FCC型低熵至高熵合金之冷軋與退火集合組織演化及機械性質之研究

Investigation of Cold-rolled and Annealed Texture Evolution and Mechanical Properties from FCC-structured Low-entropy to High-entropy Alloys

林冠豪(Kuan-Hao Lin)

指導教授 ： 葉均蔚

國立清華大學/工學院/材料科學工程學系/博士(2021年)

摘要

本研究旨在探討低熵、中熵及高熵合金之集合組織演化與機械性質異向性，設計了五種單相FCC型金屬，分別為Ni、Ni–2at.% W、Ni–4at.% W、CrFeNi及CoCrFeMnNi。五種金屬的製備係透過真空電弧熔煉後固化成型，隨後經均質化處理24小時使呈現單一相結構。均質化處理後，五種金屬皆施以70%厚度減量之冷軋，並於不同溫度進行1小時之退火處理，經維氏硬度量測得各金屬的退火軟化曲線。
各金屬在不同溫度退火之巨觀集合組織演化係採用X光繞射極圖量測分析。結果顯示，屬於低熵的Ni、Ni–2at.% W及Ni–4at.% W材料具有較高的疊差能，其冷軋集合組織為Copper type，經過退火後，其方位分佈函數顯示再結晶階段具有相對較明顯的再結晶集合組織分佈，但隨著溫度提升，進入晶粒成長階段，便不再具有較明顯的優選方位；而屬於中、高熵的CrFeNi及CoCrFeMnNi合金具有較低的疊差能，冷軋集合組織為Brass type，但其方位分佈函數顯示再結晶與晶粒成長階段不具有明顯優選方位。此外，透過熱場發掃描式電子顯微鏡搭配背向散射電子繞射進行微結構分析，發現Ni、Ni–2at.% W及Ni–4at.% W具有不均勻的冷軋延微結構，大部份的剪移帶分佈於<111>//RD及<112>//RD方位的晶粒內部，故其潛在再結晶核的位置也分佈不均，以致再結晶階段容易具有明顯優選方位；而CrFeNi及CoCrFeMnNi則具有甚均勻的冷軋延微結構，主要原因為大量差排與變形雙晶的交互作用，造成微結構各處的變形程度較均勻，故其潛在再結晶核的位置分佈亦會較均勻，使再結晶階段造成許多不同晶粒方位的晶粒出現，進而抑制了優選方位晶粒的形成，而呈現隨機方位的分佈。
五種金屬的室溫拉伸性質表現明顯不同。其中，材料的降伏強度分別以固溶強化、晶格摩擦力強化及幾何必須差排強化進行探討。理論計算之結果顯示，多元成份合金之CrFeNi及CoCrFeMnNi具有較高的固溶強化及晶格摩擦力強化，主要為因為基地具有嚴重晶格扭曲及較強的鍵結強度所致；Ni–2at.% W及Ni–4at.% W具有相對低的固溶強化及晶格摩擦力強化，主因是其溶質W原子濃度低，晶格扭曲較小。經過理論計算，本研究首先提出了適用於低濃度到高濃度固溶合金的固溶強化通式（universal equation），並與實驗數據吻合。
此外，CrFeNi及CoCrFeMnNi比Ni–2at.% W及Ni–4at.% W具有較高抗拉強度及較大的延伸率，主要是因為其拉伸變形過程中形成大量的變形奈米雙晶，提供較高的應變硬化速率。整體而言，具有高晶格扭曲的高濃度合金比低濃度合金具優越的機械性質組合。

關鍵字

傳統合金 ； 高熵合金 ； 冷軋 ； 退火 ； 塑性變形 ； 機械性質 ； 微結構 ； 集合組織 ； 極圖 ； 背向散射電子繞射

並列摘要

This study is mainly focused on the texture evolution and anisotropy of mechanical properties between low-entropy, medium entropy and high entropy alloys. Five single-phased FCC metals were designed: Ni, Ni–2at.% W, Ni–4at.% W, CrFeNi and CoCrFeMnNi. These five metals were prepared by vacuum arc melting and casting. They were homogenized for 24 h to have a single-phased structure. After homogenization, these five metals were cold-rolled with a thickness reduction of 70%. They were then annealed at different temperatures for 1h. Vickers hardness measurements were performed to obtain annealing softening curves.
The X-ray pole figure measurements were also conducted to characterize the evolutions of macrotexture as a function of annealing temperature. The results showed that the low-entropy Ni, Ni–2at.% W and Ni–4at.% W with higher stacking fault energy exhibited the copper type rolling texture. After annealing, more evident recrystallized textures can be observed during recrystallization by the orientation distribution functions. However, their preferred orientations would become unobvious in the grain growth stages as the annealing temperatures increased. The medium-entropy CrFeNi and high-entropy CoCrFeMnNi with lower stacking fault energy showed the brass type rolling texture. Their orientation distribution functions showed no obvious preferred orientations during recrystallization and grain growth stages. Additionally, the microstructure of each metal was analyzed by thermal field emission scanning eletron microscope equipped with electron backscattering diffraction. It can be found that Ni, Ni–2at.% W and Ni–4at.% W had inhomogeneous cold-rolled grain structures, and most of shear band located in the <111>//RD and <112>//RD grains.This demonstrated that the distributions of their potential recrystallized nuclei would be inhomogeneous. On the other hand, the CrFeNi and CoCrFeMnNi had more homogeneous cold-rolled grain structure. The main reason is that the higher amount accumulated dislocations and earsier formation of deformation nanotwinning in preferentially deformed grains have higher work hardening and easier to activate the deformation of those grains with less-preferential orientation. As a result, more homogeneous deformation level occurred everywhere. This induced many grain nuclei with different orientations during recrystallization stage and thus inhibited the propensity of preferred orientations.
The present five metals performed significantly different room-temperature mechanical properties. The yield strength was contributed by the strengthening of solid solution, lattice friction and geometrically necessary dislocations. The multielement CrFeNi and CoCrFeMnNi had higher strengthening of solid solution and lattice friction. This is mainly because they had more severe lattice distortion and higher bonding strength than those of Ni–2at.% W and Ni–4at.% W with low solute concentration. Furthermore, the CrFeNi and CoCrFeMnNi had larger ultimate strengths and elongations, which is mainly because they were easier to induce deformation twins and thus larger strain hardening rates during tensile doformation. As a whole, we demonstrated the mechanisms why concentrated CrFeNi and CoCrFeMnNi alloys had superior combinations of strength and elongation as compared to dilute Ni–2at.% W and Ni–4at.% W.

並列關鍵字

Conventional alloy ； High-entropy alloy ； Cold roll ； Anneal ； Plastic deformation ； Mechanical property ； Microstructure ； Crystallographic texture ； Pole figure ； Electron backscatter diffraction

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