元素Ti 及Mo 添加對於CoCrNi 中熵合金顯微結構、 機械性質與拉伸變形機制之影響

Abstract

本研究透過高真空電弧熔煉技術製備一系列(CoCrNi)98–xTixMo2 (x = 1, 2, 3, 4)、(CoCrNi)98–yTi2Moy (y = 1, 2, 3, 4)及(CoCrNi)100–2zTizMoz (z = 0, 1, 2, 3)中熵合金並進行微結構與機械性質的分析，透過添加不同含量的過度元素鈦(Ti)和/或鉬(Mo)來探討對CoCrNi中熵合金系統造成的微結構與機械性質影響。基於XRD、EBSD與TEM的結果顯示，隨著鈦元素添加量上升並固定鉬元素含量為2 at.%，晶體結構從單相FCC轉為FCC + η + σ。隨著鉬元素添加量上升並固定鈦元素含量為2 at.%，晶體結構從單相FCC轉為FCC + σ。η相析出物主要富含鎳及鈦，而σ相析出物主要富含鉻及鉬。晶粒尺寸隨著溶質元素的添加略為減小，但在析出物形成時則顯著減小；合金強度及硬度隨著溶質元素的添加些微增加，但在析出物形成時則顯著增加，晶粒尺寸與硬度的變化趨勢證明了溶質元素貢獻了固溶強化、而析出物則貢獻了析出強化的效果。奈米壓痕分析結果顯示，FCC基體、η相析出物與σ相析出物的奈米硬度分別約為6、9及12 GPa。Ti3Mo3在本研究中展現出最佳的機械性質，其降伏強度為953.70 MPa、最大拉伸強度為1319.09 MPa、斷裂伸長率為22.09%、以及微硬度為424 HV。本研究引入了包含固溶強化、晶界強化以及析出強化等在內的協同強化機制來提升材料的機械性質，其量測與計算的降伏強度之間存在著高度相關性。 為了研究元素添加在不同溫度下的機械性質及塑性變形機制的演變，將由單相FCC結構的Ti0Mo0及Ti2Mo2進行室溫(298 K)及低溫(173 K)拉伸試驗。拉伸試驗結果表明在低溫下具有較佳的強度及延展性組合。XRD結果顯示在室溫下，Ti0Mo0及Ti2Mo2在變形前後均維持單一FCC相，而在低溫下則發現了FCC到FCC + HCP相轉。EBSD結果顯示隨著局部應變的增加，<111>織構在平行拉伸方向上的強度增加；相較於室溫拉伸，在低溫拉伸下發現了大量的變形雙晶，暗示著不同溫度下具有不同的變形機制。透過TEM分析近一步了解Ti0Mo0及Ti2Mo2在室溫及低溫拉伸下，不同局部應變下的微結構演變。在室溫下存在明顯的成分相關變形機制轉變，Ti0Mo0主要變形機制為差排滑移，而Ti2Mo2在中及高局部應變下，出現了少量疊差(SF)及變形雙晶，暗示元素鈦及鉬的添加可有效減少疊差能(SFE)，進而導致孿晶誘導塑性(TWIP)效應的產生；在低溫下則表現出顯著的溫度相關變形機制轉變，在低局部應變下除了差排滑移外，也發現大量疊差及變形雙晶的形成；隨著應變量增加，多重塑性變形機制包含疊差、變形奈米雙晶以及HCP層狀結構，表明了在低溫下引入了孿晶誘導塑性(TWIP)及相轉誘導塑性(TRIP)效應。這些現象是主導塑性變形及加工硬化行為的重要機制，透過引入額外的晶界或是相界，不僅可以降低差排移動的平均自由程來提升材料的強度，即所謂的動態Hall-Petch效應，額外的雙晶晶界及相界亦可以作為差排滑移的額外途徑，共同延遲頸縮的發生，提升低溫下的延展性。 總而言之，我們的研究探討了溶質原子鈦及鉬添加對具有協同強化機制和塑性變形機制的CoCrNi中熵合金在室溫和低溫下的微觀結構與機械行為的影響，其結果為未來在室溫及低溫應用中實現了卓越機械性能組合的材料設計提供了潛在的途徑。

Series of (CoCrNi)98–xTixMo2 (TixMo2, x = 1, 2, 3 and 4), (CoCrNi)98–yTi2Moy (Ti2Moy, y = 1, 2, 3 and 4) and (CoCrNi)100–2zTizMoz (TizMoz, z = 0, 1, 2 and 3) medium entropy alloys (MEAs) were fabricated using high-vacuum arc melting to investigate the effects of transition elements, Ti and Mo, on the microstructures and mechanical properties of the CoCrNi-based MEAs. Phase transformations from single face-centered cubic (FCC) phase to FCC + η + σ phases were observed in TixMo2 and TizMoz, while phase transformations from single FCC phase to FCC + σ phases were observed Ti2Moy, based on the results of X-ray diffraction (XRD), electron backscatter electron (EBSD) and transmission electron microscopy (TEM) analyses. The η precipitates were rich in Ni and Ti while the σ precipitates were rich in Cr and Mo. The grain size decreased slightly with the increasing solute atoms but significantly in the presence of precipitates, while the strength and hardness increased slightly with the increasing solute atoms but significantly in the presence of precipitates, which indicated the introduction of solid solution strengthening by the addition of solute atoms and precipitation strengthening by the formation of precipitates. The nanohardness of FCC matrix, η and σ precipitates were approximately 6, 9 and 12 GPa, respectively. The optimum mechanical properties of yield strength, ultimate tensile strength, fracture elongation, and microhardness of 953.70 MPa, 1319.09 MPa, 22.09% and 424 HV, respectively, were obtained in Ti3Mo3 compared to 417.74 MPa, 813.14 MPa, 66.28%, and 226 HV in Ti0Mo0. The synergistic strengthening mechanisms including solid solution strengthening, grain boundary strengthening and precipitation strengthening were examined, and there was a high correlation between the measured and calculated yield strengths. In this research, we also investigated the microstructural evolution and plastic deformation mechanisms of Ti0Mo0 and Ti2Mo2 MEA with single FCC phase at 298 K and 173 K. There is a pronounced composition-dependent transition of deformation mechanism at 298 K, from the typical dislocation slip of Ti0Mo0 to the cooperative plastic deformation of stacking faults and deformation nano-twins of Ti2Mo2. This suggests an effective reduction of stacking fault energy through Ti/Mo co-doping, resulting in the emergence of twinning-induced plasticity (TWIP) effect. Remarkably, TizMoz exhibits temperature dependent mechanical behavior with concurrent increases of strength and ductility at cryogenic temperature. There is a notable transition from the dislocation slip at 298 K to the synergistic plastic deformation of stacking faults, deformation nano-twins and hexagonal close-packed (HCP) lamellar structures at 173 K. The activations of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP), resulting from the hierarchical deformation nano-twins and HCP structures. The findings served as significant mechanisms for plastic deformation mechanisms and work hardening behaviors and collaboratively delay the onset of necking for improved ductility at 173 K. Overall, our research investigated the effects of solute atom Ti and Mo additions on microstructural and mechanical behaviors of CoCrNi MEA with synergistic strengthening mechanisms and plastic deformation mechanisms at room temperature and cryogenic temperature. Our results offered potential avenues for future material design of superior mechanical property combination at both room-temperature and cryogenic applications.