CrCoNi中熵合金異質結構中退火雙晶的演變及相關機械性質研究

The evolution of annealing twin in CrCoNi medium entropy alloy with heterogeneous microstructure and its mechanical property

陳思妤(SHIH-YU CHEN)

指導教授 ： 楊哲人

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202001184

摘要

CrCoNi中熵合金具有優異的強度和延展性，在低溫環境下強度高達1.3 GPa和超過70％的伸長量，是近幾年熱門的研究材料。然而，根據文獻的研究，中熵合金的降伏強度約300 MPa，容易產生塑性變形，雖可透過晶粒細化的強化方式解決，但會犧牲延展性。因此，本實驗透過冷軋和中低溫退火的方式產生非均勻的部分再結晶結構，觀察不同退火時間的部分再結晶結構差異，及該結構中退火雙晶的發展和相關機械性質。
室溫拉伸結果發現，此方法顯著提高降伏強度（至~1065 MPa），相較文獻提升了三倍，且保持不錯的延展性（總伸長量~15 ％）。由於雙晶結構在低溫環境下較容易形成，所以在-50 ℃、-100 ℃、-150 ℃三個溫度下進行低溫拉伸測試，實驗結果發現，隨著溫度降低，強度、延展性、加工硬化率都隨之提升，在 -150 ℃可達到最佳的機械性質 (降伏強度~1326 MPa、抗拉強度~ 1491 MPa、總伸長量34 %)。由穿透式電子顯微鏡進行室溫間斷拉伸試片的微觀結構分析，在塑性變形初期（應變量~2 ％），變形機制主要為差排滑移和疊差，隨著變形量增加，在應變量2~6 ％ 間會啟動雙晶變形機制; 由於 -150 ℃的低溫拉伸試片可以達到較大的變形量(均勻應變量27 %)，除了機械雙晶外還觀察到HCP相變。
最後，本實驗進行快速撞擊實驗 (應變速率3000 s-1)，和拉伸實驗 (應變速率10-3 s-1)進行顯微結構比較。在TEM影像中，快速撞擊使試片產生密集的機械雙晶，其雙晶寬度較厚，且雙晶間距狹窄，而兩組交錯機械雙晶的角度在拉伸實驗中平均為70°，接近理論值的70.52°; 快速撞擊試片的雙晶交錯角度平均為53°，大幅偏離理論值，產生的HCP相變量也比低溫拉伸更多。上述微結構差異說明快速撞擊使試片變形較嚴重，因此無論在雙晶的密度或是形貌上都與拉伸實驗中的雙晶有較大的差異。

關鍵字

FeCoNiCrMn高熵合金 ； CrCoNi中熵合金 ； 退火雙晶 ； 機械雙晶 ； 部分再結晶結構 ； 間斷拉伸 ； 低溫拉伸 ； 霍普金森快速撞擊

並列摘要

CrCoNi Medium-entropy alloy (MEA) is a branch of CrMnFeCoNi high-entropy alloy (HEA). MEA has better mechanical properties than HEA. Most of medium-entropy and high-entropy alloys are limited by the modest yield strength ( ~300 MPa). Although grain refinement method can enhance yield strength, it requires significant compromises to ductility. Therefore, this experiment uses cold-rolling and low-temperature annealing to produce a non-uniform and partially recrystallized microstructure. The mechanical properties and the evolution of annealing twin are examined.
The tensile test at room temperature shows that partially recrystallized microstructure significantly enhances the yield strength ( ~1065 MPa) and still remain ductility (~15%). Due to low stacking fault energy at a lower temperature, twin structure is easily formed. Cryogenic tensile tests were performed at -50 ℃, -100 ℃ and -150 ℃. With the decrease in temperature, both strength and ductility increase. In this experiment, the best tensile test condition can be achieved at -150 ℃. The yield strength is ~1326 MPa, tensile strength ~1491 MPa, and total strain 34%.
The microstructure of MEA was observed by TEM after tensile test. The deformation mechanisms differed with the strain level. At small strain levels (~ 2%), the deformation substructure mainly consisted of planar dislocation slip and stacking faults. At larger strain levels, additional substructures including deformation twins ( 2 ~ 6%) and a new phase HCP also appeared (strain ~ 27%).
To observe the microstructure change under different strain rate, Hopkinson bar test (strain rate 3000 s-1) was performed. From TEM image, there are dense mechanical twins with a thicker width, and forming a narrowly spacing twin strip. The degree of intersectional twins is 53 °, which deviated greatly from the theoretical value 70.52°. The HCP phase transformation of Hopkinson bar test specimen is also more than the tensile test specimen. The difference in microstructure indicates that Hopkinson bar test specimen deformed more seriously. Both twin density and morphology are quite different from those in the tensile experiment.

並列關鍵字

CrCoNi medium entropy alloy ； annealing twin ； Cantor alloy ； TWIP effect ； Partially recrystallized structure ； Tensile test ； Hopkinson bar test ； Interrupted tensile test

參考文獻

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