CoCrFeMnNi等莫耳比高熵合金在先前已有許多論文提到低溫至高溫皆呈現單一 FCC 相，且表現高應變硬化速率而具有高伸長率及抗拉強度。而從 Ni、CoNi、CoFeNi、CoCrFeNi至五元的CoCrFeMnNi，疊差能愈低，也越強越韌。此疊差能愈低歸因於晶格扭曲使能量提高且Suzuki effect使溶質原子容易在疊差缺陷偏析降低能量。本研究之目的即開發更低疊差能的高熵合金，在已知等莫耳CoCrFeMnNi基礎，提高Co及Cr比例、降低Ni含量和添加微量元素，降低疊差能，並找最佳化的比例。
本實驗合金，分別為Co25、Co30、Co35。其中小晶粒Co25室溫拉伸的伸長率為70%，已超越等莫耳CoCrFeMnNi的50%，在強度表現上也較為優異。從EBSD觀測，前者延性優異的原因可歸因於應力誘發相轉換，使部分FCC相轉換為HCP，因而提升高應變硬化速率。接著以此款合金當作母成分，藉由微量添加Al、Si、Ti、Ge及V等來觀察高應變硬化速率的提升。
最後發現 Co25合金微量添加 Si 至6 at.%，不但可以維持FCC 相不使合金脆化，強度及延性跟著提升。而添加 Al、Ti、Ge、V等元素皆無法使疊差能有效下降，促進應變硬化率提升。
在低溫拉伸的表現上，本實驗採用室溫拉伸最優異的合金Co25，分別添加4及6 at.%的Si作比較，結果顯示此兩合金皆無法拉伸至頸縮而提前斷裂，原因為此兩合金在低溫下，FCC 太快相轉換至 HCP，由於 HCP 的滑移系統較少，使整體延性遠不及 CoCrFeMnNi。又由於Fe 為產生HCP的來源，故接著將Fe含量降低，換取Si的添加，分別為 Si_2^*、Si_4^* 及 Si_6^*，其中 Si_2^* 在低溫拉伸表現上強度可達1217.5 MPa，延性為60%，且有明顯的頸縮點。

High-entropy alloy CoCrFeMnNi has been reported to have a low stacking fault energy and excellent mechanical properties. It showed that CoCrFeMnNi is single FCC from low to high temperature. The stacking fault energy of CoCrFeMnNi is lowest among Ni, CoNi, CoFeNi, CoCrFeNi and CoCrFeMnNi. The low stacking energy is attributed to the lattice distortion which increases the energy and the Suzuki effect which makes it easy to reduce the energy in stacking fault defect by solute segregation. The purpose of this study is to develop high-entropy alloys with even lower stacking fault energy. By increasing the ratio of Co to Cr, decreasing the content of Ni, and adding small mount of element, stacking fault energy is expected to be reduced. From this, we will find optimal proportion with the lowest energy based on CoCrFeMnNi.
The alloys designed in this experiment were Co25, Co30 and Co35. The tensile behavior of Co25 with small grains shows 70% elongation at room temperature, which exceeds 50% elongation of CoCrFeMnNi. Its strength is higher than CoCrFeMnNi at least 100 MPa as well. The reason for the superior ductility of the former can be attributed to the transformation-induced plasticity, and thereby enhanced strain hardening rate. Co25 was used as a base composition accordingly. The stacking fault energy, strain hardening rate, strength and ductility were assessed by adding Al, Si, Ti, Ge, and V in a small amount.
It was found that Co25Si with 2, 4 or 6 at.% Si still remains a single FCC phase. The stacking fault energy decreases as Si content increases. Tensile strength and the ductility at room temperature is better than that of Co25. However, adding elements Al, Ti, Ge, or V cannot effectively reduce the stacking fault energy and promote strain hardening rate.
In the cyrogenic tensile test, Co25, Co25-4Si and Co25-6Si were compared. The results showed that the alloy can’t be stretched to neck point and fractured without warning. The reason is that FCC is converted to HCP quickly at cyrogenic temperature. As slip systems of HCP is lesser than FCC, its ductility becomes far less than that of CoCrFeMnNi. Moreover, as Fe is the source of HCP, the direct deduction of Fe in exchange for the addition of Si is expected to reduce phase transformation. Among Si_2^*, Si_4^* and Si_6^*, tensile strength of Si_2^* is up to 1217.5 MPa, the elongation is 60% at cyrogenic temperature, and there is an obvious necking point.

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