Vol 61, No 7-8 (2019)

Special features of the microrelief of fractures of notched specimens of steel 20 tested under shear, opening and mixed loading modes are studied by x-ray diffraction analysis and analysis of fracture surfaces. The degree of the damage of the specimens is assessed. It is shown that increase in the contribution of the shear component lowers the degree of distortion of crystal structure on the fracture surface, which agrees with reduction of the area of the plastic zone, of the depth of the fracture microrelief, and of the relative width of the shear region. The physical broadening of the x-ray diffraction line obtained from the fracture surface is shown to be correlated with the mode of loading of the specimen.

The microstructure, mechanical properties and fracture behavior of magnesium alloy Mg – 10% Gd – 3% Y – 0.6% Zr is studied in cast condition and after T4 and T6 treatments. It is shown that in cast condition the alloy contains an α-phase (a magnesium-base solid solution) and a Mg₂₄(Gd, Y)₅ eutectic phase. After the solution treatment the eutectic phase dissolves in the α-matrix containing cuboid particles of a phase enriched with gadolinium and yttrium. The hardness and the tensile strength criteria are used to determine the optimum treatment for the cast alloy, i.e., 8-h solution treatment at 500°C and 16-h aging at 220°C (T6). This mode of heat treatment provides the best mechanical properties.

A model of precipitation hardening of an Al – Mg – Si alloy due to precipitation of several phases formed under different aging conditions is presented. The diameters of the precipitated particles and their relative contents are determined by the method of thermodynamic simulation. The characteristics of the precipitated phases are allowed for, which raises the prediction accuracy. The rate of the precipitation of the β- and β′-phases is shown to increase with the aging time, and the rate of growth of their particles is shown to increase with the temperature.

The effect of solution treatment and preliminary cold working on the high-temperature tensile behavior of a high-nitrogen nickel-free corrosion-resistant austenitic steel at 875 – 950°C and different deformation rates (from 10 ^{– 2} to 10 ^{– 4} sec ^{– 1} ) is studied. It is shown that the steel has high mechanical properties after the solution treatment. After the cold rolling the strength of the specimens increases and the ductility decreases due to strong strain hardening. At a high temperature, the stress-strain curves exhibit serrations at the lower deformation rates. The strength of such specimens is lower than after the solution treatment.

Functional characteristics of bar semiproducts from shape memory NiTi alloys obtained by equal channel angular pressing, worm forging, and their combination are determined. The functional characteristics are measured and the phase transitions are studied under the conditions of three-point bending and thermocycling at a constant load. It is shown that the bar specimens subjected to warm forging at 350°C and to a combination of equal channel angular pressing at 450°C and warm forging possess the best level of functional properties.

The effects of carbides and nonmetallic inclusions on formation of fatigue microcracks in steels for high-load bearings are determined. The running time is shown to affect the growth of parameter N_c/N_t for all the steels studied. The size of the carbides is shown to affect the degree of propagation of fatigue microcracks; the fraction of carbides with fatigue microcracks grows with their size. The results obtained are helpful in choosing the direction of optimization of the structure of carbide phases in bearing steels.

The microstructure, impact toughness and melting parameters of cold-resistant steel 20GL are studied. The impact toughness of the steel at negative temperature (KCV^-60) is shown to be affected the most by the size of the primary (natural) grains of the metal. The size of the natural grains is shown to be dependent on the content of oxygen and (to a less degree) of silicon. Introduction of an elevated amount of slag-forming materials and aluminum into liquid metal provides the required (at most 0.005–0.008 wt.%) content of oxygen and an impact toughness (KCV^-60) no less than 0.167 MJ/m². Influence of the nonmetallic inclusions, of the size of actual grains, and of the content of pearlitic phase on KCV^-60 in normalized steel has not been detected or was not obvious.

The evolution of the structure and mechanical properties of sheets from alloy Al – 4.7% Mg – 0.32% Mn – 0.21% Sc – 0.09% Zr during warm rolling and subsequent annealing is studied. It is shown that extended fibers with a thickness of 50 – 100 nm form inside the deformed grains in the rolling process. Post-rolling annealing of the sheets at 150 and 200°C results in substantial lowering of the elongation after a hold for 3 h; then the ductility recovers after a hold for 5 h. The lowering of the ductility is associated with precipitation of a fine β -phase over grain boundaries during the annealing. The phase grows and its volume fraction decreases when the annealing time is prolonged. The possibility of formation of high-level mechanical properties after warm rolling and subsequent annealing is shown.

The kinetics of decomposition of supersaturated magnesium solid solution and of the accompanying hardening of the latter at 200°C in cast alloy IMV7-1 of the Mg – Y – Gd – Zr system alloyed with two rare-earth metals, i.e., holmium and ytterbium, is studied. Holmium and ytterbium are introduced into IMV7-1 by substituting two other rare-earth metals, i.e., yttrium and gadolinium. It is shown that the presence of holmium and ytterbium instead of yttrium and gadolinium does not change the decomposition behavior of the supersaturated solid solution but decelerates this process and lowers the effect of age hardening.

A hardness model employing the end quench Jominy method is developed for steels C25, EN8, EN19, EN31 and EN24. The time-temperature data are obtained from four thermocouples mounted at critical places of a specimen. The heat flux during the quenching is determined from the cooling curve obtained with the help of the thermocouple closest to the end of the specimen (quenching place). The two-dimensional axisymmetric equation of heat conduction is solved and used jointly with the models of decomposition of austenite to obtain the distribution of microstructure at the places used to plot the cooling curves. The computed distribution of microstructure and the chemical compositions of the steels are used to estimate the hardness. The computed hardness values agree well with those determined experimentally over the length of the specimen.

Results of fractographic analysis and Auger electron spectroscopy of materials of vessels of VVER-1000 reactors obtained at the “Kurchatov Institute” Research Center are systematized. Comparative analysis of the effect of the operating factors on the level of grain boundary embrittlement in the matrix metal and welded joints is performed. The contribution of grain boundary embrittlement into the total radiation-induced embrittlement of the weld metal of the vessels is determined.

Regular features of the effect of recrystallization of austenite under hot deformation on the formation of structure in thermal deformation treatment (with subsequent hold in the furnace and without it) of rolled sections from corrosion-resistant austenitic chromium-nickel steels 10Kh17N13M2T (EI-447) and 12Kh18N19MT are investigated. The kinetics of the austenite recrystallization is studied by metallography and measurement of the hardness of specimens and of the microhardness of recrystallized and not recrystallized grains. The volume of the recrystallized grains and their mean size are determined. The effect of the homogenizing modes in heating for quenching, and of accelerated and decelerated cooling after rolling in a 1050/950 rolling mill of the “Dneprospetsstal’” Company on the impact toughness of the steels is studied.

Results of a study of the effect of a modifier (boron carbide) on the microstructure and mechanical properties of a castable titanium pseudo-α-alloy are presented.

The elemental composition, the mechanical and thermomechanical characteristics of specimens cut from different zones of cast and pressed bars are determined for assessing the possibility of commercial application of shape memory alloys of the Ti – Ni – Nb system. Statistical analysis of the results is performed using the STADIA 7.0 software. The effect of inhomogeneity of the composition of the alloy in different zones of the bars on nonuniformity of mechanical and thermomechanical properties is determined in the initial condition and after vacuum annealing. The results obtained are used in design and development of technologies for objects of the nuclear power industry.

The methods of design of experiment (DOE), analysis of variance (ANOVA) and response surface (RS) are used to study the effect of heat treatment on the microstructure and mechanical properties of a novel AlSi11MgMn pressure-cast alloy with an addition of Al₂Ca. After the high-pressure casting, the specimens are solution treated at 490°C for 15 min and quenched in cold water. After the solution treatment, the morphology of the eutectic silicon changes, the elongation increases, and the yield strength and the ultimate strength decrease. The effect of the aging temperature and time on the tensile mechanical properties is studied by 4 × 4 full factorial design of experiment. The response surface is plotted for the ultimate and yield strengths and for the elongation; three equations predicting satisfactorily the mechanical properties of the alloy as a function of the aging temperature and time are derived.