Growth of intragranular ferrite in Fe-Ni-P alloys
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INTRODUCTION
A S early as 1947, Hultgren ~ in a study of isothermal transformation of austenite in alloy steels pointed out that the separation of ferrite is assumed to be associated with diffusion and partitioning of C and the alloying element. Since that time several investigators have pointed out the role of diffusion of the alloying elements in determining the growth kinetics of proeutectoid ferrite. 2-~~Although all the work has been done with grain boundary allotriomorphs, the arguments will hold just as well for the growth of intragranular ferrite if the kinetics are diffusion controlled. Bradley and Aaronson 5 classified the partitioning behavior of the alloying element as follows: 1. Full equilibrium: The alloying element partitions fully between ferrite and austenite, and there is chemical equilibrium at the interface. 2. Local equilibrium: Ferrite forms without the partitioning of the alloying element, but local equilibrium exists at the interface. 3. Paraequilibrium: Ferrite inherits the alloy content of the parent phase, and there is no pile-up of the alloying element at the interface, i.e., no local equilibrium. The three types of equilibrium are illustrated in Figure 1 where the concentration of the alloying element X is plotted against distance across the ferrite/austenite interface. In the case of full equilibrium the growth is controlled by the diffusion of the alloying element while for local and paraequilibrium, diffusion of C is the rate controlling step. Hillert, 6 Purdy et al., 7 Coates, 8 Sharma et al., 9 and Gilmour et al. 10 have developed a theoretical basis to understand the growth of both equilibrium and nonequilibrium ferrite under different isothermal heat treatment conditions. Their models examine the nature of chemical equilibrium at a / y interfaces, and they explain the kinetics in terms of the partitioning of the alloying element. These investigators conclude that for large undercoolings it is possible to grow no-partition ferrite with diffusion of C as the rate controlling step, while for low undercoolings ferrite
C. NARAYAN is with T.J. Watson Research Center, IBM, P.O. Box 218, Yorktown Heights, NY 10598. J.I. GOLDSTEIN is Professor, Department of Metallurgy and Materials Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted August 19, 1983. METALLURGICALTRANSACTIONS A
growth will be accompanied by the redistribution of the alloying element and the kinetics will be dictated by the slower diffusing alloying element. Redistribution of the alloying element during the austenite to ferrite transformation has been experimentally studied by several investigators. 7'H-~3 In these ferrite growth experiments the extent of partitioning could be measured, although the presence or absence of chemical equilibrium at the interface could not be detected because of the limited resolution of the electron microprobe. As a result it was not possible to differentiate between local equilibrium and paraequilibrium (see Figure 1). Bradley and Aaronson 5 recently conducted a
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