arXiv subjects

N. P. Kobelev

Publications and source records attributed to N. P. Kobelev.

13 recordsLinked to original sources

arXiv2026

Relationship between heat effects and shear modulus relaxation during structural relaxation of a telluride glass

We performed parallel measurements of heat effects and shear modulus relaxation for glassy Te$_{75}$Ge$_{15}$Ga$_{10}$ taken as a representative of practically important non-metallic glasses with covalent bonding. It is shown that the heat effects occurring upon heating are quantitatively linked to the shear moduli in the glassy and crystalline states and their temperature derivatives as implied by Eq.(1), which was originally derived for metallic glasses. This relationship provides a good description of exo- and endothermal reactions using shear modulus relaxation data as an input. This is the first application of this approach to a non-metallic glass with directional interatomic bonding. The obtained results suggest that relaxation phenomena are governed by elastic dipoles -- atomic configurations with the symmetry lower than that of surrounding matrix.

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arXiv2025

Thermodynamic potentials of metallic alloys in the undercooled liquid and solid glassy states

We first present a comparative analysis of temperature evolution of the excess thermodynamic potentials (state functions), the enthalpy $\Delta H$, entropy $\Delta S$ and Gibbs free energy $\Delta \Phi$, determined for \textit{i}) undercooled melts using literature data and \textit{ii}) solid glassy state calculated on the basis of calorimetry measurements using an approach proposed recently. Three metallic alloys were taken as an example for data analysis. It is found that temperature dependences $\Delta H(T)$, $\Delta S(T)$ and $\Delta G(T)$ calculated with both approaches coincide in the supercooled liquid range (i.e. at temperatures $T_g<T<T_x$, where $T_g$ and $T_x$ are the glass transition and crystallization onset temperatures, respectively). However, the necessary conditions for this coincidence is the introduction of important changes to the above approach \textit{i}), which are related to the calculation of the melting entropy. We also introduce and calculate a dimensionless order parameter $\xi$, which changes in the range $0<\xi<1$ and characterizes the evolution of the structural order from liquid-like to crystal-like one. It is shown that the order parameter $\xi_{scl}$ calculated for the end of the supercooled liquid range (i.e. for a temperature just below $T_x$) correlates with the melt critical cooling rate $R_c$: the smaller the order parameter $\xi_{scl}$ (i.e. the closer the structure to that of the equilibrium liquid), the smaller $R_c$ is.

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arXiv2025

Relationship of structural disorder and stability of supercooled liquid state with glass-forming ability of metallic glasses

We performed calorimetric studies of 26 metallic glasses and calculated the excess entropy and excess enthalpy with respect to their counterpart crystals. On this basis, we introduced a dimensionless entropy-based parameter {\sigma}scl, which characterizes structural disordering and stability of the supercooled liquid state upon heating. A very good correlation of {\sigma}scl with literature data on the critical cooling rate Rc and critical diameter Dmax of metallic glasses is shown. We also introduced another dimensionless parameter {\eta}scl based on the excess enthalpy of glass and showed that {\eta}scl provides equally good correlation with Rc and Dmax. Possible relationship of structural disordering and glass-forming ability in the supercooled liquid range with the defect structure of glass is discussed. The obtained results provide a new window for the understandingof the glass-forming ability of metallic glasses.

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arXiv2025

Non-isothermal stress relaxation in conventional and high-entropy metallic glasses and its relationship to themixing and excess entropy

We performed calorimetric and torsion stress relaxation measurements upon linear heating of six conventional and high-entropy metallic glasses with the mixing entropy {\Delta}Smix ranging from 0.86R to 1.79R (R is the universal gas constant). It is shown that high-entropy metallic glasses ({\Delta}Smix > 1.5 R) exhibit significantly greater resistance to stress relaxation. Based on calorimetric data, we calculated the excess entropy of glass relative to the counterpart crystalline state and introduced an entropy-based dimensionless parameter {\Delta}S, which characterizes the rise of the entropy and structural disordering of glass in the supercooled liquid region. It is shown that the depth of stress relaxation at a given temperature decreases with {\Delta}Smix but increases with {\Delta}S. Possible reasons for this relationship are discussed.

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arXiv2025

On the nature of the glass transition in metallic glasses after deep relaxation

We performed parallel study of calorimetric and high-frequency shear modulus behavior of Zr-based metallic glasses after deep relaxation just below the glass transition. It is shown that deep relaxation results in the appearance of a strong peak of the excess heat capacity while the shear modulus is moderately affected. A theory assuming high-frequency shear modulus to be a major physical parameter controlling glass relaxation is suggested. The energy barrier for these rearrangements is proportional to the shear modulus while its magnitude, in turn, varies due to the changes in the defect concentration (diaelastic effect). Both dependences lead to the occurrence of heat effects. The excess heat capacity calculated using experimental shear modulus data demonstrates very good agreement with the experimental calorimetric data for all states of glasses. It is argued that the glass transition behavior after deep relaxation of glass is close to a phase transition of the first kind.

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arXiv2024

Relationship between the shear modulus and volume relaxation in high-entropy metallic glasses: experiment and physical origin

We performed parallel measurements of the high-frequency shear modulus $G$ and relative volume $ΔV/V$ for high-entropy Ti$_{20}$Zr$_{20}$Hf$_{20}$Be$_{20}$Cu$_{20}$ and Ti$_{20}$Zr$_{20}$Hf$_{20}$Be$_{20}$Ni$_{20}$ glasses upon heating from room temperature up to the complete crystallization. The changes of these properties due to structural relaxation both below and above the glass transition temperature are singled out. It is shown that these changes for both initial and preannealed samples can be well described within the framework of the Interstitialcy theory. It is found that the whole relaxation process in the full temperature range of the experiments for both samples' states can be characterized by a single dimensionless temperature-independent parameter $K_i=dln\;G/dln\;V$, which equals to -44 and -53 for the above glasses, respectively, and strongly points at interstitial-type defects as a source of relaxation. We also show that the relaxation of the relative volume linearly depends on the defect concentration. This behavior can be described by another dimensionless temperature-independent parameter, which is related with the relaxation volume of defects. Possible contribution of vacancy-like defects into the relaxation is discussed.

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arXiv2024

Fast relaxation in metallic glasses studied by measurements of the internal friction at high frequencies

We performed high-frequency (0.4 to 1.7 MHz) measurements of the internal friction (IF) on 14 bulk metallic glasses (MGs). It is found that 12 of these MGs display relaxation IF peaks at temperatures T= 400-500 K, which are weakly affected by heat treatment within the amorphous state. The corresponding relaxation time is about 0.3 microseconds. This fast relaxation is reported for the first time in the literature. The apparent activation enthalpy for 4 MGs is determined.

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arXiv2024

High entropy metallic glasses, what does it mean?

We performed calorimetric measurements on 30 bulk metallic glasses differing with their mixing entropies DSmix. On this basis, the excess entropies DS and excess enthalpies DH of glasses with respect to their maternal crystalline states are calculated. It is found that the excess entropy DS on the average decreases with increasing mixing entropy DSmix. This means that so-called "high-entropymetallic glasses" (i.e. the glasses having high DSmix) actually constitute glasses with low excess entropy DS. We predict that such glasses should have reduced relaxation ability. We also found that the excess enthalpy DH of glass linearly increases with its excess entropy DS, in line with a general thermodynamic estimate.

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arXiv2023

Relation of the thermodynamic parameter of disordering with the width of structure factor and defect concentration in a metallic glass

In this work, we show that above the glass transition there exists a strong unique interrelationship between the thermodynamic parameter of disorder of a metallic glass derived using its excess entropy, diffraction measure of disorder given by the width of the X-ray structure factor and defect concentration derived from shear modulus measurements. Below the glass transition, this relationship is more complicated and depends on both temperature and thermal prehistory.

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arXiv2023

Effect of the entropy on the shear viscosity of metallic glasses near the glass transition

We measured the shear viscosity of 14 metallic glasses differing with their mixing entropy $ΔS_{mix}$. It is found that the viscosity at the glass transition temperature $T_g$ significantly increases with $ΔS_{mix}$. Using calorimetric data, we calculated the excess entropy of all glasses $ΔS$ with respect to their maternal crystalline states as a function of temperature. It is shown that the excess entropy $ΔS$ both at room temperature and at $T_g$ \textit{decreases} with $ΔS_{mix}$. It is concluded that glasses with "high mixing entropy" $ΔS_{mix}$ correspond to MGs with \textit{low} excess entropy $ΔS$. The origin of the increased shear viscosity at $T_g$ of glasses with high $ΔS_{mix}$ is determined by their reduced excess entropy $ΔS$.

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arXiv2023

Dimensionless parameter of structural ordering and excess entropy of metallic and tellurite glasses

Using a notion on the excess entropy of glass with respect to the counterpart crystal, we introduce a simple dimensionless order parameter $ξ$, which changes from $ξ\rightarrow 0$ to $ξ\rightarrow 1$. The former case corresponds to a strongly disordered liquid-like structure while the latter situation describes a highly ordered crystal-like glass. This approach is applied to 13 metallic and 2 tellurite glasses. We found that $ξ$ is strongly sensitive to structural state and/or chemical composition. It can be also used for a comparison of the order in glasses belonging to different classes and appear to represent a new way of structural analysis.

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arXiv2023

Critical behavior of the fluctuation heat capacity near the glass transition of metallic glasses

The high-frequency shear modulus of five Zr-, Pd-, Cu-based conventional and two high-entropy bulk metallic glasses was measured in a wide temperature range up to the beginning of crystallization. Using these data and general thermodynamic relations, the "fluctuation" heat capacity $ΔC_f$ determined by local structural fluctuations in the defect regions is introduced and calculated. It is found that $ΔC_f$ temperature dependence for all metallic glasses has a large peak located slightly below or above the glass transition temperature but clearly lower than the crystallization onset temperature. The form of this peak resembles the characteristic $λ$-peak typical for order-disorder phase transitions. It is suggested that this $ΔC_f$-peak reflects certain underlying critical phenomenon. The critical temperature $T_0$ (peak temperature) and corresponding critical index $α$ are determined. Averaged over all seven metallic glasses under investigation in the initial and relaxed states, the critical index $α=0.26$. The results obtained indicate that the fluctuations of thermal energy near the glass transition bear the marks of a continuous phase transition. However, the derived critical index is between those corresponding to a second-order phase transition ($α\approx 0.1$) and a critical transition characterized by a tricritical point ($α\approx 0.5$).

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arXiv2023

Defect-induced ordering and disordering in metallic glasses

On the basis of shear modulus measurements on a Pt-based glass, we calculated temperature dependence of the defect concentration c using the Interstitialcy theory. This temperature dependence is compared with temperature dependence of the normalized full width at half maximum (FWHM) gamma of the first peak of the structure factor S(q) for the same glass available in the literature. It is found that above the glass transition temperature Tg linearly increases with c in the same way for both initial and relaxed (preannealed) samples providing the evidence of defect-induced disordering in the supercooled liquid region independent of glass thermal prehistory. For both states of the samples, the derivative d(gamma)/dc is close to unity. Below Tg, the interrelation between gamma and c is entirely different for initial and relaxed samples. In the former case, strong defect-induced ordering upon approaching Tg is observed while relaxed samples do not reveal any clear ordering/disordering. Possible reasons for these observations are discussed. To further investigate the relationship between the normalized FWHM and defect concentration, we performed molecular dynamic simulation of gamma(c)-dependence in a high-entropy FeNiCrCoCu model glass. It is found that gamma also linearly increases with c while the derivative d(gamma)/dc is again close to unity just as in the case of Pt-based glass.

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