La對於CoCrNi中熵合金顯微結構與機械性質之影響

Effects of La addition on the microstructures and mechanical properties of CoCrNi medium-entropy alloy

詹舒霓(Shu-Ni Chan)

指導教授 ： 薛承輝

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/NTU202102748

摘要

本研究中添加稀土元素La進CoCrNi中熵合金，從微結構及機械性質兩個面向去探討La添加帶來的影響，並從塊材與薄膜去分別作討論。CoCrNiLa合金塊材是以電弧熔煉製備，再經過均質化、熱軋及再結晶處理，得到成分為(CoCrNi)100-xLax (x = 0, 0.10, 0.24, 0.32, 0.48) 的中熵合金塊材；CoCrNiLa合金薄膜則是以磁控濺鍍系統將(CoCrNi)100-xLax膜鍍在單晶(111)的矽基板上，透過調控La靶材的功率 (0–60 W) 來獲得不同La含量的(CoCrNi)100-xLax (x = 0, 1.6, 3.1, 4.8, 6.7, 9.0) 中熵合金薄膜。塊材方面， La ≤ 0.24 at.%時為FCC相，在La添加量達0.32 at.%以上時除了FCC相以外會有HCP相的LaNi5析出物出現。微量La添加時會有晶粒細化與晶格扭曲的效果，但在析出物出現後即減弱，此趨勢與母相中的La含量相符。機械性質在微量La添加會有提升的效果，並在x = 0.24時達最佳值，硬度、降伏強度與極限強度分別為235.5 HV、608 MPa及1110 MPa。薄膜方面，結構一開始呈現單一FCC相，隨著La含量增加出現了HCP相的LaNi5析出物與HCP的CoCrNi相，最終變為非晶相。由(111)繞射峰推估晶粒尺寸，晶粒尺寸將隨著La增加而從24.80 nm下降至1.39 nm，TEM可看到在x = 0, 1.6, 3.1時為純柱狀晶結構，其寬度隨著La含量增加而降低；x = 4.8時則呈現柱狀晶與等軸晶共存，柱狀晶的寬度也是比x = 3.1的更小，而等軸晶則是晶粒尺寸約100至200 nm；x = 6.7時為微量奈米晶分佈在非晶的母相中；x = 9.0時則轉變成完全非晶。奈米壓痕結果指出硬度值及楊氏模數隨著La含量增加而增加，於x = 3.1時達到最大值9.66 GPa及199.50 GPa，在更高La含量時則兩者皆下降。

關鍵字

中熵合金 ； 中熵合金薄膜 ； 鑭添加 ； 微結構 ； 機械性質

並列摘要

In this study, different amounts of La were added to CoCrNi medium entropy (MEAs) and CoCrNi medium entropy alloy films (MEAFs) in order to investigate the effect of La on the microstructure and mechanical behaviors of the CoCrNi MEAs and the MEAFs. For the CoCrNi MEAs, the structures of La-0, La-0.1 and La-0.2 were FCC solid solutions, while LaNi5 HCP precipitates formed in the FCC matrix for La-0.3 and La-0.5. The presence of La in FCC matrix not only enhanced the lattice distortion but also reduced the grain size of the matrix. However, in the presence of LaNi5 precipitates resulted in decreased hardness and tensile strength in La-0.3 and La-0.5. The optimum mechanical properties with hardness, yield strength and ultimate tensile strength of 235.3 HV, 608 MPa and 1110 MPa were obtained with small amounts of La addition in La-0.2. For the CoCrNi MEAFs, the structures retained single FCC phase in La0 and La1.6. HCP LaNi5 phase was found in La3.1 and La4.8, and HCP CoCrNi phase was shown in La4.8 in addition to FCC phase. The structure of La6.7 transformed to mainly amorphous structure with small amounts of FCC nanocrystals embedded in the amorphous matrix. Eventually, La9.0 became totally amorphous. La0, La1.6 and La3.1 showed columnar grain structure, and the grain refinement effect was observed with La addition. Both columnar grains and equiaxed grains coexisted in La4.8. With excessive La addition, the structure transformed to amorphous structure. Both hardness and Young’s modulus increased with increasing La content. The maximum hardness and Young’s modulus of 9.66 GPa and 199.50 GPa were achieved in La3.1, while the mechanical properties declined with further La addition.

並列關鍵字

medium entropy alloys ； medium entropy alloy films ； La addition ； microstructure ； mechanical properties

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