THE c/r RATIO OF MARTENSITIC PLATES IN Fe-Ni ALLOYS

Abstract
The mean dimensions of martensitic plates formed at the burst temperature in ten iron-nickel alloys have been measured. The burst transformation proceeds predominantly by the spreading of nucleation through autocatalytic stimulation across grain boundaries rather than by filling-in of plates within partially transformed grains. The mean semithickness-to-radius ratio, [MATH], of plates is found to decrease linearly from 0.11 at 250 K to 0.04 at 80 K. Extrapolation of this linear relationship to temperatures near 0 K yields a very small [MATH] ratio, that is characteristic of the thin-plate morphology. The martensitic plates are partially twinned about the midrib, the extent of twinning increasing with decreasing burst temperature. The mean twinned semithickness-to-radius ratio, [MATH] increases with decreasing burst temperature, reaches a maximum of 0.05 at 160 K, and decreases thereafter. Extrapolation of [MATH] to zero yields 285 K as the temperature of transition from the partially-twinned plate to the fully-lath morphology.