Al0.5CoCrCuFeNi 高熵合金變形及退火行為與機械性質之研究

Deformation and Annealing Behaviors, and Mechanical Proerties of High-Entropy Alloy Al0.5CoCrCuFeNi

蔡哲瑋(Che-Wei Tsai)

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(2009年)

https://doi.org/10.6843/NTHU.2009.00787

摘要

本研究旨在探討高熵合金變形行為、退火行為與機械性質，以瞭解與傳統合金行為差異之處，並找出奈米晶的可行製程。首先對FCC為主的高熵合金Al0.5CoCrCuFeNi 施以900 oC熱鍛，而後在1100 oC真空下均質化24 h後爐冷。接著在室溫施以不同滾軋加工量，以觀察微結構與機械性質的演變。另取50 %滾軋態試片以900 oC退火不同時間，以觀察退火對微結構與機械性質的影響。研究結果發現，此合金即使在900 oC熱鍛仍呈現明顯加工硬化，意指在高溫下差排不易滑動，具有較低的動態回復，此歸因於多主元素效應，因基地固溶元素多種且高濃度，故產生固溶強化，阻礙差排移動，且由於疊差能下降與空孔擴散下降，差排交叉滑移不易產生。
此外發現合金於室溫滾軋加工初期即以奈米雙晶為主要變形機制，而當加工量增加時，奈米雙晶相互交叉切割，產生奈米晶粒，並加工硬化，此為獨特發現，即本合金採簡單的冷軋即可得奈米晶金屬塊材。此歸因於爐冷過程中基地相產生針狀富銅相析出與有序化結構阻礙差排的移動，且多元素固溶效應降低疊差能，使奈米雙晶易形成，促進雙晶變形。
滾軋態試片在900 oC下卻需要5小時才能完全退火，顯示再結晶緩慢，此歸因於合金因晶格扭曲使疊差能及晶界能降低，進而降低再結晶驅動能，而緩慢擴散效應亦有延滯晶界的移動與阻礙差排滑移的效果。
本研究另對此合金鍛造態的機械性質及微結構加以探討，首先將此合金在1000 oC均質化6 h後採水淬處理，而後在室溫滾軋加工，另對部分加工試片施以不同溫度之退火處理，以探討滾軋態與退火態，從室溫到900 oC臨場的拉伸表現。研究結果發現，在室溫中，滾軋態即有相當高的降伏強度(1284 MPa)與伸長率(7.6 %)，退火態以900 oC 10 分鐘退火具最佳的組合：降伏強度(1021 MPa)與伸長率(15.2 %)，此與極細微米晶的形成有關。長時間退火因回復或晶粒成長，強度下降較多。低於900oC的退火，雖有BCC相的析出強化，但伸長率都很低( < 3%)。
但在300 至 600 oC 間，不論是滾壓態或退火態，因BCC 相的析出強化，使伸長率度大幅降低，呈現類似不鏽鋼的中溫脆性現象。當溫度超過700 oC時，因晶粒滑移介入，伸長率逐漸上升。此結果顯示本合金熱處理或使用應避開300 至 600 oC的溫度範圍。

關鍵字

高熵合金 ； 冷加工 ； 機械性質 ； 變形 ； 退火 ； 奈米双晶

並列摘要

The thesis studies the deformation annealing behaviors and mechanical properties of FCC-type Al0.5CoCrCuFeNi high-entropy alloy, and finds the difference from traditional alloys and the possible method to produce nanocrystalline bulk alloys. In the first part, cast Al0.5CoCrCuFeNi was homogenized in vacuum at 1100 oC for 24 h with subsequent furnace cooling. The as-homogenizes samples were rolled with different thickness reductions at ambient temperature. The 50%-rolled sample was also annealed at 900 oC for different time. All the microstructural evolutions and mechanical properties were investigated. The results show that the alloy displayed significant work hardening and thus low dynamic recovery even during 900 oC forging. It is attributable to the multi-principal-element effect. The matrix with concentrated solute atoms had solution hardening to resist dislocation movement. In addition, dislocation cross-slip was difficult to operate because stacking fault energy and vacancy diffusion were both largely reduced. The initial deformation of Al0.5CoCrCuFeNi is accompanied by a lot of nano-twinning. This is attributable to the nano-precipitates in the matrix, which increases the stress for slip, and the low stacking fault energy which decreases the stress for twinning, respectively. Upon further deformation, the nanotwins intersected each other, forming nanograins. This is unique suggesting that a bulk nanocrystalline alloy can be obtained by simple rolling. In the annealing experiments, fully-annealed state was achieved after annealing for 5 h at 900 oC, suggesting large resistance to recrystallization. This is attributed to the low twin boundary energy and grain boundary energy which give a low driving force for recrystallization. In addition, sluggish diffusion effect also slows down the movement of grain boundary and dislocations.
In the second part, cast Al0.5CoCrCuFeNi was homogenized in air at 1100 oC for 24 h with subsequent water quenching. The homogenized samples were then cold-rolled with 80 % thickness reduction. Some samples were further annealed at different temperatures. The results show that the as-rolled sample had high yield strength (1284 MPa) and moderate elongation to failure (7.6 %). After 900 oC for 10 min, the strength and elongation combination is optimized: elongation doubled to 15.2 % and yield strength (1021 MPa) decreased by 20 %. Longer annealing at 900 oC significantly decreased the strength due to further recovery and recrystallization. Lower temperature annealing below 800 oC increased the strengths but reduced the ductility due to the precipitation of BCC phase. From tensile testing, the elongation was quite low between 300 and 600 oC revealing intermediate-temperature embrittlement as seen in stainless steels. This phenomenon is attributed to the formation of BCC phase. The elongation gradually increases when the testing temperatures is higher than 700 oC presumably due to the activation of grain-boundary sliding. The above results suggest that the present alloys should avoid the heat treatment or applications between 300 and 600 oC.

並列關鍵字

high-entropy alloys ； cold rolling ； mechanical property ； deformation ； annealling ； nano-twin

參考文獻

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