個人論文摘要

Creep and thermo-mechanical fatigue (TMF) properties of newly developed ferritic stainless steels (Crofer 22 APU and Crofer 22 H) are investigated at 25^oC-800^oC for use in planar solid oxide fuel cell (pSOFC) interconnect. Tensile properties of both Crofer 22 APU and Crofer 22 H steels are evaluated at temperatures of 25^oCto 800^oC. Creep properties of the given steels are evaluated by constant-load tests at 650^oC to 800^oC. Several creep lifetime models are applied to correlate the creep rupture time with applied stress or minimum creep rate. Comprehensive comparisons between Crofer 22 APU and Crofer 22 H steels are made on the tensile strength and creep resistance so as to characterize the influence of additions of refractory elements (Nb and W). Out-of-phase TMF tests as well as TMF-creep interaction tests under various combinations of cyclic mechanical and thermal loadings are conducted at a temperature range of 25^oC-800^oC for Crofer 22 H to study its long-term durability for applications in pSOFCs.

Experimental results show the variation of yield strength with temperature in Crofer 22 APU can be described by a sigmoidal curve for different deformation mechanisms. According to the creep stress exponent, activation energy, and microstructural observations, a diffusion-controlled dislocation creep mechanism is involved in the creep behavior of Crofer 22 APU steels at 650^oC-800^oC, while a power-law dislocation creep mechanism interacting with an in-situ precipitation strengthening mechanism is involved in the creep behavior of Crofer 22 H steels at 650^oC-800^oC. A significantly improved tensile and creep strength of Crofer 22 H over Crofer 22 APU for pSOFC interconnect is observed and attributed to a precipitation strengthening effect of the Laves phase. A significant coarsening of the Laves phase is responsible for a reduced improvement of creep resistance in Crofer 22 H at the low-stress, long-term region of 800^oC. In addition, creep rupture time of the Crofer 22 APU and Crofer 22 H steels can be described by a Monkman-Grant relation. The relation between creep rupture time and normalized stress for both steels is well fitted by a universal simple power law for all of the given testing temperatures. Larson-Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for both steels. Fractographic and microstructural observations indicate a ductile, dimpled fracture pattern with considerable necking is identified for the Crofer 22 APU and Crofer 22 H specimens after creep test.

Experimental results of Crofer 22 H steels under TMF loadings show the number of cycles to failure for non-hold-time TMF loading is decreased with an increase in the minimum stress applied at 800^oC. There is very little effect of maximum stress applied at 25^oC on the number of cycles to failure. The non-hold-time TMF life is dominated by a fatigue mechanism involving cyclic high-temperature softening plastic deformation. A hold-time of 100 h for the minimum stress applied at 800^oC causes a significant drop of number of cycles to failure due to a synergistic action of fatigue and creep. Creep and creep-fatigue interaction mechanisms are the two primary contributors to the hold-time TMF damage. The creep damage ratio in the hold-time TMF damage is increased with a decrease in applied stress at 800^oC and an increase in number of cycles to failure.



中文摘要：
	本研究針對平板式固態氧化物燃料電池連接板用兩款肥粒鐵系不銹鋼(Crofer 22 APU及Crofer 22 H)之潛變與熱機疲勞性質進行分析。高溫拉伸試驗分別在室溫、600^oC、650^oC、700^oC、750^oC及800^oC的溫度環境中進行，以建立Crofer 22 APU和Crofer 22 H不銹鋼在上述不同溫度環境下的應力－應變關係。而兩款不銹鋼之潛變試驗分別在650^oC、700^oC、750^oC及800^oC進行，以建立基本的潛變應變—時間曲線及應力—斷裂時間曲線，並進一步求得不同應力下第二階段的最小應變率。另外，導入幾種壽命評估分析模式，找出潛變斷裂時間與施加應力或最小應變率的關聯性，以預估兩款不銹鋼的潛變壽命。並比較Crofer 22 APU及Crofer 22 H兩款不銹鋼拉伸及潛變性質，以了解多添加Nb及W微量元素的Crofer 22 H鋼對拉伸性質及潛變機制之影響。此外，Crofer 22 H不銹鋼之異相熱機疲勞試驗在週期性溫度(25^oC-800^oC)及異相應力作用下進行，而熱機疲勞—潛變交互作用試驗則在800^oC最小施加應力給予額外持時100小時，以探討Crofer 22 H不銹鋼於平板式固態氧化物燃料電池長期運轉條件下之耐久機械行為，並定量分析各種損傷機制對此款不銹鋼熱機疲勞壽命的影響程度。研究結果顯示Crofer 22 APU鋼材之降伏強度隨溫度的變化可藉由S型關係式伴隨不同變形機制得到不錯的描述，其降伏強度在300^oC至500^oC主要是受到高溫軟化及動態應變時效機制影響，而在700^oC以上主要是受到高溫軟化及動態析出機制的交互作用。此外，根據潛變應力指數、活化能及微結構觀察，得知Crofer 22 APU的潛變變形機制主要為擴散控制之差排潛變，而Crofer 22 H的潛變機制乃是差排潛變伴隨即時析出強化效應。比較此兩款金屬連接板材料，發現Crofer 22 H比Crofer 22 APU有較佳的拉伸及潛變強度，此可歸因於Crofer 22 H鋼中Laves相的析出強化效果。而Laves相的粗大化為Crofer 22 H在800^oC長時間低應力下抗潛變能力減弱的主要原因。在潛變壽命評估方面，發現利用Monkman-Grant關係式來描述Crofer 22 APU及Crofer 22 H之潛變行為有相當不錯的結果。另外，將不同溫度下的施加應力以抗拉強度正規化後，發現以此正規化參數來預測兩款不銹鋼潛變壽命的結果相當不錯。而利用Larson-Miller關係式來整合兩款不銹鋼之潛變壽命、施加應力與溫度也有不錯的效果。此外，Crofer 22 APU及Crofer 22 H試片經潛變試驗後，破斷面具有許多韌窩之延性破裂特徵。 Crofer 22 H鋼之熱機疲勞研究結果顯示，未持時熱機疲勞負荷下循環壽命數會隨著800^oC施加應力的增加而減少，而與室溫時施加應力值幾乎無關。此外，未持時熱機疲勞壽命主要受到週期性高溫軟化塑性變形機制的影響。在持時熱機疲勞負荷方面，於800^oC施加應力持時100小時會導致循環壽命數明顯減少，主要歸因於疲勞與潛變機制的加乘作用所致。另外，持時熱機疲勞損傷主要是由潛變與潛變—疲勞交互作用兩種機制所造成，其中的潛變損傷比率會隨著800^oC施加應力減少而增加，而隨著循環壽命數增加而上升。

Abstract：
	Creep and thermo-mechanical fatigue (TMF) properties of newly developed ferritic stainless steels (Crofer 22 APU and Crofer 22 H) are investigated at 25^oC-800^oC for use in planar solid oxide fuel cell (pSOFC) interconnect. Tensile properties of both Crofer 22 APU and Crofer 22 H steels are evaluated at temperatures of 25^oCto 800^oC. Creep properties of the given steels are evaluated by constant-load tests at 650^oC to 800^oC. Several creep lifetime models are applied to correlate the creep rupture time with applied stress or minimum creep rate. Comprehensive comparisons between Crofer 22 APU and Crofer 22 H steels are made on the tensile strength and creep resistance so as to characterize the influence of additions of refractory elements (Nb and W). Out-of-phase TMF tests as well as TMF-creep interaction tests under various combinations of cyclic mechanical and thermal loadings are conducted at a temperature range of 25^oC-800^oC for Crofer 22 H to study its long-term durability for applications in pSOFCs. Experimental results show the variation of yield strength with temperature in Crofer 22 APU can be described by a sigmoidal curve for different deformation mechanisms. According to the creep stress exponent, activation energy, and microstructural observations, a diffusion-controlled dislocation creep mechanism is involved in the creep behavior of Crofer 22 APU steels at 650^oC-800^oC, while a power-law dislocation creep mechanism interacting with an in-situ precipitation strengthening mechanism is involved in the creep behavior of Crofer 22 H steels at 650^oC-800^oC. A significantly improved tensile and creep strength of Crofer 22 H over Crofer 22 APU for pSOFC interconnect is observed and attributed to a precipitation strengthening effect of the Laves phase. A significant coarsening of the Laves phase is responsible for a reduced improvement of creep resistance in Crofer 22 H at the low-stress, long-term region of 800^oC. In addition, creep rupture time of the Crofer 22 APU and Crofer 22 H steels can be described by a Monkman-Grant relation. The relation between creep rupture time and normalized stress for both steels is well fitted by a universal simple power law for all of the given testing temperatures. Larson-Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for both steels. Fractographic and microstructural observations indicate a ductile, dimpled fracture pattern with considerable necking is identified for the Crofer 22 APU and Crofer 22 H specimens after creep test. Experimental results of Crofer 22 H steels under TMF loadings show the number of cycles to failure for non-hold-time TMF loading is decreased with an increase in the minimum stress applied at 800^oC. There is very little effect of maximum stress applied at 25^oC on the number of cycles to failure. The non-hold-time TMF life is dominated by a fatigue mechanism involving cyclic high-temperature softening plastic deformation. A hold-time of 100 h for the minimum stress applied at 800^oC causes a significant drop of number of cycles to failure due to a synergistic action of fatigue and creep. Creep and creep-fatigue interaction mechanisms are the two primary contributors to the hold-time TMF damage. The creep damage ratio in the hold-time TMF damage is increased with a decrease in applied stress at 800^oC and an increase in number of cycles to failure.