|Abstract: ||本研究細分為兩部份；第一是探討FeCoNiCrSix五元高熵合金(其中x = 0, 0.25, 0.5及1.0；以下分別簡稱H4, H5-0.25Si, H5-0.5Si及H5-1.0Si ) 在700 ~ 900 oC空氣下的氧化行為，第二是探討H5-0.5Si 及 H5-1.0Si在900oC不同氧分壓(Po2 = 10-4 ~ 1 atm)下的氧化特性。研究結果顯示，四種合金在所有溫度下的氧化動力學皆遵守拋物線率，且合金的氧化速率隨溫度上升而加快，但隨Si含量增加而減緩，其中，H5S-1.0Si合金的氧化速率常數(kp值)最具抗氧化特性；經數據比較發現，在700 ~ 900oC下，五元合金比四元FeCoNiCr等莫耳合金皆有較低的氧化速率值，在900oC下H5-1.0Si氧化速率常數(kp值)最為顯著，與H4相較可以降低約0.8個數量級；說明添加Si有助於提升合金之抗氧化特性。同時，藉由白金指標實驗得知，合金的氧化反應主要是由陽離子的外擴散所主導，而從顯微結構分析得知，三種合金經氧化後皆生成Cr2O3與SiO2的混合層，且隨著合金的Si含量增加，SiO2的含量也相對地增加。 此外，H5-0.5Si及H5-1.0Si合金在900℃四種不同氧分壓(10-4 ~ 1 atm)下的氧化動力學亦皆遵守拋物線律，且氧化速率會隨氧分壓增加而加快，並呈現kp ∝ Po_2^n的正比關係；其中，n值約在0.120 ~ 0.136之間，顯示合金以化後的生成物具有p型半導體特徵。另一方面，顯微結構分析得知，其氧化物組成與空氣下相似，仍以Cr2O3與SiO2之混合層為主。綜合結果顯示，隨著合金中Si含量的增加能提升合金的抗氧化能力，推測生成Cr2O3及SiO2為合金氧化速率降低之主因。|
The main goal of this thesis is to investigate high temperature oxidation of three quinary FeCoNiCrSix high-entropy alloys (HEAs, where x=0, 0.25, 0.5, and 1.0, as named as H4, H5-0.25Si, H5-0.5 and H5-1.0) in two various parts. In the first part of the study, the oxidation behavior of the four HEAs was studied at 700~900℃ in dry air；Secondly, the oxidation behavior of H5-0.5Si and H5-1.0Si was studied in four oxygen-containing atmospheres over the oxygen partial pressure range from 10-4 to 1 atm at 900℃. The results showed that the oxidation kinetics of all the HEAs followed the parabolic-rate law, with their oxidation rate constants (kp values) increased with increasing temperature, but decreased with increasing Si content, and the most pronounced reduction of kp values for H5-1.0Si was around 0.8 order of magnitude slower than those of H4 at 900℃. Pt-markers were alwaye formed in the scale / subarate interface, indicating that the oxidation mechanism of the HEAs was predominant by outward diffusion of cations. The scales formed on all the alloys were composed of an irregular intermixed scale-layer of Cr2O3 and SiO2, and the amount of SiO2 increased with increasing Si content. The oxidation behavior of FeCoNiCr-0.5Si (H5-0.5Si) and FeCoNiCr-1.0Si (H5-1.0Si) was also studied in four oxygen-containing atmospheres at 900 oC. The oxidation kinetics of the alloys followed the parabolic rate law with their kp values increasing with increasing oxygen pressure. A positive dependence of kp ∝ Po_2^n was obtained for the alloys, having an n values of 0.120 ~ 0.136, indicative of a typical oxide-scale exhibiting a p-type semiconductivity for the alloys. The scales formed on the two alloys in various oxygen pressures are similar to those in dry air. In summary, the formation of intermixed Cr2O3 and SiO2 scales is responsible for the lower oxidation rates of FeCoNiCrSix HEAs.