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標題: | CrCoNiSi0.15中熵合金在冷軋延後退火下之異常硬化現象及顯微結構演變之研究 The Annealing-Induced Hardening Effect and Microstructure Evolution of CrCoNiSi0.15 Medium Entropy Alloy During Post-rolling Annealing |
作者: | 林宜嫺 Yi-Xian Lin |
指導教授: | 楊哲人 Jer-Ren Yang |
關鍵字: | 中熵合金,冷軋延,變形後退火,退火誘導硬化現象,微帶,變形雙晶,奈米退火雙晶, medium entropy alloy,cold rolling,post-deformation annealing,annealing-induced hardening,microband,deformation twin,nano-annealing twin, |
出版年 : | 2023 |
學位: | 碩士 |
摘要: | 一般而言,對變形後的金屬材料施以退火時,再結晶抑或是回復過程中缺陷的消弭都會導致材料的軟化。然而,對一些經歷了嚴重塑性變形的合金(包括高熵合金)進行退火後,可以觀察到稱為「退火誘導硬化」的異常硬化現象。縱使已有數種針對「退火誘導硬化」現象的解釋被提出,確切的機制仍未有定論。
本研究對CrCoNiSi0.15中熵合金進行80%的冷軋延,接著分別在300 ~ 1000 °C退火1小時,量測其硬度變化。在退火溫度介於300 ~ 550 °C的回復階段下,可以觀察到試片在退火後硬度不降反升,即發生所謂的退火誘導硬化現象。其中,試片在500 °C 退火1小時後硬度達到最大值:563.6±28.4 HV,與冷軋延試片的硬度相比高出了將近20%。為了探討退火誘導硬化現象背後的原因,本研究使用電子背向散射繞射(EBSD)和穿透式電子顯微鏡(TEM)來分析冷軋延和後續退火過程中顯微結構的演變。 分析冷軋延試片的顯微組織後發現,CrCoNiSi0.15中熵合金在冷軋延過程中會啟動兩種變形機制:變形雙晶和微帶形成。在500 °C 退火1小時後,主要由變形雙晶組成的區域不受影響,但在剪切帶處可觀察到,由變形雙晶和板條狀微帶所構成的組織,在退火過程中藉由差排的重新排列與消弭,被分節、分割成長條狀的次晶粒。這些次晶粒中可以觀察到疊差和變形雙晶等缺陷。退火後所形成的奈米級次晶粒結構,以及回復階段可動差排源的減少、晶界能量的鬆弛,皆被推測為引起CrCoNiSi0.15中熵合金發生退火誘導硬化的原因。冷軋延試片在600 °C或是更高的溫度退火1小時後,試片進入再結晶階段,使得退火誘導硬化的現象消退。被保留在次晶粒中的疊差及變形雙晶,有可能做為成核點,促進再結晶階段奈米級退火雙晶的形成,使得CrCoNiSi0.15中熵合金的再結晶組織中,有相當高的退火雙晶密度。 In most deformed metals, annealing leads to softening due to either recrystallization or the annihilation of defects during recovery. However, an abnormal hardening effect, referred to as “annealing-induced hardening”, has been observed in several alloys, including high entropy alloys, following severe plastic deformation and post-deformation annealing. Though several possible explanations have been suggested to elucidate the annealing-induced hardening phenomenon, the exact mechanism remains uncertain. In this study, CrCoNiSi0.15 medium entropy alloy was cold rolled to 80% thickness reduction and subsequently annealed at temperatures ranging from 300 ~ 1000 °C for 1 h. An annealing-induced hardening phenomenon was observed as the hardness of the cold-rolled samples increased after annealing at medium temperatures 300 ~ 550 °C during the recovery stage. A maximum value of 563.6±28.4 HV is reached after annealing at 500 °C for 1 h, exceeding the hardness of the cold-rolled sample by nearly 20%. The microstructural evolution during the cold rolling and post-deformation annealing process was investigated by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). It was observed that during cold rolling, two deformation mechanisms: deformation twinning and microbands formation were both activated. After annealed at 500 °C for 1 h, the regions of deformation twins remained, while shear bands comprising both deformation twins and thin lath-like microbands underwent segmentation and splitting, resulting in the formation of elongated subgrains containing defects like stacking faults and deformation twins. The presence of these remained defects in nanoscale subgrains, along with the reduction in mobile dislocation sources and the relaxation of the subgrain boundaries' energy during the recovery stage, are presumed to be the main factors contributing to the annealing-induced hardening phenomenon in the CrCoNiSi0.15 medium entropy alloy. This phenomenon subsided as recrystallization was initiated upon annealing the samples at temperatures of 600 °C and above. The stacking faults and deformation twins present in the subgrains may serve as nuclei for nano-annealing twin formation in the newly recrystallized grains, resulting in a high density of annealing twins in the recrystallized microstructure of the CrCoNiSi0.15 medium entropy alloy. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/89034 |
DOI: | 10.6342/NTU202303722 |
全文授權: | 同意授權(限校園內公開) |
顯示於系所單位: | 材料科學與工程學系 |
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