  The temperature driven segregation of Cr to the surface of the tungsten-based
WCrY alloy is analysed with low energy ion scattering of He+ ions with an
energy of 1 keV in the temperature range from room temperature to 1000 K. Due
to the high surface sensitivity, these measurements probe only the composition
of the outermost monolayer. The surface concentration of Cr increases slightly
when the temperature of the sample is increased up to 700 K and exhibits a much
stronger increase when the sample temperature is further raised. The
segregation enthalpy for Cr is obtained from the Langmuir-McLean relation and
amounts to 0.7 eV. The surface concentration of Y shows a similar behaviour to
the Cr concentration. The temperature thresholds between slow and accelerated
surface density increases for Cr and Y are nearly the same. At a temperature of
1000 K the low energy ion scattering detects almost no W on the surface. The
modified surface composition due to the segregated species, i.e. the mixed Cr/Y
layer, stays stable during cool-down of the sample. Preferential sputtering is
investigated using ion bombardment of 250 eV D atoms, resulting in an increase
of the W surface density at room temperature. This effect is counteracted at
elevated temperatures where segregation replenishes the lighter elements on the
surface and prevents the formation of an all-W surface layer. The flux of
segregating Cr atoms towards the surface is evaluated from the equilibrium
between sputter erosion and segregation.
