arXiv subjects

Josef Stráský

Publications and source records attributed to Josef Stráský.

8 recordsLinked to original sources

arXiv2026

Designing Homogeneous Ti-Nb-Fe-Sn $β$ Titanium Alloys by PBF-LB: A Pre-Alloyed Powder Blend Strategy

Metastable $β$ titanium alloys are attractive for biomedical and structural applications owing to their low elastic modulus, high specific strength, and excellent corrosion resistance. Laser powder bed fusion (PBF-LB) enables complex-shape production and controlled compositional variation through powder blending. However, processing elemental Ti-Nb blends often results in chemical heterogeneity from incomplete dissolution of Nb-rich particles and non-equilibrium phase formation. To address this, low-modulus Ti-Nb-Fe-Sn alloys were produced by PBF-LB using Ti-42Nb, Ti-20Nb-15Fe, and Ti-20Nb-20Sn master-alloy powders blended with commercially pure Ti. Ti-23Nb-3Fe-4Sn, Ti-26Nb-2Fe-4Sn, Ti-29Nb-1Fe-4Sn, and Ti-32Nb-4Sn were fabricated using an uncommonly large 70 $μ$m layer thickness with layer remelting, followed by heat treatment at 1000 $°$C for 2 h and water quenching. After heat treatment, all alloys exhibited low porosity, homogeneous chemical distribution, and single $β$-phase microstructures with predominantly equiaxed grains and weak crystallographic texture. Thermodynamic calculations indicated that solidification descriptors alone could not explain the non-monotonic grain-size evolution, which was attributed to inherited solidification structure, transient TiFe-like phase formation, Nb/Sn partitioning, and/or solute-drag-controlled $β$-grain growth. Hardness and yield strength decreased with decreasing Fe and increasing Nb contents, from 268 to 224 HV and 691 to 468 MPa, respectively. Young's modulus, determined by resonant ultrasound spectroscopy, ranged 63-81 GPa. These results demonstrate that pre-alloyed master-alloy blends combined with remelting and heat treatment provide an effective route for producing chemically homogeneous Ti-Nb-Fe-Sn $β$ alloys while revealing how small compositional changes govern grain-growth behavior and mechanical response.

cond-mat.mtrl-sci↗

arXiv2026

Phase Equilibria of the Al-Ti-Nb-Zr-Ta System

Phase equilibria in the Al-Ti-Nb-Zr-Ta refractory complex concentrated alloy system were investigated using a high throughput experimental approach. A pseudo-ternary section of the quinary compositional space was prepared by a honeycomb type powder metallurgy design, consolidated by spark plasma sintering and subsequently homogenized at 1400 °C for 168 h. Phase constitution and chemical partitioning were characterized by SEM/EDS, XRD, EBSD, and TEM, supported by a custom EDS phase clustering workflow. Equilibrium microstructures consisting primarily of BCC, B2, and secondary phases were identified across the sampled compositions, with nanoscale precipitates forming in Zr and Ta rich regions. Measured phase compositions were compared with CALPHAD predictions, revealing both agreements and systematic deviations linked to CALPHAD database limitations. The results provide new experimental insight into phase stability and microstructural trends in Al-Ti-Nb-Zr-Ta alloys and demonstrate the effectiveness of high throughput combinatorial approaches for mapping complex multicomponent systems.

cond-mat.mtrl-sci↗

arXiv2026

The effect of Nb and O on the martensitic transformation in the Ti-Nb-O alloys

This study examines the influence of niobium and oxygen on phase stability, crystal structure, and martensitic transformation pathways in Ti-Nb-O alloys. A series of Ti-(8-28)Nb-(0-3)O (at.%) alloys were prepared and solution-treated in the $β$-phase field. Microstructure and crystallography were characterized by X-ray diffraction, electron microscopy, and reciprocal-space mapping. A 2D-XRD orientation simulation approach was applied to distinguish all 12 crystallographically equivalent $α"$ martensitic variants originating from a single prior $β$ grain, enabling detailed diffraction analysis. This method further allowed quantitative evaluation of the atomic shuffle parameter y, describing the $β\rightarrowα"$ transformation. The results demonstrate that Nb primarily governs $α"$ martensite evolution. Increasing Nb stabilizes the $β$ phase and shifts the $α"$ structure toward higher symmetry, as reflected by systematic changes in lattice parameters and increasing shuffle parameter y, indicating suppression of transformation toward the hexagonal $α'$ phase. Oxygen, in contrast, modifies transformation pathways. At lower Nb contents, it suppresses the $ω$ phase formation and promotes $β\rightarrowα"$ transformation, while at higher Nb levels it inhibits long-range martensitic transformation, resulting in retained $β$ or competing $ω$ phase. These effects are attributed to local lattice distortions induced by interstitial oxygen.

cond-mat.mtrl-sci↗

arXiv2025

Phase transformations in metastable $β$ Zr15Nb alloy revealed by in-situ methods

This study examines the phase transitions occurring during linear heating of the Zr15Nb alloy through a comprehensive, multi-technique methodology comprising in-situ high-energy synchrotron X-ray diffraction (HEXRD), electrical resistance measurements, differential scanning calorimetry (DSC), and thermal expansion analysis, supplemented by ex-situ transmission electron microscopy (TEM). The findings reveal a complex sequence of phase transformations and corresponding structural changes over a broad temperature range (from room temperature up to 800 $°$C). Two distinct body-centered cubic (bcc) $β$ phases - $β_{Zr}$ and $β_{Nb}$ - with closely related lattice parameters are identified. At room temperature, the microstructure is characterized by a mixture of the metastable $β_{Zr}$ + $ω_{ath}$ phase. Upon heating, $β_{Zr}$ progressively decomposes, giving rise to the formation of $β_{Nb}$. TEM observation revealed the cuboidal shape of the $ω_{iso}$ particles resulting from the high lattice misfit between $β$ and $ω$ phase. The $ω$ solvus temperature is determined to be approximately 555 $°$C, as evidenced by in situ HEXRD and abrupt changes in the alloy's thermal and electrical properties. The growth of the $α$ phase occurs after the dissolution of the $ω$ phase, resulting in a pronounced increase in thermal expansion.

cond-mat.mtrl-sci↗

arXiv2025

Microstructure and phase stability within the AlMoNbTiZr system: design tools and compositional boundaries for a high-entropy alloy

This study explores Ti-containing complex concentrated alloys (CCAs) within the AlMoNbTiZr system, focusing on compositions located in regions of the Bo-Md diagram characterized by low bond order (Bo) and d-orbital energy level (Md). Four alloys were designed near the line predicting stress-induced martensite formation in conventional Ti alloys, then cast, annealed at 1200°C, and water quenched. Their microstructures and phases were analyzed and compared against phase prediction tools commonly applied to high-entropy alloys (HEAs), namely empirical parameters and CALPHAD simulations. Results highlight the strong influence of chemical affinity, particularly the roles of Al, Mo, and Zr concentrations, on solid-solution stability. A maximum Al content of 10 at% was identified as the threshold for achieving a single-phase microstructure, observed in the 10Al15Mo10Nb35Ti30Zr alloy. This alloy exhibited a bcc/b2 structure with high compressive strength (above 1300 MPa), low Young's modulus (28 GPa), and limited strain (<6%), but lacked the transformation-induced strengthening mechanisms expected for Ti alloys with comparable Bo-Md values.

cond-mat.mtrl-sci↗

arXiv2025

Stress-induced phase transformations in Ti-15Mo alloy at elevated temperature

Controlled mechanical loading was applied to Ti-15Mo alloy during annealing at 550 $^\circ$C. Massive formation of the $ω_{\textrm{iso}}$ phase from the parent $β$-phase occurred during annealing at 550 $^\circ$C without external stress or with stress well below the yield stress. Moreover, a massive $α$ phase precipitation takes place under simultaneous annealing and plastic deformation. Plastic deformation plays a key role in $β\rightarrowα$ transformation and achieving refined $α+β$ type microstructure resulted in improved mechanical properties. Studying phase transformations during plastic deformation is critical for understanding and optimizing thermomechanical processing of metastable $β$-Ti alloys.

cond-mat.mtrl-sci↗

arXiv2024

Phase Composition of AlTiNbMoV, AlTiNbTaZr and AlTiNbMoCr Refractory Complex Concentrated Alloys: A Correlation of Predictions and Experiment

Designing complex concentrated alloys (CCA), also known as high entropy alloys (HEA), requires reliable and accessible thermodynamic predictions due to vast space of possible compositions. Numerous semiempirical parameters have been developed for phase predictions over the years. However, in this paper we show that none of these parameters is a robust indicator of phase content in various refractory CCA. CALPHAD proved to be a more powerful tool for phase predictions, however, the predictions face several limitations. AlTiNbMoV, AlTiNbTaZr and AlTiNbMoCr alloys were prepared using blended elemental powder metallurgy. Their phase and chemical composition were investigated by the means of scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. Apart from the minor contamination phases (Al2O3 and Ti(C,N,O)), AlTiNbMoV and AlTiNbMoCr exhibited single-phase solid solution microstructure at the homogenization temperature of 1400 °C, while Al3Zr5 based intermetallics were present in the AlTiNbTaZr alloy. None of the simple semiempirical parameter was able to predict phase content correctly in all three alloys. Predictions by CALPHAD (TCHEA4 database) were able to predict the phases with limited accuracy only. Critical limitation of the TCHEA4 database is that only binary and ternary phase diagrams are assessed and some more complex phases cannot be predicted.

cond-mat.mtrl-sci↗

arXiv2018

Ordered array of $ω$ particles in $β$-Ti matrix studied by small-angle X-ray scattering

Nanosized particles of $ω$ phase in a $β$-Ti alloy were investigated by small-angle X-ray scattering using synchrotron radiation. We demonstrated that the particles are spontaneously weakly ordered in a three-dimensional cubic array along the $\langle 100\rangle$-directions in the $β$-Ti matrix. The small-angle scattering data fit well to a three-dimensional short-range-order model; from the fit we determined the evolution of the mean particle size and mean distance between particles during ageing. The self-ordering of the particles is explained by elastic interaction between the particles, since the relative positions of the particles coincide with local minima of the interaction energy. We performed numerical Monte Carlo simulation of the particle ordering and we obtained a good agreement with the experimental data.

cond-mat.mtrl-sci↗