FeCoNiCr四元高熵合金慢速拉伸及快速撞擊實驗之機械性質與顯微結構研究

Mechanical Property and Microstructure of Low-stain-rate Tensile Deformation and High-strain-rate Compressive Deformation in the FeCoNiCr Quaternary High-entropy Alloy

林佑諠(Yo-Shiuan Lin)

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/NTU202001267

摘要

本研究係觀察均質化完之Fe-Co-Ni-Cr四元高熵合金施以70%冷加工量後在650°C進行退火，以不同退火時間(10分鐘至4小時)觀察退火後的退火雙晶(Annealing twin)成長情形，並發現在四小時內的退火處理除了退火雙晶外仍可以發現原先冷軋過程殘存尚未再結晶的大晶粒，而這些大晶粒內有冷軋時產生的變形雙晶(Deformation twin)，因此材料的晶粒分布為部分再結晶(Partially recrystallized)的情形，預期以相對低溫的熱處理設計使材料在變形時，退火雙晶及原先的變形雙晶一同阻擋差排移動，使材料擁有更好的機械性質。接著製作前述條件之拉伸試片在室溫及低溫下做慢速拉伸試驗，由拉伸曲線觀察其強度及延性等機械性質；以及製作退火800°C(小晶粒)及1100°C(大晶粒)持溫1小時的霍普金森快速撞擊試片，在室溫及低溫下進行高應變速率的霍普金森快速撞擊試驗。並藉由EBSD (Electron Backscatter Diffraction )探討不同退火溫度及時間的再結晶程度及晶粒大小分佈情形，以及利用TEM (Transmission Electron Microscopy)觀察慢速拉伸試片及快速撞擊試片變形區的顯微結構變化。
在慢速拉伸實驗結果得知部分再結晶的拉伸試片擁有比完全再結晶試片好的強度，因為在未再結晶的滾軋晶粒中內含變形雙晶可阻擋差排移動，但犧牲些許延性。而低溫拉伸後擁有比室溫更好的強度及延性，原因為低溫下變形易於新的變形雙晶形成。
在室溫的快速霍普金森撞擊實驗結果得知高應變速率下易於變形雙晶的形成；在低溫的快速霍普金森撞擊實驗結果得知隨實驗的溫度下降材料的強度會上升，原因為低溫下變形易於新的變形雙晶形成，且小晶粒S的強度比大晶粒L來的好，原因為小晶粒擁有晶粒細化的效果。相較於慢速拉伸而言，在高應變速率的撞擊實驗中可以發現到變形雙晶隨處可見，且有許多地方可以觀察到第二組的變形雙晶(Secondary deformation twins)形成、變形雙晶之間交錯(twin-twin interaction)以及在奈米級退火雙晶出現兩方向變形雙晶的現象，這也是高應變速率撞擊實驗較低應變速率拉伸實驗擁有更好強度的原因。

關鍵字

四元高熵合金 ； 慢速拉伸實驗 ； 快速撞擊實驗 ； 機械雙晶 ； 退火雙晶

並列摘要

The present study was carried out to observe the growth situation of the annealing twins of a homogenized Fe-Co-Ni-Cr quaternary high entropy alloy with 70% cold working at 650°C heat treatment from 10 minutes to 4 hours. In addition to the annealing twins, it was found that large grains remaining from the original cold rolling process could still be found within four hours of annealing, and these large grains contained the deformation twins produced during cold rolling, so the grain distribution of the material was partially recrystallized. It is expected that the relatively low-temperature heat treatment design will allow the annealing twins and the original deformation twins to resist dislocations movement as the material is deformed, resulting in better mechanical properties. Next, the Fe-Co-Ni-Cr quaternary high-entropy alloy will take the low-stain-rate tensile deformation experiment and high-strain-rate compressive deformation experiment at room and cryogenic temperature to observe the mechanical properties. Last, by EBSD (Electron Backscatter Diffraction), the degree of recrystallization and grain size distribution at different annealing temperature and time are investigated, and TEM (Transmission Electron Microscopy) is used to observe the change of microstructure in the deformation area of the low strain rate tensile specimens and high strain rate compressive specimens.
In the low strain rate tensile test, the results show that the partially recrystallized tensile specimens have better strength than fully recrystallized tensile specimens because the deformation twins which were contained in the original cold-rolled grains can impede the dislocations movement but sacrifice some ductility. Besides, the tensile specimens tensiling at cryogenic temperature have better strength and ductility than the tensile specimens tensiling at room temperature because the deformation at cryogenic temperature is easy to form new deformation twins.
In the high strain rate Split Hopkinson Pressure Bar compressive test at room temperature, the results show that high strain rate deformation is easy to form deformation twins. And the results of this experiment at cryogenic temperature show that the strength of the material will increase as the temperature decreases because the cryogenic temperature is also easy to form new deformation twins. Besides, the strength of the small grain is better than that of the large grain because the small grain has better effect of grain refinement. Compared with low strain rate tensile test, the deformation twins can be found everywhere, and there are many places where the formation of the secondary deformation twins, the twin-twin interaction, and the formation of two variants of deformation twins in nano-annealing twins can be observed in the high strain rate compressive test. Hence, these are also the reasons why the Fe-Co-Ni-Cr quaternary high-entropy alloy material in high strain rate compressive test has better strength than in the low strain rate tensile test.

並列關鍵字

Fe-Co-Ni-Cr quaternary high-entropy alloy ； Low strain rate tensile test ； High strain rate compressive test ； Deformation twins ； Annealing twins

參考文獻

[1] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural development in equiatomic multicomponent alloys, Materials Science and Engineering: A 375-377 (2004) 213-218.

[2] S.-K.C. Jien-Wei Yeh, Su-Jien Lin, Jon-Yiew Gan, Tsung-Shune Chin, Tao-Tsung Shun, Chun-Huei Tsau, and Shou-Yi Chang, Nanostructured High‐Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes, Advanced Engineering Materials (2004).

[3] D.B. Miracle, O.N. Senkov, A critical review of high entropy alloys and related concepts, Acta Materialia 122 (2017) 448-511.

[4] M. Beyramali Kivy, M. Asle Zaeem, Generalized stacking fault energies, ductilities, and twinnabilities of CoCrFeNi-based face-centered cubic high entropy alloys, Scripta Materialia 139 (2017) 83-86.

[5] F. Otto, Y. Yang, H. Bei, E.P. George, Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys, Acta Materialia 61(7) (2013) 2628-2638.