新世代輕量中熵合金之合金設計、微結構及其性質分析之研究;The study of alloy design, microstructure and properties characterization of novel lightweight Medium Entropy Alloys

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Title:	新世代輕量中熵合金之合金設計、微結構及其性質分析之研究;The study of alloy design, microstructure and properties characterization of novel lightweight Medium Entropy Alloys
Authors:	廖俞欽;Liao, Yu-Chin
Contributors:	機械工程學系
Keywords:	高熵合金;中熵合金;輕量材料;熱機處理;異構組織;機械性質;high-entropy alloys;medium-entropy alloys;lightweight;thermomechanical treatment;heterogeneous structure;mechanical property
Date:	2021-08-04
Issue Date:	2021-12-07 13:27:09 (UTC+8)
Publisher:	國立中央大學
Abstract:	近年來，由高熵合金以及中熵合金所構成的多元合金為材料領域熱門研究話題，打破既有合金設計規則，將合金世界的組成原則徹底翻轉。此研究應用創新的多元合金組成概念，開發國際首創輕量化新世代中熵合金，除了擁有低密度特性且具有優異的機械性質。透過合金改質可以有效的提升鑄造合金之性質，不只在機械性質有顯著的提升且更是展現富鈦中熵合金的無窮潛力。更出色的是，透過熱機處理可以使合金比強度提升至可衝擊國際期刊之水準。最後也進行大尺寸塊材的開發，與實驗室尺寸相對應提供可靠性，為未來產業應用鋪下一條康莊大道。此研究是未來國際上研究富鈦中熵合金發展之藍圖，將成為材料領域之領頭羊，應用創新材料以及創新強化理論推廣至台灣產業端，促進國家科技發展，引領台灣產業走向國際舞台。;Recently, multi-principal elements alloys (MPEAs) become a one of the breakthroughs in the news class of materials world due to the extremely complex composition and the plenty of excellent properties. They are generally called as high entropy alloys(HEAs) and medium entropy alloys(MEAs), and also widely known as complex-concentrated alloys (CCAs). Most of well-known and well-studied MPEAs contain large proportion of heavy transition metal elements. In contrast, less attraction was held in lightweight MPEAs areas due to the limited selections of elements. However, development of lightweight materials with high strength and ductility is the hot issue in the structural materials community not only for energy transformation efficiency but the fuel consumption. Hence, designing a lightweight (~ 5 g/cm3) MPEAs with well-balanced strength-ductility synergy (1500 MPa, 20%) is the core goal of this research. Firstly, a non-equiatoic quaternary alloy system, Tix(AlCrNb)100-x, was designed by using CALPHAD (acronym of the calculation of phase diagrams). The simulation results show that a single body centered cubic (BCC) phase can be formed, being stable at temperatures higher than 950?C. All of these alloys exhibit superior mechanical properties with high yield strength and high plasticity (more than 30% plastic strain) at room temperature. Meanwhile, the Ti-65 alloy demonstrated a tensile elongation up to 32% plastic strain and 1,200 MPa fracture strength for the sample with homogenization treatment for 24 h. The specific tensile strength can reach to 0.243 GPa?cm3/g. Secondly, solid-solution strengthening was used to expand the potential of alloys designing and also improve the mechanical properties of cast Ti-65 alloys. Lightweight nonequiatomic Ti60(Al)x(VCrNb)40-x (x = 6, 8, 10, 12, and 18 at.%) medium-entropy alloys (MEAs) were designed by CALPHAD. The density of these cast alloys decreased with an increase in Al concentration from 5.45 to 4.79 g/cm3. All of these cast alloys exhibited a body-centered cubic (BCC) microstructure. However, a nano-sized ordered B2 phase was identified in the cast alloys with higher Al concentrations (Al-12 and Al-18). The prediction of BCC phase formation using CALPHAD was consistent with the experimental results. These BCC-structured alloys can withstand over 50% strain at room temperature, which demonstrates excellent compressive ductility. Moreover, the results demonstrate that the as-cast Al-6 and Al-8 samples had superior plasticity under tensile testing, with a tensile strength of 1120 MPa and approximately 30% plastic strain. Furthermore, with an increase in Al concentration, the alloys exhibited a notable trend in yield strength and a decreasing trend in plastic strain. The change in mechanical properties of these MEAs caused by the formation of B2 nanoparticles was also investigated. Thirdly, thermomechanical treatment was conducted on Ti65(AlCrNb)35 medium-entropy alloy(Ti-65) to further improve the mechanical properties by dislocation strengthening. Ti-65 ingots were produced by arc-melting and drop casting in a water cooled-copper mold. Then these alloy ingots were treated by the process sequence of homogenization, hot rolling, cold rolling, and recrystallization. The effect of thermomechanical treatment (TMT) on microstructures and mechanical properties were investigated using X-ray diffraction (XRD), Electron back-scattered diffraction analysis in scanning electron microscope (SEM-EBSD), and mechanical testing instrument. Results of XRD show that Ti-65 alloy maintained BCC structure after 50% hot-rolling, 70% cold-rolling, and recrystallization at 900℃, 1000℃ and 1100℃, respectively. The fully recrystallized sample has 80% smaller grain size than the as-cast sample. Meanwhile, the cold-rolled Ti-65 alloy specimen exhibited high tensile strength of 1620 MPa. Moreover, in comparison with the tensile strength of as-cast Ti-65 sample (1100 MPa), a significant increase in tensile strength (1380 MPa) for the Ti65 alloy after partial recrystallization annealing. The enhancement of tensile strength is attributed its hetero-structure composed of deformed bands and smaller recrystallized grains. This demonstrated that excellent ductility-strength synergy of this Ti-65 alloy can be achieved through various thermomechanical treatment. In order to break through the limits of strength-ductility trade-off, rapid annealing treatment was further optimized to obtain the heterogeneous structured Ti-65 MEA with extraordinary mechanical tensile properties (strength of 1500 MPa, ductility of 25%). Finally, large dimension Ti-65 MEAs (2 kg) are produced by induction skull melting. The microstructure and the mechanical hardness of these cold-rolled and annealed specimens are explored. The similar results reveal that the Ti-65 MEA appears to be a promising material for industrial application. Overall, the aim of this research is to design the novel lightweight Ti-rich MEAs systems with promising mechanical tensile properties. Traditional and innovative strengthening mechanisms were used to push the limits of mechanical properties of these investigated MEAs. In addition to exploring the effect of strengthening mechanisms, this work provides the blueprint for developing the heterogeneous structured lightweight Ti-rich MEAs from alloys design, optimization of mechanical properties to commercial production. Undoubtedly, this is only the beginning, the various lightweight MEAs systems with extraordinary mechanical tensile properties will be developed in the future, and these alloys with immense potential absolutely will become the game changers in the structural materials community.
Appears in Collections:	[Graduate Institute of Mechanical Engineering] Electronic Thesis & Dissertation

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