In this paper, the TCP phase in the second-generation Ni-based superalloy DD5 is studied and the precipitation and evolution of the TCP phase are found and summarized. However, there is little work on the detailed precipitation behavior of the P phase and R phase. Many structural defects have been found in the phase in some superalloys or intermetallic compounds, such as stacking faults, twin bands, microtwins, and second-phase symbiosis. In the past few decades, the precipitation behavior of the phase has been extensively studied. The antiprism can usually be regarded as some simple structural units, and using these structural units to characterize the structure of the TCP phase will greatly simplify the process of analysis. The structure of the TCP phase is composed of pentagonal or hexagonal antiprism arranged side by side. These so-called Kasper polyhedrons have equilateral triangle surfaces and four atomic coordination numbers of 12, 14, 15, and 16, but in practice, polyhedron surfaces often deviate from equilateral triangles. Only by knowing more about the precipitation rules and characteristics of the TCP phase, we can avoid the precipitation of the TCP phase reasonably and effectively, so as to better optimize the design of the alloy.įrank and Kasper first investigated the crystal structure characteristics of TCP phases and characterized the TCP phase by coordination polyhedron.
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So how to avoid the precipitation of the TCP phase is an important aspect of alloy design. It is generally accepted that the TCP phase will deteriorate the creep properties of the alloy. The TCP phases of Ni-based single-crystal superalloys are, , P, and R. Because of their special dense structures, these phases are generally referred to as the topologically close-packed (TCP) phase. The addition of excessive alloy elements such as Cr, Mo, W, and Re to the superalloy will cause the precipitation of intermetallic phases which have complex crystal structures rich in these refractory elements during long-term heat exposure or service. However, the addition of excessive refractory elements will significantly reduce the stability of the alloy structure. In particular, Re is recognized as the alloy element with outstanding strengthening effect. Presently, the known refractory elements such as Cr, Mo, W, and Re are good creep-strengthening elements. In order to continuously improve its high temperature creep resistance, more and more refractory elements have been added to the superalloy. Nickel-based single-crystal superalloys have been widely used as jet engines and industrial gas turbine blades. Superalloy is a kind of high-alloying iron-based, nickel-based, or cobalt-based metal material, which can withstand large complex stress above 600☌ and has certain surface stability. It is difficult to precipitate at the interdendrite/grain boundary, which is caused by the segregation of the constituent elements of the TCP phase to the dendrite. The TCP phases precipitate preferentially in the dendrite.
Furthermore, during long-term aging, the needle-like TCP phases gradually decompose and transform into globular and short rod-like phases. The angle between needle-like TCP phases and phase channels is 45°, but the angle between the needle-like TCP phases of the adjacent grains is equal to the misorientation of the adjacent grains.
The results showed that the crystallographic orientations are consistent with that of adjacent grains however, the direction of the needle-like TCP phases is not consistent with that of the phase channels. The precipitation behaviors of the topologically close-packed (TCP) phases in the bicrystal DD5 superalloy have been investigated.
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