arXiv subjects

Hanuš Seiner

Publications and source records attributed to Hanuš Seiner.

12 recordsLinked to original sources

arXiv2026

Ultra-transient grating spectroscopy for visualization of surface acoustics

Ultrasonic wave propagation across material surfaces reveals essential information about the materials' elastic behavior. The elastodynamic response of the surface is characterized by the Green's function that fully captures all its direction-dependent and frequency-dependent features. Here we present the first direct experimental visualization of the frequency-domain angular-resolved Green's function, including all its complex details resulting from elastic anisotropy. We achieve this visualization using transient grating spectroscopy (TGS), which is a method otherwise well established for measuring Rayleigh-type surface acoustic waves (SAWs). But here we focus on early-time thermoacoustic phenomena in the TGS experiment, revealing that, along with the transient standing-wave patterns of SAWs, there also emerge oscillations with at least an order of magnitude shorter lifetimes. These oscillations superpose into dynamic displacement patterns that are transient with respect to the classical transient timescales in TGS; the optical diffraction signal from these 'ultra-transient' gratings enables capturing the surface acoustic response with exceptional detail, and the resulting experimental angular dispersion maps strikingly replicate the theoretical frequency-domain Green's functions. By utilizing this feature, ultra-transient grating spectroscopy (UTGS) becomes a powerful tool for detailed contactless characterization of anisotropic solids, opening new pathways for studying single-crystalline or nanostructured materials.

cond-mat.mtrl-sci↗

arXiv2026

On the proposed concept of mechanical phasons in Ni-Mn-Ga modulated martensite

We discuss modulation phasons as a possible source of unusual elastic behavior of five-layer modulated (10\,M) martensite of the Ni-Mn-Ga shape memory alloy. This material exhibits anomalous macroscopic shear compliance along specific planes perpendicular to the modulation vector, and this compliance disappears when the modulations become incommensurate. Using a simple mechanical model, we show that modulation phasons in Ni-Mn-Ga can have macroscopic mechanical manifestations, and that the resulting 'mechanical phasons' can relax external shear loadings for commensurate and weakly incommensurate modulations, but not for strongly incommensurate modulations. The model merges ideas from the adaptive martensite theory and electronic-structure considerations, and enables straightforward explanations of several properties of the 10\,M lattice, such as spontaneous monoclinic distortion or easy formation and propagation of $a/b$ twins.

cond-mat.mtrl-sci↗

arXiv2026

A high-dimensional neural network potential for finite-temperature phenomena in NiTi martensite

We present a high-dimensional neural network potential (HDNNP) for the martensitic phase of the NiTi shape-memory alloy trained to density functional theory (DFT) data. A central aspect of this work is the systematic validation of the potential with respect to the underlying DFT reference method for key properties governing structural evolution, including equilibrium crystal structures, elastic constants, generalized-stacking fault energies, and vibrational spectra. The HDNNP accurately describes the relative stability of the B19$^\prime$ and B33 phases, including subtle energy differences on the order of meV/atom. The predicted stacking-fault energy landscape is strongly anisotropic and reveals a preferential shear pathway, providing atomistic insight into deformation and twinning mechanisms. Finite-temperature molecular dynamics simulations further enable the investigation of unconstrained structural evolution as a function of temperature. Overall, the developed HDNNP provides a robust basis for atomistic simulations of the complex structural and functional behavior of martensitic NiTi systems containing hundreds of thousands of atoms on nanosecond time scales.

cond-mat.mtrl-sci↗

arXiv2026

Study of the acoustic and thermal response of an elastically anisotropic solid to a sub-nanosecond laser pulse in transient grating spectroscopy

Transient grating spectroscopy (TGS) is a material characterization technique based on laser-induced thermoelastic excitation of thermal and acoustic gratings. On opaque samples, these gratings are dynamic surface displacements that reflect the sample's elastic and thermal properties, enabling both types of parameters to be determined from a single experiment. Here, we develop a detailed finite element model (FEM) of the TGS experiment that fully captures the coupling between the thermal and mechanical fields, as well as the optical detection of surface displacement using a heterodyning approach. Using custom-designed two-dimensional elements, the model is particularly suitable for analyzing TGS measurements on anisotropic media, for which analytical theory is insufficient. The simulation captures not only the anisotropic relaxation of the thermoelastic field but also several acoustic features that arise at very short (ultra-transient) timescales and provide additional information about the elastic properties of the examined material. The model offers new opportunities for the in silico testing of various modifications of TGS experiments and their applications to a broad class of materials.

cond-mat.mtrl-sci↗

arXiv2025

In-situ observation of elastic instability of stress-induced B19$^\prime$ martensite in thin NiTi wires

A laser-ultrasonic approach was used to measure elastic properties of a superelastic nickel-titanium wire with the aim to evaluate their evolution with stress and temperature in stress-induced martensite. It was observed that this evolution can be well described by a single smooth surface in the stress-temperature space, with the values of Young's modulus ranging from 30 to 50 GPa. The evolution of the modulus was then monitored in-situ with further heating under fixed strain, that is, during the shape setting. The results revealed that the martensite phase experienced further softening during this process, reaching Young's modulus of nearly 10 GPa at high temperatures and high stresses. In addition, the measurement enabled a direct detection of the initiation and termination temperatures of the shape setting from the elasticity data, which was used to show that it occurs in the same temperature interval for tension-induced and torsion-induced martensite.

cond-mat.mtrl-sci↗

arXiv2025

Magneto-elastic softening in cold-sprayed polycrystalline nickel studied by resonant ultrasound spectroscopy

Cold-sprayed metallic deposits are additively manufactured materials containing high levels of compressive residual stress. Here we show that the presence and intensity of this stress can be analyzed using laser-ultrasonics, provided that the sprayed material is ferromagnetic and magnetostrictive, as in the case of pure nickel. Contactless resonant ultrasound spectroscopy is used to monitor the evolution of shear modulus and internal friction parameter of two polycrystalline Ni deposits with temperature over the Curie point, which enables a direct assessment of the strength of magneto-elastic softening that is known to be strongly stress-dependent. In addition, the proposed methodology is also shown to be suitable for in-situ observation of the recrystallization process in the vicinity of the Curie point, as well as inspecting the homogeneity of the residual stress level across the thickness of the cold-sprayed deposit. Finally, a methodology for room-temperature probing of the magnetoelastic coupling is proposed and tested on the examined materials.

cond-mat.mtrl-sci↗

arXiv2025

Atomic Topology and Magnetic Microstructure of Highly Mobile Type I and Supermobile Type II Twin Boundaries in 10M Ni-Mn-Ga Single Crystal

The atomic topology and magnetic microstructure of individual, highly mobile Type I and Type II twin boundaries in 10M Ni-Mn-Ga martensite were investigated by transmission electron microscopy (TEM). The twin boundaries established in a bulk single crystal showed twinning stresses of ~1 MPa for Type I and ~0.1 MPa for Type II twin boundaries. TEM lamellae with a (010) cross-section, their c-axis (easy-magnetization direction) lying in-plane, were prepared by focused ion-beam milling, each containing a single twin boundary of specific type. High-resolution TEM confirmed an atomically sharp Type I twin boundary oriented along the rational (101) plane. The Type II boundary was also atomically sharp, apart from occasional single-atomic-plane steps. This contrasts with previous suggestions of its diffuse nature. Lorentz TEM showed 180° domain walls within martensite variants. The magnetic induction reorients sharply on both twin boundaries, forming 90°-like magnetic domain walls that follow the c-axis easy-magnetization direction.

cond-mat.mtrl-sci↗

arXiv2025

Elastic constants of single-crystalline NiTi studied by resonant ultrasound spectroscopy

Contactless, laser-based resonant ultrasound spectroscopy was utilized to monitor changes in elastic properties in single-crystalline NiTi shape memory alloy. It was observed that the elastic behavior of the temperature-induced B19$^\prime$ martensite, which is formed by a fine mixture of variants, adopts the symmetry elements of the parent austenite phase, and thus, the changes over the transformation temperature can be represented by the temperature evolution of three cubic elastic coefficients. The experiments confirm that the transition during the cooling run is preceded by pronounced softening of the $c_{44}$ elastic coefficient, which leads to nearly complete vanishing of elastic anisotropy prior to the transition. Below the transition, this coefficient remains soft, and the character of anisotropy switches from $c_{44}/c^\prime>1$ to $c_{44}/c^\prime<1$. We rationalize this behavior from the mechanical instability of the B19$^\prime$ lattice with respect to shears along the (001)$_{B19^\prime}$ plane, which is known from first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2024

Shear Deformation of Nonmodulated Ni$_2$MnGa Martensite: An Ab Initio Study

The impact of shear deformation in $(1\,0\,1)[1\,0\,\bar{1}]$ system of non-modulated (NM) martensite in Ni$_2$MnGa ferromagnetic shape memory alloy is investigated by means of ab initio atomistic simulations. The shear system is associated with twinning of NM lattice and intermatensitic transformation to modulated structures. The stability of the NM lattice increases with increasing content of Mn. The most realistic shear mechanism for twin reorientation can be approximated by the simple shear mechanism, although the lowest barriers were calculated for pure shear mechanism. The energy barrier between twin variants further reduces due to spontaneous appearance of lattice modulation or, in other words, the nanotwins with thickness of two atomic planes. Such nanotwins appear also on the generalized planar fault energy (GPFE) curve calculated using a newly developed advanced procedure and exhibits even lower energy than the defect free NM structure. These nanotwin doublelayers are also basic building blocks of modulated structures and play an important role in intermartensitic transformation.

cond-mat.mtrl-sci↗

arXiv2024

Compliant Lattice Modulations Enable Anomalous Elasticity in Ni-Mn-Ga Martensite

High mobility of twin boundaries in modulated martensites of Ni-Mn-Ga-based ferromagnetic shape memory alloys holds a promise for unique magnetomechanical applications. This feature has not been fully understood so far, and in particular it has yet not been unveiled what makes the lattice mechanics of modulated Ni-Mn-Ga specifically different from other martensitic alloys. Here, results of dedicated laser-ultrasonic measurements on hierarchically twinned five-layer modulated (10 M) crystals fill this gap. Using a combination of transient grating spectroscopy and laser-baser resonant ultrasound spectroscopy, it is confirmed that there is a shear elastic instability in the lattice, being significantly stronger than in any other martensitic material and also than what the first-principles calculations for Ni-Mn-Ga predict. The experimental results reveal that the instability is directly related to the lattice modulations. A lattice-scale mechanism of dynamic faulting of the modulation sequence that explains this behavior is proposed; this mechanism can explain the extraordinary mobility of twin boundaries in 10 M.

cond-mat.mtrl-sci↗

arXiv2024

Scanning acoustic microscopy characterization of cold sprayed coatings deposited on grooved substrates

The effect of non-planar substrate surface on homogeneity and quality of cold sprayed (CS) deposits was studied by scanning acoustic microscopy (SAM). Fe coatings were cold sprayed onto Al substrates containing artificially introduced grooves of square- and trapezoid-shaped geometries, with flat or cylindrical bottoms. The Al substrates were either wrought or cold sprayed, to comprehend their prospective influence on the Fe coatings build-up. SAM was then used to assess morphological properties of the materials from the cross-view and top-view directions. The microstructure below the surface of the studied samples was visualized by measuring the amplitudes of the reflection echoes and the velocity of the ultrasonic waves. The SAM analysis revealed that the regions of coating imperfections around the grooves are larger than what is suggested by standard scanning electron microscopy (SEM) observations. Furthermore, we found that the seemingly non-influenced coating regions that appear perfectly homogeneous and dense in SEM do, in fact, possess heterogeneous microstructure associated with the individual CS nozzle passes.

cond-mat.mtrl-sci↗

arXiv2023

Kwinking as the plastic forming mechanism of B19' NiTi martensite

Irreversible plastic forming of B19$^\prime$ martensite of the NiTi shape memory alloy is discussed within the framework of continuum mechanics. It is suggested that the main mechanism arises from coupling between martensite reorientation and coordinated $[100](001)_{\rm M}$ dislocation slip. A heuristic model is proposed, showing that the ${(20\bar{1})_{\rm M}}$ deformation-twin bands, commonly observed in experiments, can be interpreted as a combination of dislocation-mediated kink bands, appearing due to strong plastic anisotropy, and reversible twinning of martensite. We introduce a term 'kwinking' for this combination of reversible twinning and irreversible plastic kinking. The model is subsequently formulated using the tools of nonlinear elasticity theory of martensite and crystal plasticity, introducing 'kwink interfaces' as planar, kinematically compatible interfaces between two differently plastically slipped variants of martensite. It is shown that the ${(20\bar{1})_{\rm M}}$ kwink bands may be understood as resultsing from energy minimization, and that their nucleation and growth and their pairing with $(100)_{\rm M}$ twins into specific patterns enables low-energy plastic forming of NiTi martensite. We conclude that kwinking makes plastic deformation of B19$^\prime$ martensite in polycrystalline NiTi possible despite only one slip system being available.

cond-mat.mtrl-sci↗