FeCoNiCr中熵合金在室溫及低溫變形下缺陷顯微結構演變之研究

Microstructure Evolution of FeCoNiCr Medium-entropy Alloy during Deformation at Room Temperature and Cryogenic Temperature

李侑霖(Yu-Lin Li)

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(2021年)

https://doi.org/10.6342/NTU202101569

摘要

本研究將在室溫(25℃)進行70%冷輥軋後的FeCoNiCr中熵合金進行850℃持溫1小時的退火熱處理以得到完全再結晶(Fully recrystallized)之顯微結構，接著分別在室溫(25℃)以及低溫(-150℃)下進行應變速率為10-3 s-1的間斷拉伸實驗，以觀察不同溫度下缺陷顯微結構演變的過程。之後同樣在室溫(25℃)以及低溫(-150℃)下進行應變速率為9000 s-1的霍普金森快速撞擊實驗，並與間斷拉伸實驗結果做比較，以觀察不同應變速率下缺陷分布以及形貌上的差異，主要透過穿透式電子顯微鏡(Transmission Electron Microscope, TEM)來進行缺陷顯微結構的分析。
由實驗結果可以得知不論是在25℃還是-150℃，FeCoNiCr中熵合金在塑性變形初期主要都是透過差排滑移(Dislocation glide)來產生變形，直到應力達到啟動雙晶所需的臨界應力值，才會開始有變形雙晶(Deformation twin)的形成。在應變速率為10-3 s-1的實驗條件下，FeCoNiCr中熵合金在25℃下即便在真實應變量為38%也就是在抗拉強度的應變階段下，僅能觀察到一組變形雙晶(Single variant of deformation twin)的形成，推測是因為應力不足所致；在-150℃下則大約在真實應變量為12%之應變階段即可觀察到兩組變形雙晶(Two variants of deformation twin)的形成。而在應變速率為9000 s-1的實驗條件下，FeCoNiCr中熵合金在25℃且真實應變量為28.8%之應變階段下，即可觀察到兩組變形雙晶的形成，而在-150℃且真實應變量為24%之應變階段下，可以在許多奈米退火雙晶(Annealing nanotwin)內部觀察到密集分布的變形雙晶，而這些密集分布的變形雙晶其厚度大部分皆小於2 nm。最後在雙晶面符合Edge-on configuration的條件下進行變形雙晶寬度(厚度)之統計，由結果可以得知不論是在25℃還是-150℃，變形雙晶之平均寬度皆隨著應變量的增加而些微下降。且若是在相同真實應變量之條件下，不論是在25℃還是-150℃下FeCoNiCr中熵合金在經過快速撞擊後相較於慢速拉伸所生成的變形雙晶皆具有較細之平均寬度。

關鍵字

FeCoNiCr中熵合金 ； 間斷拉伸實驗 ； 霍普金森快速撞擊實驗 ； 穿透式電子顯微鏡 ； 奈米退火雙晶 ； 變形雙晶

並列摘要

In this study, the FeCoNiCr medium-entropy alloy was annealed at 850°C for 1 hour after cold rolling with a thickness reduction of 70% at room temperature (25°C) to obtain the fully recrystallized microstructure. The specimens were then subjected to interrupted tensile test with a strain rate of 10-3 s-1 at room temperature (25°C) and cryogenic temperature (-150°C) to observe the evolution of the defects at different temperatures. Afterwards, the split Hopkinson pressure bar (SHPB) test with a strain rate of 9000 s-1 was also performed at room temperature (25°C) and cryogenic temperature (-150°C), and compared with the results of the interrupted tensile test to observe the differences in the distribution and morphology of the defects at different strain rates, mainly by transmission electron microscope (TEM).
From the experimental results, it can be seen that whether at 25°C or -150°C, deformation of the FeCoNiCr medium-entropy alloy occurs by dislocation plasticity at the beginning of plastic deformation until the stress reaches the critical stress for twinning, then the formation of the deformation twin will occur. Under the experimental conditions with a strain rate of 10-3 s-1, single variant of deformation twins can only be observed in the FeCoNiCr medium-entropy alloy at 25°C even at a true strain of 38%, i.e., at the strain level of ultimate tensile strength (UTS), presumably due to insufficient stress condition. At -150°C, two variants of deformation twins can be observed at a true strain of 12%. Two variants of deformation twins can be observed at a strain rate of 9000 s-1 for the FeCoNiCr medium-entropy alloy at 25°C and a true strain of 28.8%, and at -150°C and a true strain of 24%, densely distributed deformation twins can be observed inside many annealing nanotwins. Finally, the average width (thickness) of deformation twins was measured under the edge-on configuration of the twinning plane, and the results show that the average width of deformation twins slightly decreases with increasing strain at both 25°C and -150°C. The average width of the deformation twins generated during split Hopkinson pressure bar test is smaller than that of the deformation twins generated during tensile test at the same temperature and strain level.

並列關鍵字

FeCoNiCr medium-entropy alloy ； Interrupted tensile test ； Split Hopkinson pressure bar (SHPB) test ； Transmission electron microscope (TEM) ； Annealing nanotwin ； Deformation twin

參考文獻

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