Neutron-Induced Spinodal-Like Decomposition of Fe-Ni and Fe-Ni-Cr Alloys

Abstract

Spinodal decomposition is not known to occur in the iron-rich regime of Fe-Ni and Fe-Ni-Cr fcc systems. During neutron irradiations conducted in the range 450 to 600°C, however, a spinodal-like separation occurs in the invar compositional range. Ion irradiation studies show a similar behavior and indicate that the separation process occurs as least as high as 725°C. The spinodal-like process is characterized by compositional microoscillations between ∼25 and ∼50% nickel with temperature-dependent wavelengths on the order of hundreds of nanometers. These oscillations exhibit a dependence on crystallographic orientation and cause significant hardening. Comparison of these oscillations with those produced in the Santa Catharina meteorite (Fe-35Ni) leads to the conclusion that Fe-Ni invar alloys decompose very slowly (many thousands of years) in the absence of radiation. During irradiation, the spinodallike decomposition and its subsequent coarsening are accelerated by enhanced diffusion and at lower irradiation temperatures by segregation of nickel via the inverse Kirkendall effect at radiation-produced sinks.