The self-diffusion of the constituents in this Laves phase was studied by using the tracer method, with the radio-isotopes 57Co and 95Nb; together with sputtering and mechanical sectioning. The temperature dependences of the Co and Nb diffusivities in various compositions were determined (tables 5 and 6). It was found that all of the activation enthalpies were of similar magnitude. The Nb diffusivity in Co2Nb was significantly lower than that of Co. The latter was itself considerably faster in alloys with a higher Co content. The data (figure 3) could be described by:

57Co in Co69Nb31 (1143-1523K):     D (m2/s) = 2.84 x 10-5 exp[-294.7(kJ/mol)/RT]

57Co in Co71Nb29 (1141-1423K):     D (m2/s) = 3.32 x 10-4 exp[-279.5(kJ/mol)/RT]

95Nb in Co69Nb31 (1143-1569K):     D (m2/s) = 9.24 x 10-7 exp[-292.0(kJ/mol)/RT]

The results were explained in terms of the structure of, and atomic defects in, Co2Nb. It was proposed that diffusion was mediated by thermal vacancies.

Diffusion in the C15-Type Intermetallic Laves Phase NbCo2. M.Denkinger, H.Mehrer: Philosophical Magazine A, 2000, 80[5], 1245-63

Figure 3

Diffusivity of Co and Nb in Co2Nb

(a: 57Co in Co71Nb29, b: 57Co in Co69Nb31 c: 95Nb in Co69Nb31)

Table 5

Diffusivity of 57Co in Co2Nb

Table 6

Diffusivity of 95Nb in Co2Nb