The diffusion of 51Cr in alloys with 8, 8.4 or 10.2wt%Cr was measured by using residual activity and serial sectioning techniques (table 56). It was found that the grain boundary diffusivities of 51Cr in the various alloys were essentially the same (table 57), and also agreed well with published data for higher temperatures. It was noted that the temperature dependence of Db could be described by:

Db(m3/s) = 3.4 x 10-9exp[-244(kJ/mol)/RT]

No downward curvature of the Arrhenius dependence at low temperatures was observed. The grain boundary diffusivities in the most complex alloy were always lower, and this was attributed to the segregation of alloying elements to grain boundaries. The Arrhenius expression for grain boundary diffusion in the latter alloy was:

D(m3/s) = 2.3 x 10-10exp[-241(kJ/mol)/RT]

The relatively high values of the activation energy almost reached the value of 259kJ/mol which had been deduced for Cr grain boundary diffusion in Fe-9.1wt%Cr. They were attributed to the additive activation enthalpy which was needed for the breaking of interactions between diffusing Cr atoms and C atoms which were segregated at the grain boundaries. The C was present as an impurity or as an alloying element.

Table 58

Self-Diffusion of 51Cr in Fe-50wt%Cr