The plastic deformation of ferromagnetic face-centred cubic Fe-Ni-C alloys

Abstract

When face-centred cubic alloys with the physical properties typical of Invar are strained over a sufficient range of deformation variables, peculiar deformation characteristics become apparent. For example, there is an unusually large temperature-dependence of the flow stress, completely atypical of f.c.c. metals. Factors such as compositional departure from the Invar range, the intentional formation of martensite, arranged prohibition of martensite, and compositional control of Curie temperatures, were manipulated to determine the origin of the deformation behaviour. Only the Invars reacted as a class, and ferromagnetism appeared causal both to the onset of the special deformation and to its nature. It was observed that the increase of flow stress produced by cooling below the Curie temperature can be completely reversed upon heating, that the formation of martensite is irrelevant, and that the nucleation of martensite is inhibited by the spontaneous ferromagnetism. Observations such as these isolate the importance of ferromagnetism and suggest that the basic dislocation reactions in these alloys change with ferromagnetism.