At present no dislocation theories can answer the above question, and we suggest that a modified interpretation of K, is dispensable to account for the yielding behavior of pressurized iron.
Another question is left unanswered which is about the result of coarse grained specimens.
The increase of free dislocations must decrease frictional stress as predicted from Johnston-Ghan yield point theory.(a) This is not in accordance with the present result on coarse grained specimens. We assume that the increase of free dislocations produced by pressurizing is not enough to affect frictional stress but it changes K, because K, is more sensitive to the density of free dislocations than oi is.
As to the discrepancy between the result on Bullen et d's high purity iron and the present results, no clear explanation has been given in the above discussion, but we believe that the amount of carbon content is not so significant compared with the factors associated with the specimen preparation such as the selection of grain size and cooling rate after annealing. Recently we detected the above mentioned pressurizing effect even in the higher purity iron than the one used here. In summary it is emphasized that the yielding behavior of pressurized iron can be explained by neither the recent theories of yielding(4*5) nor the classical unpinning concepts of Cottrellc6J.
lt is suggested that a modified interpretation of K, in the Petch's formula is dispensable to account for the behavior of pressurized iron.
The authors take this opportunity to thank Prof. S. Sakui and associate Prof. T. Nakamura of Tokyo Institute of Technology for their helpful discussions. They are indebted to Dr. S. Maeda for support and permission to publish.
M. YAJIMA M. ISHII