The mechanical stability of precipitated austenite in 9Ni steel
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I.
INTRODUCTION
IN a previous paper,~
we reported a detailed study of the stability of precipitated austenite against the martensitic transformation and how this stability is related to the ductileto-brittle transition of 9Ni steel. In essence, the reduction in stability of the austenite during isothermal tempering leads to an increase in the ductile-to-brittle transition temperature. Microstructural features which affect the austenite stability were examined. Changes in the chemical composition of the austenite during tempering were too small to account for the large reduction in the stability of the austenite; the reduction in carbon concentration could account for a change in MD of only 50 ~ and the increasing Ni concentration of the austenite should actually stabilize the austenite with further tempering. Another microstructural feature which may affect the austenite stability is the coherency of the austenite/martensite interface. The energetics associated with the reduction in interface coherency during tempering are rather small. However, the nucleation of the transformation of an austenite particle may be influenced by changes in the dislocation structure at its surface. Such an effect is difficult to estimate quantitatively, and furthermore it may be true that the loss of interface coherency during tempering is a consequence, rather than a cause, of the transformation of austenite particles. In this paper we explore a third microstructural feature which affects the mechanical stability of austenite particles. This feature is the type of dislocation structure that forms around austenite particles as they transform to martensite. Detailed observations of these dislocation structures around fresh martensite particles are reported in this paper, and we show how changes in these dislocation structures reduce the austenite stability during isothermal tempering. We propose that the energy of formation of these dislocation structures is the basis for the "mechanical stability" of austenite particles.
II.
E X P E R I M E N T A L DATA ANALYSIS
Experimental techniques for material preparation and characterization were described in Reference 1. In this section we describe further analysis of the X-ray diffraction
B. FULTZ is Assistant Professor of Materials Science, Keck Laboratory, California Institute of Technology, Pasadena, CA 91125. J.W. MORRIS, Jr. is Professor, Department of Materials Science and Mineral Engineering, University of California, Berkeley, and Faculty Senior Scientist, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720. Manuscript submitted August 31, 1984. METALLURGICAL TRANSACTIONS A
lineshapes. We show how this analysis provides information about the dislocation structures which result from the transformation of austenite particles. The broadening of both austenite and martensite X-ray diffraction peaks was analyzed to provide information on the internal strain distributions and the average size of the coherently diffracting domains. The "method of multiple orders
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