熱均壓及碳含量對微細鑄造CM-681LC超合金顯微組織及機械性能之研究

Abstract

本研究針對微細鑄造CM-681LC超合金較佳製程及合金成份進行研究探討，主要分為兩大部份：第一部份探討熱均壓對微細鑄造CM-681LC超合金顯微組織及機械性能之影響；第二部份探討為碳含量對微細鑄造CM-681LC超合金顯微組織及機械性能之影響。 在熱均壓對微細鑄造CM-681LC超合金顯微組織及機械性能之影響方面，本研究採用微細鑄造法澆鑄出晶粒大小為80 μm之細晶試桿，研究結果顯示：無熱均壓之細晶試桿，因微細鑄造製程金屬液凝固時間短，產生大量微縮孔，進而造成拉伸強度及伸長率的急遽下降。熱均壓可顯著消除細晶試桿之微縮孔，微縮孔面積分率由0.2%大幅減少為0.06%。此外，CM-681LC超合金晶界上MC碳化物經熱均壓後部份轉變成M23C6碳化物，其形態由長條狀轉變成不連續顆粒狀，顯示熱均壓有細化及球化碳化物的趨勢。由於熱均壓減少微縮孔及細化碳化物，可有效提升細晶試桿室溫及高溫之拉伸強度約9%及伸長率10%以上。在拉伸破壞模式方面，無熱均壓之細晶試桿，內部微縮孔及晶界上長條狀碳化物是造成破斷之主要因素；經熱均壓之細晶試桿，由於微縮孔大量減少及碳化物之細化，其破壞模式為典型之沿晶破壞。由本研究結果證實，熱均壓可消除微縮孔及細化碳化物，有效提升拉伸強度及伸長率，進而達到細晶鑄件優良的性質。 在碳含量對微細鑄造CM-681LC超合金顯微組織及機械性能之影響方面，本研究採用微細鑄造製程及添加碳含量之方式，澆鑄出兩種不同碳含量(0.11wt%及0.15wt%)之細晶試桿，研究結果顯示：對CM-681LC超合金而言，無論碳含量為0.11wt%或0.15wt%，其主要碳化物均為骨架型及塊狀富鉭、鉿的MC型碳化物及顆粒狀富鉻的M23C6型碳化物，且添加碳含量對CM-681LC超合金之熔點並無明顯改變，以本研究採用相同澆鑄條件而言，不會造成晶粒大小之改變，晶粒大小同樣為80 μm，但碳化物面積分率由0.91%顯著增加至1.57%。然而，由於微細鑄造製程凝固時間較短，限制碳化物之成長，統計結果顯示碳化物平均長度由12.84 μm改變為13.82 μm，而長短軸比由1.85變為1.96，因此兩種碳含量之合金具相似之碳化物大小及形狀。此外，由於共晶相形成元素(鉭、鉿)及析出位置被較多碳化物析出所消耗及佔據，因此碳含量添加造成γ-γ’共晶相含量由8.6%減少為5.3%。在機械性能方面，適度由0.11wt%添加碳含量至0.15wt%可有效提升細晶試桿之拉伸強度2%∼8%及拉伸延伸率超過22%，並可有效提升982℃/200MPa潛變壽命將近兩倍。在拉伸破壞模式方面，無論測試溫度高低，0.11wt%碳含量之拉伸破壞模式為典型沿晶破壞，而0.15wt%碳含量之拉伸破壞模式屬於穿晶及沿晶混合破壞模式。在982℃/200MPa潛變破壞模式方面，無論碳含量為0.11wt%或0.15wt%，潛變破壞模式均為沿晶破壞，但0.11wt%碳含量細晶試桿之裂紋起始於晶界附近之γ-γ’共晶相，而0.15wt%碳含量細晶試桿之裂紋則起始於晶界碳化物附近。綜上所述，適度添加碳含量，可有效提升細晶試桿之拉伸及高溫低應力潛變性能，進而達到細晶鑄件優良的性質。因此，對微細鑄造製程而言，CM-681LC超合金之碳含量可添加至0.15wt%，以改善其機械性質。

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.