- 學位論文
合金元素微調對CM 247 LC超合金 細晶鑄件影響之研究
Effect of Micro-Alloy on Microstructure and Mechanical Properties of Cast Fine-Grain CM 247 LC Superalloy
摘要
本研究以CM 247 LC超合金為素材，使用真空精密鑄造爐進行細晶試件鑄造，探討析出相微結構、鋯元素、硼元素及錸元素對CM 247 LC超合金微細晶鑄件機械性能之影響。鑄造過程中添加不等量之鋯、硼及錸元素，以控制鑄造參數法澆鑄成70μm左右之等軸細晶試件，所有試件經熱均壓及熱處理後，進行微結構分析及機械性能測試。 實驗中藉由不同之熱處理條件以改變CM 247 LC超合金細晶鑄件中之碳化物型態及顯微結構。研究結果顯示，CM 247 LC超合金之拉伸降伏強度隨著固溶溫度之提高而提升。在760℃/724MPa中溫高應力潛變測試方面，低溫固溶處理(1221℃/2hr)試件之破斷壽命及延伸率(67-78hr,3.8﹪)優於多階段高溫固溶(1254℃/2hr)處理者(22-32hr,2.6﹪)，主要原因是由於1221℃/2hr固溶處理之試件在晶界及晶界碳化物周圍產生γ'包覆膜，有利於晶界應力釋放或變形調節，具抑制延晶裂紋的成核及生長之效果。而在982℃/200MPa 高溫低應力潛變測試方面，多階段高溫固溶之試件由於析出較高體積分率及適當尺寸之γ'強化相與碳化物，可有效抑制差排之滑移與爬升，因而具有較佳之潛變壽命。 添加鋯元素(0.015-0.15wt% Zr)之影響研究方面，研究顯示適量添加鋯元素可提升室溫及760℃降伏強度及延伸率，並對760℃/724MPa中溫高應力，927℃/345MP高溫中應力及982℃/200MPa高溫低應力潛變壽命具有提升作用。由SEM觀察及AES與EPMA分析證明，適量鋯元素能偏析至晶界及固溶於碳化物與γ相中，使碳化物型態改變及鈍化，有利於晶界及相界之應力調節，並抑制裂縫之形成及成長，具有提高拉伸強度及潛變性能之效果。 在硼元素(0.015-0.15wt% B)影響方面，由SEM觀察及AES與EPMA分析證明，適量硼元素能偏析至晶界、相界與形成硼化物，使碳化物型態改變及鈍化，改變晶界及相界之結合能，因此有利於晶界及相界之應力調節，抑制裂縫形成及成長。拉伸測試結果顯示，添加硼元素之CM 247 LC超合金細晶鑄件，不但可有效增加室溫抗拉強度及延伸率，並對760℃之降伏強度有明顯提升作用，並提高760℃/724MPa中溫高應力、927℃/345MPa高溫中應力及982℃/200MPa高溫低應力條件下之潛變壽命及延展性。 而在錸元素添加之研究方面，拉伸測試結果顯示，添加錸元素之CM 247 LC超合金細晶鑄件，不但有效增加室溫及760℃降伏強度及抗拉強度，並對降低760℃/724MPa、927℃/345MPa及982℃/200MPa之潛變率有明顯作用，主要原因是高熔點之錸元素能分別固溶於γ基地及γ'相，有效提高基地及析出相之強度，並降低原子擴散速率，抑制差排之滑移及爬升。 本研究結果證實，CM 247 LC 超合金不但有極佳之微細晶鑄造特性，且經由不同之熱處理條件，CM 247 LC超合金鑄件在不同操作條件下，都能提供極佳之強度及潛變性能。此外在CM 247 LC超合金細晶鑄件中適量添加錸元素、硼元素與鋯元素不但能有效提升試件強度，滿足渦輪盤中溫高應力操作要求，同時能夠降低高溫潛變率，符合渦輪葉片高溫低應力之環境，因此能夠提升整體鑄造渦輪轉子之操作溫度、提升轉速及延長其高溫壽命，因而增加飛行距離。
並列摘要
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.