A microstructural investigation of the origin of brittle behavior in the transverse direction in Mo-based alloy bars
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INTRODUCTION
M O L Y B D E N U M and its alloys often provide a unique combination of high temperature strength and corrosion resistance. For these reasons they are selected for many hightemperature aerospace applications instead of columbium or tantalum alloys. ~'2 More extensive use of molybdenumbased alloys, however, is often limited by their brittle behavior at ambient temperature] This brittleness can be of a number of forms but is usually initiated by intergranular fracture, either during fabrication or in service. The nature of the origin of brittle behavior in Mo has been the subject of much discussion, and a number of important factors has been identified. Kumar and Eyre, 3 for example, recently demonstrated that under certain conditions extremely small amounts of oxygen (about 6 atomic ppm) could cause grain boundary embrittlement in "bamboo" samples of Mo. In their work, the amount of carbon in the alloys was shown to be an important consideration in determining whether or not segregation of oxygen to grain boundaries occurred. Increasing the amount of carbon, for a fixed oxygen level, was shown to diminish the degree of oxygen segregation and hence to promote ductile behavior. Also, Suzuki et al. 4 as well as Tsuya and Aritomi, 5 have claimed that carbon segregation to grain boundaries is associated with improved longitudinal ductility of coarse-grained Mo alloys. If the carbide precipitation is too severe at grain boundaries, however, as is often the case in cast or lightly-worked structures, then the effect of carbon is deleterious and the semi-continuous network of carbide can lead to brittle behavior. 5 Brosse et al.,6 meanwhile, have suggested that even in Mo of ultra high purity, intergranular brittle behavior would be a problem because of an inherent weakness of grain boundaries in Mo. In the present paper, the mechanical properties of extrudate and bar stock of commercial alloys of Mo are discussed. Although ductility in the longitudinal direction of J. WADSWORTH and C.M. PACKER are with the Lockheed Palo Alto Research Laboratory, 3251 Hanover Street, Palo Alto, CA 94304; P.M. CHEWEY is with Lockheed Missiles & Space Co., Inc., 1111 Lockheed Way, Sunnyvale, CA 94086; and W.C. COONS is Metallographic Consultant, 533 Water Witch Way, San Jose, CA 95117. Manuscript submitted January 13, 1984. METALLURGICALTRANSACTIONS A
these alloys is usually excellent, extremely poor transverse ductility is always observed. This latter property can be the limiting factor in components used in aerospace applications such as liners, washers, flanges, nuts, sleeves, pistons, etc. Traditionally, these components are manufactured by machining from available product forms such as extrudate or bar stock. Acceptability tests for such product forms are, however, based on tensile tests on specimens machined from the longitudinal direction. In service at ambient temperature, the components described above are often stressed in a circumferential or radial manner, and it is in these cases that brittle failures can occur
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