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Carsbring, Amanda. "Heat Treatment and Secondary Phase Formation in FeCrNi Medium Entropy Alloys." Thesis, KTH, Materialvetenskap, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-291251.
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The topics of high entropy alloys (HEA) and medium entropy alloys (MEA) have been heavily researched in recent years. A HEA usually consists of five or more base elements, and a MEA would have three or four base elements. These types of alloys are multi-principal element alloys (MPEA) that have been thought to have interesting properties due to their high configurational entropy, which was thought to be the reason for stabilized simple solid solution phase in the HEA. The high entropy effect contributing to stable single phase in these alloys has been discussed and has not been found to be a predicament to which MPEA that will present as single phase at lowered temperatures. Still, some of the HEA and MEA investigated have interesting properties such as high ductility and good thermal properties, as is the case for the commonly researched CoCrFeMnNi HEA and the CoCrNi MEA which are both solid solution FCC phase at lower temperatures. This master thesis aims to investigate one of the less commonly researched MEA: equimolar FeCrNi. This alloy has been studied previously, and it was found there might be a possibility of precipitation hardening the alloy. To further study this alloy system, three FeCrNi alloys in the close-to equimolar range were produced and underwent a series of aging heat treatments to study the amount of precipitated secondary phase with composition changes and different aging temperatures. The objective is to evaluate and interpret the data found in the different CALPHAD databases used in Thermo-Calc and FactSage software and make comparisons to the experimental results. This to discuss the possibilities of hardening this alloy through aging treatment. The alloys selected and produced are 33Fe33Cr33Ni, 40Fe30Cr30Ni and 45Fe30Cr25Ni, all in mol%. Through experimental investigation using x-ray diffraction (XRD) analysis, it is found that Cr-rich BCC phase is formed in all alloys after most of the aging treatments performed. The volume fraction of BCC was quantified through the reference intensity ratio (RIR) method. From quantification, the largest volume fraction BCC is found in the equimolar 33Fe33Cr33Ni alloy, and the lowest fraction BCC is shown in the 40Fe30Cr30Ni alloy. The increased volume fraction of BCC coincides with an elevated hardness in all three alloys. It is also found that out of the three equilibrium phase calculations used in this project, the ThermoCalc steel database TCHEA4 seems to give results that are in closest agreement with the experimental results. For future studies in this subject, the recommendation is to further study the mechanical properties of the FeCrNi MEA and assess possibilities for application.<br>Högentropilegeringar och mediumentropilegeringar har studerats närmre de senaste åren på grund av deras intressanta mekaniska egenskaper. En högentropilegering består vanligtvis av fem eller flera baselement, och en mediumentropilegering har tre eller fyra baselement. Detta skiljer dem från konventionella legeringar som i regel har ett, maximalt två, baselement. Dessa typer av multi-baslegeringar har ansetts ha intressanta egenskaper på grund av deras höga konfigurationsentropi, vilken tros vara orsaken till stabilisering av fast lösningsfas i legeringarna. Högentropieffekten som bidrar till stabil enfas i dessa legeringar har diskuterats och har emellertid inte visat sig vara tillförlitligt för att förutsäga vilka multi-baslegeringar som blir fast lösningsfas vid ett lägre temperaturintervall. Ändå har några av de undersökta legeringarna intressanta egenskaper som hög duktilitet och goda termiska egenskaper, vilket är fallet för högentropilegeringen CoCrFeMnNi och mediumentropilegeringen CoCrNi som båda är enkel FCC-fas vid lägre temperaturer. Detta examensarbete syftar till att undersöka en av de mindre omnämnda mediumentropilegeringarna: ekvimolär FeCrNi. Denna legering har studerats tidigare, och det visade sig att det kan finnas en möjlighet att utskiljningshärda legeringen. För att ytterligare studera detta legeringssystem producerades tre FeCrNi-legeringar i ett sammansättningsintervall nära ekvimolär sammansättning, som sedan genomgick en serie åldringsbehandlingar för att studera mängden utskild sekundär fas beroende på sammansättning och åldringstemperatur. Målet är att utvärdera och tolka data som finns i de olika CALPHAD-databaserna som används i beräkningsverktygen Thermo-Calc och FactSage, och därefter göra jämförelser med experimentresultaten. Detta för att diskutera möjligheterna att härda dessa legeringar med partikelutskiljning. De legeringar som valts och tillverkats är 33Fe33Cr33Ni, 40Fe30Cr30Ni och 45Fe30Cr25Ni, alla angivna i molprocent. Genom experimentella undersökningar med röntgendiffraktionsanalys hittades att BCC-fas med höga halter Cr bildats i alla legeringar efter majoriteten av åldringsbehandlingarna som utförts. Volymfraktionen av BCC kvantifierades genom beräkningar från referensintensitetsförhållande (RIR). Från kvantifiering fås den största volymfraktionen BCC i den ekvimolära 33Fe33Cr33Ni-legeringen, och den lägsta fraktionen BCC i 40Fe30Cr30Ni-legeringen. Högre volymfraktion BCC sammanfaller med en förhöjd hårdhet i alla tre legeringarna. Det visar sig också att utav de tre jämviktsfasberäkningarna som används i detta projekt så är det ThermoCalc-ståldatabasen TCFE10 som gett resultat som överensstämmer med experimentresultaten. För framtida undersökningar inom detta område rekommenderas att studera de mekaniska egenskaperna hos FeCrNi och bedöma möjligheterna för tillämpning av materialet.
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Yoshida, Shuhei. "Microstructure and mechanical properties of face-centered cubic high/medium entropy alloys：From a viewpoint of heterogeneity on atomic-scale." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263618.
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Slone, Connor. "Influence of composition and processing on the mechanical response of multi-principal element alloys containing Ni, Cr, and Co." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555522223986934.
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Gao, Peng. "Effects of grain size, strain rate and temperature on the mechanical properties and microstructures of CrCoNi medium-entropy alloys." Thesis, The University of Sydney, 2021. https://hdl.handle.net/2123/25042.
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In conventional alloys, a single metallic element is used as the dominant constituent. For example, Fe is the major constituent of steels. Further alloying with small amounts of other elements is a common way to obtain different mechanical properties. About two decades ago, high-entropy alloys (HEAs) were introduced as a new class of alloys. It is believed that mixing multi-principal elements with different crystallographic structures and atomic sizes to a high degree can achieve single crystallographic structures. For example, one of the most commonly studied HEAs, CrMnFeCoNi, contains five equimolar constituents and shows a single face-centred-cubic (FCC) structure. As a new class of alloys, HEAs show promise in achieving outstanding mechanical properties. In 2014, a CrMnFeCoNi alloy with an exceptional combination of strength and fracture toughness was reported. Its ductility is higher at 77 K than at room temperature due to extensive deformation twinning activity. Then, in 2016, a medium-entropy alloy (MEA), CrCoNi, was reported to exhibit even better strength and ductility at cryogenic temperatures than HEAs. Deformation twinning also plays an important role in its exceptional mechanical properties. Since then, CrCoNi alloys have been studied with great interest.
Although the ductility of MEAs is outstanding at ambient and cryogenic temperatures, their yield strengths are relatively low, being ~ 450 MPa at room temperature and ~ 700 MPa at 77 K. Grain refinement is arguably the most effective way to further improve strength. It has been reported that nanocrystalline structures in MEAs can provide outstanding strength while maintaining reasonably good plasticity.
Mechanical properties are influenced not only by grain size but also by temperature and deformation strain rate. However, there are very few reports on the dynamic mechanical behaviours of MEAs, with most previous studies having focused on dynamic deformation at room temperature. Their strength and ductility at cryogenic temperatures are largely unknown.
In this study, CrCoNi MEAs with nanocrystalline structures are used as model materials to investigate their outstanding overall mechanical properties. The mechanical performance of bulk CrCoNi alloys is explored at cryogenic temperatures and high strain rate. Microstructural evolution at various strain rates and temperatures is characterised.
Chapter 1 provides a background to H/MEAs. Key aspects of these alloys are reviewed, including their classification, core features and major alloying elements. The main synthesis methods are introduced. Factors that influence mechanical properties are reviewed, such as grain size, twinning, temperature and strain rate. Research gaps and goals related to CrCoNi alloys are identified based on the literature review.
Chapter 2 presents the main experimental principles and detailed procedures used in this thesis, including material synthesis and preparation, mechanical tests and microstructural characterisation. Nano-structured samples made with a sputtering magnetron were tested via in-situ compression. Bulk samples with coarse grains were tested at dynamic strain rates and cryogenic temperatures.
Chapter 3 reports on an ultra-strong CrCoNi MEA with a columnar nanostructure. The mechanical behaviours of the alloy were explored by compression tests of micro-pillars in different directions. Pillars compressed at 90° to the grain-growth direction presented an exceptional yield strength of 5 ± 0.5 GPa, which is the highest reported among FCC pillars. This record high strength is mainly ascribed to the presence of nanotwins and columnar grain boundaries.
Chapter 4 describes a coarse-grained CrCoNi alloy possessing a combination of superior strength and ductility at a high strain rate and cryogenic temperatures. While its strength was dramatically enhanced at a higher strain rate and lower temperatures, its ductility also increased. At 77 K, the yield strength, ultimate tensile strength (UTS) and true strain reached 872 MPa, 1,809 MPa and 54%, respectively. This alloy exhibits the best cryogenic dynamic mechanical properties ever reported.
Chapter 5 describes microstructural evolution in CrCoNi alloys at various temperatures and strain rates. There was a combined positive effect of strain rate and cryogenic temperature on twin formation. The density of twins at a temperature of 77 K and strain rate of 2000 s-1 was significantly higher than that at room temperature under quasi-static testing. In addition to extensive twinning, there was significant grain refinement in fracture areas due to dislocation slip and dynamic recrystallisation.
Chapter 6 presents the conclusions of this thesis and summarises important future tasks.
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Moravčík, Igor. "Metal Matrix Composites Prepared by Powder Metallurgy Route." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2017. http://www.nusl.cz/ntk/nusl-367507.
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Ve všeobecnosti, poznatky o design slitin, jejich výrobě a výběru legujúcich prvků sú omezené na slitiny s jedním základním prvkem. Tento fakt ale výrazně limituje možnosti a volnost výběru prvků pro dosáhnuti speciálních vlastností a mikrostruktur. V poslední dekádě se ukázalo, že materiálová věda a inženýrství nejsou ještě zdaleka prozkoumané v důsledku objevu nové třídy materiálů nazvané vysoko entropické slitiny (HEA high entropy alloys). Jejich objev upoutal pozornost vědecké komunity. Základní koncept pro jejich design je, že namísto jednoho, nebo dvou základních prvků obsahují minimálně 5 prvků v podobných atomových koncentracích. V posledních letech se objevila skupina materiálů odvozená od HEA, nazvaná slitiny so střednou entropii (MEA medium entropy alloys). Na rozdíl od HEA ale obsahují 3, nebo 4 prvky. Táto práce je věnovaná studiu přípravy a charakterizaci HEA, MEA a jejich kompozitů s pomocí metod práškové metalurgie. V této práci byli dohromady zkoumány tři kompozice: AlCoCrFeNiTi0.5, Co1.5Ni1.5CrFeTi0.5 a CoCrNi, kompozity s kovovou matricí (MMC metal matrix composites) vyztužené částicemi B4C s CoCrNi jako matricí. Hloubková mikrostrukturní a mechanická analýza těchto materiálů byla provedena pomoví metod rastrovací a transmisní elektronové mikroskopie spojené s tahovými a ohybovými zkouškami. V průběhu celé studie se objevovaly problémy s kontaminací kyslíkem, co se projevilo vznikem značného množství oxidů v připravených materiálech. U Slitiny AlCoCrFeNiTi0.5 byla naměřena tvrdost přesahující 800 HV. Její houževnatost ale byla velice omezena. V její mikrostruktuře byly identifikovány částice in-situ TiC v důsledku přítomnosti organického, anti-aglomeračního činidla (metanolu) v mlecí misce. Tato reakce může být použita v budoucnu k přípravě MMC se záměrnou disperzí TiC. Na druhé straně, slitina CoCrNi ukázala vysoké hodnoty tažnosti (26%) a meze pevnosti přes 1000 MPa. Mikrostruktura obsahovala majoritní FCC fázi s BCC precipitáty. Tahle slitina byla z důvodu vysoké tažnosti zvolena pro přípravu kompozitu s výztuží B4C. V průběhu slinování ale došlo k reakci mezi přítomným Cr a B4C, které výsledkem byl Cr5B3 borid. Tento kompozit mel pevnost v tahu 1400 MP a extrémne jemnozrnnou strukturu. Celková tažnost ale klesla na 1.9 %. Slitina AlCoCrFeNiTi0.5, která mela strukturu složenou jen z FCC tuhého roztoku dosáhla nejlepší kombinaci mechanických vlastností s pevností přesahující 1300 MPa a dostatečnou tažností 4%. Prášková metalurgie se ukázala jako vhodná metoda pro přípravu HEA a MEA slitin a jejich kompozitů, s dobrou kombinací pevnosti a tažnosti. Tato metoda dovoluje měnit mikrostrukturní parametry připravených materiálů jednoduchou úpravou parametrů procesu.
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Havlíček, Štěpán-Adam. "Vysoce entropické slitiny Cantorova typu zpevněné disperzí nitridů." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2020. http://www.nusl.cz/ntk/nusl-432460.
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High Entropy Alloy (HEA) is a class of construction steels based on the mixing of five or more elements in approximately equimolar ratios. Despite the ambiguity of their future use, HEAs represent a significantly new group of construction materials that are currently receiving a great deal of attention. Single-phase HEAs fail when used at elevated tempera-tures. The improvement of their high-temperature resistance was achieved by introducing a dispersion of oxides Al2O3 and Y2O3. To generalize the positive effect of dispersions on the mechanical properties at elevated temperatures, particles of a similar nature were cho-sen. These were dispersed particles of nitrides: hardness-incompatible AlN and hardness-compatible BN. The particles were evenly distributed inside the alloys by mechanical al-loying and compacted by SPS (Spark Plasma Sintering). The new structural alloy reached a density higher than 96.5 % and brought an increase in yield strength at room tempera-ture of up to 67 % and 40 % at elevated temperatures, while maintaining a homogeneous distribution of input powders.
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Liao, Yu-Chin, and 廖俞欽. "Alloy design and microstructure evolution of the medium-low density high entropy alloys." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/99773525423293429433.
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碩士<br>國立中央大學<br>機械工程學系<br>105<br>In this study, we tried to design the medium-low density high entropy alloys (HEAs) (contains at least 5 elements and the density of the alloys are below 5 g/cm3) by empirical phase-formation rules. Meanwhile, these HEAs should remain a structure of single phase or two phases after heat treatment. Therefore, we start to study the HEAs from the TiAlX alloy (3 elements), then add the fourth and fifth elements step by step based on the parameter calculation which related to the requirement of HEA. The high entropy alloys ingots were firstly prepared by vacuum arc-melting and drop casting, and then homogenizing at 800℃for 24h in vacuum. The as-cast and heat treated samples were characterized by XRD analysis, OM and SEM/ESD examination, and Vickers’ hardness test. The XRD results reveal that the as-cast TiAlVCrFe (=5.72 g/cm3), TiAlVCrMn (=5.55 g/cm3), TiAlVCrCu (=5.61 g/cm3) HEAs showed the BCC single structure with hardness around 670 Hv. Moreover, the hardness of TiAlVCrMn and TiAlVCrFe HEAs can reach to 800 Hv after annealing at 800°C for 24 hr in vacuum due to the precipitation hardening. It is believed that the goal of medium-low density for the 5-components HEAs can be achieved by increasing the Ti and Al molar ratio in our further study.
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Chen, Chun-Cheng, and 陳俊丞. "Mechanical Deformation Behavior of Nickel-Titanium Compound-Based Low-, Medium- and High-Entropy Alloys." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/vzcumb.
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Chen, Wen-Ju, and 陳玟儒. "Mechanical Properties and Deformation Behavior of Body-centered Cubic Low-, Medium- and High-entropy Alloys." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/kw9nkv.
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Huang, Tai-Jan, and 黃泰然. "Anisotropic Mechanical Properties and Deformation Behaviors of Face-centered Cubic Low-, Medium- and High-entropy Alloys." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/zdfk55.
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碩士<br>國立清華大學<br>材料科學工程學系<br>105<br>Metal has always been a crucial part of human society with prestigious status in the field of material science development history. However the abundant research done on metal and alloy also resulted in the lack of marginal breakthrough in modern days. During last decade, a brand new alloying system called “High Entropy Alloys” (HEAs) was presented and gradually gaining attention from all associations, its special mechanical properties and element composition has dawned a new era of alloy materials. Due to apparent differences of formation rules between HEAs and traditional alloys, current researches toward HEAs are still far from successfully providing results to explain the core effect and compositional theory of HEAs. This research was done on a series of FCC alloys consists of traditional low-entropy NiW alloys, medium-entropy alloy NiCrFe and HEA NiCrFeMnCo by conducting mechanical and structural experiment respectively in both macro and micro scales. Macro scale experiments included Vickers hardness and density measurement. Micro scale experiments included nano indentation, in-situ SEM micro pillar compression and in-situ TEM nano pillar compression, respectively performed on (100), (110) and (111) crystal planes of alloys with different entropy status. Micro scale crystal structure differences between traditional alloys and HEAs was obtained via high intensity X-ray diffraction. By analyzing mentioned experiment results with theoretical information, we found numerous distinct differences in both deformation behavior and crystal structure between traditional alloys and HEAs, providing further insight for future study of HEA’s core effect and compositional theory.
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Su, Che-Hsuan, and 蘇哲萱. "A study on Hall-Petch relationship and grain growth of ternary to quinary FCC-structured medium/high entropy alloys." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/6ab79a.
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碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>106<br>In this study, recrystallization behavior and grain growth characteristics of homogenized and 80% cold-rolled FeCoNiCrMn、FeCoNiCrPd、FeCoNiCr、FeCoNiMn、CoNiCr and CoNiMn multi-component equiatomic medium / high entropy alloys(MEAs/HEAs) heated at different annealing time and temperature are investigated. The values of δ、∆H_mix and VEC in these six alloys all indicate the formation of single-phase FCC structure. The hardness of recrystallized alloys decreases as the grain size increases, and obeys the Hall-Patch equation, Hv=H0+KHd-1/2. The ternary CoNiCr alloy has the highest KH value, whereas the quinary FeCoNiCrPd alloy has the lowest one. This phenomenum is caused by the largest stacking fault energy and smallest shear modulus exhibited in FeCoNiCrPd alloy. After 80% cold rolling, the surface hardness of the ternary CoNiCr alloy is hardest, indicating that solid solution hardening in MEAs/HEAs is not determined by the number of soluted elements but by the kind of the element. Furthermore, all six alloys have the grain growth exponents 1/n higher than 2 with FeCoNiCrPd alloy having the highest activation energy of grain growth, say 831.9kJ/mol which is much higher than that in the conventional alloys. This characteristic suggests that the effects of the solute drag and the sluggish diffusion control the grain boundary motion. Moreover, the self-diffusion activation energy of the element exhibited in the alloy also affects the MEAs/HEAs activation energy of grain growth.
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"Computational Design of Compositionally Complex 3D and 2D Semiconductors." Doctoral diss., 2020. http://hdl.handle.net/2286/R.I.62929.
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abstract: The structural and electronic properties of compositionally complex semiconductors have long been of both theoretical interest and engineering importance. As a new class of materials with an intrinsic compositional complexity, medium entropy alloys (MEAs) are immensely studied mainly for their excellent mechanical properties. The electronic properties of MEAs, however, are less well investigated. In this thesis, various properties such as electronic, spin, and thermal properties of two three-dimensional (3D) and two two-dimensional (2D) compositionally complex semiconductors are demonstrated to have promising various applications in photovoltaic, thermoelectric, and spin quantum bits (qubits).3D semiconducting Si-Ge-Sn and C3BN alloys is firstly introduced. Density functional theory (DFT) calculations and Monte Carlo simulations show that the Si1/3Ge1/3Sn1/3 MEA exhibits a large local distortion effect yet no chemical short-range order. Single vacancies in this MEA can be stabilized by bond reformations while the alloy retains semiconducting. DFT and molecular dynamics calculations predict that increasing the compositional disorder in SiyGeySnx MEAs enhances their electrical conductivity while weakens the thermal conductivity at room temperature, making the SiyGeySnx MEAs promising functional materials for thermoelectric devices. Furthermore, the nitrogen-vacancy (NV) center analog in C3BN (NV-C3BN) is studied to explore its applications in quantum computers. This analog possesses similar properties to the NV center in diamond such as a highly localized spin density and strong hyperfine interactions, making C3BN suitable for hosting spin qubits. The analog also displays two zero-phonon-line energies corresponding to wavelengths close to the ideal telecommunication band width, useful for quantum communications.
2D semiconducting transition metal chalcogenides (TMCs) and PtPN are also investigated. The quaternary compositionally complex TMCs show tunable properties such as in-plane lattice constants, band gaps, and band alignment, using a high through-put workflow from DFT calculations in conjunction with the virtual crystal approximation. A novel 2D semiconductor PtPN of direct bandgap is also predicted, based on pentagonal tessellation.
The work in the thesis offers guidance to the experimental realization of these novel semiconductors, which serve as valuable prototypes of other compositionally complex systems from other elements.<br>Dissertation/Thesis<br>Doctoral Dissertation Materials Science and Engineering 2020
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Lu, Che-Wei, and 呂哲維. "Electrochemical properties of CoCrFeMnNi high entropy alloy- a comparison with Fe5Mn3CoCr medium entropy alloy." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/6z56du.
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碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>106<br>In recent years, Fe5Mn3CoCr medium entropy alloy (MEA) has been reported as an alloy system which shows transformation-induced plasticity effect during deformation. The excellent strength and ductility combination of this alloy is even better than CoCrFeMnNi high entropy alloy (HEA), but the corrosion behavior is still unknown. In this study, the corrosion behavior of CoCrFeMnNi HEA and Fe5Mn3CoCr MEA were investigated. The environments chose for the cyclic polarization measurement were 3.5wt% NaCl solution with different pH values (pH=3, 6, 9, 12) and temperatures (10oC, 40oC, 70oC) and 0.1M H2SO4 solution. The results show that, compared with CoCrFeMnNi HEA, Fe5Mn3CoCr MEA the had smaller passivation region in every test environment, but the two substrate had almost the same corrosion current density value in 3.5wt% NaCl solution without any adjustment. In 0.1M H2SO4 solution, the corrosion current density of Fe5Mn3CoCr MEA is higher than CoCrFeMnNi HEA. The surface morphologies of CoCrFeMnNi HEA after polarization measurement in 3.5wt% NaCl solution exhibited a pitting appearance, which was resulted from the disintegration of inclusions on surface; Fe5Mn3CoCr MEA showed “lath-shaped” corrosion morphologies which were caused by the connection of pits. After cyclic polarization in 0.1M H2SO4 solution, CoCrFeMnNi HEA showed intergranular corrosion and galvanic corrosion morphologies; Fe5Mn3CoCr MEA showed “branch-like” corrosion morphologies which is attributed to the dissolution of ε martensite.
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Ye, Han-Ting, and 葉翰庭. "Micro/nano-scaled creep mechanism of TiAlV medium/light weight high entropy alloy." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/5kw55t.
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碩士<br>國立中山大學<br>材料與光電科學學系研究所<br>106<br>The high entropy alloys (HEAs) is a new emerging class of metallic alloys which are frequently considered as potential structural materials for high-temperature applications. Nowadays, several refractory HEAs compositions based on transition element have demonstrated remarkable compressive strength at elevated temperatures. But their high density over 8.0 g/cm3 would restrict the application range and the development foreground of HEAs mostly.
In this study, we use novel medium-density TiAlV HEA as material. The theoretical density of TiAlV equiatomic alloy is about 4.44 g/cm3 which is much lower than that of conventional Fe based HEAs. The TiAlV alloy is firstly investigated by characterizing its microstructure and mechanical properties. In addition to the room temperature (RT) properties, the high temperature (HT) performance is studied by using nanoindenter to extract creep behavior. The results were compared with the traditional FeCoNiCrMn and FeCoNiCrMn-Al high density high entropy alloys, as well as with commercial Ti-6Al-4V alloys.
The experimental results reveal that TiAlV possess high hardness in room temperature. The Vickers hardness of TiAlV is about 543±14 Hv and the nano-scaled hardness is about 7.2±0.1 GPa. On the other hand, the results of creep testing show that at the temperature regime from 400 to 600oC, under a normalized stress level (σ/E) of 1.3x10-2, the stress exponents for TiAlV are found to be 4.7 from 400 to 450οC and 3.5 from 500 to 600oC, or the strain rate sensitivities are about 0.21 and 0.29 separately. The results indicate the dislocation climb power law creep as the dominant creep mechanism. In addition, although the TiAlV high entropy alloy has higher strength and hardness at room temperature, the creep activation energy of TiAlV (208±7 kJ/mol) is lower than those of FeCoNiCrMn (259±10 kJ/mol), FeCoNiCrMn-Al (260±8 kJ/mol) and Ti-6Al-4V (280 kJ/mol) in high temperature creep experiments. In terms of activation volume, TiAlV also has lower activation volume than those of FeCoNiCrMn, FeCoNiCrMn-Al and Ti-6Al-4V.
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Lu, Yi-Sheng, and 盧易聖. "Electrochemical Properties and Passive Film Characterization of Fe50Mn30Co10Cr10 Medium Entropy Alloy in Acid Environment." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/g35pc6.
Full textAbstract:
碩士<br>國立臺灣大學<br>工程科學及海洋工程學研究所<br>107<br>The design of high-entropy alloy is based on the “cocktail effect”, which can yield the alloy with superior properties for the combination of mechanical performance and anti-corrosion resistance because every element can provide their specific natures. Although past studies have emphasized that high-entropy alloys tended to form a single-phase solid solution, they are limited to the ideal case of the maximum mixing entropy. The Fe50Mn30Co10Cr10 medium entropy alloy (MEA) is a novel entropy alloy proposed in recent years, whose plastic deformation behavior revealed high-strength and high- ductility because of “transformation induced plasticity (TRIP)" effect. It was superior to equiatomic CoCrFeMnNi high entropy alloy (HEA). Regarding MEA applied to pipelines or workpieces, it is necessary to establish the corrosion resistance of MEA in acid solutions. In this study, the dilute sulfuric acid with a molarity of 0.1 molars was used to simulate the acidic environment, and then the electrochemical behavior of MEA and its passive film were investigated to evaluate the corrosion resistance of the material, compared with HEA. Finally, the composition of the passive film was characterized and relationship between the composition and corrosion resistance was explored.
The polarization curve showed that MEA had relatively higher corrosion density and lower corrosion potential than HEA, however, they displayed the similar passive current densities. After polarization, the surface of both alloys exhibited intergranular corrosion attacking, especially MEA occurred intense corrosion obviously distinguishable along the ε-martensite plates. Besides, the Galvanic corrosion effect occurred around the inclusions embedded in HEA. On the other hand, under the condition of high potential, the manganese dioxide film precipitated on the surface of both alloys led to the current density decreasing again, which is known as “secondary passivation behavior”. The existence of the MnO2 film was verified by XPS.
After passivation two hours at passive potential, a comprehensive evaluation of the anodic passive films was performed. Both HRTEM and the depth profile of the AES revealed the passive films on MEA is slightly thicker than that on HEA, but the passive films on HEA has higher both stability and corrosion resistance from the result of potential decay, and EIS. High point defects densities of MEA and HEA were carried out through the Mott–Schottky analysis, and MEA with higher doping concentration demonstrated the unstable passive film on the surface. These phenomena were the grounds that passive films had higher chromium oxide content and lower manganese content. Therefore, this study suggests that the first passivation (at low potential) depended on chromium oxides or hydroxides causing passivation behavior. When the potential rose, the second passivation behavior pertained to the generation of manganese oxides or hydroxides to cause current density reducing. In short, addition of manganese is unfavorable to the stability of the first passive film, but it is dominant species of secondary passivation behavior at high working potential.