Alloying in a system with positive heat of mixing: MD simulations of Ag-Cu

Based on molecular dynamics (MD) simulation results, we present two schemes to enhance solid solubility in positive-heat-of-mixing (+ΔH) systems at low temperatures. The Ag-Cu system, which is virtually immiscible below about 600 K, is used as a model for systems in which the relatively large atomic size mismatch (13%) between the two constituent elements contributes significantly to the +ΔH of the crystalline solid solution. We show two cases where the atomic configurations involved lead to easier accommodation of the alloying atoms such that the magnitude of the +ΔH can be significantly reduced. The first strategy is to go to free surfaces with under-coordinated atoms such that in the surface layers the apparent +ΔH is less positive relative to its value in the bulk. A solubility enhancement is thus observable at moderately elevated temperatures (≥600K), representing an extension of the experimentally known phenomenon of surface alloying at very low coverages. The second approach takes the system through the liquid state that also costs less strain energy for alloying compared with the crystalline phase. Our approach differs from the well known melt quench processes, however, in that it is an interdiffusional mixing reaction between two elemental melts induced mechanically through high strain rate deformation of crystals at relatively low temperatures.