Low Temperature Ductility of Ultrafine Grain α Iron and Its Substitutional Solid Solutions

Abstract

A study was made of the effect of substitutional solute elements on the ductility of ultrafine grain α iron which was prepared by cold rolling and subsequent annealing to various stages of recovery and recrystallization. The addition of Si or Ti was found to have a remarkable effect in improving the low temperature ductility of the alloy recrystallized into the ultrafine grain structure which in an unalloyed condition showed a very small uniform elongation owing to the plastic instability. Al or Ni addition, however, was not very efficient. Ni was found to suppress the intergranular fracture which was one of the most important causes of low temperature failure of iron having relatively coarse grain structure.
Lüders strain was also measured and it was found that a critical grain size, d_c, at which Lüders strain exceeded uniform elongation and a transition from stable to unstable plastic flow occurred was reduced by the addition of substitutional solute elements.
Possible causes for reducing Lüders strain or d_c were briefly discussed in terms of slip character and interstitial distribution which were expected to undergo a considerable change by the presence of those substitutional elements.