Browsing by Author "Wei, Chao-Nan"

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Effect of hot isostatic pressing on microstructure and mechanical properties of CM-681LC nickel-base superalloy using microcast
(2008-01-01)
Wei, Chao-Nan
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Bor, Hui-Yun
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Chang, Li
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材料科學與工程學系
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Department of Materials Science and Engineering
This study investigates how hot isostatic pressing (HIP) affects the microstructure and fracture modes of CM-681LC superalloy. As-cast test bars with grain size of 80 mu m were prepared using the fine-grain process followed by HIP. Experimental results indicate that micropores formed during solidification and contraction degrade the tensile strengths and elongations of the fine-grain CM-681LC superalloy before HIP. The area fraction of micropores was reduced from 0.2% to 0.06% following HIP. Scriptlike MC carbides decompose into particlelike M23C6 carbides during HIP, revealing that HIP refines and spheroidizes the carbides. Eliminating the micropores and refining the carbides increases 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 scriptlike carbides at the grain boundaries are the main causes of the fracture of the test bars before HIP. The fracture mode of the fine-grain test bars following HIP, according to the tensile test, 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.
Influence of microstructure and its evolution on the mechanical behavior of modified MAR-M247 fine-grain superalloys at 871 degrees C
(2012-03-30)
Liao, Jian-Hong
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Bor, Hui-Yun
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Wei, Chao-Nan
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Chao, Chuen-Guang
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Liu, Tzeng-Feng
This study added Re to Mar-M247 fine-grain superalloy and investigated the influence of microstructure and its evolution on the mechanical behavior at 871 degrees C. Mar-M247 and modified Mar-M247 alloys consisted of dual precipitation of primary and secondary gamma' particles in the gamma matrix. An increase in Re content led to a reduction in the primary gamma' phase with the gamma' phase becoming finer; however, the addition of Re did not have an obvious influence the secondary gamma' phase. Tensile and creep tests (871 degrees C/379 MPa) showed that both tensile and creep strength increased with an increase in Re content up to a maximum of 3 wt%. In the early creep stage, the addition of Re improved creep resistance by increasing the strength of the gamma matrix and decreasing the inter-particle spacing. In the later creep stage, the linear fraction of the gamma' raft in stress axial direction increased following the addition of Re, resulting in improved creep resistance. Observation of microstructure evolution indicated that only secondary gamma' phase coarsened directionally into a rafting structure and the primary gamma' phase was unaffected by creep. An interrupted creep tests verified that the gamma' rafting structure initiated in the primary creep stage and completed in the secondary creep stage. The addition of excessive quantities of Re, such as 5 wt%, resulted in the formation of needle-like P phase, which damaged the tensile and creep properties. During tensile and creep tests, cracks initiated and propagated along grain boundary (GB) in alloys containing 0-3 wt% Re; and propagated along both the GB and P/gamma interface in an alloy containing 5 wt% Re. In conclusion, optimal results were obtained for fine-grain Mar-M247 following the addition of 3 wt% Re. (C) 2012 Elsevier B.V. All rights reserved.
The effects of carbon content on the microstructure and elevated temperature tensile strength of a nickel-base superalloy
(2010-06-25)
Wei, Chao-Nan
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Bor, Hui-Yun
;
Chang, Li
;
材料科學與工程學系
;
Department of Materials Science and Engineering
This study investigates how carbon content affects the microstructure and elevated temperature tensile strength of fine-grain CM-681LC superalloy. Experimental results indicate that increasing the carbon content from 0.11 to 0.15 wt% increases the total area fraction of carbides considerably. However, 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 gamma-gamma' eutectic phases from 8.6 to 5.3% because the eutectic phase forming elements are consumed by (Ta, Hf)C carbides and the carbides occupy the position of gamma-gamma' eutectic phases during the solidification. Further, the carbon addition improves the mechanical strength by about 6% and the elongation by about 50% in tensile tests at 760 degrees C and yields favorable tensile performance at elevated temperature. (C) 2010 Elsevier B.V. All rights reserved.
The influence of carbon addition on carbide characteristics and mechanical properties of CM-681LC superalloy using fine-grain process
(2011-05-05)
Wei, Chao-Nan
;
Bor, Hui-Yun
;
Chang, Li
;
材料科學與工程學系
;
Department of Materials Science and Engineering
This study investigates how carbon addition affects the carbide characteristics and mechanical properties of CM-681LC nickel-base superalloy with grain size of 80 mu m prepared using the fine-grain process. Experimental results indicate that carbon addition from 0.11 wt% to 0.15 wt% greatly increases the amount of carbides, but the shape and size of carbides are similar due to the short solidification time of the fine-grain process, which limits the growth of carbides. The increase of carbides by proper carbon addition effectively improves the tensile strength by about 5% and the tensile elongation by over 30% at room and moderate temperatures (21-760 degrees C). The fracture analyses reveal that the carbon addition in fine-grain CM-681LC superalloy can changes the fracture modes from typical intergranular fracture modes to transgranular and intergranular mixed modes. The better carbon content of CM-681LC superalloy applied in the fine-grain process is 0.15 wt%, which has better mechanical properties. (C) 2011 Elsevier B.V. All rights reserved.