I. M. Safarov

Strength and impact toughness of low-carbon steel with fibrous ultrafine-grained structure

The effect of severe warm rolling on the structure and mechanical characteristics of the 12GBA low-carbon steel has been studied. A fibrous ultrafine-grained structure has been formed; the average transverse size of structural elements was 0.5 μm and the length of fibers in the longitudinal section was 20–25 μm. An analysis of this ultrafine-grained structure in transverse and longitudinal sections was carried out using the method of electron backscatter diffraction. It has been shown that the formation of the fibrous structure results in a twofold increase in the strength of the steel in comparison with the original coarse-grained state, with retaining satisfactory ductility. The additional annealing of the steel leads to a slight decrease in its strength characteristics, but results in the complete restoration of its ductile characteristics to the values typical of the coarse-grained state. Impact tests at negative temperatures have shown that, after rolling, the cold-brittleness threshold shifts toward a lower temperature range (from −30 to −60°C) in comparison with the coarse-grained state. It has been found that the formation of the ultrafine-grained state leads to a decrease in the cold-brittleness threshold of the 12GBA steel, as well as to an increase in its impact toughness in the low-temperature range and in the contribution of the crack-propagation work to the total work of fracture of a specimen.

Effect of submicrocrystalline state on strength and impact toughness of low-carbon 12GBA steel

The effect of intense warm deformation on the structure and mechanical properties of low-carbon 12GBA steel was investigated. A submicrocrystalline (SMC) structure with an average element size of 0.3 μm was formed in the steel by isothermal overall forging. The formation of the SMC structure resulted in a sharp increase in strength by a factor of two to three in relation to the initial coarse-grained state while retaining a sufficient level of plasticity and impact toughness. After further annealing, steels exhibit an improved set of properties; i.e., as the strength decreases slightly, the plasticity increases sharply. Impact tests at low temperatures have shown the significant advantage of the SMC state of the steel over the coarse-grained state in the impact toughness. It is established that the cold resistance in the SMC state increases because the crack propagation prevails in the overall sample fracture.

Effect of grain boundaries on the electron work function of nanocrystalline nickel

The electron work function of nickel with various grain sizes has been studied. It has been shown that the work function decreases as the specific length of grain boundaries in nickel increases with decreasing average grain size. It has been found that the transformation of grain boundaries from a nonequilibrium to equilibrium state leads to an increase in the electron work function by 0.15 eV.

Development of martensitic transformation induced by severe plastic deformation and subsequent heat treatment in polycrystalline Ni52Mn24Ga24 alloy

The results of a study of the martensitic transformation in the polycrystalline Ni 52 Mn 24 Ga 24 alloy in different structural states are presented in the article. In the initial state in the alloy magnetic and martensitic phase transformation is observed with the following characteristic points: M S =25°C; M F =15°C; A S =35°C; A F =45°C; T C =127°C. The alloy in the initial state, after intensive plastic deformation by high pressure torsion and after the stepped annealing of the deformed material at temperatures of 400°C, 500°C and 600°C was investigated. The microstructure of the alloy was investigated by means of scanning electron microscope equipped by the detector sensitive to orientation contrast of material. Analysis of the microstructure of the alloy in the initial state shows that the average grain size is 270 µm. In the alloy after plastic deformation and subsequent annealing at 400°C, this value is 180 nm. Annealing at 500°C and 600°C leads to an increase in the average grain size up to 1.08 µm and 2.33 µm, respectively. The results of the study of the microhardness of the alloy in different structural states are presented. As a result of plastic deformation of the alloy the microhardness increases from 2.1 GPa in the initial state to 5.1 GPa in the deformed state. After annealing, a gradual decreasing of the microhardness down to 2.8 GPa is found. The temperature dependence of magnetization of the alloy in different structural states shows that as a result of severe plastic deformation the ferromagnetic order is destroyed and the martensitic transformation is suppressed. After stepped annealing at 400°C, 500°C and 600°C, there is a gradual recovery of ferromagnetic order. The martensitic transformation is observed in the alloy only after annealing at 500°C. In this case the average grain size is increased to 1 µm.

Fine-grained structure and properties of a Ni2MnIn alloy after a settling plastic deformation

The structure and properties of a polycrystalline Ni–Mn–In Heusler alloy have been studied after a plastic deformation by upsetting. An analysis of points of a martensitic and magnetic phase transformations shows that the martensite transformation takes place at temperatures lower than the Curie point. At high temperatures in the range 930–1110 K the alloy undergoes the phase transformation of ordered phase L21 to disordered phase B2, and the melting temperature of the alloy is 1245 K. The flow curves of the alloy cylindrical samples at temperatures 773, 873, and 973 K have been built. An analysis of the alloy microstructure after upsetting at a temperature of 773 K leads to the conclusion that many macrocracks are initiated in the sample. The treatment at 873 and 973 K causes a fragmentation of the grains with grain sizes from several to several dozen micrometers. However, the upsetting at 873 K leads to insignificant scatter in the grain sizes, and the microstructure is more homogeneous and worked out.

Effect of severe plastic deformation on the properties of the Fe-36% Ni invar alloy

Temperature dependences of the magnetization and thermal expansion coefficient of the Fe-36% Ni invar alloy in different structural states obtained upon plastic deformation by torsion under a quasihydrostatic pressure followed by annealing at different temperatures are studied. A decrease in the thermal expansion coefficient of the invar in a temperature range of 20–100°C and the formation of ordered precipitates (with a superstructure) were found to result from severe plastic deformation.

An anomaly of the temperature dependence of the impact strength of low-carbon steel with an ultrafine-grain structure

The effect of all-round isothermal forging on the structure and mechanical properties of low-carbon steel 12GBA is investigated.

Influence of the structure of the Ti-49.8 at % Ni alloy on its thermal expansion during the martensitic phase transformation

The temperature dependence of thermal expansion of the Ti-49.8 at % Ni alloy has been measured after rolling at temperatures of 470, 570, 670, 770, and 870 K. The maximum dilatation jump during the martensitic phase transition has been observed for the samples rolled at 570 and 670 K. A fragmented structure, in which the phase transformations are hampered, is formed during the low-temperature rolling. An increase in the rolling temperature to 770–870 K leads to the return processes and dynamic recrystallization of the material; as a result, the slope of dilatation curves changes and the range of phase transitions narrows.

Effect of upsetting deformation temperature on the formation of the fine-grained cast alloy structure of the Ni–Mn–Ga system

The plastic behavior during deformation by upsetting and its effect on the microstructure in the polycrystalline Ni2.19Fe0.04Mn0.77Ga alloy are studied. The temperatures of martensitic and magnetic phase transformations were determined by the method for analyzing the temperature dependence of the specific magnetization as MF = 320 K, AS = 360 K, and TC = 380 K. Using differential scanning calorimetry, it is shown that the phase transition from the ordered phase L21 to the disordered phase B2 is observed in the alloy during sample heating in the temperature range of 930–1070 K. The melting temperature is 1426 K. An analysis of the load curves constructed for sample deposition at temperatures of 773, 873, and 973 K shows that the behavior of the stress–strain curve at a temperature of 773 K is inherent to cold deformation. The behavior of the dependences for 873 and 973 K is typical of hot deformation. After deforming the alloy, its microstructure is studied using backscattered scanning electron microscopy. Plastic deformation of the alloy at study temperatures results in grain structure fragmentation in the localized deformation region. At all temperatures, a recrystallized grain structure is observed. It is found that the structure is heterogeneously recrystallized after upsetting at 973 K due to the process intensity at such a high temperature. The alloy microstructure after plastic deformation at a temperature of 873 K is most homogeneous in terms of the average grain size.

Effect of plastic deformation on the structure and mechanical properties of an ultra-low carbon interstitial-free steel in the monolithic material and as a component of a sandwich composite

The structure and mechanical properties of ultra-low carbon interstitial-free (IF) steel in the annealed state, after warm and cold rolling, and as a component of seven-layer steel–aluminum composite have been studied. A comparative analysis of the results of structural studies using optical microscopy and scanning and transmission electron microscopy have revealed the possibility of the formation of an ultrafinegrained structure in a steel layer during rolling at temperatures ranging from room temperature to 520°C. It has been found that the seven-layer composite has higher strength properties as compared to monolithic samples of the IF steel after analogous regime of the warm rolling.