Local lattice distortions in refractory high-entropy alloys
Doctoral thesis, 2025
Refractory high-entropy alloys (RHEAs) are a novel class of alloys known for their exceptional mechanical properties at high temperatures, making them promising candidates for next-generation aerospace applications. RHEAs typically composed of multiple principal elements in (near-)equiatomic concentrations. A key factor contributing to their strength is local lattice distortion (LLD), which arises from atomic size mismatch, charge transfer effects and force constant variations among constituent elements. LLDs have been shown to contribute to solid solution strengthening and phase stabilisation, making them vital for mechanical performance and reliable processing of RHEAs. However, LLD remains poorly understood, particularly regarding quantification due to experimental challenges.
Neutron and synchrotron X-ray total scattering are the primary techniques used in this thesis, which enables simultaneous probing of both long-range order and local disorder. Quantitative determination of LLDs was done through small-box modelling of the pair distribution functions in real space, and through Rietveld refinement of diffraction patterns in reciprocal space. Molecular dynamics (MD) simulations provide the vibrational density of states (VDOS), allowing separation of scattering from dynamic (or thermal) and static atomic displacements. Specific heat measurements analysed using Debye approximation offer an alternative route for estimating thermal contributions.
It is shown that LLDs in bcc-structured RHEAs can be accurately quantified using both reciprocal- and real-space methods. A comprehensive methodology, combining variable-temperature neutron total scattering experiments with VDOS from MD simulations, revealed a negative temperature dependence of LLDs in a HfNbTaTiZr RHEA, which was further confirmed and extended to NbTaTiZr and MoNbTaW. Additional studies on alternative quantification methods and the effect of chemical heterogeneity establish a framework for understanding LLDs in RHEAs.
Neutron and synchrotron X-ray total scattering are the primary techniques used in this thesis, which enables simultaneous probing of both long-range order and local disorder. Quantitative determination of LLDs was done through small-box modelling of the pair distribution functions in real space, and through Rietveld refinement of diffraction patterns in reciprocal space. Molecular dynamics (MD) simulations provide the vibrational density of states (VDOS), allowing separation of scattering from dynamic (or thermal) and static atomic displacements. Specific heat measurements analysed using Debye approximation offer an alternative route for estimating thermal contributions.
It is shown that LLDs in bcc-structured RHEAs can be accurately quantified using both reciprocal- and real-space methods. A comprehensive methodology, combining variable-temperature neutron total scattering experiments with VDOS from MD simulations, revealed a negative temperature dependence of LLDs in a HfNbTaTiZr RHEA, which was further confirmed and extended to NbTaTiZr and MoNbTaW. Additional studies on alternative quantification methods and the effect of chemical heterogeneity establish a framework for understanding LLDs in RHEAs.
thermal vibration
synchrotron X-ray diffraction
atomic displacements
refractory high-entropy alloys
neutron scattering
local lattice distortion
total scattering
pair distribution function
Author
Yao Hu
Chalmers, Physics, Microstructure Physics
Quantifying local lattice distortions in refractory high-entropy alloys
Physical Review Materials,;Vol. 8(2024)
Journal article
Effect of chemical segregation on accuracy of local lattice distortions determination by pair distribution functions
AIP Advances,;Vol. 14(2024)
Journal article
Yao Hu, Paul Erhart, Lewis R. Owen, Gabriel E. Pérez, Helen Y. Playford, František Lukáč, Severin Jakob, Mattias Thuvander, Janez Dolinšek, Sheng Guo, Magnus Hörnqvist Colliander. Temperature dependence of local lattice distortions in the refractory high-entropy alloy HfNbTaTiZr.
Yao Hu, Gabriel J. Cuello, Henry E. Fischer, Janez Dolinšek, Jiatu Liu, Lewis R. Owen, Sheng Guo, Magnus Hörnqvist Colliander. Effect of composition on the temperature dependence of local lattice distortions in refractory medium-entropy alloys.
Refractory high-entropy alloys are a novel type of metallic material made by mixing several elements in equal amounts. They can withstand extremely high temperatures (over 1000°C) and are seen as promising candidates for future aerospace applications, potentially surpassing the performance limits of today's Ni-based superalloys in turbine components. With materials that are more heat-resistant, the combustion temperatures in turbines can be increased, leading to higher efficiencies and lower emissions.
This exceptional high-temperature strength has been hypothesised to come from tiny distortions in their structure at the atomic scale, where atoms are slightly shifted from their ideal positions, cause local strains in the materials, and thereby affect their resistance to deformation. To understand and design better alloys, we need accurate measurements of these distortions. This is, however, very difficult, since the distortions are very small and difficult to separate from displacements due to the inherent vibration of atoms.
In my thesis, I use powerful neutron and high-energy X-ray beams to study these alloys at the atomic scale. By analysing how the neutrons and X-rays scatter, I can determine how far atoms are shifted from their ideal positions. Combining this with simulations and specific heat measurements makes it possible to separate thermal vibrations of atoms from true structural distortions.
The results show that these local distortions can be measured accurately, and can reach significant magnitudes in the alloys studied. They depend strongly on alloy composition, and even change with temperature. The knowledge gained in this work helps to understand the structure-property relationships at atomic level, and builds a basis for designing more reliable, more heat-resistant materials for next-generation aerospace applications.
This exceptional high-temperature strength has been hypothesised to come from tiny distortions in their structure at the atomic scale, where atoms are slightly shifted from their ideal positions, cause local strains in the materials, and thereby affect their resistance to deformation. To understand and design better alloys, we need accurate measurements of these distortions. This is, however, very difficult, since the distortions are very small and difficult to separate from displacements due to the inherent vibration of atoms.
In my thesis, I use powerful neutron and high-energy X-ray beams to study these alloys at the atomic scale. By analysing how the neutrons and X-rays scatter, I can determine how far atoms are shifted from their ideal positions. Combining this with simulations and specific heat measurements makes it possible to separate thermal vibrations of atoms from true structural distortions.
The results show that these local distortions can be measured accurately, and can reach significant magnitudes in the alloys studied. They depend strongly on alloy composition, and even change with temperature. The knowledge gained in this work helps to understand the structure-property relationships at atomic level, and builds a basis for designing more reliable, more heat-resistant materials for next-generation aerospace applications.
Subject Categories (SSIF 2025)
Metallurgy and Metallic Materials
Infrastructure
Chalmers Materials Analysis Laboratory
Chalmers e-Commons (incl. C3SE, 2020-)
Areas of Advance
Materials Science
DOI
10.63959/chalmers.dt/5810
ISBN
978-91-8103-353-3
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5810
Publisher
Chalmers
PJ-salen, Fysik Origo Building, Kemigården 1, Gothenburg
Opponent: Dr. Matthew Tucker, Neutron Scattering Division, Oak Ridge National Laboratory, USA
Related datasets
Local lattice distortions in a HfNbTaTiZr refractory high entropy alloys; Resolving local lattice distortions in a refractory high entropy alloy; Investigating local lattice distortions in refractory high-entropy alloys via temperature-dependent PDF [dataset]
DOI: 10.5286/ISIS.E.RB2000207; 10.5286/ISIS.E.RB2220499-1; 10.5291/ILL-DATA.1-04-274