Er 對於 CoCrNi 中熵合金顯微結構與機械性質的影響

Abstract

本研究專注在塊材和薄膜兩個不同尺度下，稀土元素Er添加進入CoCrNi中熵合金對於微結構和機械性質的影響，在塊材方面，是以真空熔煉爐製備不同Er成分的(CoCrNi)100-xErx樣品(x = 0, 0.1, 0.3, 0.5, 0.7, 1.0, 1.3)，再經由均質化等熱處理得到用於分析的試片；在微結構觀察中發現添加Er元素將導致在FCC相的CoCrNi基材中形成HCP相的ErNi5 (CaCu5型)析出物，而HCP相析出物的體積分率隨著Er含量的增加而增加。而晶粒大小則由18.1 µm減小到 5.1 µm。在機械性質方面，極限抗拉強度(UTS)、降伏強度(YS)和維氏硬度均隨Er添加量增加而增加，但延展性降低。與 CoCrNi中熵合金相比，(CoCrNi)98.7Er1.3的 UTS和維氏硬度分別從 878 MPa 增加到1143 MPa和212 HV到263 HV。此外，析出物和CoCrNi基材的奈米壓痕量測表明，HCP相析出物具有高硬度(~7.87 GPa)的特性，幾乎是 FCC 相基材(~4.72 GPa)的兩倍。在塊材(CoCrNi)100-xErx機械性質的提升是由於固溶強化、晶界強化和析出強化。在薄膜方面，CoCrNiEr合金薄膜則是利用磁控濺鍍機以CoCrNi合金靶和Er靶共鍍的方式將(CoCrNi)100-xErx薄膜鍍在單晶矽(111)的基板上，透過調控施加在Er靶上的功率來得到不同Er含量的(CoCrNi)100-xErx薄膜(x = 0, 0.89, 1.93, 2.57, 3.35, 4.08)，和塊材的結果有所不同，在CoCrNiEr薄膜的微結構中並無觀察到HCP相的ErNi5析出物，結構維持單一的FCC相，直到最終形成完全的非晶相，由TEM 明場相的觀察，在x = 0, 0.89, 1.93, 2.57, 3.35呈現柱狀晶的結構，而柱狀晶的寬度則隨著Er含量增加而減少，奈米壓痕數據顯示硬度值和楊氏係數隨著Er含量增加而增加，由於固溶強化以及細晶粒尺寸強化使得在x = 1.93兩項數據分別達到最大值12.64 GPa和223.48 GPa，但由於在x > 1.93逐漸出現了結構較為鬆散的非晶相和inverse Hall-Petch effect因此此時硬度和楊氏係數皆下降，相同的強度變化趨勢也可以在微型柱壓縮試驗中看到。

In this study, the effects of Er content on the microstructures and mechanical properties of Er-doped CoCrNi medium entropy alloys (MEAs) and CoCrNi medium entropy alloy films (MEAFs) were systematically investigated. In the bulk alloy part, a series of Er-doped CoCrNi MEAs with 0, 0.1, 0.3, 0.5, 0.7 ,1.0, and 1.3 at.% Er was fabricated by vacuum arc melting. It was found that Er element addition led to the formation of Er-Ni riched hexagonal phase (CaCu5 type) precipitates embedded in the original FCC matrix. The volume fraction of the HCP phase increased with the increase in the Er content. The grain sizes decreased from 18.1 to 5.1 µm as the Er content increased from 0 to 1.3 at.%. Both the ultimate tensile strength (UTS) and Vickers hardness increased with the increasing Er content, while the ductility decreased. Compared to the CoCrNi MEA, the UTS and Vickers hardness of (CoCrNi)98.7Er1.3 MEA increased from 878 MPa to 1143 MPa and 212 HV to 263 HV, respectively. In addition, the nanoindentation measurements on different phases indicated that the HCP phase had a high hardness ( ~ 7.87 GPa), which was almost two times of the FCC phase ( ~ 4.72 GPa). The strengthening mechanisms of the present alloys included solid-solution strengthening, grain boundary strengthening, and precipitation strengthening. In the thin film part, a series of (CoCrNi)100-xErx (x = 0, 0.89, 1.93, 2.57, 3.35, and 4.08) MEAFs was fabricated by radio frequency magnetron (RF) co-sputtering. A crystal structure transition from single FCC phase to amorphous structure could be observed from XRD and TEM results with the increasing Er amounts from 0 to 4.08 at.%. According to TEM results, the SAED pattern of Er1.93 revealed a crystal-amorphous dual-phase structure. Nanoindentation tests indicated an initial increase in hardness and Young’s modulus, reached a maxima value at x = 1.93, and then decreased with further Er addition. Similar trends were also shown in the yield strength and compressive strength. The initial increment of mechanical properties could be ascribed to the solid solution strengthening and grain refinement strengthening with small Er additions. However, the excess Er addition would be accompanied by the inverse Hall–Petch effect, leading to the softening.