Grain-Coarsening Resistance and the Stability of Second-Phase Dispersions in Rapidly Solidified Steels
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GRAIN-COARSENING RESISTANCE AND THE STABILITY OF SECOND-PHASE DISPERSIONS IN RAPIDLY SOLIDIFIED STEELS G. B. OLSON, H. C. LING*, J. S. MONTGOMERY, J. B. VANDER SANDE, AND M. COHEN MIT, Cambridge, MA; *now at Western Electric Engineering Research Labs., Princeton, NJ, USA ABSTRACT Control of alloy composition and processing to achieve grain coarsening resistance in rapidly solidified alloys is examined via the theory of grain boundary pinning and particle coarsening. The principles are illustrated for the case of manganese sulfides in steels. A thermodynamic survey of potential stable dispersed phases identifies TiN and rare-earth sulfides as particularly promising for alloy development via rapid solidification. INTRODUCTION An investigation of the tempering and grain growth behavior of rapidly solidified Ni-Co and Mo steels has revealed a remarkable resistance to high temperature grain coarsening [1]. At 1200C where conventionally processed steels coarsen to austenitic grain sizes of several hundred microns, the rapidly solidified (RSP) steels retain a grain size of 10-20 -pm. Electron microscopy suggests that the coarsening resistance is due to finely dispersed stable inclusions, principally sulfides, which can maintain a Q0.1 lpm particle size during high temperature austenitizing. This coarsening resistance is of potential benefit to the mechanical properties of steels for which high austenitizing temperatures have been found to improve sharp-crack fracture toughness (KIC) but with losses in other fracture properties associated with grain coarsening. Preliminary toughness measurements on the rapidly solidified steels have revealed KIC values equal to or superior to conventionally processed material, and substantial increases with high austenitizing treatments have been obtained without deleterious grain coarsening [2]. In view of the potential importance of this phenomenon, the control of boundary pinning dispersions is here assessed in light of the theory of grain boundary pinning and particle coarsening, and the relative thermodynamic stability of potential dispersed phases in steels is surveyed to provide alloy design guidelines for rapid solidification processing. GRAIN BOUNDARY PINNING AND PARTICLE COARSENING Several detailed treatments [3-5] have extended the original model of Zener [6] for the pinning effect of second-phase particles on grain boundaries. These models can be used to predict a limiting grain size at which the driving force for grain growth is balanced by the particle pinning force. The model of Gladman [3], which has been quantitatively applied to the grain coarsening of austenite [7], predicts a limiting grain size expressed by:
2R =ir 3
v where R is the limiting grain radius, F~ is the mean particle radius, fv
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the particle volume fraction, and Z is a grain size distribution parameter (ratio of the largest to the average grain size) determining the driving force for grain growth. As in all such pinning models, the limiting grain size is proportional to F and inversely proport
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