Microstructure and Mechanical Properties of Two-Phase Cr-Cr 2 Ta Alloys
- PDF / 899,852 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 53 Downloads / 214 Views
I.
INTRODUCTION
THE alloys based on refractory metal Laves phases have been identified as having potential as high-temperature structural materials and, as such, have been the subject of extensive research.[1–5] However, the desirable high-temperature strength of the refractory metal Laves phases is accompanied by low fracture toughness values (~1.5 MPam),[6,7] which makes them unacceptable as monolithic materials in structural applications. Efforts to mitigate this deficiency have focused on coupling with a tougher phase, typically a metallic solid solution that exists in thermodynamic equilibrium with the Laves phase. Among these materials, attention has been focused on the alloys comprising a chromium solid solution and a chromium-rich refractory metal Laves phase in the hope of realizing a material that also possesses adequate oxidation resistance.[8–10] The systems investigated to date include Cr-Cr2Nb,[11–13] Cr-Cr2Ta,[14,15] Cr-Cr2Hf,[16,17] and Cr-Cr2Zr.[18] Typically, these materials exploit the eutectics that exist between the two principle phases to produce the microstructures comprising a lamellar eutectic intergrowth of the Cr-solid solution and the Cr2X Laves phase. They may also be directionally solidified to produce in situ composites. However, compositions including proeutectic phases have also been considered. The constituent Laves phase in these alloys might exist in three major polytypic forms viz. the hightemperature hexagonal C14 (prototype Zn2Mg) with a XY’XY’…-type stacking sequence, the low-temperature cubic C15 form (prototype Cu2Zn) with a XYZXYZ…-type AYAN BHOWMIK, Ph.D. Student, and HOWARD J. STONE, Assistant Director of Research, are with the Rolls-Royce UTC, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted July 8, 2011. Article published online March 23, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
stacking, and an intermediate dihexagonal C36 (prototype Ni2Mg) having XY’X’Z…-type stacking (C14, C15 and C36 are the Structurbericht notations of the crystal structures and may, alternatively, be referred to using the Ramsdell notation, as 2H, 4H, and 3C, respectively). Based on the periodicity in the stacking sequence, other complex or intergrown polytypes may form, namely 6H, 8H, 5R, 9R, etc.[19,20] Transformation from the C15 to C14 structures or vice versa has been suggested to be assisted by a synchroshear mechanism and takes place via the formation of the C36 structure.[19,21] Depending on the extent of transformation, other complex intermediate polytypes of the Laves phase may also form.[20] The synchroshear mechanism does not, however, explain the varying kinetics of the transformation observed in different systems. For example, two-phase Cr-Cr2Nb and Cr-Cr2Ti alloys show a completely transformed twinned C15 structure in the as-cast condition and after annealing at 1273 K (1000 C) for 24 hours respectively. However in other systems, the hexagonal form is metastably re
Data Loading...
