The relationship between toughness and microstructure in Fe-high Mn binary alloys
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I.
INTRODUCTION
GIVENthe increasing demand for structural materials for low or very low temperature service, new high-Mn steels have recently been developed. 1.2 In commercial alloys, Cr, Ni, N, C, and other elements are usually added in addition to Mn to provide good combinations of strength and toughness.3 6 These combinations are, of course, controlled not only by the chemical compositions of the alloys but also by their microstructures. However, since the alloys are complex, their microstructure-property relations are difficult to decipher. For example, intergranular fracture or quasicleavage is observed in some high-Mn steels, 3 but the reason for their occurrence is not clear. Although they have been studied previously,4 the phase transformations that occur during deformation and their effects on the mechanical behavior need further clarification. In this study, the influence of the microstructure on toughness is discussed in light of the strong influence of Mn content on the microstructure of Fe-Mn binary alloys.
II.
EXPERIMENTAL PROCEDURE
The alloys used in this investigation were inductionmelted in an argon gas atmosphere, homogenized, and hotforged into plates. They were solution-treated at 1273 K for 1 hour in an argon gas atmosphere and water quenched. ASTM standard V-notch Charpy impact test specimens were cut along the rolling direction and notched throughthickness. Charpy impact tests were performed at various temperatures from 77 to 523 K. Ethyl alcohol and isopentane cooled by liquid nitrogen were used to control test temperatures between 77 K and room temperature, while a silicon oil bath was used for tests above room temperature. The fracture surface morphology was characterized through SEM observations on the central regions of the impact specimens. Fracture profiles of specimens fractured at 77 K were Y. TOMOTA is Professor, Ibaraki University, Faculty of Engineering, Hitachi-Shi, Japan. M. STRUM, Graduate Research Assistant, and J. W. MORRIS, Jr., Professor, are with the Department of Materials Science and with the Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94703. Manuscript submitted August 8, 1985. METALLURGICAL TRANSACTIONS A
examined with optical and SEM microscopy. For these studies the impact specimens were Ni-plated, sectioned longitudinally, polished, and etched with Villela's reagent. Notched round bar specimens with 32 mm length and 3.68 mm diameter were also fractured in situ for Auger spectroscopic analysis using a PHI model 590 scanning Auger microscope. The chemical compositions of the specimens are listed in Table I. The alloys are identified by their nominal Mn contents. III.
EXPERIMENTAL RESULTS
A. Microstructure
The as-annealed microstructures of the alloys and the changes in their microstructures during tensile deformation were reported elsewhere. 7 The results may be summarized as follows. The volume fractions of the constituent phases in the as-annealed alloys and in specimens fractured in tensile tests at 77 K are given as a function of
