Studies on Cr-Mo binary alloys II. Cross-over in surface segregation for chromium-rich alloys

The equilibrium segregation behaviour of an 83.4 at% Cr-Mo alloy has been studied by Auger electron spectroscopy. The high chemical affinity of Cr, particularly for nitrogen, and its high vapour pressure made cleaning of this alloy difficult. Several cleaning techniques were tried, but the only successful procedure was a tedious sequence of 530 sputter-anneal cycles, which we have optimised by microcomputer control. The changes in the Auger spectra on annealing this clean alloy give the impression that, just as was observed for Mo-rich alloys, even in this Cr-rich alloy it is the Cr which segregates to the surface. However the initial sputtered surface is depleted in Cr by preferential sputtering and calculation of the segregation behaviour taking this effect into account, shows that, as predicted, there has been a cross-over to Mo as the segregating element, the first such experimental observation. The extent of segregation to the first molecular layer has been calculated and the enthalpy of segregation of Mo is found to be −7.5 kJ mol−1. These results have been compared with theoretical predictions. The regular solution model gives poor agreement but the unified segregation model, which includes a large strain energy term for segregation of the oversized solute Mo, gives satisfactory agreement. However, the magnitude of the strain energy correction seems excessive and the merits of a modified regular solution model are discussed.