臺灣博碩士論文加值系統

本研究針對先進的高熵合金材料進行合金設計，以開發具高溫應用價值之新合金成分。參考目前最廣泛使用的高溫應用超合金材料，新型高溫合金的微結構也控制成面心立方基地相(γ)搭配均勻散佈有序L12析出強化相(γ′)之兩相結構，以兼顧良好的高溫機械強度及延展性。本研究首先提高有序Ni3Al相的固溶量，並研究其相穩定性及強度，結果發現高度固溶對γ′相的強化效果相當顯著，其原因來自較高的防相界能量(anti-phase boundary energy)；但高固溶所造成的混亂原子排列則會降低γ′的高溫穩定性。接著我們進一步針對γ - γ′兩相結構之合金進行高熵化的探討，結果發現熵值(entropy)提高的同時，也要顧及L12相的焓(enthalpy)值，否則此有序相的高溫穩定性也會明顯下降。經過系統化之合金設計，此種新開發超合金之基地相已跳脫傳統以鎳(Ni)、鈷(Co)、鐵(Fe)為基底之想法，而是以鎳－鈷－鐵基為主體，相較傳統超合金是屬於新的研究領域。進一步的研究證實，此較高混亂度的基底相能固溶更多的元素添加，且維持良好的高溫相穩定性，因此我們將其命名為「高熵超合金」。為了減少高溫時晶界脆弱的問題，我們成功將高熵超合金進行方向性鑄造，再進行各式高溫機械強度的研究。結果顯示此鑄造型合金能展現與傳統超合金相匹配的高溫硬度、高溫拉伸強度及高溫潛變特性，其原因可歸功於：優異的高溫相穩定性、高體積百分比的析出強化相、高的防相界能量(anti-phase boundary energy)以強化L12相及低的疊差能(stacking fault energy)以阻礙差排之爬升移動；高熵超合金也表現出良好的表面穩定性，在高溫氧化及熱腐蝕之環境中，緻密的氧化鋁或氧化鉻保護層能迅速生成，有效減緩侵蝕；此外與傳統超合金相比，高熵超合金的耐火強化元素添加量較少，因此具有低密度及低成本等優勢。但是我們也發現，此種合金在高溫下的析出相粗化現象無助於提昇潛變組抗能力，且基底相的強度較傳統超合金來的弱，因此仍有合金設計的優化空間。總結來說，本研究開發之高熵超合金，具有獨特的成分配比、良好的高溫性質及更佳的性能價格比，因此有潛力成為新世代的高溫應用材料。

In this study, high entropy alloys have been developed toward high temperature applications. According to the most widely used high temperature material superalloys, the microstructure of face-centered cubic (FCC) γ matrix with uniformly distributed L12 γ′ precipitates implies the more balanced high temperature strength and ductility. So, the thermal stability and strength of highly alloyed Ni3Al were studied initially. The strengthening effect on developing a γ′ composition toward higher entropy is significant, due to higher anti-phase boundary energy of the order phase. However, the order-disorder transition temperature would be decreased with the more random atomic distribution in γ′ lattice. The microstructure stability of the γ - γ′ alloys with medium to high mixing entropy were then studied. It was found that the high temperature alloys cannot be solely designed by entropy term, but should also enhance the ordering enthalpy of γ′ phase, to avoid lowering the thermal stability of γ′ phase. Through alloy designs, we have also found that present alloys are quite different from the conventional Ni-, Co- or Fe-based alloy design, but is within a range of stable (Ni, Co, Fe)-rich system. This composition space has rarely been studied through the development of superalloys. In addition, such highly-soluted (Ni-Co-Fe) matrix can exhibit an enlarged solubility of alloying contents, while remains good phase stability till high temperatures. Therefore, they have been named as high entropy superalloys (HESA). Since grain boundaries might be drawbacks to the thermal properties, HESAs have been successfully casted into the directionally-solidified (DS) structure by Bridgeman method. In terms of the high temperature mechanical properties, HESAs can exhibit comparable high temperature hardness, tensile strength and creep resistance to that of commercial superalloys due to the stable γ - γ′ microstructure, high volume fraction of γ′ precipitates, high anti-phase boundary energy for γ′ strengthening and low stacking fault energy to hinder dislocation climb. Good surface stability of HESA in high temperature oxidizing and corrosive environments were also demonstrated, which can be attributed to the rapid formation of continuous Chromia or Alumina for surface protection. Furthermore, with less alloying of refractory elements, HESAs exhibit the apparent advantages in lower density and cost of materials. Nevertheless, there are still concerns such as the directional coarsening of γ′ for HESAs cannot contribute to the creep resistance, and the strength of γ matrix is still lower than that of superalloys. As a result, further rooms for composition optimization of HESA exist. To summary, the novel high entropy superalloys are with unique composition, good thermal properties and improved cost-performance, thus can be promising as a new type of high temperature alloy.

摘要 I
Abstract III
List of Tables VII
List of Fifures VIII
1. Background & Introduction 1
2. Literature Review 4
2.1 The development and challenges of Ni-based superalloys 4
2.2 High entropy alloys (HEA) 6
2.3 Early studies on the high temperature mechanical properties of HEA 9
2.4 Single FCC matrix HEA 10
2.5 Precipitation strengthened HEA 14
2.6 L12 γ' precipitation strengthened HEA 15
3. Research scope 21
4. Increase the entropy of the ordered L12 phase
23
4.1 Abstract 23
4.2 Introduction 24
4.3 Materials and Methods 24
4.4 Results 25
4.5 Discussion 34
5. Increase the entropy of the γ + γ' alloys (High Entropy Superalloy) 37
5.1 Abstract 37
5.2 Introduction 38
5.3 Materials and Methods 38
5.4 Results 40
5.5 Discussion 46
6. The High Temperature Hardness of High Entropy Superalloy 51
6.1 Abstract 51
6.2 Introduction 52
6.3 Materials and Methods 52
6.4 Results 56
6.5 Discussion 64
7. The High Temperature Tensile and Creep Properties of High Entropy Superalloy 67
7.1 Abstract 67
7.2 Introduction 68
7.3 Materials and Methods 68
7.4 Results 70
7.5 Discussion 76
8. The Oxidation and Hot Corrosion Properties of High Entropy Superalloy 80
8.1 Abstract 80
8.2 Introduction 81
8.3 Materials and Methods 81
8.4 Results 83
8.5 Discussion 94
9. Conclusions 103
10. Future Work 105
11. References 106
Journal Publications 122

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