Degeneration of Primary MC Carbide in a Cast Ni-Base Superalloy

Abstract

During long-term thermal exposure, the degeneration of primary MC carbide is a diffusion-controlled process. With the element exchange between the primary MC carbide and the γ matrix, three MC degeneration reactions, that is, γ + MC → γ' + M23C6 (Reaction I), MC + γ →η+ M23C6 (Reaction II) and MC + γ →η+ M6C (Reaction III), sequentially operates. Of them Reaction III has never been reported in the previous references and is kinetically most difficulty. It is also shown that the grain boundary MC decomposes much more rapidly than the grain interior MC and that Reaction III is not as often seen at the grain boundary as in the grain interior.

Effects of Long-Term Thermal Exposure on the Microstructure and Properties of a Cast Ni-Base Superalloy

“…During long-term thermal exposure, these carbides deteriorates severely and various transformation products are sequentially present on their peripheries with exposure, as illustrated in Figure 4. [15] Initially, a thin layer of c¢ decorated with small and discrete Cr-rich M 23 C 6 particles emerges at the MC/c interface, suggesting that a classical decomposition reaction, that is MC + c fi M 23 C 6 + c¢, is operative there (Figure 4(a)). As the reaction proceeds, the MC acts as the source of C and Ti, while the c matrix serves as the source of Ni, Al, and Cr.…”

“…[21] However, the following aspects determine the inappreciable influence of M 23 C 6 on mechanical properties: (1) the population of M 23 C 6 is small, less than 1.3 pct in volume fraction; (2) the precipitation and coarsening of M 23 C 6 -c¢ cells remove much of solid solution strengthening elements, that is, Cr, W, Mo, Al, and Ti, from the c matrix and weaken the alloy; [5,22] and (3) M 23 C 6 particles mainly distribute in the interdendritic regions and often crowd in the form of loops, chains, or clusters, as demonstrated in Figure 6, so that their resistance to dislocation motion is discounted severely. …”

“…Their average size is both temperature and time dependent: For a given exposure temperature, the size increases and gradually approaches a critical value with time; and, the higher the exposure temperature, the larger the critical size. [21] The precipitation of M 23 C 6 carbides rich in Cr, W, and Mo leads to the local enrichment of c¢ forming elements, such as Ni, Al, and Ti and then the presence of the c¢ precipitates, forming an M 23 C 6 -c¢ eutectoid cell. [21] In the cell, initially the M 23 C 6 carbide is dendrite-shaped and the c¢ precipitates are embedded in the carbide dendrite.…”

“…[21] The precipitation of M 23 C 6 carbides rich in Cr, W, and Mo leads to the local enrichment of c¢ forming elements, such as Ni, Al, and Ti and then the presence of the c¢ precipitates, forming an M 23 C 6 -c¢ eutectoid cell. [21] In the cell, initially the M 23 C 6 carbide is dendrite-shaped and the c¢ precipitates are embedded in the carbide dendrite. Under different thermal exposure conditions, the M 23 C 6 carbide undergoes various degrees of morphological changes: At 800°C, it basically exhibits a dendritical appearance through the entire period of exposure; while at 850°C, its morphology tends to transform from flowerlike dendrite to irregular block.…”

“…Under the applied load, these interfaces, because of high stress concentration and interfacial energy, tend to become the favored initiation sites and propagation paths of microcracks. [15] Because microcracks promote the fracture of superalloys and contribute to the reduction of service life, [5,[18][19][20] MC degeneration, which results in more frequent cracking than the intact MC, is undoubtedly deleterious to the thermally exposed K452 alloy. A considerable amount of degenerated, coarse, and irregular MC carbides distributed at the GBs are especially more detrimental than when they are in the GIs.…”