合金元素微調對CM 247 LC超合金細晶鑄件影響之研究

Abstract

An investigation on fine-grained casting process and mechanical behavior of CM 247 LC superalloy was carried out in this study. The alloy was remelted and cast to obtain the desired fine-grain test bars by controlling casting parameters. The objective of this study is to estimate the feasibility of producing a fine-grain CM 247 LC superalloy and improving its mechanical properties. After casting, the specimens were through the processes of hot isostatic pressing and vacuum heat treatment. Then, the effect of heat treatment and Zr, B and Re minor addition were investigated. This research employs the heat treatment at various temperatures to change the carbide characteristics and alloy microstructure. Tensile test results reveal that the yield strength of the alloy after multi-step 1254℃/2hr solution-treatment is higher than those after 1221℃/2hr solution-treatment. Under the creep condition of intermediate temperature and high stress (760℃/724MPa), the fine-grain alloy solution-treated at 1221℃ shows greater creep life and elongation than those solution-treated at 1254℃. The reason for improved mechanical properties is related to the formation of γ' film coated on the GB carbide which is beneficial to the stress accommodation at GB and retards the crack initiation and propagation. On the contrary, the alloy solution-treated at 1254℃ demonstrates better creep life at high temperature and low stress (982℃/200-345MPa). This is due to the high volume fraction of γ', proper γ' size and refinement of carbide, which inhibit the gliding and climbing of dislocations. This study investigates the effect of Zr minor additions, with Zr content from 0.015 to 0.15wt%, on microstructure and mechanical properties of fine-grain (70μm) CM 247 LC superalloy. Tensile test results indicate that minor addition of Zr to fine-grain CM 247 LC superalloy can dramatically improve the yield strength at 25℃ as well as both the yield strength and the elongation at 760℃. Under creep conditions of 760℃/724MPa or 927℃/345MPa, the Zr additions drastically increased the rupture life, creep rate and elongation. The AES and EPMA observation reveal that Zr may enrich at grain-boundary and boride/matrix interface, and dissolve in matrix, carbide or γ' phase. Thus, Zr may change the primary MC carbide characteristics and inhibit the script-like MC carbide formation. Moreover, Zr is apparently to increase the effects of the cohesive energy of both the precipitated phase/matrix interface and the grain boundaries, also it is beneficial to stress accommodation and retards the crack initiation and propagation. In the study of boron minor addition effect, the content of boron ranges from 0.015 to 0.15wt﹪in CM 247 LC superalloy. The AES and EPMA observation reveal that boron may enrich at grain-boundary and γ' phase/γ matrix interface and form boride or BC compound at grain-boundary. Thus, boron may change the primary MC carbide characteristics and inhibit the script-like MC carbide formation. Moreover, boron is apparently to increase the effects of the cohesive energy of both the precipitated phase/matrix interface and the grain boundaries, also it is beneficial to stress accommodation and retards the crack initiation and propagation. Tensile test results indicate that minor addition of boron to fine-grain CM 247 LC superalloy can dramatically improve the yield strength and the elongation at 760℃. Under creep conditions of 760℃/724MPa, 927℃/345MPa or 982℃/200MPa, the boron additions drastically increase the rupture life and elongation. Thus, boron may change the primary MC carbide characteristics and inhibit the script-like MC carbide formation. Moreover, boron is apparently to increase the effects of the cohesive energy of both the precipitated phase/matrix interface and the grain boundaries, also it is beneficial to stress accommodation and retards the crack initiation and propagation. This study also investigates the effect of Re minor additions, with Re content from 0.0 to 3.0wt%. Tensile test results indicate that addition of Re to fine-grain CM 247 LC superalloy can dramatically improve the yield strength at 25℃and 760℃. Under creep conditions of 982℃/200MPa or 927℃/345MPa, the Re additions (1.0wt％) drastically increased the rupture life and creep rate. The reason for improved mechanical properties is related to the solid solution strengthening effect of Re in the matrix and γ' phase. The high melting points and low diffusion coefficients of Re lead to an increase in melting temperature of the superalloy, as well as reduced velocity of γ' morphology changes, thereby hindering dislocation movement. Finally this investigation indicates that the CM 247 LC superalloy has an excellent castability to form a fine grain structure. In addition, this alloy exhibits superior creep behaviors at various operation environments after a suitable heat treatment. Moreover, the optimal Zr, B, and Re additions promote the performance of fine-grain CM 247 LC superalloys in small gas turbine engines, by increasing yield strength at operating temperature of the hub, protecting the parts from bursting, and increasing creep strength under operating condition of the blade.