Phase change in CoTi₂ induced by MeV electron irradiation

Abstract

The phase change induced by MeV electron irradiation in the intermetallic compound E9₃–CoTi₂ was investigated using high-voltage electron microscopy. Under MeV electron irradiation, CoTi₂ was first transformed into an amorphous phase and, with continued irradiation, crystallite formation in the amorphous phase (i.e. formation of crystallites of a solid-solution phase within the amorphous phase) was induced. The critical temperature for amorphisation was around 250 K. The total dose (dpa) required for crystallite formation (i.e. that required for partial crystallisation) was high (i.e. 27–80 dpa) and, even after prolonged irradiation, the amorphous phase was retained in the irradiated sample. Such partial crystallisation behaviour of amorphous Co₃₃Ti₆₇ was clearly different from the crystallisation behaviour (i.e. amorphous-to-solid solution, polymorphous transformation) of amorphous Cr₆₇Ti₃₃ reported in the literature. A possible cause of the difference is discussed.

Keywords:

• Electron irradiation
• partial crystallisation
• in situ electron microscopy
• high-voltage electron microscopy
• intermetallic compounds

Acknowledgement

We thank Helen McPherson, PhD, for editing a draft of this manuscript.

Notes

1. The word ‘monotonically’ was used in this paper to emphasise that the transition occurred directly from a C^e with a low Gibbs free energy to a Cⁿ with a higher one. Note that, strictly speaking, in the final stage of the transformation into the Cⁿ, there may be an ‘overshoot’ in the increment in the Gibbs free energy, which would account for the nucleation barrier (i.e. the energy required for the nucleation of Cⁿ in the C^e matrix) to the transformation.

2. The activation energy for the process responsible for the nucleation of crystallites under MeV electron irradiation was estimated as follows. It was first assumed that the total atom flux necessary for nucleation remains constant, irrespective of temperature. Based on this premise, the values of 1/(total dose), measured at 103, 140 and 220 K, were plotted against 1/T. The slope of the Arrhenius plot gave 0.01–0.02 eV as the activation energy. This value is low compared to those known for atom migrations, therefore the adjective ‘weak’ was used here.

3. Quasicrystals were excluded from Table 1.

4. The value was for 298 K.

5. If we confine ourselves to diffusional transformations (except for the displacive transformation), only these two reports on this subject are available in the literature.