Deformation of Two-Phase Alloys Based on C15 Laves Phase HfV 2 + Nb
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posed as the operative deformation mode in the C15 Laves phase, although, the twinning mechanism has not yet been clearly identified. Since atomic shuffles are unlikely to be operative at very low temperatures, if deformation twinning occurs in this temperature regime it is unlikely to be the result of shuffles and therefore the synchroshear mechanism is the more reasonable mechanism.
V Alloy MPl:14Hf 64V 22Nb Matrix:Cl55
A Alloy MP3:
~
16Hf 70V14Nb
Alloy MP2:6.5Hf 68V 25.5Nb Matrix: (V. Nb)
o
fC
"
*O
C5
17 "f57V 26Nb
2
NI..
... 0
W
2....... Is o t he rm a t 12 7 3 K
Chu, 1993
-I /,F. Hf
60
50
40 30 20 ,- Hafnium (at%)
10
0
Nb
Figure 1. Isotherm of the Hf-V-Nb ternary system at 1273 K evaluated by Chu et al. [2], and back-scattered electron images of the C1 5/bcc two-phase alloys, MP1 and MP2.
EXPERIMENTAL PROCEDURES
Alloys were prepared by arc melting in an argon gas atmosphere using high purity raw materials, 99.9% Hf, 99.7% V, and 99.9% Nb. Microstructural observation and phase identification were conducted by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy. The mechanical behavior of the C 15/bcc two-phase alloys was investigated using compression tests at temperatures ranging from room temperature down to 4.2 K. The compression tests were performed on an Instron universal testing machine equipped with a double-dewar for immersing the sample in a constant temperature bath. The test temperature was monitored by carbon resisters attached to the apparatus above and below the specimen and with a thermocouple on the specimen. The compression test specimens of dimensions 3 x 3 x 6 and 2.5 x 2.5 x 5 mm were prepared by electron-discharge machining (EDM). The strain rates were 7.1 and/or 8.6 x 10-4 s-1 . The microstructures of deformed and as-homogenized materials were observed by means of transmission electron microscopy (TEM), using an accelerating voltage of 400 kV or 200kV. Thinning and perforation of EDM-cut disks was accomplished using a conventional argon ion mill operating at 77 K or on a Precision Ion Polishing System (PIPS) operating at ambient temperature. The argon ion gun emission angle on the PIPS was 4 degrees with an energy of 5 keV. Prior to final thinning on the ion mill or the PIPS, the disks were mechanically polished and dimpled on one side until the center of dimple was approximately 30 g.m thick. Phase relations involving C 15 HfV2+Nb and bcc (V, Nb) solid solution were investigated using optical and scanning electron microscopic observations and the differential thermal analysis (DTA) on the alloys. The DTA measurements were performed in an argon gas atmosphere at heating and cooling rates of 2 to 10 K/min. KK7.5.2
RESULTS AND DISCUSSION The C15/bcc two-phase microstructure
The isotherm at 1273K of the Hf-V-Nb ternary system, from Chu et al.[2], is shown in FIG.1. The compositions of the alloys tested in the current study are shown on the diagram. The wide compositional range of the C15/bcc two-phase field allows us to select a wide range
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