Hot explosive compaction of Mo-Ti alloys
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I. INTRODUCTION
REFRACTORY alloys continue to present considerable challenges to the powder metallurgy industry. Because of their high melting points, alloys, especially those with transition metal elements, are most effectively produced by liquidphase sintering (LPS).[1] Over the last 10 to 15 years, LPS has focused on the development of two-phase tungsten (W) heavy alloys with a nickel-iron or copper-nickel matrix for use in military applications. Renewed interest in the extension of LPS technology has centered on matrix metals such as titanium (Ti), Ti alloys; zirconium (Zr), hafnium (Hf), and steel.[2] However, brittle intermetallics in the systems with Zr, Hf, and, iron[3] limit their applicability and utilization. In addition to LPS, solid-state sintering, extrusion, and mechanical alloying,[4] the use of explosive compaction to consolidate and sinter the powdered precursors has been examined.[5] During the past three decades, explosive compaction has been extensively applied to metals and ceramics. When an explosive is detonated near a powder sample, a shock wave propagates through and densifies the powder bed. Bonding is produced via localized heating between adjacent particles as they move past one another. Due to the rapid passage of the shock wave (v 5 36,000 to 8,000 m/s), the samples often suffer from low nonuniform densities, poor interparticle bonding, and severe cracking. The rise in temperature associated with the irreversible work occurring during the consolidation of distended solids has frequently been found to be insufficient to develop adequate bonding.[6] To overcome some of these problems, hot explosive compaction (HEC) has been suggested,[7–12] where preheating is used to decrease the yield strength of the powder, thereby improving its ductility. The W-Ti alloy billets have been fabricated with a variant of HEC at the U.S. Army Research Laboratory.[5] This technique, referred to hereafter as combustion synthesis assisted LASZLO J. KECSKES, Research Physical Scientist, is with the U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5066. Manuscript submitted September 9, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
(CSA)-HEC, uses an in situ self-propagating high-temperature (SHS) reaction (i.e., “chemical furnace”) to preheat the powder sample prior to compaction. The chemical furnace is a highly exothermic mixture of reactant powders that, when initiated, will generate heat and provide high temperatures. Self-propagating high-temperature synthesis has been extensively researched in the former Soviet Union, Japan, and the United States.[13,14,15] Chemical heating[16] and thermite mixtures[17] have already been used to heat samples prior to shock loading. Admixed exothermic mixtures have also been shock initiated to locally enhance sample properties.[11,12] The CSA-HEC experiments at ARL focused on the 95W5Ti wt pct (83W-17Ti at. pct) alloy because of its moderately high density, 16.55 g/cm3. The billets could be divided into an inner region surrounded by an annular periph
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