The effect of temperature and extrusion speed on the consolidation of zirconium-based metallic glass powder using equal-
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I.
INTRODUCTION
OVER the last two decades, the science and engineering of metallic glasses has progressed rapidly. The promising characteristics of this class of materials include high tensile strength, hardness, corrosion resistance, and impact fracture energy, as well as viscous deformability.[1,2] Metallic glasses are appealing materials for applications in many commercial industries, including medicine, sports, telecommunications, and machining.[2] It has also been shown that metallic glasses fail by shear localization during ballistic impact conditions, making them appealing for kinetic-energy-penetrator applications.[3] Considerable efforts have been made to produce bulk metallic glasses (BMGs). In the past, it was only possible to produce metallic glasses as thin ribbons due to the rapid cooling rates necessary to achieve an amorphous state. In the last decade however advances in the selection of compositions of glass-forming alloys have made it possible to produce cast bulk materials. Some Zr- and Cu-based alloys can achieve vitrification with cooling rates as low as 1 K/s.[1] However, the dimensions achievable with casting methods are still considerably smaller than what is required for some structural applications, where the use of BMGs is appealing. One technique for the fabrication of bulk glasses is powder consolidation. The potential of consolidation lies in the fact that this method does not require a substantial thickI. KARAMAN, Assistant Professor, J. ROBERTSON, J.-T. IM, and S.N. MATHAUDHU, Graduate Research Assistants, and K.T. HARTWIG, Professor, Department of Mechanical Engineering, and Z.P. LUO, Research Scientist, Microscopy and Imaging Center, are with Texas A&M University, College Station, TX 77843. Contact e-mail: [email protected] Manuscript submitted December 30, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
ness of amorphous metal to cool rapidly from the melt. There are two methods that have been used to consolidate metallic glass powder: hot pressing and conventional areareduction extrusion. In the case of hot pressing, it is difficult to achieve full density on large dimensions.[4,5] Conventional extrusion, on the other hand, has shown promise for fabricating bulk material that is near-full density, has good particle-to-particle bonding, and exhibits tensile strengths comparable to cast amorphous material.[4,5,6] However, the area reduction required to achieve sufficient bonding between particles may be a limiting factor for conventional extrusion consolidation. As an alternative method of powder consolidation, the authors have recently utilized equal-channel angular extrusion (ECAE) to consolidate Cu-based metallic glass powders in the supercooled liquid region.[7] The ECAE method subjects a volume of material to simple shear deformation by forcing it through two intersecting channels. This has some advantages over conventional extrusion: the workpiece experiences nearly uniform strain, the cross section of the workpiece remains constant, and ECAE can be accomplished at lo
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