| 1. |
- Huang, Shuo, et al.
(författare)
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Phase stability and magnetic behavior of FeCrCoNiGe high-entropy alloy
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:25
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Tidskriftsartikel (refereegranskat)abstract
- We report an alternative FeCrCoNiGe magnetic material based on FeCrCoNi high-entropy alloy with Curie point far below the room temperature. Investigations are done using first-principles calculations and key experimental measurements. Results show that the equimolar FeCrCoNiGe system is decomposed into a mixture of face-centered cubic and body-centered cubic solid solution phases. The increased stability of the ferromagnetic order in the as-cast FeCrCoNiGe composite, with measured Curie temperature of 640 K, is explained using the exchange interactions.
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| 2. |
- Huang, Shuo, et al.
(författare)
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Thermal expansion in FeCrCoNiGa high-entropy alloy from theory and experiment
- 2017
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 110:24
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Tidskriftsartikel (refereegranskat)abstract
- First-principle alloy theory and key experimental techniques are applied to determine the thermal expansion of FeCrCoNiGa high-entropy alloy. The magnetic transition, observed at 649 K, is accompanied by a significant increase in the thermal expansion coefficient. The phase stability is analyzed as a function of temperature via the calculated free energies accounting for the structural, magnetic, electronic, vibrational and configurational contributions. The single-and polycrystal elastic modulus for the ferro-and paramagnetic states of the face-centered and body-centered cubic phases are presented. By combining the measured and theoretically predicted temperature-dependent lattice parameters, we reveal the structural and magnetic origin of the observed anomalous thermal expansion behavior. Published by AIP Publishing.
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| 3. |
- Molnar, David, et al.
(författare)
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Deformation properties of austenitic stainless steels with different stacking fault energies
- 2018
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Ingår i: Materials Science Forum. - Switzerland : Trans Tech Publications Ltd. - 0255-5476 .- 1662-9752. ; 941, s. 190-197, s. 190-197
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Tidskriftsartikel (refereegranskat)abstract
- In FCC metals a single parameter – stacking fault energy (SFE) – can help to predict the expectable way of deformation such as martensitic deformation, deformation twinning or pure dislocation glide. At low SFE one can expect the perfect dislocations to dissociate into partial dislocations, but at high SFE this separation is more restricted. The role of the magnitude of the stacking fault energy on the deformation microstructures and tensile behaviour of different austenitic steels have been investigated using uniaxial tensile testing and electron backscatter diffraction (EBSD). The SFE was determined by using quantum mechanical first-principles approach. By using plasticity models we make an attempt to explain and interpret the different strain hardening behaviour of stainless steels with different stacking fault energies.
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| 4. |
- Molnár, Dávid Sándor, et al.
(författare)
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Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel
- 2019
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 759, s. 490-497
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Tidskriftsartikel (refereegranskat)abstract
- The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial dislocation ribbon, and shows strong correlation with the leading plastic deformation modes. Based on the SFE, one can estimate the critical twinning stress of the system as well. The SFE mainly depends on the composition of the system, but temperature can also play an important role. In this work, using first principles calculations, electron backscatter diffraction and tensile tests, we show a correlation between the temperature dependent critical twinning stress and the developing microstructure in a typical austenitic stainless steel (316L) during plastic deformation. We also show that the deformation twins contribute to the strain hardening rate and gradually disappear with increasing temperature. We conclude that, for a given grain size there is a critical temperature above which the critical twinning stress cannot be reached by normal tensile deformation, and the disappearance of the deformation twinning leads to lower strain hardening rate and decreased ductility.
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| 5. |
- Molnár, Dávid Sándor, et al.
(författare)
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Experimental study of the γ-surface of austenitic stainless steels
- 2019
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 173, s. 34-43
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a theory-guided experimental approach to study the γ-surface of austenitic stainless steels. The γ-surface includes a series of intrinsic energy barriers (IEBs), which are connected to the unstable stacking fault (USF), the intrinsic stacking fault (ISF), the unstable twinning fault (UTF) and the extrinsic stacking fault (ESF) energies. The approach uses the relationship between the Schmid factors and the effective energy barriers for twinning and slip. The deformation modes are identified as a function of grain orientation using in situ electron backscatter diffraction measurements. The observed critical grain orientation separating the twinning and slip regimes yields the USF energy, which combined with the universal scaling law provides access to all IEBs. The measured IEBs and the critical twinning stress are verified by direct first-principles calculations. The present advance opens new opportunities for modelling the plastic deformation mechanisms in multi-component alloys.
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| 6. |
- Molnár, Dávid Sándor, 1990-
(författare)
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Generalised stacking fault energy and plastic deformation of austenitic stainless steels
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Austenitic stainless steels are primarily known for their exceptional corrosion resistance. They have the face centred cubic (FCC) structure which is stabilised by adding nickel to the Fe-Cr alloy. The Fe-Cr-Ni system can be further extended by adding other elements such as Mn, Mo, N, C, etc. in order to improve the properties. Since austenitic stainless steels are often used as structural materials, it is important to be able to predict their mechanical behaviour based on their composition, microstructure, magnetic state, etc.In this work, we investigate the plastic deformation behaviour of austenitic stainless steels by theoretical and experimental approaches. In FCC materials the stacking fault energy (SFE) plays an important role in the prediction of the deformation modes. Based on the magnitude of the SFE different deformation modes can be observed such as martensite formation, deformation twinning, dissociated or undissociated dislocation glide. All these features influence the behaviour differently, therefore it is desired to be able to predict their occurrence. Alloying and temperature have strong effect on the SFE and thus on the mechanical properties of the alloys. Several models based on the SFE and more recently on the so called generalised stacking fault energy (GSFE or γ-surface) are available to predict the alloy's affinity to twinning and the critical twinning stress representing the minimum resolved shear stress required to initiate the twinning deformation mechanism. One can employ well established experimental techniques to measure the SFE. On the other hand, one needs to resort to ab initio calculations based on density functional theory (DFT) to compute the GSFE of austenitic steels and derive parameters like the twinnability and the critical twinning stress. We discuss the effect of the stacking fault energy on the deformation behaviour for two different austenitic stainless steels. We calculate the GSFE of the selected alloys and based on different models, we predict their tendency for twinning and the critical twinning stress. The theoretical predictions are contrasted with tensile tests and electron backscatter diffraction (EBSD) measurements. Several conventional and in situ tensile test are performed to verify the theoretical results. We carry out EBSD measurements on interrupted and fractured specimens and during tensile tests to closely follow the development of the microstructure. We take into account the role of the intrinsic energy barriers in our predictions and introduce a new and so far unique way to experimentally obtain the GSFE of austenitic stainless steels. Previously, only the SFE could be measured precisely by well-designed experiments. In the present thesis we go further and propose a technique that can provide accurate unstable stacking fault energy values for any austenitic alloy exhibiting twinning.
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| 7. |
- Molnár, Dávid Sándor, 1990-
(författare)
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Stacking fault energy and deformation behaviour of austenitic stainless steels: a joint theoretical-experimental study
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Austenitiska rostfria stål är främst kända för sin exceptionella korrosionsbeständighet. De har en ytcentrerad kubisk (FCC) struktur som stabiliseras genom att nickel, mangan eller kväve tillsätts till Fe-Cr legeringen. Fe-Cr-Ni-systemet kan utökas ytterligare genom tillsats av andra element såsom Mo, Cu, Ti, C, etc. för att förbättra egenskaperna. Eftersom austenitiska rostfria stål ofta används som konstruktionsmaterial är det viktigt att kunna förutsäga deras mekaniska egenskaper baserat på deras sammansättning, mikrostruktur, magnetiska tillstånd, etc. I denna avhandling är det plastiska deformationsbeteendet hos austenitiska rostfria stål undersökt med teoretiskt och experimentellt. I FCC material spelar staplingsfelsenergin (SFE) en viktig roll vid förutsägelsen och beskrivning av deformationsmekanism. Baserat på storleken av SFE kan olika deformationsmekanismer observeras, såsom martensitbildning, tvillingbildning, eller dislokationsglidning. Alla dessa funktioner påverkar beteendet, därför är det önskvärt att förutsäga och kontrollera deras förekomst. Legering och temperatur har stark inverkan på SFE och därmed legeringarnas mekaniska egenskaper. Flera modeller, baserade på SFE och mer nyligen på den så kallade generaliserade staplingsfelenergin (GSFE eller γ-surface), är tillgängliga för att förutsäga legeringens affinitet till tvillingbildning och den kritiska spänning som representerar den minsta upplösta skjuvspänningen som krävs för att initiera tvillingbildning. Man kan använda ab initio beräkningar baserade på täthetsfunktionalteori (DFT) för att beräkna GSFE för austenitiska stål och härleda parametrar som twinnability och kritisk tvillingsspänning.Korrelation mellan staplingsfelenergi och deformationsbeteendet för fyra olika austenitiska rostavstavning stål diskuteras i detta arbete. SFE för de valda legeringarna erhålls genom ab initio beräkningar och baserat på olika modeller, deras tendens till tvillingbildning och den kritiska tvillingsspänningen kan förutsägas. Deras mekaniska beteende och affinitet till tvilling och martensitisk transformation kartläggs över ett brett temperaturområde (−70°C to +500°C) för de fyra legeringarna. De teoretiska förutsägelserna jämförs med resultat från dragprov och bakåtspridd elektrondiffraktion (EBSD). Flera konventionella och in situ dragprov utfördes för att verifiera de teoretiska resultaten. Vi utförde EBSD-mätningar på dragprov som avbrutits vid olika töjningar och efter brott samt med in situ dragprov för att följa utvecklingen av mikrostrukturen noggrant. Vi tar hänsyn till de inre energibarriärernas roll i våra förutsägelser och presenterar ett nytt sätt att experimentellt få GSFE av austenitiska rostfria stål. Tidigare kunde endast SFE mätas tillförlitligt genom väl utformade experiment. I den aktuella avhandlingen går vi vidare och föreslår en teknik som kan ge noggranna värden för den instabila staplingsfelenergin för alla austenitiska legeringar som uppvisar tvillingbildning på låga spänningsnivåer. Betydelsen av temperatur och mellanliggande legering på mekaniskt beteende undersökt också i detta arbete.
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| 8. |
- Vida, Adam, et al.
(författare)
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Effects of the sp element additions on the microstructure and mechanical properties of NiCoFeCr based high entropy alloys
- 2016
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 669, s. 14-19
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Tidskriftsartikel (refereegranskat)abstract
- The effects of the sp (Al, Ga, Ge, Sn) element additions on the microstructure and mechanical properties of equimolar NiCoFeCr High Entropy Alloys (HEAs) are investigated. The results of X-ray diffraction measurements combined with scanning electron microscopy SEM investigations, as well as the results of nanoindentation test revealed that while the structure of the basic alloy is full FCC, the addition of sp elements has changed it to a multiphase containing both FCC and BCC components, but in different scales. Accordingly, the addition of sp elements strongly increases the strength of the basic state, especially in the case of alloys where the BCC phase is dominant in the microstructure. The physical properties as the Young's- and shear moduli of the investigated HEAs were also determined using ultrasound methods. The correlation between these two moduli suggests a general relationship for metallic alloys.
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| 9. |
- Vida, Adam, et al.
(författare)
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Evolution of the phase structure after different heat treatments in NiCoFeCrGa high entropy alloy
- 2018
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 743, s. 234-239
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Tidskriftsartikel (refereegranskat)abstract
- The non-equilibrium nature of a two-phase NiCoFeCrGa high entropy alloy (HEA) is studied by analyzing its microstructures after different heat-treatment conditions. The microstructure - containing a mixture of well-separated FCC and BCC regions - of the as-cast alloy has changed strongly by heat-treatment for 1 h at 1150 K, significantly changing the ratio of the volume fraction of the FCC and BCC phases. Needle like BCC phase particles evolved inside the original FCC regions when cooling the heat-treated samples to room temperature by rapid or medium rates. In the case of slow cooling, the original BCC regions transformed into a mixture of a matrix and cube-like BCC phase precipitates, which can be attributed to the effect of the transition between para-and ferromagnetism around the Curie-point. There is also an unambiguous correlation between the cooling rate and mechanical properties of the heat-treated alloys.
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