Relaxation kinetics of mechanically alloyed powders. Fe-Ni-Si-P: A case study

Powders obtained by mechanical alloying show a relaxation or recovery of the stresses, which are related to the milling process, when submitted to heat treatment. The kinetics of relaxation of the as milled 'state' is studied. The analysis is performed by the coupling of experimental data obtained by differential scanning calorimetry under different working conditions: continuous heating and isothermal heat treatments. The modelling of the kinetics of the relaxation process is grounded on the consideration of a wide spectrum of relaxation times or activation energies. It is shown that two different mechanisms can be distinguished for the relaxation process. Evidence is facilitated by the experimental set-up employed: i) Isothermal annealing allows the relaxation of processes whose relaxation time is lower than the annealing time or equivalently, whose activation energy is restricted to values that depend on annealing temperature and time. ii) Continuous heating, on the contrary, allows the relaxation of processes that have activation energy progressively increasing with temperature. In general, since relaxation processes occurs in a large temperature interval, they are overlapped with other thermally activated transformations of the powdered materials. Examples related to the relaxation processes in Fe-Ni-Si-P alloys are presented. The as-milled powdered materials contain a mixture of nanocrystals and amorphous phase, whose relative amounts vary with milling time and alloy composition. Depending of the heat treatment applied to the sample, relaxation is overlapped with both growth of nanocrystals and nucleation of new crystalline phases. The several contributions of all these transformations in the overall calorimetric signal are discussed.