Physical Mesomechanics最新文献

Coarse-grained and fine-grained M1 copper is experimentally investigated using a new methodology for processing the results of anvil impact tests. Dynamic test data are supplemented by yield strength measurements in static tests and microstructural studies before and after impact testing. Key parameters characterizing the material response to high-speed loading are evaluated within the incubation time approach, which provides new insights into the dynamic behavior of copper with different grain structures. Comparison is made between the results of classical Taylor tests and the data derived within the new approach, which allows the yield strength to be predicted under both impulse and high-speed loading. It is shown that coarse-grained or fine-grained specimens can exhibit higher strength depending on the loading rate.

A theoretical model is proposed that describes the formation micromechanism of a zigzag-shaped deformation twinning structure in aluminum nanowires covered with an amorphous aluminum oxide layer. Within the framework of a two-dimensional model, this structure is formed by successive nucleation of twins in the aluminum nanolayer between the interfaces with the amorphous oxide films. In this case, the primary twin is formed at a stress concentrator near one of these interfaces, secondary twins nucleate in the stress concentration sites at the ends of the primary twin, and then this process repeats cyclically, i.e., new twins nucleate at the ends of the previous ones. Contour plots of von Mises shear stresses and elastic energy density of the primary twin are generated. Using these plots, the nucleation sites and growth directions of secondary twins are determined. According to the results of the model, the successive nucleation of deformation twins should lead to the formation of a zigzag twinning structure. It is concluded that the main reason for the formation of such a structure in aluminum nanowire is the presence of an oxide amorphous layer on its surface, which prevents the escape of the twins to the free surface of the nanowire, and as a result new stress concentration sites emerge in their locking points.

A comprehensive study is performed to investigate the effect of zinc concentration on structural and phase transformations in metastable (α + β) Cu-Zn-based shape memory alloys. The start and finish temperatures of forward and reverse martensitic transformations in Cu-xZn alloys (x=38, 39.5, and 41 wt %) are determined using electrical resistance measurements. Structure and phase transformations are studied by optical, scanning and transmission electron microscopy as well as by X-ray phase analysis. It is found that critical temperatures of thermoelastic martensitic transformations decrease with an increase in the zinc concentration. Diffuse effects in the selected area electron diffraction patterns are analyzed depending on the zinc concentration in metastable Cu-Zn binary alloys. An increase is observed in the dislocation density under thermal cycling due to the martensitic transition.

The structural-phase state, mechanical and inelastic properties of Ti_49.8Ni_50.2 alloy after severe plastic deformation by abc pressing at 573 K were investigated. It was found that abc pressing of the Ti_49.8Ni_50.2 alloy specimens at a relatively low temperature (573 K) to high true strain (e = 7.43) did not affect the start temperature of B2→R martensite transformation (MT). At the same time, the start and finish temperatures of direct MT to the B19' phase and reverse MT decreased by only 14 ± 2 degrees. After abc pressing at 573 K to е = 7.43, the martensitic shear stress τ_m sharply decreased (from 230 MPa to 100 MPa), and the pseudo-yield plateau significantly shortened (from 7.5 to 3.5%). In addition, the stress of transition from the strain hardening stage to the active plastic deformation stage sharply increased (from 835 to 1300 MPa). It was found that the dependences of the shape memory effect γ_SME, superelasticity γ_SE, and plastic strain component γ_pl on the total strain γ_t both in the initial state and after abc pressing had a similar pattern and were close in magnitude. It was shown that abc pressing to e = 7.43 induced the total inelastic strain γ_TIS = γ_SE + γ_SME of about 8% in the specimens, which recovered after unloading due to the superelasticity and shape memory effects.

The variation in martensitic transformation temperatures, electrical resistivity, and dislocation density were studied during 500 thermal cycles in Ti_40.7Hf_9.5Ni_44.8Cu₅ alloy with grain sizes ranging from 130 to 0.16 µm. The maximum transformation temperature change was observed in the sample with an average grain size of 130 µm, and the minimum temperature change was found in the sample with a grain size of 0.16 µm. It was shown that during thermal cycling of Ti_40.7Hf_9.5Ni_44.8Cu₅ alloy the transformation temperatures can decrease, increase, or remain constant, independently of the grain size. Analysis of the dislocation density variation measured by the Williamson–Hall method and the resistivity variation determined in the austenitic and martensitic states showed that these quantities change nonmonotonically with increasing number of thermal cycles. In Ti_40.7Hf_9.5Ni_44.8Cu₅ samples with grain sizes of 16 and 0.16 µm, the dislocation density and resistivity increased during the first 50–200 cycles and then decreased, without any effect on the transformation temperature variation. The obtained results show that the change in dislocation density is not the only reason for the change in transformation temperatures during thermal cycling.

Rubber-metal springs are widely utilized in industrial applications, particularly as vibration absorbers, due to their ability to mitigate dynamic loads. The dynamic stiffness of rubber-metal springs plays a crucial role in determining the natural frequency of a system, as natural frequency is directly linked to dynamic stiffness. Therefore, the accurate determination of dynamic stiffness is essential when selecting an appropriate rubber-metal spring for a given application. However, the assessment of dynamic stiffness presents a significant challenge due to the complex interaction between rubber and metal components, particularly when considering the viscoelastic properties of rubber and the geometric properties of the spring. Rubber’s viscoelastic response and how it changes under different strain rates is fundamentally rooted in the micro- and meso-scale configuration of polymer chains, filler particles, and their bonding to metal components. Consequently, dynamic stiffness is often approximated using static stiffness measurements, which simplifies the problem but may lead to inaccuracies in predicting the true dynamic behaviour of the spring. In this paper, we present an experimental method for dynamic stiffness assessment using an electrodynamic shaker, which allows for a more accurate characterization of the spring’s response to dynamic loading. This method is compared to an analytical approach based on static stiffness, highlighting the limitations of the latter approach. Furthermore, we propose an improved range for calculating dynamic stiffness from static stiffness, enhancing the predictive accuracy for dynamic behaviour.

High-chromium martensitic steels with low nitrogen and high boron contents are promising materials for the manufacture of thermal power units operating at ultra-supercritical steam parameters, which must have high creep resistance and good impact resistance. In all the studied steels, regardless of alloying and heat treatment, a lath structure with a high dislocation density is formed, which is stabilized by the M₂₃(C, B)₆, M₆C and NbX particles. The addition of rhenium together with a change in the tungsten/molybdenum and carbon contents ensures a decrease in the number density of grain boundary M₂₃(C, B)₆ particles, which allows reducing the ductile-brittle transition temperature by 15–20°C. The addition of copper leads to the formation of copper clusters/particles, which, on the contrary, increases the ductile-brittle transition temperature by 25–30°C. Increasing the quenching temperature does not affect the position of the ductile-brittle transition for low-copper steels alloyed with copper, tungsten, and molybdenum, although this shifts the Charpy curve towards lower energies due to coarsening of the prior austenite grains. For the rhenium-containing high-copper steel, increasing the quenching temperature reduces the ductile-brittle transition temperature by 5–10°C due to a decrease in the number of copper clusters/particles. The modification of alloying by increasing the content of rhenium, tungsten, and copper together with the change in heat treatment improves significantly the creep resistance, while the resistance to impact loads remains at a sufficiently high level (above 100 J × cm^-2 at room temperature), which meets the requirements for boiler materials and steam turbine blades.

In this work, manganese-doped (Cr_1-xMn_x)₂AlC phases were synthesized by self-propagating high-temperature synthesis. The synthesis was carried out in a 3 L reactor under an initial argon pressure P₀ = 5 MPa with the calculated x values of 0.05, 0.15, and 0.30. The diffraction peaks of the MAX phase in the manganese-containing samples were found to be shifted relative to the positions of the Cr₂AlC peaks, indicating the substitution of a part of chromium atoms by manganese atoms. The manganese content in the MAX phase was estimated by energy-dispersive X-ray spectroscopy (EDS) after purifying the phase in hydrochloric acid. The measured manganese concentration corresponded to x = 0.015, 0.035, and 0.15. The synthesized (Cr_1-xMn_x)₂AlC phases were compacted to a residual porosity of 20% and then infiltrated with copper melt. The infiltration was performed by depositing a melt droplet onto the sample surface and holding it at 1150°C in a vacuum of 10^-3 Pa. The examination of the infiltrated phase structure revealed complete or partial decomposition of the MAX phases and the fusion of individual grains, resulting in a mechanically robust sample. The resulting submicron structure consists of a nanoscale chromium carbide skeleton infiltrated with Cu(Al, Cr, Mn) bronze. The mechanical properties were evaluated by measuring the microhardness both inside and outside the infiltrated region. It was shown that manganese reduces the hardness of the composite structure and, at high content, suppresses the formation of chromium carbide. The resulting composite structures with low or no manganese content show high potential for applications as wear- and corrosion-resistant conductive materials.

The application of arc, plasma, laser, or electron beam welding methods to high-strength heat-treatable aluminum alloys, such as AA7075 belonging to a Al-Zn-Cu-Mg system, makes little sense because of the formation of weak heat-affected zones and hot cracks. Attempts were made to produce strong joints in these alloys using friction stir welding, which is a low heat input method. However, there are still problems with overaging and the formation of weak thermomechanically affected zones. In this research, we employ liquid flow cooling of the weld zone to eliminate overaging. It is found that the microhardness in the stir zone and the joint strength are maximum due to cooling at a medium liquid flow rate of ~4–6 l/min. All samples were subjected to artificial aging to eliminate weak thermomechanically affected zones, which further increased the microhardness and strength of joints. Structural studies reveal that the contribution from the dispersion strengthening mechanism under internal aging, which can be further enhanced by artificial aging, is maximum at medium cooling rates.

The paper studies the microstructure and mechanical characteristics, in particular, fatigue strength, of the Zn-1%Mg-0.1%Ca alloy subjected to high-pressure torsion (HPT) at room temperature. The study shows that a mixed microstructure is formed in the alloy after HPT, which consists of α-Zn grains 700–900 nm in size, nanograins of a magnesium-rich phase 50–100 nm in size, and finely dispersed particles ~30–50 nm in size. The formation of such a structure leads to an increase in the yield stress of the alloy by ~2.5 times and in the ultimate tensile strength by ~2.7 times with a simultaneous significant growth in ductility (approximately by 13 times). An increase in the static strength is also accompanied by an increase in the fatigue strength by 1.7 times. However, the increase in fatigue strength is less pronounced compared to the increase in static strength. The fatigue strength increases by almost 70% above that of the initial alloy (from 65 MPa to 110 MPa, respectively). Upon reaching 2.8 × 10⁶ loading cycles by the repeated tension scheme, fracture of the specimen is accompanied by the formation of microcracks directed perpendicular to the fatigue crack front and located near the final fracture zone.