Participation of helium in discontinuous transformations in metallic alloys

Abstract

The influence of helium on the diffusion at a migrating boundary has been investigated during discontinuous precipitation in a Co–14 at.-%Al alloy and during discontinuous dissolution in an Al–15 at.-%Zn alloy. The helium was introduced into the alloys via a plasma treatment using various fluxes. Two opposing processes have been observed depending on the flux. One, below a critical flux, led to an increase in the linear growth rate of discontinuous precipitation and discontinuous dissolution owing to an enhanced concentration of vacancies, which acted as carriers of solute atoms in the diffusion process. The other process, above the critical flux, resulted in the appearance of helium bubbles, which acted as mechanical obstacles to the moving discontinuous precipitation and discontinuous dissolution front and as sinks for the excess vacancies. The effect of helium on the diffusion ahead of the advancing front during discontinuous precipitation and discontinuous dissolution has been well described using a parameter D_b^{Al or Zn}/D_V^He proposed by the authors.

MST/1531