The effect of composition and temperature upon Ni bulk self-diffusion was studied in monocrystalline specimens, having well-defined compositions which ranged from 36.8 to 56.6at%Ni, at temperatures ranging from 1050 to 1630K (table 6, figure 1). The data could be described by:

46.8at%Ni:     D (m2/s) = 2.34 x 10-5 exp[-2.97(eV)/kT]

48.7at%Ni:     D (m2/s) = 2.39 x 10-5 exp[-2.97(eV)/kT]

49.7at%Ni:     D (m2/s) = 1.67 x 10-5 exp[-2.92(eV)/kT]

50.0at%Ni:     D (m2/s) = 2.98 x 10-5 exp[-2.99(eV)/kT]

51.8at%Ni:     D (m2/s) = 4.80 x 10-5 exp[-2.99(eV)/kT]

54.6at%Ni:     D (m2/s) = 4.36 x 10-5 exp[-2.88(eV)/kT]

56.6at%Ni:     D (m2/s) = 1.03 x 10-6 exp[-2.39(eV)/kT]

The diffusion penetration profiles of Ni were determined by using 2 different techniques of profile detection. Radiotracer experiments were carried out by using 63Ni tracers, serial sectioning techniques; and a sensitive liquid scintillation counter at high temperatures. At lower temperatures, the diffusion profiles were analyzed by using secondary ion mass spectrometry and the highly enriched stable isotope, 64Ni. In contrast to published data on Ni self-diffusion in NiAl, the present measurements revealed an unexpected concentration-dependence of the Ni diffusion coefficient; with almost constant diffusivities for stoichiometric and Al-rich alloys, and increasing diffusivity values with increasing Ni content on the Ni-rich side of the NiAl composition range (figure 2). The effective diffusion activation enthalpy was equal to about 3.0eV for Al-rich, stoichiometric and slightly Ni-rich alloys. In the case of samples having higher Ni contents, a decrease was observed with increasing Ni content. For example, the effective diffusion activation enthalpy was equal to 2.39eV for Ni56.6Al43.4. The present results implied that the same diffusion mechanism operated on both sides of stoichiometry. This was suggested to be the triple-defect mechanism. Its contribution was independent of composition. An activation energy of 3.18eV was estimated for the triple defect mechanism by using empirical potentials. The decrease in effective diffusion activation enthalpy at high Ni concentrations on the Ni-rich side was attributed to an additional contribution from the anti-structure bridge mechanism, with an activation energy of 1.73eV.

Ni Tracer Diffusion in the B2-Compound NiAl - Influence of Temperature and Composition. S.Frank, S.V.Divinski, U.Södervall, C.Herzig: Acta Materialia, 2001, 49[8], 1399–411

Figure 1

Diffusivity of Ni in NiAl

Figure 2

Diffusivity of Ni in NiAl

Table 6

Bulk Self-Diffusivity of Ni in NiAl