High-Temperature deformation of B2 NiAl-base alloys
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INTRODUCTION
POLYCRYSTALLINE nickel aluminides based on B2 crystal structure have received much attention due to their low density, microstructural stability, potential for good creep resistance, and oxidation resistance at elevated temperatures. Although this material is generally brittle under ambient conditions, it has been suggested that brittleness may be reduced by grain refinement, u,21 For this purpose, techniques for producing materials of fine grain size, such as rapid solidification processing and thermomechanical processing, are of considerable interest. One approach toward this end is the utilization of grain-boundary pinning by fine particles. The room- and elevated-temperature mechanical properties of NiA1 have been studied extensively.12-9] The resuits indicate that high-temperature creep strength of NiAt is rather low. There have been several attempts in the past to improve high-temperature strength by the development of a second phase through the addition of a third element to the NiA1 matrix, t~~ However, high-temperature stability of such ternary precipitate compounds is always of concern; thus, dispersoid particulates, e.g., TiB2 and HfC, were considered reasonable candidates for enhancing the high-temperature strength of NiA1. These strengthening refractory phases were preselected because they are generally stable at high temperatures, t~2,~31 Furthermore, since these particles promote a grain-boundary pinning effect that produces fine grain structure, improved low-temperature toughness was expected. The poor tensile ductility in polycrystalline B2 NiAI has been attributed to the lack of a sufficient number of independent slip systems to satisfy von Mises' criterion IN-GYU LEE, formerly Graduate Student, The University of Michigan, is with the Ceramics Division, Korea Institute of Science & Technology, Cheongryang, Seoul, Korea. AMIT K. GHOSH, Professor, is with the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48106. RAN JAN RAY, President is with Marko Materials, Inc., North Billerica, MA 01862. SUNIL JHA, formerly with Marko Materials, Inc., is with Texas Instruments, Attleboro, Ma. Manuscript submitted October 31, 1990. METALLURGICAL AND MATERIALS TRANSACTIONS A
for compatibility in polycrystalline deformation. At room temperature, only the {110}(001) slip system is known to operate, so the number of slip systems is insufficient for flow. Above 673 K, this material shows considerable ductility, because (111), (110), and (100) slip are possible. 114-2~ It is possible that sufficient grain refinement may lead to noncube slip in the vicinity of grain boundaries to improve ductility even at room temperature. The present study was undertaken to characterize the deformation of NiAl-base alloys, to examine the microstructural changes that take place during deformation and the effects of TiB2 and HfC dispersoids on these changes, and to understand the effects of these changes on mechanical properties at low and elevated temperatures. Hot compressi
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