Amorphization by mechanical alloying in metallic systems with positive Gibbs energy of formation

Fe-W has been chosen as a model system to investigate amorphization during mechanical alloying in systems With positive Gibbs energy of formation. Depending on the overall compositions, amorphous phases and supersaturated body-centered-cubic (bcc) Fe-W solid solutions were obtained by mechanical alloying. The thermodynamics of the system was determined by the calculation-of-phase-diagram (CALPHAD) method. The analysis shows that the Fe-W system exhibits a positive heat of mixing in the bcc: solid solution, and a negative heat of mixing in the amorphous phase. As a result, the amorphous phase has a lower (however, still positive) Gibbs energy of formation than the solid solution in the concentration range of 17-42 at. % W, indicating that amorphization in this range is favored, in agreement with the experiments. The mechanism which allows amorphization and formation of solid solutions upon milling to occur in the Fe-W system is discussed. It is proposed that the elastic contribution to the excess enthalpy which originates from coherent interfaces between the elemental components provides a major driving force for alloying in the Fe-W system.