Microstructures and Mechanical Properties of Two-Phase Alloys Based on NbCr 2
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The same alloy composition was prepared by arc-casting for the melt-spin process. Pieces were then loaded into a BN crucible. The melt-spinning chamber was evacuated to < 1.3 Pa and back-filled with ultra-high purity helium three times. The operating pressure in the chamber was -8.4 x I04 Pa. Using induction heating, the alloy was heated (within five minutes) to the pour temperature of 1800'C. A helium gas pressure of 20.7 x 103 Pa ejected the molten alloy through the orifice onto the spinning copper wheel to form ribbons. Partial reaction of the crucible resulted in boron additions in the alloy. The meltspun ribbons were chopped into smaller pieces by a blender and pressed into a Ta can. The hot isostatic pressing (HIP) procedure involved initially pressurizing the chamber to 7 ksi (50 MPa). Based on the alloy decomposition signatures from differential thermal analyses (DTA), the material was ramped up to 1300'C in 90 minutes, held for an hour at pressures of about 30 ksi (200 MPa), and then furnace cooled. The alloys were characterized by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction (XRD). Compositions were analyzed by electron microprobe, energy dispersive spectroscopy (EDS), and atom location by channeling enhanced microanalysis (ALCHEMI). Quasi-static compression tests were conducted on cylindrical samples (5 mm x 5 mm), and at least two samples were used for each test temperature. Tests within the temperature range of 25°C-600°C were performed in air, while tests at the higher temperatures were done in vacuum. A strain rate of 0.001/sec was used. RESULTS AND DISCUSSION Microstructures
The arc melted alloy retains a metastable microstructure of supersaturated bcc phase and roughly 25 vol% Laves phase. The Laves phase forms an almost continuous network of small particles (900'C), precipitation of the Laves phase occurs within the bcc grains. The melt-spun alloy experiences extremely fast cooling rates, and is an attempt to use metastable solidification pathways to create a compositionally-homogeneous bcc solid solution [9]. A cross-sectional view of the melt-spun ribbon is shown in Figure 2, and depicts the fine-scale of the microstructure. The ribbons have a thickness of about 50 rtm, and display submicron dendritic structure on the "air side" of the ribbon. The "chill side" of the ribbon is almost entirely metastable bcc. The lattice constants of the bcc phase give some indication of the degree of supersaturation (Table I). Since the Cr atom is the smallest of the ternary elements, Cr supersaturated in the bcc phase (as opposed to forming the more complicated (Nb,Ti)Cr 2 Laves phase) results in a smaller bcc lattice constant. Upon annealing treatments or testing at elevated temperatures, the amount of Laves phase increases by precipitation, while the bcc composition becomes Cr-poor and the bcc lattice constant increases. Microprobe, EDS, and ALCHEMI work confirm the analysis [5].
Figure 1. Backscattered electron image of the as-cast a
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