The Effect of an Inert Oxide Particle Dispersion on the Morphology of Martensite in Fe-27Ni-0.025C Alloys
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INTRODUCTION
THE austenite-to-martensite
transformation in iron-based alloys occurs by a diffusionless, shear type transformation. Body centered martensites take on two distinct morphologies generally known ~ as lath and plate. Lath martensite typically forms in parallel packets or blocks of heavily dislocated needle-like units that lie along planes closely parallel to {1 l l}a. In contrast, plate martensite forms as individual lens-shaped units containing many fine internal twins. Plate martensite is characterized by an irrational habit plane with subsequent plates generally taking on nonparallel variants. Reported habit planes for plate martensite include {225}a, {259}a, and {3,10,15}a. Many workers have tried to establish which variables control the martensite morphology. Variables reported to influence morphology include: transformation temperature (Mj,~ 4 interstitial and substitutional solute additions. ~56 thermodynamic driving force, 7-~ austenite stacking fault energy, t2'~3't4 quench rate above Mo. ~5~6 and austenite shear strength. 5"6 The morphology control controversy is complicated by the fact that these variables are generally interrelated. Pinpointing a controlling variable has, therefore, proven to be quite difficult. In as-quenched steels, five variables have received primary focus in morphology studies: M~ temperature, carbon content, thermodynamic driving force, austenite stacking fault energy, and austenite yield strength at Ms. M~ temperature was one of the first metallurgical vanables associated with the morphology transition. Studies by Kelly and Nutting, 2 and more extensive work by Krauss and Marder, ~ report that a decrease in Mo due to alloying additions favors a shift toward plate morphology. Carbon content strongly influences the kinetics of the martensite transformation in steels. The occurrence, howP.J. BROFMAN is Senior Associate Engineer with IBM Corporation. East Fishkill Facility, Hopewell Junction, NY 12533. G.S ANSELL. Dean of Engineering, and G. JUDD, Vice Provostand Graduate Dean. are both with Rensselaer Polytechnic Institute, Troy, NY 12181 Manuscript submitted April 20, 1981. METALLURGICALTRANSACTIONSA
ever, of a morphology transition in substitutional systems (such as FeNi) eliminates carbon content as a primary morphology controlling variable. Modifying the Ms argument, Bell and Owen, 7 and later Pascover and Radcliffe, 8 noted that the magnitude of the thermodynamic driving force (which is a function of both alloy chemistry and Ms temperature) was higher in plateforming alloys than in lath formers of the same alloy series. Strife 9 found evidence supporting this trend in his study of a series of Fe-Ni-Cr-C alloys. Low austenite stacking fault energy (SFE) has been consistently associated with lath morphology in alloys similar to the austenitic stainless steels. Works by Kelly and Nutting, 12 Breedis, ~3and most recently by Carr et al, ~4have all strongly suggested that stacking fault energy is an important factor in morphology selection, at least for the very low
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