| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 99 === This study investigates the optimum procedure and compositions of CM-681LC superalloy using Microcast, and it can be divided into two parts, (1) how hot isostatic pressing (HIP) affects the microstructure and tensile properties of fine-grain CM-681LC superalloy, and (2) how carbon content affects the microstructure and mechanical proformance of fine-grain CM-681LC superalloy.
In the study of how hot isostatic pressing (HIP) affects the microstructure and tensile properties of fine-grain CM-681LC superalloy, the test bars with grain size of 80 μm can be obtained using the Microcast process followed by HIP. Experimental results indicate that micropores formed during solidification and contraction degrade the tensile strengths and elongations of the CM-681LC superalloy using Micarcast before HIP. The area fraction of micropores was reduced from 0.2% to 0.06% following HIP. Script-like MC carbides decompose into particle-like M23C6 carbides during HIP, revealing that HIP refines and spheroidizes the carbides. Eliminating the micropores and refining the carbides increase the mechanical strength by up to about 9% and the elongation by over 10% in room- and high-temperature tensile tests. The fracture analyses after tensile tests of the fine-grain test bars reveal that the microporosity and the Script-like carbides at grain boundaries are the main causes of the fracture of the test bars before HIP. According to the tensile test, the fracture mode of the fine-grain test bars after HIP, is typically intergranular because the micropores are eliminated and the carbides are refined. Since the elimination of the micropores and refinement of the carbides by HIP effectively improves the tensile strength and elongation, the fine-grain casting yields favorable mechanical properties.
In the study of how carbon content affects the microstructure and mechanical properties of fine-grain CM-681LC superalloy, the fine-grain test bars with different carbon content (0.11wt% and 0.15wt%) were fabricated using Microcast process with carbon addition. Experimental results indicate that script-like and blocky MC carbides rich in Ta and Hf and particle-like M23C6 carbides rich in Cr coexist in CM-681LC superalloys with 0.11wt% and 0.15wt% carbon. An increase in carbon content from 0.11wt% to 0.15wt% produces no apparent effect on the melting point of CM-681LC superalloy. Further, no difference in grain size can be observed in this study, and the grain sizes are both 80 μm. Increasing the carbon content from 0.11wt% to 0.15wt% increases the total area fraction of carbides from 0.91% to1.57% considerably. However, the statistical results reveal that the average carbide length increases from 12.84 to 13.82 μm, while the aspect ratio increases from 1.85 to 1.96. Microstructures in the two alloys exhibit similar carbide shapes and sizes, probably because the short solidification time in the fine-grain process limits the growth of carbides. Besides, carbon addition significantly reduces the area fraction of γ-γ’ eutectic phases from 8.6 to 5.3 % because the eutectic phase forming elements are consumed in (Ta, Hf)C carbides, and the carbides occupy the position of γ-γ’ eutectic phases during the solidification. The carbon addition improves the tensile strength by about 2～8% and the tensile elongation by over 22%. Furthermore, the carbon addition also enhances creep behavior under 982℃/200MPa; in particular, creep life almost doubles. The fracture analyses reveal that the carbon addition from 0.11wt% to 0.15wt% in fine-grain CM-681LC superalloy changes the fracture mode from typical intergranular fracture mode to transgranular and intergranular mixed modes in room- and high-temperature tensile tests. Further, the creep fracture for both alloys under 982℃/200MPa are both typical intergranular fracture. The γ-γ’ eutectic phase near grain boundaries (GBs) are the main causes of fracture of CM-681LC superalloys of 0.11wt% carbon, whereas the cracks mainly initiate along GB carbides and propagate along GBs in the superalloys of 0.15wt% carbon. From the above results, the proper carbon addition can effectively enhance the tensile and the high temperature/low stress creep performance and the fine-grain casting yields favorable mechanical properties. Hence, the carbon content of CM-681LC superalloy applied in Microcast process can increase to 0.15wt%, to improve the mechanical properties.
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