Atomic-Scale Design for Enhanced Low Temperature Twinning in ZrCr 2 -Based Laves Phases
- PDF / 1,532,950 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 34 Downloads / 149 Views
EXPERIMENTAL PROCEDURES
The purities of the raw materials used in the present work were 99.99wt.%Cr, 99.8wt.%Zr, 99.8wt.%Hf, 99.8wt.%Nb, 99.8wt.%Ta and 99.9wt.%Ti, respectively. Small ingots of 15mm diameter were prepared by the non -consumable arc-melting technique on a copper hearth in an argon atmosphere. The as-arc-melted ingots were annealed at 1373 K for 1 week in vacuum, followed by furnace cooling to room temperature. Microstructures were observed using optical microscopy (OM) and transmission electron microscopy (TEM). The volume fractions of constituent phases were measured using a computer-aided image analyzer. Phase analysis was performed using x-ray diffraction (XRD). Compression specimens with a dimension of 2x2x5 mm were cut by electro-discharge machining and mechanically polished using SiC paper and alumina powder. Compression tests were carried out in room-temperature air at an initial strain rate of 4.2x10 4 s-. To observe the deformation microstructure, TEM thin foils were prepared from the deformed specimens by twin-jet electropolishing in a solution of 5%HC10 4 and 95%CH 3OH at about 240K. TEM observations were carried out using a JEOL-4000 operated at 400 kV. RESULTS AND DISCUSSION Microstructures and Phase Analysis In Figure 1, optical micrographs of the microstructures of the Hf-alloy (a and b of the figure) and the Ta-alloy (c of the Figure) are shown.
Figure 1 Optical micrographs of the microstructures of the Zr10Cr80Hff0 (a), Zrl 5Cr 75Hf10 (b) and Zr10Crs0Tal 0 (c) alloys heat treated at 1373 K for 1week. The hypoeutectic microstructures are composed of a primary Laves phase and a eutectic structure of fine Laves phase with a Cr-rich solid solution. The result is consistent with the binary alloy phase diagrams (Zr-Cr, Hf-Cr, Ta-Cr), which all contain a eutectic transformation to the Cr side of the binary Laves phase[10]. With the exception of the Ti alloy system, all of the ternary systems are expected to have a eutectic reaction to the Cr side of the Laves phase field. With decreasing Cr content, the volume fraction of the Laves phase increased markedly due to the increasing volume fraction of the primary Laves phase and decreasing eutectic volume fraction.
100
cc
ZrlOCr8ONblO
ZrlOCr8OTalO
0
20
30
40
A" N
A
Zr5C7T•1
50
60
70
80
90
Degree (20) Figure 2 XRD spectra for four Laves phase alloys heat treated at 1373 K for lweek. Note that the indicated arrow corresponds to the C36 106 peak and the C14 103 peak. The results indicate that the alloys are actually composed of the three Laves structures (C15, C14 or C36) and the Cr-rich bcc solid solution. While most diffraction peaks are common to the three Laves phases, the 106 peak of the C36 phase and the 103 peak of the
C14 phase allow their presence to be uniquely determined. It is well known that AB2 Laves phase alloys with the C15, C36 or C14 structures frequently display a phase transformation from one structure to the other with temperature change. In most Laves alloys, the C14 structure is stable at high temperatur
Data Loading...
