| 其他摘要 | In this dissertation, precipitation behavior and role of the Mu (a topologically close packed (TCP) phase-μ) phase are investigated with the well-known commercial single crystal (SX) superalloy CMSX-4 and three candidate SX superalloys developed by IMR and KIMS. The candidate alloys were designed by varying the contents of Re, W and Cr to some extent with respect to the composition of the alloy CMSX-4. The candidate alloy R0 was obtained by substituting 1.5%wt. Cr and 3%wt. W for the 3%wt. Re in alloy CMSX-4, while the candidate alloy R1 was made by adding 1%wt. Re to alloy R0 and the candidate alloy R3 by adding 3%wt. Re. This dissertation focused on the transformation process of μ, the factors (temperature-time, composition and stress) which can affect its transformation and its role during creep deformation. Many experimental facilities such as high temperature furnace, creep deformation tester, optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer and electron probe microanalysis have been used.
The μ phase was mainly composed of Ni, Cr, Co, W and Re, and W was the determinant forming element of μ phase. In the composition of μ phase, Cr and Co contents were relatively stable, while Ni, Re and W contents varied to some extent. The μ phase contained more refractory alloy elements at higher temperature or after longer thermal exposure than at lower temperature or after shorter thermal exposure. W and Re located the A positions of μ prototype B7A6, Cr and Co the B positions and Ni both the A and B positions.
Addition of the Re increased the phase instability of the alloys but did not change the type of the precipitation. All the TCP phase found in the alloys was characterized as μ phase. Moreover, addition of the Re increased the forming temperature of μ phase, accelerated the precipitation of μ phase near nose temperature (TTT curve) and at low temperature (850oC), but relatively depressed the precipitation of μ phase between these temperatures due to the low diffusivity of Re.
Except for Mo, addition of Re has not significantly changed the dendrite segregation of other alloying elements. However, it could stabilize or increase the segregation abilities of other alloying elements in the γ/γ΄ phases. With adding Re to the alloy, the W content increased in γ phase, but the Cr content decreased, which is the reason that addition of the Re enhanced the phase instability of the alloys.
The γ' evolution could affect the nucleation, growth morphology, and the fracture behavior during creep deformation of the μ phase. Nucleation of the μ phase in superalloys depended more on the temperature than growth of the μ phase. Therefore, at low temperature the μ particles presented large plate-like morphology due to low nucleation rate and high growth rate, but at high temperature rod-like and blocky μ particles increased obviously because of the high nucleation rate and interaction of growth. Morphologies of the μ particles and the orientation relationship between them and matrix were related:
Plate-like —— <1-100>μ//<110>γ or <11-20>μ//<110>γ or <11-20>μ//<112>γ, due to relative high coherency at {0001}μ//{111}γ；
Rod-like —— nucleated and grew based on both the low index planes {001}γ/γ'、{011}γ/γ'、{112}γ/γ' or {111}γ/γ' and {1-210}μ or {1-100}μ，with growth direction of <0001>μ and <110>γ and zone of <1-100>μ was about 2.26~2.5o from zone of <110>γ due to relative low coherency at {1-100}μ与{110}γ；
Irregular blocky —— there is no specific orientation relationship.
Precipitation amount, size and morphology of the μ phase could affect the creep properties of superalloys. Precipitation of μ phase could lower the solution of alloying elements in γ matrix; big plate-like μ particle could disrupt the continuity of γ/γ′ microstructure and then caused the inhomogeneous deformation during creep; role of the rod-like μ particles during the creep deformation was dependent on the creep rate which is closely related to the main strengthening phase γ΄. The rod-like μ particles were not fractured by stress concentration and their effect on the creep properties by softening γ phase or disrupting the γ/γ΄ structure was much smaller compared to the dramatic γ΄ evolution; big sized μ particles could cause stress concentration but not strong enough to initiate the crack. The μ phase affected the creep properties at the early stage of creep rather than at the later stage. However, the μ phase could decrease the creep elongation.
During high temperature thermal exposure, sign and magnitude of the applied stress, internal stress caused by dendrite segregation and γ΄ evolution all could affect the μ phase precipitation. Due to the internal stress, precipitation of the μ phase varied significantly when much low applied stress was loaded. Later, with increasing the tensile applied stress precipitation of the μ phase relatively increased, while contrary for the compressive applied stress. Applied stress depressed or accelerated the precipitation of μ phase through influencing the lattice misfit between γ matrix and μ phase. |
修改评论