Effect of the α-γ Phase Transition on the Stability of Dislocation Loops in bcc Iron

Body-centered-cubic iron develops an elastic instability, driven by spin fluctuations, near the α-γ phase transition temperature T_c=912 °C that is associated with the dramatic reduction of the shear stiffness constant c^′=(c₁₁-c₁₂)/2 near T_c. This reduction of c^′ has a profound effect on the temperature dependence of the anisotropic elastic self-energies of dislocations in iron. It also affects the relative stability of the a⟨100⟩ and a/2⟨111⟩ prismatic edge dislocation loops formed during irradiation. The difference between the anisotropic elastic free energies provides the fundamental explanation for the observed dominant occurrence of the a⟨100⟩, as opposed to the a/2⟨111⟩, Burgers vector configurations of prismatic dislocation loops in iron and iron-based alloys at high temperatures.