The TCP phase is closely packed by different atoms in size, in comparison with the structure which is closely packed by the atoms in same size. In crystallography, four different coordination polyhedrons with coordination numbers 12, 14, 15 and 16 respectively, pack closely to form the TCP phase. Intermetallic compounds with TCP structure have been extensive application these years. But the TCP phases precipitated in superalloy and alloy steel are usually hard 、 brittle and effect the plastic properties of these materials detrimentally, so the formation of TCP phase must be avoided. Since 1970, the intermetallic compounds as new functional materials and structure materials have been deeply investigated. Nowadays the laves phase compounds have been chosen as high temperature structure materials because of their high melting point 、good high temperature intensity 、 high oxidation resistance and creep resistance.
SEM observation shows that a large amount of σ phase precipitate from the superalloy K44 aged at 800℃ for 10000h. These σ phases dispersed homogeneously in the matrix with needle-like shape. EDS analysis shows that σ phases enrich more Cr and Co than matrix. From HREM analysis, it is learned that the σ phases have two intergrowth phases: H phase and F phase, and the relationships between them have been determined. Translation domains and rotation domains are found in σ phases and a systemic analysis has been done to these translation domain boundaries and rotation domain boundaries. EDS results show that more heavy elements segregate at the domain boundaries than other areas.
The Cr2Ta sample was prepared by vacuum furnace melting. The XRD results show that the structure of sample is C14, and there are no C15 and C36 diffraction apexes detectable. Atomic projection of hexagonal Cr2Ta Laves phase has been directly imaged by Z-contrast imaging at an atomic scale, which provides much more detailed information compared with the conventional coherent high-resolution imaging. Stacking faults with sequence of …stst’st’stst’…are identified, where the three-layer stacks (Ta-Cr-Ta) switch from t to t’ type.
修改评论