Microstructure stabilization in a rapidly solidified
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I.
INTRODUCTION
T H E benefits of rapid solidification processing of metallic alloys are known to include chemical homogeneity, fine microstructures, extended solid solutions, and metastable phases, u'21 However, there is a concern that these benefits may be reduced or even erased after exposure to relatively high temperatures, such as during powder consolidation or any subsequent thermomechanical treatments. Rapid solidification processed (RSP) type 304 stainless steel (SS) powders, consolidated by several methods (hot extrusion, hot isostatic pressing, and dynamic compaction using explosives) had nearly equivalent high tensile properties after exposure to temperatures used for solution annealing conventionally processed (ingot metallurgy) type 304 S S . |31 The grain growth was similar for the three consolidation methods but significantly retarded compared to that of the wrought ingot metallurgy (IM) counterpart. These observations suggest that some feature or features have been formed in the powders during RSP that control the microstructure. The strengths (yield and ultimate) associated with these materials are significantly higher than those of their IM counterpart, t31 This study was undertaken to determine the microstructural features which could provide the improvements in tensile properties observed for the RSP type 304 SS material compared to its IM counterpart. [31 II.
EXPERIMENTAL
A. Description of Materials The RSP type 304 SS powders used for the study were processed by centrifugal atomization (CA). The processing parameters included a superheat of - 1 0 0 ~ and JOHN E. FLINN and GARY E. KORTH are with Materials Technology, Idaho National Engineering Laboratory, EG&G Idaho, Inc., Idaho Falls, ID 83415-2218. JUNG CHAN BAE, formerly with the Department of Materials Science and Engineering, University of Wisconsin-Madison, is with the Advanced Materials Center, Korea Academy of Industrial Technology, Seoul, Korea. THOMAS F. KELLY, Associate Professor, is with the Department of Materials Science and Metallurgical Engineering, University of Wisconsin-Madison, Madison, WI 53706. Manuscript submitted February 19, 1990. METALLURGICAL TRANSACTIONS A
a rotating cup speed of 24,000 rpm. The molten droplets were cooled by forced convection with flowing helium. Powder characterization involved particle size distribution measurements, microscopy and X-ray diffraction (XRD), study of surface films, and gas analysis for entrained helium contents. The CA type 304 SS powders were consolidated by hot extrusion in hermetically sealed type 304 SS cans at 900 ~ with an extrusion ratio of 8 : 1. The IM type 304 SS used for tensile behavior comparisons was rolledwrought, mill-annealed plate. The compositions of the two materials are shown in Table I. B. Testing and Examinations The tensile specimens were oriented along the extrusion axis for the CA material and parallel to the rolling direction for the IM plate. The tensile tests were conducted at temperatures of 24 ~ 600 ~ and 800 ~ with a constant crosshead speed of 0
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