Papers - Reactions in the Solid State, I-Initial Course of Subcritical Isothermal Diffusion Reactions in Austenite in an Alloy Steel

Of late considerable experimental and theoretical interest has been shown in the rates of transformation from a single phase, usually a supercooled solid solution, into one or more phases. Such reactions may involve both diffusion and transformation of the major constituent, as in iron alloys, or it may simply involve diffusion, as in superstructure formation and in age-hardening systems. Most of the work has been confined to ferrous alloy1-9† although two investigations have been made on copper alloys10*". That such work may be of considerable practical importance has been demonstrated by Bain12, by the workers at the Kaiser Wilhelm Institut fur Eisen-for~chung'13, 14, and by the everyday use of precipitation and age-hardening principles15. Experiments in this country have generally followed Davenport and Bain's technique of rapid supercooling followed by isothermal treatment. In Germany16, 17 the work has been extended to the study of the influence of accurately controlled and measured rates of cooling. The work on steel indicates that there are two subcritical temperature ranges where reaction starts sooner after quenching and proceeds at a more rapid rate than at other temperatures. At suberitical temperatures above Ar" (temperature of martensite formation) where reaction is dependent upon atomic migration there is a period of time of which the length depends upon the alloy content and the temperature, before which no reaction has been detected. Davenport and Bain' find that different sorts of atoms in solid solution in gamma iron affect the length of this " prereaction" period differently, manganese probably being the most effective in lengthening it. It is very difficult to believe that nothing occurs within these "prereaction" regions, for if this were so there is no reason why any reaction at all should be observed. In the light of experimental facts and theoretical considerations it is certainly