No 3 (2020)

The development of composite materials based on polymers modified with carbon nanostructures (CNS) is a promising area of scientific research as their application allows one to more efficiently improve the functional properties of polymers in comparison with other modifiers. The paper deals with the study of the electrically conductive properties of an epoxy resin modified with the expanded graphite compound (EGC), which was previously modified with the phenol-formaldehyde resin (PFR) by ultrasonic treatment in an aqueous solution. The authors flocculated the resulting concentrated dispersion of EGC with PFR with the acetic acid, filtered and washed with water. The PFR-modified EGC aqueous paste was introduced into the ER matrix by the mechanical stirring in a three-roll mill. The study proved that the modification of RSF with FFS before introduction into the epoxy matrix contributes to the better distribution of the carbon material, as well as to the decrease in the size of its particle aggregates. The authors carried out the investigations of the electrical conductivity of the composites obtained by hot curing, according to which they found that samples based on epoxy resin containing 9 wt.% graphite modified with PFR had the maximum electrical conductivity of 6.2×10^-4 S×cm^-1, which was 2 orders of magnitude higher than the electrical conductivity of samples using graphite without preliminary processing. The percolation threshold was observed at 3 wt.% graphite in the epoxy composite. The obtained results prove that the use of EGC modified with PFR as the epoxy resin filler allows achieving higher electrical conductivities than when using untreated EGC. Moreover, the use of PFR for the EGC modification has an advantage over other surfactants as, due to the interaction of active PFR molecules with the epoxy resin molecules, there is no need in further removal of the surface-active substance (SAS) from a composite.

The authors studied the rate dependence of the deformation behavior of the circular section samples prepared from the Ti-3.5Al-1.1Zr-2.5V alloy under the uniaxial tension at room temperature. Samples 200 mm long were divided into three groups of five pieces each. The authors tested the first group of samples at a traverse rate of 0.05 mm/min, the second group - at a rate of 5mm/min, and the third group - at a rate of 500 mm/min. The evaluation of the titanium alloy microstructure in the undeformed state showed that the average grain size of the titanium α-phase was about 7 μm, and the grain boundaries were mostly angular, i.e. the neighboring grains were disordered by more than 15°. The mechanical tests showed that the nature of the titanium alloy deformation behavior did not depend on the loading rate. Despite this, the yield and strength limit increased with an increase in the strain rate, while the total strain value decreased. At the place of sample fracture, a neck was observed. The contraction coefficient did not depend on the tensile speed. The authors did not observe any qualitative changes in the mechanical behavior nature and the morphology of the surface of sample fractures (a cup fracture typical for viscous fracture). The study of samples microstructure justifies an increase in the deformability of samples with a decrease in the tensile rate. The width of the diffraction peaks of the samples tested at a lower speed was greater. The fluctuation of the obtained values of the diffraction lines’ width relative to the approximating straight line indicates the speed sensitivity of the grains of “hard” and “soft” orientations. This indicated the existence of the slip system activation sequence. Thus, first of all, “soft” grains are loaded, which are favorably oriented for easy prismatic sliding in the (100) and (110) planes. Then they harden, which contributes to the redistribution of the load on the “hard” grains with basic normals close to the axis of loading, which, at the initial stage, were deformed elastically.

The existing numerous experimentally-built phase equilibrium diagrams of nonferrous alloys reflect the specific character of interaction of the components at their different ratios and different temperatures and give an idea of the so-called “metallographic” structure of alloys. In general, the literature sources establish a rather good relation between the structure and the properties, which allows controlling properties, predicting their possible change when varying the components concentrations and the structure forming conditions. However, the applied criteria, which sometimes allow explaining and predicting the level of the achieved properties according to phase equilibrium diagram appearance, do not make it possible to explain the nature of a rather large number of existing anomalies of physicomechanical properties of the industrially used nonferrous alloys. Based on the study of numerous literature data, the author identified the regularity, which allows establishing a relationship between the anomalies in the physicomechanical properties of nonferrous alloys and phase equilibrium diagrams. The author introduced the concept of phase equilibrium diagram as the concentration dependence of the qualitative changes in the crystallization (recrystallization) intervals, which makes it possible to associate the phase equilibrium diagram with the extreme values of physicomechanical properties, which cannot be explained by the peculiarities of the phase composition or structure. The author developed the technique that allows associating anomalies in the properties of alloys with phase equilibrium diagrams based on the first established criterion - a qualitative change (temperature extension) of the crystallization (recrystallization) interval (Q∆LS), as well as with a difference in the structural heredity (genealogy) of the component atoms that make up the dual system. The joint analysis of the anomalies in the properties of binary alloys with state diagrams (based on the established criterion (Q∆LS)) allows relating the latter to the presence of intermediate phases in the Cu-Zn, Cu-Sn, Cu-Si, Al-Cu, Al-Si, Al-Mg, Al-Cu-Mg, Cu-Mn systems. Based on the identified regularity of the relationship between the anomalies of physicomechanical properties of alloys and the qualitative changes in the crystallization (recrystallization) interval (Q∆LS), the author proposes an alternative version of Kurnakov’s law.