FeCoNiCrMnx (x=1,0.3) 高熵合金之1050 ℃氧化行為

Oxidation Behavior of FeCoNiCrMnx(x=1,0.3) High Entropy Alloys at 1050 ℃

陳詠鈴(Yung-Ling Chen)

指導教授 ： 林招松

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103770

摘要

高熵合金起初被設計為含有等莫耳或接近等莫耳比例的多主要成分合金，其擁有高混合熵可提供穩定性以形成大量無序的固溶狀態。作為設計新穎的合金，有寬廣的成分範圍與多元素可潛在符合應用需求，在高溫下仍有足夠的機械強度、韌性與熱穩定性。自2004年Cantor et al.與Yeh et al.發表相關研究，使高熵合金的研究成指數性增長，成為現今熱門研究的主題。若此材料應用於高溫，無可避免地會經歷高溫氧化，表面氧化使得底材損失，降低底材的使用效率和壽命，並增加後續去除氧化層的製程成本，大幅限制其在高溫氧化之應用。
本研究使用真空感應熔煉50 公斤級的大型鑄材，熱軋至原厚的20%，接著進行均質化處理後得到成分分佈均勻的FeCoNiCrMnx(x=1,0.3)高熵合金。探討兩種合金於1050 ℃持溫25分鐘到24小時的短期至長期高溫氧化，且用示差熱掃描儀估算合金熔點與硬度試驗得知氧化層與基材的硬度以搭配解釋氧化特性，由刮痕試驗可知氧化數小時的氧化層有較佳附著性。在兩合金的短期氧化動力學皆為線性；長期氧化皆遵循拋物線性，FeCoNiCrMn0.3有較低的氧化速率常數，表示氧化相對較慢，其可有效減緩氧化速率因素可能為降低錳元素含量(7 at%)、保有鉻含量(23 at%)，仍可維持緩慢擴散與晶格扭曲效應及在氧化層內層形成薄且連續緻密的Cr2O3保護層。經由OM、SEM、EPMA、XRD、EBSD和TEM分析氧化層表面與橫截面形貌、氧化相與成元素分佈結果，可知氧化層外層為富錳的Mn3O4；氧化層內層為富鉻的Cr2O3；Cr2O3兩側為含有一些錳與鉻的MnCr2O4; MnCr2O4來自Mn3O4與Cr2O3形成擴散偶與Cr2O3做為元素擴散阻礙層。底材靠近界面形成錳與鉻的匱乏，使剩餘元素填補，且隨氧化時間增加，氧化層受熱應力與成長應力導致剝落與再氧化。故Mn及Cr的氧化物是決定氧化行為的主導因素。

關鍵字

高熵合金 ； 氧化動力學 ； 短期高溫氧化 ； 長期高溫氧化 ； 非等莫耳

並列摘要

High entropy alloys were originally designed as multi-component alloys with equal molar or close to equal molar ratios. The high mixing entropy provides stability to form a large number of disordered solid solution state. As a novel alloy design, it has a wide composition range and multi-element. It can potentially meet application requirements, and still have sufficient mechanical strength, toughness and thermal stability at high temperature. Since Cantor et al. and Yeh et al. published related studies in 2004, research on high entropy alloys has grown exponentially and has become the subject of popular research today. If the material is applied to high temperature, it will inevitably undergo high temperature oxidation. Surface oxidation will cause the loss of the substrate, reduce the use efficiency, life of the substrate and increase the cost of the subsequent oxide removal process. It greatly limits its application in high temperature oxidation.
In this study, vacuum induction smelting of 50 kilograms of large casting was used, undergoing hot rolled to 20% of the original thickness, and homogenized to obtain FeCoNiCrMnx (x=1,0.3) high entropy alloy with uniform composition. Discuss the short-term to long-term high temperature oxidation of two alloys at 1050 ℃ for 25 minutes to 24 hours. Use differential scanning calorimetry to estimate the alloy melting point and hardness test. The hardness of the oxide layer and the substrate are matched to explain the oxidation characteristics. It can be seen that the oxide layer oxidized for several hours has better adhesion from Scratch test. The short-term oxidation kinetics of the two alloys are linear ; the long-term oxidation follows the parabolic law. FeCoNiCrMn0.3 has a lower oxidation rate constant, which means that the oxidation is relatively slow. The factor that can effectively slow down the oxidation rate may be to reduce the manganese content ( 7 at%), retain the chromium content (23 at%), can still maintain the slow diffusion and lattice distortion effects and form a thin and continuous dense Cr2O3 protective layer in the inner layer of the oxide layer.Through OM, SEM, EPMA, XRD, EBSD and TEM analysis of the oxide layer surface and cross-sectional morphology, oxidation phase and element distribution results, it can be seen that the outer layer of the oxide layer is manganese-rich Mn3O4; the inner layer of the oxide layer is chromium-rich Cr2O3; The both sides of Cr2O3 are MnCr2O4 containing some manganese and chromium; MnCr2O4 comes from Mn3O4 and Cr2O3 forming a diffusion couple and Cr2O3 as the element diffusion barrier layer. The substrate is close to the interface to form a shortage of manganese and chromium, so that the remaining elements are filled. As the oxidation time increases, the oxide layer is subjected to thermal stress and growth stress to cause peeling and re-oxidation. Therefore, the oxides of Mn and Cr are the dominant factors that determine the oxidation behavior.

並列關鍵字

high entropy alloys ； oxidation kinetics ； short-term high-temperature oxidation ； long-term high-temperature oxidation ； non-isomolar

參考文獻

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