Alloying behaviour in nanocrystalline materials during mechanical alloying

The alloying behaviour in a number of systems such as Cu-Ni, Cu-Zn, Cu-Al, Ni-Al, Nb-Al has been studied to understand the mechanism as well as the ki netics of alloying during mechanical alloying (MA). The results show that n anocrystallization is a prerequisite for alloying in all the systems during MA. The mechanism of alloying appears to be a strong function of the entha lpy of formation of the phase and the energy of ordering in case of interme tallic compounds. Solid solutions (Cu-Ni), intermetallic compounds with low ordering energies (such as NI3Al which forms in a disordered state during MA) and compounds with low enthalpy of formation (Cu-Zn, Al3Nb) form by con tinuous diffusive mixing. Compounds with high enthalpy of formation and hig h ordering energies form by a new mechanism christened as discontinuous add itive mixing. When the intermetallic gets disordered, its formation mechani sm changes from discontinuous additive mixing to continuous diffusive one. A rigorous mathematical model, based on iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixin g in binary systems during MA. Based on the results of Cu-Ni, Cu-Zn and Cu- Al systems, an effective temperature (T-eff) has been proposed that can sim ulate the observed alloying kinetics. The T-eff for the systems studied is found to lie between 0.420-0.52T(1).