Mechanisms of radiation-induced surface composition modification in VCr alloys at high and low temperature

The effect of 0.5 to 3 keV argon ion sputtering on the surface composition of several alloys of vanadium and chromium has been investigated at temperatures in the range 300 to 950 K. At room temperature, sputtering depletes the alloys in chromium and the relative V/Cr sputter rate constant ratio is found to be 1.20, consistent with the ratio of the pure element sputter yields. Annealing clean alloys at temperatures in the range 550 to 670°C further enriches the surface in vanadium and also oxygen segregates to the surface. This behaviour is attributed to co-segregation of oxygen and vanadium, an interpretation supported by the observation of vanadium chemical shifts analogous to those observed in vanadium oxides, the depth profiles of the segregated surfaces and the bond energies in the VCrO system. Sputtering the alloys at temperatures in the range 410 to 620°C enriches the surface in chromium to a depth which increases with sputter annealing time. Similar behaviour has been reported on sputtering a V/Cr alloy at these temperatures but with much higher energy V+ ions, and has been attributed to radiation-induced segregation of chromium. The results presented in this paper offer a different interpretation. Annealing enriches the surface in vanadium which co-segregates with oxygen. Thus on sputter- annealing the surface will become depleted in vanadium at a rate which is probably enhanced by an increased sputter yield of VO over either V or Cr. This interpretation is further supported by the observation that the alloy is still depleted in vanadium when the sputtering occurs at room temperature when performed in a cycle with intervening annealing intervals.