REFRACTORY METAL ALLOYS Metallurgy and Technology Editors I. Machlin Materials and Processes Branch Naval Air Systems Command Departrn.ent of the Navy Washington, D.C. R. T. Begley Astronuclear Laboratory Westinghouse Electric Company Pittsburgh, Pennsylvania atnd E. D. Weisert Materials Research Rocketdyne Division North American Rockwell CompOh/,y Canoga Park, California • '$i1YOt~~ A Publication of The Metallurgical Society of AIME REFRACTORY METAL ALLOYS Metallurgy and Technology Proceedings of a Symposium on Metallurgy and Tech nology of Refractory Metals held in Washington, D.C., April 25-26, 1968. Sponsored by the Refractory Metals Committee, Institute of Metals Division, The Metallurgi cal Society of AIME and the National Aeronautics and Space Administration, Washington, D.C. <±> PLENUM PRESS • NEW YORK • 1968 ISBN 978-1-4684-9122-7 ISBN 978-1-4684-9120-3 (eBook) 001 10.1007/978-1-4684-9120-3 © 1968 American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Softcover reprint of the hardcover I st edition 1968 Library of Congress Catalog Card Number 69-13684 Plenum Press is a division of Plenum Publishing Corporation 227 West 17 Street, New York, N. Y.10011 Preface This publication documents Proceedings of the Symposium on Metal lurgy and Technology of Refractory Metal Alloys, held in Washington, D.C. at the Washington Hilton Hotel on April 25-26, 1968, under sponsorship of the Refractory Metals Committee, Institute of Metals Division, of the Metallurgical Society of AIME, and the National Aeronautics and Space Administration. The Symposium presented critical reviews of selected topics in refractory metal alloys, thereby contributing to an in-depth understanding of the state-of-the-art, and establishing a base line for further research, development, and application. This Symposium is fifth in a series of conferences on refractory metals, sponsored by the Metallurgical Society of AlME. Publications issuing from the conferences are valuable technical and historical source books, tracing the evolution of refractory metals from early laboratory alloying studies to their present status as useful engineering materials. Refractory metals are arbitrarily defined by melting point. A melting temperature of over 35000F was selected as the minimum for this Symposium, thus excluding chromium and vanadium, which logically could be treated with other refractory metals in Groups VA and VIA of the periodic table. The Refractory Metals Committee is planning reviews of chromium and vanadium in subsequent conferences. Of eleven metals with melting points above 35000F, four have been advanced significantly in recent years, as bases for structural alloys, namely Cb, Mo, Ta, and W. The remaining seven (Re, 8f, Ir, Rh, Ru, Tc and Os) have received lesser development, a fact reflected in the Sympo sium program. Perhaps future Symposia will show a different emphasis. At the time of the first High Temperature Materials Conference in 1957 (Cleveland, Ohio) only the Mo-O.5% T1 alloy was in commercial pro duction. Currently (1968) several dozen useful Cb, Mo, Ta, and W alloys have evolved, with many obtainable in good quality as sheet, foil, tube, wire, bar, and forgings. Although complete information on aerospace, nuclear, and commercial applications was not available for presentation at the SympoSium, it appears that refractory metals are serving essential structural functions, predominantly in prototype studies of propulsion, power generation, and hypersonic flight devices, with few requirements involving large quantities. A major factor limiting the utility of refractory metals in high temperature processes continues to be the lack of alloys (coated or otherwise) reliably operable for long times (multi-hundreds ofbours) in oxidizing environments. Nonetheless, there are several environments in which refractory metals are optimum materials, including vacua, inert gases, and liquid alkali metals. In oxidizing environments coated re frac~ory metals can serve for limited periods, under appropriate con- v vi PREFACE ditions. The U. S. capability in refractory metals for structural uses has been attained with Government support. supplemented by industry. of pro grams ranging from basic research to fabrication of hardware. Much work remains. if desired improvements in oxidation-resistance. strength. ductility. weldability and fabricability are to be attained. However, it appears likely that research efforts. in the future, will be selectively directed towards solving those critical problems limiting the useful potential of refractory metals. The invited papers in this publication were intended to provide maximum information required in judging future needs and research directions. We are grateful to the National Aeronautics and Space Administration for support enabling us to publish these proceedings. Much credit is due to authors of papers, who have made significant professional contributions by directing many hours of their leisure time to hard work. Our thanks are also extended to session chairmen and to members of the Refractory Metals Committee. who partiCipated in organizing this Symposium. Distribution of this book in a timely and economical manner has been facilitated by photographic reproduction of authors' manuscripts, a pro cedure necessitating some relaxation of standards for editorial uniformity and format. We trust that the advantage of timely publication will out weigh perfection of format. It is regretted that the following planned papers were not available: Refractory Metals in Aerospace Structures, Use of Refractory Metals in Aerospace PropulSion, and Molybdenum and Moly bdenum-Base-Alloys-Recent Developments. April 1968 1. Hachlin R. T. Begley E. D. Weisert Contents Basic Strengthening Mechanism in Refractory Metals ••••••••••••••• 1 B. A. Wilcox High Temperature Creep and Fracture Behavior of the Refractory Metals 41 0.0 •• 00000 •• 0.0000000.00000.0.0000 ••• 00 R. T. Begley, D. L. Harrod, and R. E. Gold The Effect of Thermal-Mechanical Treatments on the Structure and Properties of Refractory Metals ••••••••••••••••••••• 85 R. A. Perkins Interactions in Coated Refractory Metal Systems ~................. 121 A. G. Metcalfe and A. R. Stetson Interactions of Refractory Metals with Active Gases in Vacua and Inert Gas Environments •••••••••••••••••••••••• 165 H. Inouye Hydrogen Effects in Refractory Metals 197 W. T. Chandler and R. J. Walter The Compatibility of Refractory Metals with Liquid Metals •••••••• 251 E. E. Hoffman and R. W. Harrison Refractory Metals in Space Electric Power Conversion Systems ••••• 289 J. W. Semmel, Jr. Recent Advances in Columbium Alloys 325 R. G. Frank Considerations in the Development of Tantalum Base Alloys •••••••• 373 R. W. Buckman, Jr. and R. C. Goodspeed Advanced Processing Technology and High Temperature Mechanical Properties of Tungsten Base Alloys ••••••••••••••••••••• 395 H. G. Sell vii viii CONTENTS The Less Common Refractory Metals (Rhenium, Technetium, Hafnium, Noble Metals) ••••••••••••••••••••••••••••••••••• 451 J. Maltz Index 489 0 0 •• " 0 •••••• 0 • 0 " ••••• " • 0 0 • " 0 • 0 •• " 0 0 " 0 • 0 0 " •••• 0 0 0 0 •••• 0 0 " 0 •• 0 BASIC STRENGTHENING MECHANISMS IN REFRACTORY METALS B. A. Wilcox ABSTRAcr Various mechanisms of strengthening refractory metals and alloys are discussed. Special emphasis is placed on the microstructural features which can be varied to produce strengthening, and relevant illustrations are given. The general areas reviewed include: (a) strengthening by second phase particles, (b) solid solution strength ening, (c) strain hardening and grain size refinement, (d) retaining worked structures at high temperatures, (e) dynamic strengthening, and (f) fiber reinforcement. B. A. Wilcox is a Fellow in the Metal Science Group, Battelle Memorial Institute, Columbus Laboratories, Columbus, Ohio. 1 2 B. A.WILCOX INTRODUCTION In order to capitalize fully on the potential of refractory metals in structural applications, it is necessary to strengthen the base metals. Thus, improving the strength of refractory metals has been one of the most important topics of research and development on these mate rials, and in previous AIME Refractory Metals Symposia this area has been the subject of major review papers (1,2). This paper approaches strengthening in refractory metals by considering the various mechanisms that have been employed to increase the yield, ultimate and creep strengths. The subjects treated are: (a) strengthening by second phase particles, (b) solid solution strengthening, (c) strain hardening and grain size refinement, (d) retaining worked structures at high temperatures, (e) dynamic strengthening, and (f) fiber reinforcement. Radiation hardening is a special topic which is not considered here, since it is not a technique that materials designers intentionally use to strengthen refractory metals. Also, texture strengthening is not discussed, because in the major refractory metals (W, Mo, Cr, Ta, Nb, V) the BCC structure is not amenable to pronounced strengthening by preferred orientation, as com~ pared to HCP metals. The main emphasis here is placed on strengthening at room and elevated temperatures. Many of the strengthening mechanisms, e.g., dispersion hardening, solution strengthening, working and grain size refinement, etc., also are operative at low temperatures. However, at low temperatures additional factors, such as overcoming the Peierls barrier, are important. Conrad (3) has reviewed this area including numerous results on the BCC refractory metals. For practical purposes, ductility as well as strength is important to consider in refractory metal alloy development. In many cases those factors which influence the strength of refractory metals also affect the fracture behavior. However, this topic is not treated here, since it has been well re viewed in several recent papers (4,5). A complete review of the literature on refractory metal strengthening is beyond the scope of this paper. Rather, the author has tried to give details of pertinent strengthening mechanisms, and to illustrate these with appropriate examples based on refractory metal alloys. Thus, it is the intention of this paper to provide the alloy developer with basic guidelines which will be of assistance in selecting methods to improve the strength of refractory metals. It is often desirable in practice to employ more than one strengthening mechanism. This paper considers that the various strengthening mechanisms can supplement one another and they are often interrelated. Even though the strengthening effects may not be directly additive, it is convenient to formulate a phenomenological expression such that the strength, cr, of an alloy is written as: (1)