Microstructural dependence of Fe-high Mn tensile behavior
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I.
INTRODUCTION
THE high Mn
steels have stimulated the attention of re, searchers for a variety of applications. 1,2 Of particular importance is the anstenite stabilizing influence of manganese, which allows Mn to replace relatively expensive additions of nickel and chromium. The characteristic mechanical and magnetic properties of the alloys are of great interest and have been measured by several investigators. 3'4'5 However, many aspects of the metallurgy of these alloys remain poorly understood. A deeper understanding of the Fe-Mn binary system is needed for the further development of high Mn steels. It has been observed that the microstructures of Fe-Mn binary alloys are strongly dependent on Mn content and can be characterized by the volume fractions of the austenite (3'), hcp epsilon martensite (e), and bcc martensite (a ') component phases. The mechanical properties are strongly influenced by the Mn dependence of both the initial phase composition and the phase transformations that occur during tensile deformation. It is the purpose of this paper to clarify the sensitivity of the tensile properties to the Mn content by demonstrating how the microstructure and its changes during deformation control the deformation and fracture behavior of Fe-16 to 36 wt pct Mn binary alloys.
II.
EXPERIMENTAL PROCEDURE
The alloys used in this study were prepared by inductionmelting in an argon gas atmosphere. The chemical compositions of the six alloys are shown in Table I. Throughout this paper each alloy will be referred to by designations based on the approximate Mn content. Each ingot, about Y. TOMOTA is Professor, Department of Metallurgical Engineering, Ibaraki University, Hitachi, Japan. M. STRUM is Graduate Research Assistant, Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720. J. W. MORRIS, Jr. is Professor of Metallurgy, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted June 4, 1985. METALLURGICALTRANSACTIONSA
9 kg, was homogenized at 1423 K for 24 hours followed by hot rolling at 1473 K to a plate thickness of 10 mm. Round bar tensile specimens of 6.4 mm gage diameter were machined after solution treatment at 1273 K for one hour in an argon atmosphere followed by a water quench. These specimens were used for tensile tests at room temperature and 77 K on an Instron electrohydraulic machine. Plate tensile specimens with 3.2 by 3.8 mm gage cross-sections and 25.4 mm gage lengths were cut from cold-rolled sheets along the rolling direction. They were solution treated as above and tested on a gear driven Instron machine over a range of temperatures between 77 and 553 K. The tensile strain rate was 3.3 (10-3)/S. The volume fractions of the component phases, i.e., austenite (3'), hcp epsilon martensite (e), and bcc martensite (a '), were measured by X-ray diffraction4 using the average of multiple peaks corresponding to each phase: Direct microstructural observations were condu
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