Recovery and Recrystallization of 60%-Cold-rolled Ic-218 Alloy

An annealing study was performed on cold-rolled IC-218 alloy (an advanced form of nickel aluminide) with a reduction of 60%. During isochronal annealing for 10 min, a hardness maximum was found at 475 °C; this was due to reordering, as confirmed by DSC analysis. When the annealing temperature was raised above 675 °C, recovery softening began and became more pronounced with increase of temperature, indicating a kinetic competition between strain-age hardening by reordering and recovery softening by thermally activated dislocation mobility. Recrystallization started readily above 775 °C, with strongly preferential nucleation at shear bands. However, growth of the new grains was very slow and the growth rate progressively decreased; many grains stopped growing at a size of 5 to 10 μm. This led to peculiar kinetics, characterized by a very sluggish progress of the fraction recrystallized, XV, during the stage of 20% ≤ XV ≤ 50%. If the Avrami equation XV = 1 — exp (—Btk) was formally used to describe the process in this stage, an extremely small value of the Avrami exponent k = 0.24, resulted. An Arrhenius plot for XV = 40% yielded an apparent activation energy of ≈︂230 kJ/mol, which was found to be equal to that of dislocation recovery.