A reversible phase transition in carbon segregation to W(110)

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Abstract
The equilibrium segregation of carbon to the (110) faces of a single crystal of tungsten containing the equivalent of two monolayers of carbon has been studied in the temperature range 1500 to 2000K. The enthalpy of segregation is -239 kJ mol-1 for temperatures up to 1660K and -192 kJ mol-1 for temperatures above 1800K. The change in segregation enthalpy between these two temperatures is attributed to a reversible phase transition in the surface. The high-temperature phase can accommodate only one third of the amount of carbon in the low-temperature phase. A semi-empirical analysis of the segregation kinetics in the temperature range 1200 to 1400K gave a value of 250 kJ mol-1 for the activation energy for volume diffusion of carbon in tungsten. This is an upper limit since there may be a barrier just below the surface.