Phase Equilibria Studies of Hf 44.5 Ti 5 Cu 27 Ni 13.5 Al 10 and Hf 44.5 Nb 5 Cu 27 Ni 13.5 Al 10
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Phase Equilibria Studies of Hf44.5Ti5Cu27Ni13.5Al10 and Hf44.5Nb5Cu27Ni13.5Al10 Robert H. Woodman1 and Laszlo J. Kecskes2 1 Dynamic Science, Inc. 2 Weapons and Materials Research Directorate, U.S. Army Research Laboratory AMSRD-ARL-WM-MB Aberdeen Proving Ground, MD 21005-5069 ABSTRACT We report the equilibrium phase assemblage of two bulk-metallic-glass-forming alloys, Hf44.5Ti5Cu27Ni13.5Al10 and Hf44.5Nb5Cu27Ni13.5Al10. Using thermal analysis, scanning electron microscopy, and x-ray microanalysis, we examined the phase equilibria of the major components - Hf, Cu, and Ni. We then examined the effect of additions or substitutions of Nb, Ti, and Al on equilibrium phase assemblage and melting behavior. INTRODUCTION In order to develop an alloy with glass-forming ability similar to Zr57Nb5Cu15.4Ni12.6Al10, but with a density of 11,000 kg/m3 or greater, we first examined Hf-substituted versions of glassforming Zr alloys1,2. We observed that many of the Zr alloys had a Zr:Cu:Ni ratio near a ternary eutectic composition in the Zr:Cu:Ni system.3 This is consistent with longstanding observations about glass formation in metals 4,5 and lead to an hypothesis - if the composition of a corresponding eutectic in the Hf-Cu-Ni system were identified, the glass-forming ability of the Hf alloys could be improved. Unable to locate any studies of the Hf-Cu-Ni system, we investigated it using differential thermal analysis (DTA)6. When we found a Hf-Cu-Ni composition which melted over a narrow temperature range (< 20 K), we examined the effect on melting behavior of Ti, Nb, and Al additions to the ternary alloy. This produced two alloys Hf44.5Ti5Cu27Ni13.5Al10 and Hf44.5Nb5Cu27Ni13.5Al10, with Trg of 0.61 and 0.60, respectively3. Subsequent scanning electron microscope (SEM) examination of slow-cooled (10 K/min) DTA samples revealed aspects of the microstructure that, in conjunction with thermal analysis, may be relevant to further development of glass-forming alloys in this composition family. EXPERIMENTS Alloy ingots were prepared by arc melting elemental metals under a Ti-gettered, partial-vacuum argon atmosphere. Prior to melting, hafnium and copper were pickled in a solution of 5 vol/o hydrofluoric acid, 40 vol/o nitric acid, and 55 vol/o deionized water, rinsed in ethanol, and dried. Ingots were flipped and remelted several times (typically a minimum of 6 melts). A graph of all HfCu-Ni ternary combinations prepared is in Figure 1
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Figure 1 Ternary plot of Hf-Cu-Ni ingot compositions. Quantities are in atomic percent. Thermal analysis was conducted using a Netzsch Instruments STA 409C configured with a high-temperature (1600 oC) furnace, Type S thermocouples, graphite crucibles, and an argon atmosphere. Heating rate was 10 K/min. To establish good thermal contact between crucible and sample, samples were melted and resolidified in the DTA furnace prior to the analysis scan. Samples cooled in the DTA at 10 K/min as well as furnace-cooled samples were used for microscopy studies. We used a Hitachi S-4700
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