Metal Science and Heat Treatment最新文献

Published data on positions of carbon and iron atoms in cementite are reviewed. Experimental results of the last two decades are presented together with the data obtained by ab initio modelling. The positions of carbon atoms in the iron sublattice are considered in tree types of interstice, i.e., normal, distorted prismatic, and normal octahedral ones. According to the ab initio computations, the energy differences in displacement of a carbon atom from the most energy-favorable normal prismatic positions into positions of the two remaining types do not exceed 0.3 eV and the energy of formation of a carbon vacancy is also low (0.5 eV). It is shown thermodynamically that even at a temperature of 1000 K, the proportion of the not occupied normal prismatic pores may reach 0.2. This makes it possible to treat the iron sublattice of cementite, in addition to the bcc and fcc modifications, as a manifestation of the concentration polymorphism of iron, and to treat the cementite itself as and interstitial solid solution based on this third modification.

The possibility of fabricating articles from a Co37Ni36Al27 alloy by extrusion 3D printing followed by thermal debinding and sintering is demonstrated. The printing is carried out from metal-polymer compositions (MPC) based on two different powder fractions (71 – 125 μm and < 71 μm) obtained by mechanical alloying (MA) in an attritor. The porosity and the magnetic characteristics of the samples are shown to be controllable by changing the MPC powder fraction and by varying the sintering temperature. The porosity decreases by about a factor of 1.5 and the thickness of the necks (metallic bonds between the particles) increases by about a factor of 17. The porosity is treated as the main characteristic reflecting the sintering intensity and, consequently, the quality of the sample structure. Adecrease in the coercive force by approximately 1.5 times and an increase in the saturation magnetization by approximately 1.3 times are shown to accompany the decrease in the number of structural defects. The structure of the sintered samples is represented by β-, γ-, and γ′-phases, which indicates a low rate of cooling with the furnace.

The possibilities of modifying the structure of Ti/TiC metal matrix materials by using mechanocomposites obtained by mechanochemical synthesis of titanium carbide in a reaction mixture of (Ti + C) + 70 wt.% Ti and their subsequent consolidation by pulse plasma sintering are studied using diffraction analysis and scanning electron microscopy. It is shown that in situ solid-phase synthesis of TiC_0.38 carbide in a matrix of an amorphous-crystalline α-Ti(C) solid solution is implemented within 4 – 8 min of mechanical activation in an AGO-2 planetary ball mill. With increase of the duration of the mechanical treatment from 4 to 8 min, the particle sizes of the mechanocomposites decrease by a factor of 2.6. It is shown that the sintering yields β-Ti-based composite materials with a granular structure whatever the duration of the synthesis. With increase of the duration of the synthesis from 4 to 8 min, the porosity of the material decreases from 3.14 to 2.56%, and the grain sizes of the carbide and of the β-titanium grow by a factor of 2. The materials have a Vickers hardness of 802 ± 10 HV and 749 ± 10 HV and a compressive strength of 1400 ± 50 MPa and 1600 ± 50 MPa, respectively, and a significant wear resistance. The abrasive wear coefficients of the materials attain 3.57 × 10^–5 and 2.25 × 10^–5 mm³/(N · m) respectively.

A review and comparative analysis of materials, equipment and processes of powder metallurgy and additive technologies is suggested. Their similarities, differences and developments are described. Examples of the use of the developed and successfully tested materials and processes of powder metallurgy in additive manufacturing are considered. The possibility of the use of thermal furnaces for sintering and of installations for hot isostatic pressing of powder materials is assessed in order to increase the density, homogeneity and strength properties of blanks obtained by additive technologies.

The structure and mechanical properties of low-carbon steel 09G2S (13Mn6) in quenched condition and after tempering at different temperatures are studied. Stress–strain curves in conventional and true coordinates are analyzed to determine the plasticity parameters of samples at uniform and localized deformation stages for the steel in different structural and phase states and to assess the accumulation of defects at different scale levels. Characteristic zones of fracture surfaces (fibrous, radial and shear lips) and the morphology of secondary local cracks formed under sever plastic flow both in the neck of the tensile samples and in propagation of the main crack are described. It is shown that the highly plastic state formed in steel 09G2S after tempering at 650°C promotes the appearance of a radial zone in the fracture surface, where secondary local cracks nucleate and grow by a ductile mechanism.

An analytical review of the authors’ studies revealing the possibilities of bismuth alloying of a steel surface during short-pulse laser processing and unique tribotechnical characteristics achieved (up to implementation of zero-wear friction) during laboratory tests in a pair of different materials and in bench tests of a car turbocharger is presented. An ultra-low friction coefficient (up to 0.03) and wear in friction without lubrication is obtained due to suppression of the molecular component of the friction (adhesion) as a result of bismuth alloying, formation of bismuth subcarbonate tribolayers with easy sliding, lowering of the mechanical component of friction by diamond burnishing, self-organization of the friction surface and high cyclic strength of the alloy. A new concept for obtaining wear-resistant and antifriction tribotechnical materials based on immiscible systems is suggested.

A review of bioinert titanium alloys based on metastable β-solid solution, which can be used as orthopedic implants is presented. The effect of β-stabilizers and neutral hardeners (Zr, Sn) on the properties (elastic modulus, strength, plasticity) of Ti – Nb alloys is considered. Experimental and computational data are used to show that in order to obtain a reduced elastic modulus (up to 50 – 55 GPa) in quenched condition, the Ti – Nb binary system should be alloyed additionally with Zr and/or Sn. Such alloying provides additional strengthening of the Ti – Nb alloys to up to σ_0.2 = 450 – 650 MPa at an elongation of 10 – 40%.

The works of the Department of Heat Treatment and Metal Physics of the Ural Federal University performed for over 20 years with the aim to study the laws of the evolution of special misorientations — boundaries (SMB) in metallic materials under deformation, recrystallization and phase transformations are reviewed. The first part of the review is devoted to the concept of the principal role of SMB in the changes of texture under structural transformations in metals and alloys, where they are treated as carrier “genes” of the structure and texture heredity. The second part of the review will present the results of generalization of the concept to processes of texture variation under phase transformations.

Results of the researches conducted at the Department of Heat Treatment and Metal Physics of the Ural Federal University for more than 20 years on the evolution of special misorientations-boundaries (SMB) in metallic materials during deformation, recrystallization and phase transformations are reviewed. The first part of the review was devoted to the concept of the principal role played by SMB during structural transformations in metals and alloys. In the second part, the role of SMB as “genes” (carriers) of structure and texture heredity is generalized to phase transformations. Ahypothesis on unity of the crystallographic mechanisms of all structural and phase transformations is formulated.

This main advantages of additive manufacturing compared to traditional metallurgical technologies are discussed. A classification of methods of powder bed fusion for 3D printing of metallic materials is presented. The main types of defects and methods for their elimination are considered. The effect of the energy input parameters on formation of structure and mechanical properties is analyzed using titanium alloy Ti – 6Al – 4V as an example. The works of the authors on the possibilities of creating such functional properties of titanium and iron-nickel alloys as corrosion resistance and heat resistance by tailoring the formation of crystallographic texture and the precipitation of second phases by varying the energy input parameters during powder bed fusion including selective laser melting and electron beam melting are reviewed.