Low-temperature tracer diffusion of chromium in Fe-Cr ferritic alloys

Abstract

It can be seen from Table 2 and Fig.4 that the grain boundary diffusivity of 51Cr in alloys 1 and 2 exhibit no significant difference from one another, and, further, that they agree well with the literature data [3] measured at higher temperatures. It is also obvious that the temperature dependence of δD b can be expressed by one activation enthalpy Q b = (244 ± 9) kJ mol and pre-exponential factor ( δD) ρ = (3.4 +9.1 −2.5) × 10 −9 m 3/s . No downward curvature of Arrhenius dependence at low temperatures similar to the case of W grain boundary diffusion [4] was observed. The Cr grain boundary diffusivities in alloy 3 are systematically lower which is most likely due to the segregation of alloying elements to the grain boundaries. The obtained Arrhenius parameters for the Cr grain boundary diffusion in alloy 3 are Q b = (241 ± 11) kJ mol and ( δD) gr = (2.3 +8.6 −1.8) × 10 −10 m 3/ s. The relative high values of Qb obtained in present work reaches almost the value 259 kJ mol deduced from [3] for Cr grain boundary diffusion in Fe·9.1 wt.% Cr. They are caused most likely by the additive activation enthalpy needed for the broken of attractive interaction between diffusing Cr atoms and carbon atoms segregated at the grain boundaries. Carbon is present as impurity (approx. 20 wt.ppm in alloy 1) or as alloying element in all the studied alloys.