|Abstract: ||本研究分三部分探討添加不同元素的三種FeCoNiCr基五元高熵合金，包含FeCoNiCrAl、FeCoNiCrSi及FeCoNiCrMn (以下分別簡稱H5A, H5S及H5M)，在不同條件下的氧化行為。其一是探討FeCoNiCr基五元高熵合金在700~ 950oC空氣下恆溫氧化行為，研究結果顯示，三種合金的氧化動力學皆遵守拋物線律，其中，添加Mn的五元高熵合金的氧化速率常數(kp值)皆遠大於其他合金；而添加Si或Al合金的kp值相對較低。由顯微結構分析得知，三種合金氧化後的氧化生成不同的氧化物，其中，H5A生成α-Al2O3及θ-Al2O3，H5S生成Cr2O3及SiO2；而H5M在700~ 800oC外層生成Mn2O3，中間層是生成(Mn,Cr)3O4及Cr2O3，而內層只有Cr2O3，但在900~ 950oC外層則生成Mn3O4，其他層與低溫相似。推測生成SiO2及Al2O3是五元含矽及鋁高熵合金的氧化速率遠低於五元含錳高熵合金的主因。第二部分是探討H5M合金在950oC不同氧分壓(Po2 = 10-4 ~ 1 atm)下的氧化行為。研究結果顯示，合金在所有氧分壓下的氧化動力學皆遵守拋物線律，且其氧化速率常數(kp值)隨氧分壓增加而加快，並得知帶兩或三價的含錳及鉻離子之外擴散是主導整個氧化反應的因子。最後一部分則探討FeCoNiCr基五元高熵合金在700~ 900oC空氣下循環氧化行為，其研究結果顯示，三種合金氧化速率由慢到快依序為H5S合金最慢，H5A合金次之，而H5M合金最快，此結果跟恆溫氧化略微不同，這與氧化後生成的氧化物有關。由顯微結構分析得知，H5A合金以生成-Al2O3為主，並有少量的-Al2O3，但因應力的影響導致部分氧化層剝落，在剝落處有FeAl2O4生成；H5S合金以生成Cr2O3為主，並有少量的-SiO2；而H5M合金生成三層氧化層，其組成結構和恆溫氧化時所得相同。|
The main goal of this thesis is to investigate high temperature oxidation of three quinary FeCoNiCr-based high entropy alloys (HEAs), containing FeCoNiCrAl (H5A), FeCoNiCrSi (H5S), and FeCoNiCrMn (H5M) in various tested conditions. First of all, the oxidation behavior of all the HEAs was studied at 700~ 950oC in dry air. In general, the oxidation kinetics of all the alloys followed the parabolic rate law, and the oxidation rate constants (kp values) of the alloys were strongly dependent on alloy composition. It was found that the Mn-additional alloy revealed the fastest oxidation rates over the temperature range of interest, while the kp values of the Si- and Al-additional alloys were nearly identical and much lower with respect to those of H5M. The scales formed on the H5A alloy consisted of α-Al2O3and θ-Al2O3, while Cr2O3 and SiO2 were detected on the H5S alloy. In addition, triplex scales formed on the H5M alloy, consisted of an exclusive outer-layer of Mn2O3 and an intermediate- layer of (Mn,Cr)3O4 and Cr2O3, and an exclusive inner-layer of Cr2O3 at 700~ 800oC. Triplex scales also formed on the same alloy at 900~ 950oC consisted of an exclusive outer-layer of Mn3O4 and the intermediate and inner-layers are similar to those at lower temperatures. The formation of Al2O3 and SiO2 was responsible for the lower oxidation rates of Al- and Si-containing alloys, as compared to those of the H5M alloy. Secondly, the oxidation behavior of H5M was further studied in four oxygen-containing atmospheres over the oxygen partial pressure range from 10-4 to 1atm at 950 oC. The oxidation kinetics of the alloy followed the parabolic rate law, and the kp values increased with increasing oxygen partial pressure. Triplex scales also formed on the alloy regardless of oxygen pressure, having the same scale constitution and phases described in dry air. Finally, the cyclic oxidation behavior of all the HEAs was systematically studied at 700~ 900oC in dry air. The results showed that the cyclic-oxidation kinetics of the HEAs followed by the fast to slow rank of H5M > H5A > H5S. The scales formed on the H5A alloy consisted of α-Al2O3and θ-Al2O3, however a minor amount of FeAl2O4 was observed beneath the spalled alumina scales, while those formed on the H5S and H5M alloys remained unchanged as those in the isothermal oxidation.