RECOVERY AND RECRYSTALLIZATION OF 60-PERCENT-COLD-ROLLED IC-218 ALLOY

An annealing study was performed on cold-rolled IC-218 alloy (an advan ced form of nickel aluminide) with a reduction of 60%. During isochron al annealing for IO min, a hardness maximum was found at 475 degrees C ; this was due to reordering, as confirmed by DSC analysis. When the a nnealing temperature was raised above 675 degrees C, recovery softenin g began and became more pronounced with increase of temperature, indic ating a kinetic competition between strain-age hardening by reordering and recovery softening by thermally activated dislocation mobility. R ecrystallization started readily above 775 degrees C, with strongly pr eferential nucleation at shear bands. However, growth of the new grain s was very slow and the growth rate progressively decreased; many grai ns stopped growing at a size of 5 to 10 mu m This led to peculiar kine tics, characterized by a very sluggish progress of the fraction recrys tallized, X-V, during the stage of 20%less than or equal to X-V less t han or equal to 50%. If the Avrami equation X-V = 1 - exp (-Bt(k)) was formally used to describe the process in this stage, an extremely sma ll value of the Avrami exponent k = 0.24, resulted. An Arrhenius plot for X-V = 40% yielded an apparent activation energy of approximate to 230 kJ/mol, which was found to be equal to that of dislocation recover y.