高強度Al-Cu-Mg-(Ag)鋁合金與輕量化CoCrNi-(Si)中熵合金之顯微結構與機械性質研究

Abstract

本研究探討材料之微觀結構與機械性質之間關連性，主要以鋁合金與中熵合金作為研究對象。鋁合金採用二系列鋁合金(Al-Cu-Mg)，研究添加微量Ag與否對各個時效階段中析出物之形成與材料強度之影響。透過穿透式電子顯微鏡(Transmission Electron Microscope, TEM)觀察時效初期、尖峰時效、過時效與長時間時效之析出物演化過程，如尺寸、形貌與分布情形。並藉由高解析掃描穿透式電子顯微鏡(High Resolution- Scanning Transmission Electron Microscope, HR-STEM)解析奈米析出物之原子級結構，深入了解各個析出物，搭配電子能量損失光譜( EELS )定量分析析出物之體積佔比。接著透過拉伸試驗測試個時效階段之機械性質，結合微結構分析，深入了解析出物對材料機械性質之影響。 研究發現，添加Ag後產生之Ω相能大幅提升材料強度。除了本身具有熱穩定性外，Ω相的形成能在11小時至18小時階段抑制S相粗化，使材料於過時效階段能維持強度。透過研究團簇物發現，其原因為大量生成之Ω相能加速消耗鋁基地中的Cu-Mg團簇物，使S相不易粗化。透過此實驗發現Ω相與S相之競爭關係，故進一步研究了AA2040 Al-5.1Cu-1.0Mg-0.4Ag高強度鋁合金中，其於不同時效時間點進行溫成形模擬對材料顯微結構與機械性質所造成之影響，觀察何種熱處理將表現出最佳性質。時效溫度200 °C下分別於時效前 (5 %-1h)、預時效20分鐘 (20 min-5 %-40 min)、40分鐘 (40 min-5 %-20 min)與時效結束後進行5 %壓縮變形 (1h-5 %)，四種條件總時效時間均為一小時，觀察其強度變化與對應微結構。結果顯示與一般時效強化材料相反，時效前之變形會降低此材料強度。透過Ω相體積分率與S相數量分率統計發現，時效前進行變形將抑制Ω相生成，導致強度下降。而40 min-5%-20 min表現出最高強度，表示於時效後期進行溫成形模擬，能導入差排加速後續時效中S相析出，且時效時間短不會導致粗化，結合Ω相貢獻，將達到最高強度。 本論文下半部分為CoCrNi中熵合金之研究。由於其低溫下能以變形雙晶作為變形手段，故有極佳強度與延展性，可應用於極地抑或是太空中。本研究探討了添加矽將此材料輕量化之餘，其對圍觀結構與機械性質之影響，主要利用臨場變形穿透式電子顯微鏡(in-situ TEM)進行研究。研究發現添加矽能有效降低疊差能。相較於CoCrNi中熵合金於110方向單晶變形以差排滑移作為主要機制，CoCrNi-Si中熵合金能於降伏強度後產生變形雙晶阻礙差排移動，提升加工硬化率。此外，由於目前極具爭議之短距有序結構是否對強度提升有幫助，本實驗透過in-situ TEM對此進行研究。發現均勻分布之短距有序結構能使CoCrNi中熵合金整體變形均勻，並且於劇烈變形後產生變形雙晶，有效提升材料強度

In this dissertation, the study explores the correlation between microstructure and mechanical properties of materials, with a primary focus on aluminum alloys and medium entropy alloys. The research specifically delves into the examination of 2xxx aluminum alloys (Al-Cu-Mg) and assesses the impact of introducing minor quantities of Ag on precipitate formation and material strength throughout various stages of the ageing process. The observations of precipitate evolution, size, morphology, and distribution, were conducted using Transmission Electron Microscopy (TEM) at early, peak, over-ageing, and prolonged ageing stages. The atomic structure of nano-precipitates was analyzed through High-Resolution Scanning Transmission Electron Microscopy (HR-STEM), and the volume fraction of precipitates was quantitatively assessed using EELS. Mechanical properties were evaluated at each stage through tensile tests, establishing correlations with microstructural analyses to comprehend the strengthening effects of distinct precipitates on the alloys. The present study reveals that the introduction of Ag results in the significant enhancement of material strength, primarily attributed to the formation of the Ω phase. The Ω phase, besides demonstrating thermal stability, effectively consumes abundant Cu-Mg clusters, thereby impeding the coarsening of the S phase from 11 hours to 18 hours, consequently maintaining material strength during the over-ageing stage. The competitive relationship between the Ω phase and S phase is identified, leading to an exploration of the effects of simulated warm forming at different ageing timings on the microstructure and mechanical properties of the AA2040 Al-5.1Cu-1.0Mg-0.4Ag high-strength aluminum alloy. Four distinct combinations of warm forming and artificial ageing (5%+1h, 20 min+5%+40 min, 40 min+5%+20 min, and 1h+5%) were examined to observe the strength improvement and corresponding microstructure. In contrast to conventional age-hardening materials, pre-strain resulted in reduced the alloy’s strength. The study concludes that the 40 min+5%+20 min sample exhibited the highest strength, attributed to the combined effects of Ω and S phases. The subsequent section of this dissertation focuses on the equiatomic CoCrNi medium entropy alloy (MEA). This alloy, exhibiting excellent strength and high ductility due to the formation of deformation twins at crogenic temperatures, holds potential applications in polar districts or outer space. The investigation extends beyond the lightweight effect, examining the influence of Si addition on the microstructure and mechanical properties of CoCrNi MEA through in-situ deformation transmission electron microscopy (in-situ deformation TEM). The findings indicate that Si addition effectively reduces stacking fault energy. Deformation in CoCrNi MEA, when subjected to a single crystal under the 110 direction, primarily involves dislocation movement. In contrast, CoCrNi-Si MEA deforms through deformation twins due to its low stacking fault energy, significantly enhancing strength. The study also addresses the ongoing debate regarding the contribution of short-range order structure (SRO) to strength, finding that the formation of SRO promotes more uniform deformation. Additionally, as strain increases, the generation of deformation twins contributes substantially to strength improvement.