A study on morphology and plate mean dimensions in Fe-Ni and Fe-Ni-Cr alloys
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I.
INTRODUCTION
THE factors that determine the transformation kinetics, morphology, and substructure in martensitic transformations are poorly defined. However, the fundamental factors, such as the transformation temperature (M,), the relative strength of austenite and martensite, the critical resolved shear stress for slip and twining in martensite, and the stacking fault energy of austenite, are considered to be important along with geometrical factors, such as grain size and the formation sequence of the plates during the progress of the transformationY -51 In addition to these, it has been reported that the presence of precipitates also changes the martensite morphology.t6] Much interest has been shown in recent years in the measurements of plate dimensions, t719] using quantitative metallographic techniques adopted by Fullman[20] and Smith and Guttman. t2q Datta's work IlS~was significant as the first systematic study on the plate dimensions in Fe-Ni alloys. However, a concrete conclusion about the factors responsible for the drastic change in the plate dimensions could not be drawn on the basis of experimental results. Attempts are made in the present work to sort out whether the factors related to transformation temperature or the composition are responsible for the appearance of a different morphology and the changes in the plate mean dimensions. To sort out this problem, a set of ternary alloys with their transformation burst temperature (Mb) values in a close range has been studied. Chromium, one of the 3, loop elements, which also suppresses the Mb as does Ni, has been considered as a ternary addition in Fe-Ni alloys. In order to make sure that grain size would not affect the M b temperature, I2z~a coarse grain size has been selected for all the alloys. Although earlier work It51was carried out with electrolytically pure Fe and Ni powders, they contained oxide inclusions which might have hindered the progress of P. VISVESVARAN, Representative for Power Components Technology Limited, Switzerland, is with the Physical Metallurgy Group, Department of Applied Mechanics, Indian Institute of Technology, New Delhi 110 016, India. Manuscript submitted November 22, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
the transformation just as they hinder it in the grain boundary. In order to eliminate the oxide inclusions, the current study involved a hydrogen treatment step. Fullman's technique t2~ was adopted for quantitative measurements of plate dimensions. II.
EXPERIMENTAL PROCEDURES
Electrolytically pure Fe and Ni powders and Cr beads were used as starting materials. The powders were separately hydrogen treated at 1223 _+ 5 K for an hour. During cooling down to below 873 K, only argon (grade: IOL/AR) was passed. The surface of the sintered block was chipped off thoroughly to avoid oxidation, if any. About 20 alloys of 20 g each, including both Fe-Ni and Fe-Ni-Cr alloys, were prepared in a nonconsumable electrode argon arc furnace. The weight loss was usually found to be less than 0.05 pet (this was achiev
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