臺灣博碩士論文加值系統

雙相不�袗�因為兼具強度、可銲接性及耐蝕性，所以被廣泛應用於石油、化學及核能等工業，上述之優異性質主要來自於幾乎等量的高溫肥粒鐵相及沃斯田鐵相所形成之雙相結構。雙相不�袗�在300 - 500 ℃ 溫度區間時效，由於高溫肥粒鐵相產生離相分解或成核、成長之相變態，形成體心立方結構的富鉻α΄相及體心立方結構的富鐵α相，導致材料的脆性。另一方面，雙相不�袗�在600 – 950 ℃ 溫度區間時效，由於高合金含量而加速二次相之析出，亦將造成材料的脆性，此脆性與二次析出相及基材間之晶體幾何有很大的關聯性。
本論文主要是研究2205雙相不�袗�於低溫時效對脆性之效應及相變態、同時觀察差排在離相結構中的特性;並研究二次析出相與基材間之晶體幾何關係。
低溫時效對脆性效應之實驗結果顯示: 雙相不�袗�在475 ℃時效對於脆性有最大的效應;高溫肥粒鐵相於475 ℃時效，產生離相分解相變態，形成不規則形狀且內部互相連接的網狀結構，差排於此結構中互相交錯不易移動，導致材料嚴重的脆性。
475 ℃時效差排特性觀察之實驗結果顯示:晶界相容效應對高溫肥粒鐵相及沃斯田鐵相的差排結構有很大的影響;沃斯田鐵相的活化差排源若為螺旋差排，則可能在高溫肥粒鐵相形成連續滑移的螺旋差排帶;沃斯田鐵相的活化差排源若為差排堆積應力，則可能在高溫肥粒鐵相形成不連續滑移的混合差排。在475 ℃時效64小時後的差排密度約1012/m2 ~ 1013/m2，而造成差排不易移動的最主要原因可能是鉻元素的分佈使得 Peierls-Nabarro 應力增加所導致。
二次析出相與基材間晶體幾何實驗結果顯示:τ相的析出成長方向係由方向的最小不匹配標準所主導。R相在高溫肥粒鐵相析出，由於晶格的不匹配而產生晶格扭曲，造成R相外觀的不規則，並於內部形成大量很細的平面缺陷。

Duplex stainless steels have been widely used in the oil, chemical and nuclear industries due to their high strength, good weldability, and high resistance to stress corrosion and pitting. The superior properties of these duplex stainless steels come primarily from approximately equivalent amounts of austenite (γ) and δ-ferrite. However, these types of steels are intrinsically subject to embrittlement when exposed to temperatures above 300 ℃ because of solid-state reactions within the ferrite phase. It is well known that in Fe-Cr alloys there is a miscibility gap, where the ferrite phase may decompose into an iron-rich b.c.c. phase (? and a chromium-enriched b.c.c. phase (?amp;#900;) either by nucleation and growth of ?amp;#900; precipitates or by spinodal decomposition. However, detailed high-resolution transmission electron microscopy has not been reported, and the dislocation character and the formation mechanisms of these pinned dislocations in the spinodal structure of decomposed δ-ferrite remain unclear. On the other hand, a variety of secondary phases may form in duplex stainless steels upon aging at temperatures of 600 - 950 ℃ as follows : σ, χ, R, τ, carbide, and nitride. It is well established that the precipitation of these undesirable phases causes the embrittlement of the steels. The embrittlement is connected to the crystallographic orientation and the lattice misfit. However, the connection between morphology and crystallography, as well as the interface boundary structures of χ, R, and τ phases to the matrix or adjacent grain, are not well understood. In Chapter 3, the effect of low-temperature aging embrittlement in a 2205 duplex stainless steel has been investigated. In Chapter 4, the dislocation density and character of the spinodal structure within decomposed δ-ferrite in a 2205 duplex stainless steel has been observed. In Chapter 5, the crystallography of the secondary phase with matrix in a 2205 duplex stainless steel has been studied.
The results of the effect of low-temperature aging embrittlement indicate that the phenomenon of embrittlement occurs significantly at 475 ℃ and that the aging embrittlement at 475 ℃ corresponds to the development of a two-phase modulated microstructure in the aged δ-ferrite. FEG-TEM reveals that this two-phase mixture is irregularly shaped and fully interconnected as spongelike, indicative of a typical isotropic spinodal structure. Dislocation structures with a cross-stitch pattern in the aged δ-ferrite provide strong evidence to suggest that the immobilization of dislocations in this modulated structure causes severe embrittlement.
The results of the dislocation density and character of the spinodal structure within decomposed δ-ferrite indicate that the dislocation features of γ and δ-ferrite are affected significantly by the grain boundary compatibility effect. If the activation of slip sources within γ is caused by the screw dislocation, it will exhibit a high degree of slip continuity of screw dislocation bands within δ-ferrite. If the activation of slip sources within γ are caused by pile-up stress, it will exhibit virtual discontinuity of the mixed dislocations within δ-ferrite. The dislocations in the isotropic spinodal structure of δ-ferrite are predominantly of the screw character. In this study, the dislocation densities are on the order of about 1012/m2 ~ 1013/m2. The formation mechanisms of these pinned dislocations in the spinodal structure of decomposed δ-ferrite are complicated. The immobilization of dislocations is presumably due to the higher Peierls-Nabarro stress caused by the partition of Cr and the dislocation cores of screw dislocations.
The χ precipitate nucleated at the δ/γ interface boundary and grew preferably along the <101> direction; the morphology is polygonal and has a coherent interface with the matrix. The orientation relationship between τ precipitate and δ-ferrite can be expressed as (001)τ//(001)δ, [001]τ//[001]δ, and the τ/δ interface is extremely coherent; it also shows that the directional mismatch criterion for precipitate growth is more appropriate than that of minimum planar mismatch. The R phase precipitation in δ-ferrite will obviously lead to large lattice distortions caused by the large lattice misfit; the large lattice distortion may produce the irregular shaped morphology due to reduction of the strain energy and connection with thin plane faults in the R phase.

Chapter One
General introduction.......................................1
Chapter Two
Literature survey..........................................3
2-1. Development and Microstructure of duplex stainless steels.....................................................3
2-1-1. Development of duplex stainless steels..............3
2-1-2. Microstructure of wrought duplex stainless steels...4
2-2. Phase transformation in duplex stainless steels.......5
2-2-1. Secondary austenite.................................5
2-2-2. σ phase............................................6
2-2-3. Chromium nitrides...................................6
2-2-4. Carbides (M7C3 and M23C6)...........................6
2-2-5. χ phase..........................................7
2-2-6. R phase.............................................7
2-2-7. τ phase............................................7
2-2-8. π phase............................................7
2-2-9. α΄ and G phases....................................8
2-3. Spinodal decomposition................................8
2-3-1. Theory of spinodal reactions........................8
2-3-2. Concentration fluctuations..........................9
2-3-3. Spinodal structure.................................10
2-4. Low temperature aging embrittlement..................11
2-4-1. Fracture mechanism.................................12
2-4-2. Embrittlement mechanism............................13
2-5. Crystallographic relationships.......................13
Chapter Three
The low-temperature aging embrittlement in a 2205 duplex stainless steel
3-1. Introduction.........................................22
3-2. Experimental Procedure...............................23
3-3. Results and Discussion...............................24
3-4. Conclusions..........................................30
Chapter Four
The dislocation character aged at 475 ℃ in a 2205 duplex stainless steel
4-1. Introduction.........................................50
4-2. Experimental Procedure...............................51
4-3. Results and Discussion...............................52
4-4. Conclusions..........................................55
Chapter Five
Crystallography of secondary precipitates in 2205 duplex stainless steel
5-1. Introduction.........................................75
5-2. Experimental Procedure...............................76
5-3. Results and Discussion...............................77
5-4. Conclusions..........................................84
Chapter six
General conclusions......................................117
Future works.............................................120
Appendix.................................................121
Reference................................................122

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