Russian Metallurgy (Metally)最新文献

High-resolution electron microscopy and X-ray diffraction are used to study the phase composition and structure morphology of an equiatomic high-entropy AlCuFeCoNiCr alloy prepared by arc melting and the evolution of its structure after thermal (up to 1650°C) and thermobaric action (up to 10 GPa at 1650°C). After arc melting, the alloy structure is microcrystalline dendritic. Copper predominates in dendrites. The interdendritic region has a Widmanstätten structure. B2 (in the matrix) and A1 (in dendrites) structures are formed in the ingot. The microhardness (H_v) of the alloy is ~6500 MPa. A differential thermal analysis curve measured during heating demonstrates three endothermic processes in a sample. In the course of subsequent solidification during cooling at a rate of 1°C/s, the structure characterized by two types of dendrites having a substructure is formed. The dendrites differ in size, shape, and element contents. Like in the case of the arc-melted sample, the interdendritic region exhibits a Widmanstätten structure, but the plates are larger. During crystallization, a separation occurs in the dendrites and interdendritic regions and is accompanied by the precipitation of phases differing in composition and morphology. The hardness of the alloy (H_v) decreases to 5000 MPa. After thermobaric treatment (heating to 1650°C and subsequent cooling at a rate of 1000°C/s under pressures of 3, 5, and 8 GPa), the structure of all samples is uniform and dense. The structure is finer as compared to that observed after heat treatment; there is a correlation, i.e., dendrites of two types are formed, which differ in shape, morphology, and element contents. Dendrites of one type (first type) are enriched in copper, whereas the other dendrites (second type) are enriched in aluminum. Dendrites of both types are bordered with interlayers enriched in copper. The interdendritic region contains a mixture of phases, each of the phases is a solid solution. In accordance with pressure, a structure based on a B2 solid solution or a structure consisting of a mixture of A1, A2, and B2 phases forms in the alloy. The studies showed that, at pressures of 3–4 GPa, H_v is two times lower than that of the initial sample (3300 MPa). As the pressure increases to 5–7 GPa, H_v increases (3500 MPa). At a pressure of 8–10 GPa, the phase composition of the alloy remains similar to that observed at a pressure of 5–7 GPa; almost twofold refinement of the structure takes place, and H_v increases to 4500 MPa.

A pseudobinary phase diagram is constructed using computer modeling for a heat-resistant niobium-alloyed Cr–Ni–Si austenitic steel. Based on this diagram and earlier in situ observations of solidification in highly alloyed austenitic steel, the solidification mechanisms of phases are clarified. For carbon contents up to 0.08 wt %, an identical sequence of solid–phase growth mechanisms is established for the solidification of both the weld metal and a ladle sample taken during the melting of Fe–15Cr–9Ni–3Si–Nb electrodes for electroslag remelting. A dendritic growth boundary is constructed on the phase diagram using the concept of discontinuous solid solution solidification, which enables the interpretation of the observed primary structure morphology. Prior to the peritectic reaction, a highly branched framework of skeletal δ ferrite dendrites is shown to develop into a classical dendritic form through layer-by-layer solidification of excess high-temperature δ ferrite. The composite dendrite consists of high-temperature δ ferrite and contains two internal zones separated by a high-angle boundary with different microsegregation characteristics due to dendritic and layer-by-layer solidification mechanisms. Metallographic analysis shows that the “one dendrite–one grain” principle is maintained on a macroscale during solidification for each composite dendrite. This defines the concept of a primary (as-cast) grain for the composition under study. The cubic and blocky niobium-rich micron-sized carbide phases detected in the weld metal are found to be transferred (inherited) from the welding consumables. The identical morphology of the primary solidification structures in the regions between adjacent weld beads is shown to be caused by the same solidification features occurring under far-from-equilibrium conditions.

The microstructure and thermal properties of foils melt quenched from hypoeutectic, eutectic, and hypereutectic Al–Si alloys are investigated depending on their thicknesses. The maximum achievable foil thickness decreases with increasing silicon concentration. A common pattern of layered silumin microstructure formation is established for all alloys. Specifically, the number of layers increases with the foil thickness. The change in the solidification mechanism in the layers of three-layer (thick), two-layer (medium thickness), and single-layer (thin) foils at a constant component concentration is explained. Decreasing the thickness of thin single-layer foils leads to a reduction in the initial heat transfer coefficient and the formation of a microstructure with a primary phase and eutectic grains. This microstructure is characteristic of the final solidification stage of foils of medium and maximum thicknesses. Analysis of the melting thermograms of the eutectic and hypereutectic alloy reveals that the structural phase state of the layer adjacent to the freely solidified side exerts the greatest influence on the thermal properties. This layer occupies 80–100% of the foil volume.

A die casting technique based on controlled diffusion solidification (CDS) with simultaneous mixing was proposed. In this case, the shot chamber is regarded as the mixing crucible, i.e., the mixing crucible is a horizontal cylindrical vessel, differing from the commonly-used vertical one. Taking pure Al and Al–12Si alloy as precursor alloys 1 and 2 respectively to prepare Al–8Si target alloy, the results showed that using the horizontal mixing crucible, a better mixing effect, i.e., a mixed melt with more uniform temperature and solute fields, and thus, a higher nucleation rate and a resultant microstructure with finer and more spheroidal primary grains, could be obtained, compared with the vertical counterpart. Rising the crucible temperature within a range from 573 to 873 K was beneficial for improving the mixing effect due to the decreased rate of viscosity increase caused by the decreased chilling effect of the crucible wall on the melt. In addition, a pouring position close to the left end of the crucible and a pouring angle approaching 90° with the crucible axis were helpful for achieving a good mixing because of the intensified vortex. At the optimized parameters, a casting with primary grain size of 43.8 μm and shape factor of 1.42 was obtained. These findings confirmed the feasibility of the proposed die casting technique and supplied some basic data for this technique.

The effect of alloying elements Nb and Mo on the viscosity of the 84KKhSR Co alloy melt is investigated using the oscillating crucible method. The temperature dependences of the viscosities of the melts are shown to coincide upon heating and cooling, to be monotonic, and to be described by Arrhenius-type dependences. Alloying with 3 wt % Mo or Nb is found to increase the viscosity of the base Co–Cr–Fe–Si–B alloy, with Nb alloying having a more pronounced effect. Alloying with niobium from 1 to 3 wt % is shown to be accompanied by a pronounced linear increase in the viscosity and the activation energy of viscous flow; for alloying with molybdenum, the concentration-induced change in the viscosity and the activation energy depends on its content in the alloy. For an alloy with 1 wt % Mo, the viscosity remains virtually unchanged compared to the Co–Cr–Fe–Si–B matrix alloy, and the activation energy of viscous flow even slightly decreases. A further increase in the molybdenum concentration in the alloy is also accompanied by a monotonic increase in both the viscosity and the activation energy of viscous flow. The minimum threshold values of temperature (T = 1080°C), drawing speed (V = 4 m/min), and viscosity (ν = 27 × 10^–7 m²/s) that ensure the production of amorphous wires from the 84KKhSR alloy by the Ulitovskii–Taylor method are determined. When analyzing data on the concentration and temperature dependences of viscosity, we have determined the optimum Co alloy compositions and quenching conditions: alloys with 2 wt % Nb and 3 wt % Mo have the same viscosity (27 × 10^–7 m²/s), and the drawing temperature of microwires is 1100°C. Amorphous wires 130–150 μm in diameter and more than 50 m in length with a high level of mechanical properties are fabricated. The obtained results indicate a high glass-forming ability of Co–Cr–Fe–Si–B (84KKhSR) alloys additionally alloyed with 2 wt % Nb or 3 wt % Mo.

Due to a high sensitivity to structural changes, the viscosity of a melt is often measured as an indirect method for studying the structural state of a liquid alloy. The temperature dependence of the viscosity of a metallic melt often exhibits anomalous behavior in the form of jumps, breaks, changes in the sign of the temperature coefficient or its derivative, a hysteresis of heating/cooling polytherms, and so on. These anomalies are usually related to temperature-induced changes in the structure of a melt, but there is no consensus on their nature. This is due to the contradictory data obtained by different authors. In this work, we discuss the nature and possible mechanisms of the anomalies observed by us on studying the temperature dependence of the viscosity of a metallic melt using the oscillating crucible method. In some cases, viscosity polytherm anomalies can be of a methodological nature, which is reflected in the dependence of the obtained results on experimental conditions. Methodological anomalies include the specific features of polytherms that are caused by the influence of film effects, wetting phenomena, and sedimentation of a crystalline phase in the mushy zone on measurement results. It is important to detect and exclude these anomalies, since they promote the formation of erroneous viewpoints on the structure of metallic melts and the nature of the phenomena detected in them. In addition, the specific features of the temperature dependence of the viscosity of liquid aluminum and the melts based on it with small additions of nickel, cobalt, and iron have been revealed as deviations of polytherms from the Arrhenius equation, and they are interpreted as liquid–liquid transitions. Aluminum-based melts alloyed with transition metal exhibit specific features of the temperature and time dependences of viscosity, which are caused by long relaxation processes in a melt after the crystal–liquid phase transition.

The integration of lightweight, high-performance hybrid structures is crucial for modern aerospace and automotive applications. Among these, magnesium–titanium (Mg–Ti) dissimilar joints have garnered increasing interest due to their complementary mechanical and physical properties. However, significant differences in physical characteristics pose major challenges for direct joining. This review presents a comprehensive summary of recent developments in Mg–Ti joining technologies, with particular emphasis on interfacial modification strategies (e.g., fillers and interlayers) and advanced joining techniques, including welding–brazing, friction stir welding, ultrasonic welding, and transient liquid phase bonding. Additionally, the review synthesizes key findings on mechanical performance metrics, particularly tensile strength and hardness distribution, to assess joint quality and guide future research and engineering applications.

The temperature and concentration behavior of the kinematic viscosity of Cr_88–xFe₁₂C_x (x = 11–18 at %) melts is studied by the oscillating crucible method. The temperature dependences of the viscosities of the melts are shown to coincide upon heating and cooling, to be monotonic, and to be described by Arrhenius-type dependences. The maximum that is observed for the first time in the concentration dependences of the Cr_88–xFe₁₂C_x (x = 11–18 at %) melts near 17 at % C is likely to be due to the specific features of the nonequilibrium (Cr,Fe)–Cr₇C₃ phase diagram in the concentration region under study.

Effect of a high pressure (up to 10 GPa) on the structure formation in an Al₉₅Co₅ hypereutectic alloy upon rapid cooling the melt heated to 1500°C is studied. A two-phase structure consisting of an Al(Co) solid solution and cobalt aluminide forms in all samples independent of applied pressure. As the melt solidifies under high pressure, the structure is substantially denser and finer with a higher (by a factor of ~1.1–1.4) microhardness as compared to the sample prepared at normal atmospheric pressure. At pressures higher than 5 GPa, the solidification mechanism changes: the initially hypereutectic alloy solidifies as a hypoeutectic alloy. In this case, an anomalously supersaturated Al(Co) solid solution forms: the cobalt content in aluminum exceeds the equilibrium value by a factor of 100 and is ~3 at % at pressure of 10 GPa. The alloys Al₉₇Co₃, Al₉₆Co₄, Al₉₄Co₆, and Al₉₃Co₇ solidify according to the same mechanism as Al₉₅Co₅ alloy does; in this case, the quantitative proportions of the structural components (α-Al, α-Al + Al₉Co₂ eutectic) in the alloys change.