Cu-3% Y alloys was found to be at least one order of magnitude higher than that of the course grained-Cu counterpart, and the corresponding UTS (σu) value was more than four times higher. The strengthening effect is discussed in the light of the grain size refinement, solid solution strengthening and strain hardening.