| 1. |
- Abdoshahi, Neda, et al.
(författare)
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On the energetics of the cubic-to-hexagonal transformations in TiAl plus Mo alloys
- 2022
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 240
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Tidskriftsartikel (refereegranskat)abstract
- Diffusionless transformations allow access to metastable phases and enrich the materials design portfolio. They are well suited for atomistic modeling; nonetheless, they are challenging when involving disordered systems or alloys with complex compositions. This work presents a comprehensive study of transforma-tion energetics between bcc and hcp ordered and disordered phases in the TiAl+Mo model alloy system. By employing two complementary techniques I. VASP-SQS, and II. EMTO-CPA, we can show that chemical disorder flattens the energy landscape but may introduce a small barrier. Unlike that, the energetics of ordered phases are barrier-less and hence would suggest a spontaneous transformation. Finally, we show that Mo stabilizes the bcc phases, leading to a barrier-less transformation hcp -+ bcc for both ordered and disordered states when Mo content exceeds ti 12 at.%.
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| 2. |
- Abdoshahi, Neda, et al.
(författare)
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Structural stability and mechanical properties of TiAl plus Mo alloys : A comprehensive ab initio study
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 221, s. 117427-
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Tidskriftsartikel (refereegranskat)abstract
- Titanium aluminides are technologically important intermetallic alloys with also many curious properties interesting from a basic research point of view. When alloyed with Mo, several (meta)stable phases have been reported; their properties are, however, largely unknown due to the alloy processing (e.g. fast cooling) and/or non-existence as a single-phase material. Here we employ first principles calculations to study compositional trends in structural and mechanical properties. We could show that Mo increases the density of all studied phases, leads to their chemical destabilization with the exception of the ordered bcc /3o phase, increases their ductility, and enhances the elastic anisotropy. Discrepancies between two employed ab initio methods (special quasi-random structures vs. coherent potential approximation) in the case of the /3o and B19 phases are rationalized with significant local distortions which may eventually facilitate a spontaneous phase transformation. Predictions of ordering temperatures solely based on the configurational entropy do not yield values in the experimentally expected ranges.
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| 3. |
- Aghda, Soheil Karimi, et al.
(författare)
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Ion kinetic energy- and ion flux-dependent mechanical properties and thermal stability of (Ti,Al)N thin films
- 2023
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 250
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Tidskriftsartikel (refereegranskat)abstract
- Ion-irradiation-induced changes in structure, elastic properties, and thermal stability of metastable c-(Ti,Al)N thin films synthesized by high-power pulsed magnetron sputtering (HPPMS) and cathodic arc deposition (CAD) are systematically investigated by experiments and density functional theory (DFT) simulations. While films deposited by HPPMS show a random orientation at ion kinetic energies (Ek)>105 eV, an evolution towards (111) orientation is observed in CAD films for Ek>144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD compared to HPPMS. Hence, it is inferred that formation of the strong (111) texture in CAD films is caused by the ion flux-and ion energy-induced strain energy minimization in defective c-(Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy-and orientation -dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced Frenkel defects formation and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from Ek for HPPMS films without preferential orientation. However, an ion energy-dependent elastic modulus reduction of similar to 18% for the CAD films can be understood by considering the 34% higher Frenkel pair concentration formed at Ek=182 eV upon irradiation of the experimentally observed (111)-oriented (Ti,Al)N in comparison to the (200)-configuration at similar Ek. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable c-(Ti,Al)N is investigated by differential scanning calorimetry: Ion-irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 degrees C.
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| 4. |
- Aghda, Soheil Karimi, et al.
(författare)
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Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides
- 2023
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 255
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by frequently reported deviations from stoichiometry in cubic transition metal nitride (TMNx) thin films, the effect of N-vacancy concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and MoNx (0.72 & LE; x & LE; 1.00) is systematically studied by density functional theory (DFT) calculations. The predictions are validated experimentally for VNx (0.77 & LE; x & LE; 0.97). The DFT results indicate that the elastic behavior of the TMNx depends on both the N-vacancy concentration and the valence electron concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase. These trends can be rationalized by considering vacancy-induced changes in elastic anisotropy and bonding. While introduction of N-vacancies in TiNx results in a significant reduction of elastic modulus along all directions and a lower average bond strength of Ti-N, the vacancy-induced reduction in [001] direction of VNx is overcompensated by the higher stiffness along [011] and [111] directions, resulting in a higher average bond strength of V-N. To validate the predicted vacancy-induced changes in elasticity experimentally, close-to-singlecrystal VNx (0.77 & LE; x & LE; 0.97) are grown on MgO(001) substrates. As the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray diffraction, decreases from 4.128 & ANGS; to 4.096 & ANGS;. This reduction in lattice parameter is accompanied by an anomalous 11% increase in elastic modulus, as determined by nanoindentation. As the experimental data agree with the predictions, the elasticity enhancement in VNx upon N-vacancy formation can be understood based on the concomitant changes in elastic anisotropy and bonding.
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| 5. |
- Al-Mukadam, R., et al.
(författare)
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Viscosity of metallic glass-forming liquids based on Zr by fast-scanning calorimetry
- 2021
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 221
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Tidskriftsartikel (refereegranskat)abstract
- Fast-scanning calorimetry was applied to retrieve the viscosity of supercooled liquids of the Zr-based bulk metallic glasses (BMGs) Vitreloy 105 and AMZ4 for temperatures from standard glass transition down to similar to 0.78T(g)/T. Characteristic temperatures of the glass transition were translated into viscosity values by means of composition-independent shift factors based on the equivalency between structural relaxation and viscous flow. The extended MYEGA model with a fragile term dominant at high-temperatures and a strong term dominant at low-temperatures describes the entire viscous range. The analysis revealed that Vitreloy 105 and AMZ4 are strong liquids for log(10) eta >= 4.9-5.5. In turn, the fragile-to-strong crossover is centred on 0.69T(g)/T for Vitreloy 105 and on 0.66T(g)/T for AMZ4. The extent of the fragile-to-strong transition was found to be larger for Vitreloy 105 than for AMZ4, while their values agreed well with the inverse relation between transition factor and kinetic fragility of the strong regime established for BMG-forming liquids. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 6. |
- Alzweighi, Mossab, et al.
(författare)
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The influence of structural variations on the constitutive response and strain variations in thin fibrous materials
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 203
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Tidskriftsartikel (refereegranskat)abstract
- The stochastic variations in the structural properties of thin fiber networks govern to a great extent their mechanical performance. To assess the influence of local structural variability on the local strain and mechanical response of the network, we propose a multiscale approach combining modeling, numerical simulation and experimental measurements. Based on micro-mechanical fiber network simulations, a continuum model describing the response at the mesoscale level is first developed. Experimentally measured spatial fields of thickness, density, fiber orientation and anisotropy are thereafter used as input to a macroscale finite-element model. The latter is used to simulate the impact of spatial variability of each of the studied structural properties. In addition, this work brings novelty by including the influence of the drying condition during the production process on the fiber properties. The proposed approach is experimentally validated by comparison to measured strain fields and uniaxial responses. The results suggest that the spatial variability in density presents the highest impact on the local strain field followed by thickness and fiber orientation. Meanwhile, for the mechanical response, the fiber orientation angle with respect to the drying restraints is the key influencer and its contribution to the anisotropy of the mechanical properties is greater than the contribution of the fiber anisotropy developed during the fiber sheet-making.
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| 7. |
- Ananthanarayanan, Durga, et al.
(författare)
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Phase-field study of IMC growth in Sn-Cu/Cu solder joints including elastoplastic effects
- 2020
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 188, s. 241-258
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Tidskriftsartikel (refereegranskat)abstract
- In this article, we aim to study the problem of the growth of intermetallic phases in solder joints undergoing mechanical deformation, using a phase-field model for multi-phase systems that can treat diffusion, elastic and plastic deformation. A suitable model is formulated and applied to Sn-Cu/Cu lead-free solder joints. The growth of the intermetallic layers during solid-state annealing is simulated for different strain states. We assess the values of stiffness tensors available in literature and perform ab initio calculations to support the selection of reasonable values from literature. We also perform a parametric study with different eigenstrain values and applied strains. We find that there is a significant effect of the considered eigenstrains and applied strains on the growth kinetics of the system and parabolic growth kinetics is followed in cases where the intermetallic layers grow. We thereby establish the importance of strain in the growth of intermetallic layers and the need for more targeted experiments on the role of strain in the reliability of the solder joint.
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| 8. |
- Bakhit, Babak, et al.
(författare)
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Improving the high-temperature oxidation resistance of TiB2 thin films by alloying with Al
- 2020
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Ingår i: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 196, s. 677-689
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Tidskriftsartikel (refereegranskat)abstract
- Refractory transition-metal diborides (TMB2) are candidates for extreme environments due to melting points above 3000 degrees C, excellent hardness, good chemical stability, and thermal and electrical conductivity. However, they typically suffer from rapid high-temperature oxidation. Here, we study the effect of Al addition on the oxidation properties of sputter-deposited TiB2-rich Ti1-xAlxBy thin films and demonstrate that alloying the films with Al significantly increases the oxidation resistance with a slight decrease in hardness. TiB2.4 layers are deposited by dc magnetron sputtering (DCMS) from a TiB2 target, while Ti1-xAlxBy alloy films are grown by hybrid high-power impulse and dc magnetron co-sputtering (Al-HiPIMS/TiB2-DCMS). All as-deposited films exhibit columnar structure. The column boundaries of TiB2.4 are B-rich, while Ti0.68Al0.32B1.35 alloys have Ti-rich columns surrounded by a Ti(1-x)Al(x)By tissue phase which is predominantly Al rich. Air-annealing TiB2.4 at temperatures above 500 degrees C leads to the formation of oxide scales that do not contain B and mostly consist of a rutile-TiO2 (s) phase. The resulting oxidation products are highly porous due to the evaporation of B2O3 (g) phase as well as the coarsening of TiO2 crystallites. This poor oxidation resistance is significantly improved by alloying with Al. While air-annealing at 800 degrees C for 0.5 h results in the formation of an similar to 1900-nm oxide scale on TiB2.4, the thickness of the scale formed on the Ti0.68Al0.32B1.35 alloys is similar to 470 nm. The enhanced oxidation resistance is attributed to the formation of a dense, protective Al-containing oxide scale that considerably decreases the oxygen diffusion rate by suppressing the oxide-crystallites coarsening. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 9. |
- Calamba, Katherine, et al.
(författare)
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Effect of nitrogen vacancies on the growth, dislocation structure, and decomposition of single crystal epitaxial (Ti1-xAlx)N-y thin films
- 2021
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 203
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Tidskriftsartikel (refereegranskat)abstract
- The effect of varying nitrogen vacancies on the growth, microstructure, spinodal decomposition and hardness values of predominantly single crystal cubic phase c-(Ti1-xAlx)N-y films was investigated. Epitaxial c-(Ti1-xAlx)N-y films with y = 0.67, 0.79, and 0.92 were grown on MgO(001) and MgO(111) substrates by magnetron sputter deposition. High N vacancy c-(Ti1-xAlx)N-0.67 films deposited on MgO(111) contained coherently oriented w-(0001) structures while segregated conical structures were observed on the films grown on MgO(001). High resolution STEM images revealed that the N-deficient growth conditions induced segregation with small compositional fluctuations that increase with the number of N vacancies. Similarly, strain map analysis of the epitaxial c-(Ti1-xAlx)N-y (001) and (111) films show fluctuations in strain concentration that scales with the number of N vacancies and increases during annealing. The spinodal decomposition coarsening rate of the epitaxial c-(Ti1-xAlx)N-y films was observed to increase with decreasing N vacancies. Nanoindentation showed decreasing trends in hardness of the as-deposited films as the N vacancies increase. Isothermal post-anneal at 1100 degrees C in vacuum for 120 min revealed a continuation in the increase in hardness for the film with the largest number of N vacancies (y = 0.67) while the hardness decreased for the films with y = 0.79 and 0.92. These results suggest that nitrogen-deficient depositions of c-(Ti1-xAlx)N-y films help to promote a self-organized phase segregation, while higher N vacancies generally increase the coherency strain which delays the coarsening process and can influence the hardness at high temperatures.
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| 10. |
- Cao, Peiyu, et al.
(författare)
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Ideal superelasticity in Ni-based Heusler alloys
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 210
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Tidskriftsartikel (refereegranskat)abstract
- The hysteresis that occurs during superelasticity caused by the stress-induced first-order martensite transformation is sometimes detrimental to the properties of superelastic materials. In this paper, first-principles calculations are performed to systematically investigate the effects of the chemical composition and crystal disorder on the superelasticity of Ni50-xCoxM25Ga25 (M = Mn, Fe) Heusler alloys. Calculations of the stress-strain relation in the studied alloys reproduce the recent experimental findings for nonhysteretic superelasticity within an acceptable range of composition and ordering. We evaluate the Bloch spectral function to study the Fermi surface topology in connection with nonhysteretic superelasticity. We propose the Landau-de Gennes model-dependent critical parameter P-c, which can be used to predict the composition range of nonhysteretic superelastic materials. For the ferromagnetic L2(1) Ni50-xCoxMn25Ga25 and B2 Ni50-xCoxFe25Ga25 alloys, the nonhysteretic superelasticity phenomenon theoretically occurs for Co contents over x = 16 at.% and x = 28 at.%, respectively.
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