Physical Mesomechanics最新文献

An austenitic natural composite structure reinforced with nanosized carbides (Fe_{100– x}Mn_x)₃C was first formed in classical Hadfield steel (Fe–13Mn–1.1C) under superplastic deformation. The mechanism and kinetics of dynamic strain aging of the steel under high-pressure torsion in Bridgman anvils were revealed. It was shown that increasing the strain and temperature of deformation caused an anomalous acceleration of dynamic strain aging of the steel with the formation of nanosized carbides (Fe_100–xMn_x)₃C.

The electroplastic effect is a sharp decrease in the deformation stress of metals under the action of an electric pulse. The mechanism of the electroplastic effect is still unclear. There is no explanation for the accumulated experimental data. In this paper, we propose a new mechanism of the electroplastic effect in metals, which is governed by athermal displacements of pinned dislocations during nonadiabatic Landau–Zener transitions of atoms in an open system of nuclei and electrons. The electric pulse action causes additional displacements of atoms in the specimen volume, increases the velocity of athermal displacements of dislocations and the plastic strain rate, which leads to a drop in the deformation stress. An explanation is provided for the experimental pattern of electroplastic deformation.

A calculation model is developed for earing during cylindrical drawing of metallic materials, which is based on the phenomenological criterion of plasticity taking into account the crystallographic texture of the material. The model was verified by comparing the calculated earing defects with those observed in drawing tests on an aluminum preform. Further modeling shows that deformation orientations ({112}<111>, {110}<112>, {123}<634>, and {100}<011>) lead to earing at an angle of 45° to the rolling direction, while recrystallization orientations ({100}<001> and {110}<001>) lead to earing in the rolling and transverse directions. Crystallographic orientation {110}<001> gives maximum earing, while orientation {123}<634> causes minimum one. In all the cases, the contribution of plastic anisotropy and yield stress anisotropy to earing is almost the same, with a slight predominance of the latter. The earing behavior is shown by the example of materials with a two-component ({112}<111> + {100}<001) texture: as the fraction of the {112}<111> orientation grows, ears in the rolling and transverse directions are reduced, while they form at an angle of 45° to the rolling direction. Considering such influence, the earing coefficient is minimum at 55–60% of the {112}<111> component.

The paper analyzes the stages and kinetic features of fatigue crack growth. Particular attention is paid to fatigue stage II consisting of two substages, namely, II_a and II_b. The stress intensity factor K_S is proposed to determine the boundary between them (corresponds to the stable crack length a_S under plane-strain conditions) and to characterize the cyclic fracture toughness. It is assumed that K_S corresponds to the stress intensity factor K_GY estimated by the cyclic yield stress and the length of a focal fatigue crack. Enlargement of the plastic zone at the crack tip and a transition to the plane-stress state at K ≥ K_S change the fatigue fracture pattern, which manifests itself as knee points in the K_max dependences of acoustic emission parameters, phase transformation rate in metastable steel, and fatigue striation spacing: after reaching K_S, the fatigue crack grows by the striation-per-cycle pattern. In addition, it is shown that the value of K = K_S corresponds to the pivot point of the crack growth curve plotted for the steel tested in mixed loading modes.

The paper is concerned with the effect of high-pressure torsion (HPT) on phase transformations and structure formation in near-β titanium alloy Ti-15Mo (wt.%) as well as with the dependence of the elastic modulus E and mechanical properties of the nanostructured alloy in the temperature range of 250–600°C. It is revealed that room-temperature nanostructuring of the β-quenched Ti-15Mo alloy to the von Mises strain ε ≈ 200 results in a homogeneous microstructure with a high defect density and the size of structural elements less than 100 nm. Formation of the nanostructure ensures an 80% increase in the ultimate tensile strength (UTS) of the Ti-15Mo alloy (UTS = 1550 MPa, El. = 7%) compared to that of the β-quenched alloy. It is shown that, after aging of the quenched and deformed Ti-15Mo alloy, the metastable β solid solution undergoes isothermal decomposition, resulting in the formation of the ω- and α-phases. The high defect density of the nanostructured alloy shifts the temperature range of the α-phase precipitation to lower temperatures (by 120°C on average) and has a significant effect on the volume fraction and morphology of α-phase precipitates. The latter have an equiaxed shape compared to the needle-like α-phase that precipitates during aging of the quenched coarse-grained alloy. After aging at 600°C, an equiaxed α + β structure with the average size of structural elements 380 nm is formed in the deformed alloy. Analysis of the mechanical properties after aging showed that the precipitation of dispersed ω-phase particles makes a significant contribution to precipitation hardening of Ti-15Mo alloy, significantly increases the microhardness (by 50%) compared to the quenched and deformed alloy, and can be considered as a macromechanical cause of the embrittlement of the alloy. The formation of an equiaxed α + β structure during HPT and aging at 550°C contributes to a balance between strength and ductility (UTS = 1270 MPa, El. = 10%). Changes in the structural-phase composition and phase ratios result in a nonmonotonic behavior of the elastic properties of the Ti-15Mo alloy. Studies of biological activity showed that both coarse-grained and nanostructured states of the Ti-15Mo alloy do not exhibit in vitro cytotoxicity towards blood leukocytes, indicating that these specimens are biocompatible. However, the nanostructured specimens demonstrated a pronounced inhibition of surface adhesion of S. aureus bacteria, which may potentially reduce the risk of postsurgical infectious complications following implantation of orthopedic metal devices based on the Ti-15Mo alloy in this structural state.

Sandwich structures with thin, stiff and heavy facings compared to the core are employed in civil and aerospace engineering, while those with thick, soft and lighter facings are preferred in precipitator plate applications. Insights gained into the behavior of horizontally polarized shear (SH) waves in sandwich structures can guide the design of more resilient and efficient composites, enhancing their performance under dynamic loading conditions. The dynamic behavior of a sandwich structure with symmetric facings is rigorously analyzed within the framework of the consistent couple stress model of elasticity. Harmonic wave solutions are derived, provided that they satisfy either traction-free or fixed boundary conditions on the faces, while maintaining continuity of tractions and displacements at the interfaces between the core and facings. This analysis uses the size-dependent consistent couple stress elasticity, which incorporates a length parameter (characteristic length) assumed to be of the same order as the internal microstructures of the material. Dispersion relations for the propagation of SH waves are calculated under both stress-free and fixed boundary conditions. Detailed mathematical results are provided, accompanied by graphical illustrations that show the impacts of characteristic length parameters and thicknesses of the core and facings on the phase velocity under both symmetrical and skew-symmetrical conditions.

The paper studies the effect of hafnium additives on the microstructure and mechanical properties during high-temperature annealing of high-magnesium aluminum alloys microalloyed with scandium and zirconium. The objects of investigation are two cast aluminum alloys alloyed and unalloyed with hafnium. The alloys are heat treated at 440°C for 48 h. The cast and heat-treated material is studied in mechanical tests, as well as under optical scanning and transmission microscopes. The structural-phase composition of these alloys is examined, and the effect of hafnium on the mechanical properties is analyzed. Atom probe tomography is used for a more detailed investigation of the internal structure of Al3Sc nanoparticles. It is shown that both alloys lack discontinuous precipitation of the supersaturated solid solution. The addition of hafnium decreases the size of Al3Sc nanoparticles. Like zirconium, hafnium forms a thermostabilizing shell around Al3Sc particles, thus preventing the growth of particles and contributing to their fine dispersion.

The paper reports the data of bending tests, microstructural and fractographic studies of specimens made of new unidirectional composites with the aluminum matrix reinforced with carbon fibers. These composites have increased specific strength compared to alloys and high crack resistance compared to carbon plastics due to the targeted formation of a sufficiently weak interface during their production. This is achieved by alloying the matrix with elements modifying the fiber–matrix contact layer and providing its low shear strength, as well as by optimizing parameters of the two-stage production technology. The problem under study is the influence of some production parameters on the microstructure, mechanical properties, and fracture mechanisms of the developed composites to find their optimum values ensuring higher strength and crack resistance. Consideration is given to the fracture mechanism of low shear strength materials under bending and the effect of their delamination (delamination cascade) on the fracture scenario and a significant decrease in the tensile strength revealed in bending tests. It is shown that delamination in the most loaded zone has an avalanche-like pattern, causing a very rapid increase in normal stresses and the number of fibers under maximum stress, i.e. the initiation of numerous fracture sites and rapid fracture of the entire specimen in the cross section under force. The data of three-point bending tests on specimens with different span lengths were used to propose a method for determining the shear strength-to-tensile strength ratio for a homogeneous isotropic material. The approach is also applicable to various composites with low interlaminar shear strength, in particular, to carbon-aluminum composites.

The gradient structure formed in the surface layers of Hadfield steel under dry sliding friction was studied using Mössbauer spectroscopy and electron microscopy. Structural-phase transitions are established in the contact stress zone on the steel surface. The 20-μm-thick surface layers reveal deformation-induced short-range ordering with an increased number of neighboring manganese and carbon atoms. Thinner (up to 1 μm) layers are composed of nanocrystalline carbides of the (Fe,Mn)₃C type.

Ultrahigh molecular weight polyethylene (UHMWPE) is a thermoplastic high-performance polymer, which is in high demand in biomedicine, ship and machine building, production of anthropomorphic robots and smart prostheses. Highly oriented UHMWPE fibers possess record specific strength and may be used for the fabrication of self-reinforced PE-PE composites (SRPECs). The temperature evolution study of the small-angle X-ray scattering (SAXS) signature of the supramolecular structure of UHMWPE can help reveal their important role in the mechanism of the shape memory effect in SRPECs. The laboratory XEUSS 3.0 SAXS-WAXS beamline was used for in situ studies of the nanostructure parameters in unidirectional SRPEC. In particular, the radius of gyration and the dimensionality factor were derived from 2D SAXS patterns using several fitting algorithms. These parameters change significantly in the temperature ranges corresponding to the initiation of the shape memory effect and melting. The relationship between the material anisotropy and 2D SAXS patterns is discussed in the context of the supramolecular structure evolution.