Mar-M247超合金中高溫脆性及破壞機構之研究

Abstract

Mar-M247是一種典型的鑄造用多晶鎳基超合金，由於合金設計及顯微結構的最佳化，在高溫下具有極優異的強度及抗潛變性能，但延伸率則顯著偏低，尤其是在中溫高應力 (760℃/724MPa)下，存在延伸率偏低的本質脆性。本論文即針對Mar-M247超合金中高溫脆性及其強化機構進行研究，瞭解其脆性發生的原因，並且採用物理冶金及化學冶金等方法解決中高溫脆性的問題。 本研究首先排除晶界的影響，製作Mar-M247超合金單晶，進行中高溫機械性質之測試，以研判單晶是否具有中高溫脆性，實驗結果顯示單晶Mar-M247之899℃高溫拉伸延伸率及760℃/724MPa中溫高應力下之壽命及延伸率均遠大於多晶，單晶的破裂是沿著γ-γ'共晶組織破裂，多晶則沿著晶界上或晶粒內的長條狀MC碳化物而破裂，所以單晶Mar-M247並無中高溫脆性的問題。本研究以標準澆鑄參數製作兩種晶粗大小之試桿進行測試，在760℃/724MPa中溫高應力下，Mar-M247細晶 (3mm) 較粗晶 (8mm) 具有優異的抗潛變性能，且粗晶合金無論延伸率及破斷壽命均不符合規範值要求，然而在982℃/200MPa高溫低應力下，粗晶的抗潛變性質則優於細晶。此外，本研究將Mar-M247鑄件進行熱均壓處理，消除合金凝固時所產生之孔隙，在760℃/724MPa中溫高應力潛變測試下，潛變壽命及延伸率之提昇程度有限，所以熱均壓雖排除了微縮孔造成延伸率偏低的效應，但破斷則仍是沿著脆性的骨架形碳化物破裂。因此，孔隙的存在並不是Mar-M247中溫高應力下延伸率偏低的主要因素，其原因與脆性骨架形MC碳化物析出在晶粒內或晶界上，造成裂縫沿尖銳的碳化物起始及成長有關。 本論文先採用變化熱處理程序（二次固溶熱處理）改變碳化物及基地之界面特性，實驗顯示Mar-M247經過二次固溶熱處理後，在晶粒內骨架形及晶界上顆粒狀碳化物的周圍形成一層γ包覆膜，此γ膜具有動化相界的作用，可改變碳化物周圍的應力狀態，不僅增加899℃高溫拉伸延伸率，更顯著提昇760℃/724MPa潛變破斷延伸率。此外，本論文更採用添加微量元素的方法進行研究，研究結果顯示在Mar-M247中添加30~80ppm鎂元素，歐傑能譜分析顯示鎂偏析到MC碳化物與基地之界面，抑制長條狀骨架形碳化物的析出，具有細化及球化碳化物之功效，可顯著的提昇Mar-M247超合金899℃高溫拉伸延伸率，並且有效的提升760℃/724MPa中溫高應力的潛變壽命及破斷延伸率。由破損分析結果顯示，此中溫高應力下添加微量鎂元素後，Mar-M247的破斷模式由無鎂合金的沿著骨架形MC碳化物破裂轉變成沿著具有延展性的γ-γ'共晶組織破裂，徹底的解決Mar-M247超合金中高溫脆性及延伸率偏低的問題。此外，在982℃/200MPa高溫低應力潛變測試下，Mar-M247中含30-50ppm鎂時，具有較優良的抗潛變性質，但鎂含量添加過量時 (80ppm)，在晶界上析出大量的MC碳化物，造成潛變壽命及延伸率的急遽下降。綜合不同條件的潛變測試結果及機構顯示，在Mar-M247中添加30~50ppm的微量鎂，具有較適當的顯微結構及優異的抗潛變性能。 經由本論文之研究及探討證實，Mar-M247超合金中高溫脆性發生的原因為脆性的長條狀骨架形MC碳化物析出在晶粒內或晶界上，造成裂縫的快速形成及成長所致。二次固溶熱處理及添加適量的錢元素，可顯著的提昇Mar-M247超合金的中高溫性質，並徹底解決Mar-M247超合金中高溫脆性及延伸率偏低的問題，為Mar-M247超合金之強韌化找到一種適當的熱處理程序及微合金化(Micro-alloying) 的有效途徑。本論文研究的結果可直接用於相關航空用引擎關鍵零組件材料的改良，對於系統整體性能的提昇將有實質助益。

Mar-M247 is a typical cast Ni-base superalloy for polycrystalline. Due to optimal alloy design and microstructural modification, this alloy exhibits superior strength and excellent creep resistance at elevated temperature. However, Mar-M247 demonstrates a low ductility, particularly under the creep conditions of moderate temperatures and high stresses. Low ductility of Mar-M247 superalloy has significantly affected the safety in aerospace applications. In this thesis, the reasons resulting in brittleness and the mechanisms responsible for ductilization and strengthening of Mar-M247 superalloy are systematically studied. The methods of physical and chemical metallurgy for improving the brittleness problem are employed in the present study. In the beginning of this thesis, a single crystal of Mar-M247 is prepared via Bridgman technique in order to investigate whether the alloy with single crystal has the brittleness problem or not. The experimental results show that the elongation of single crystal is much greater than those of polycrystalline both at 899℃ and under 760℃/724MPa. The rupture in single crystal is along the ductile γ-γ' eutectic, while the fracture of polycrystalline occurs primarily along the carbide/matrix interface or at carbides. Apparently, single crystal of Mar-M247 superalloy has no brittleness problem. In further research, two typical grain sizes are prepared using a standard pouring parameter. The test results show that the creep behavior of fine grain alloy (3 mm) is much better that coarse grain (8 mm) under moderate temperature and high stress, in addition, the rupture life and elongation of coarse grain alloy can not meet the requirement of EMS-55447 (Engine Material Specification). However, the alloy with coarse grain has superior creep resistance under 982℃/200MPa. In third part, the hot isostatic pressing (HIP) is adopted to eliminate the porosity in castings. The results reveal that the improvement of rupture ductility under 760℃ /724MPa is limited, and the fracture still occurs at the sites of GB carbides or the script MC within grain interior. Therefore, the carbide characteristics of Mar-M247 superalloy play a main role for the moderate temperature brittleness during creep. On this basis, this thesis employs double solution treatment and micro-allying to change the carbide characteristics. For the processing of double solution treatment, a thick γ' film surrounding the script-like carbide within grain interior and the carbides at GB can be formed. This γ' film can enhance the toughness at interphases and accommodate the stress state near carbide interphase. Double solution treatment can not only upgrade the high temperature (899℃) tensile elongation but also improve the 760℃/724MPa rupture elongation. In the research of Mg micro-alloying, the contents ranging from 30 to 80 ppm Mg are added in Mar-M247 superalloy. The optical observation and AES analyses demonstrate that Mg segregates to the carbide/matrix interface, changing the primary MC carbide characteristics and inhibiting the script-like carbide formation. The measurement using image analyzer also prove the fact of carbide refinement and spheroidization. The mechanical tests show that the microaddition of Mg can dramatically improve the rupture life and elongation. In particular, the rupture elongation of the alloy containing 30~80 ppm Mg is enhanced up to 3~5 times that of Mg-free alloy during the 760℃/724MPa creep test. The fracture analyses demonstrates that cracks are mainly initiated and propagated at the interface of script-like MC carbides in the Mg-free Mar-M247 superalloy at elevated temperature. Because of the effective refinement and spheroidization of MC carbides, a change in the crack initiation occurred from the carbide/matrix interface to that along the γ-γ' eutectic, that is, an evidence of ductile of fracture mode. Under the condition of 982℃/200MPa, the rupture life and elongation in an over-addition alloy (80 ppm) clearly decreased owing to the formation of large amount of MC carbides present at GB. Hence, the microaddition of 30-50 ppm Mg is able to obtain the optimal microstructure and excellent creep properties under various creep conditions. Through the present research and investigation, the brittleness of Mar-M247 superalloy occurred at moderate temperature is attributed to the existence of script-like MC carbides within grain interior and at GB, facilitating the crack initiation and propagation. In this thesis, the appropriate pouring parameters to control optimal grain size, the double solution treatment and Mg microaddition can effectively change the carbide characteristics and improve rupture life and elongation. The applications of present results can upgrade the performance of engine and the key components of system.