•“Accumulative HPT” – to achieve a high strain in hard materials, including BMGs.•Accumulative HPT-effective method of processing for difficult-to-deform materials.•The structure of Zr-BMG during ...accumulative HPT transforms much more significantly than in the HPT.
Studies show that with the HPT of bulk metallic glass, the actual achievable strain is significantly lower than the one predicted by the equation γ=2πnR/h. Such a discrepancy can be explained by the effect of “slippage” during the HPT. The authors proposed a new method − “accumulative HPT” − to achieve a high strain in hard materials, including BMGs. In the accumulative HPT procedure, a sample undergoes several cycles: “HPT n = 1 revolution → cutting a sample into pieces → pressing of stacked parts on the anvils and subsequent HPT n = 1”. At the last stage, the stacked segments are subjected to HPT with many revolutions (n ≥ 3) whereby the sample is consolidated into a monolithic disk. As a result, the material receives a significant total strain by pressing and torsion. The article presents the first results of the “accumulative HPT” processing of a Zr-based BMG and a crystallized Zr-based intermetallic.
The parameters of shear band evolution with deformation were examined in the Vit105 bulk metallic glass. For this purpose, two halves of disks of the bulk metallic glass were joined together and ...processed by high-pressure torsion for various strains: from compression without rotation, to rotation for 5 revolutions. The discrepancy between the experimentally observed and predicted shear strains was detected. The actual strain is significantly smaller than the predicted one. The SEM examination of the internal surfaces of two joint halves of an HPT-processed disk allowed to study the formation and accumulation of shear bands under an increased imposed strain. The maximum density of the shear bands is observed at the edges of the HPT samples and in areas adjacent to the upper anvils. An increase in strain leads to an increase in the shear bands density. The observed minimum shear band spacing is equal to 0.5 μm after HPT processing for 5 revolutions. According to the structural changes recorded by XRD (an increase in the free volume content by about 1.3%) and formation of a high density of shear bands, HPT leads to a significant structural transformation of the amorphous structure of the BMG.
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•Two joint halves BMG Vit105 was subjected to HPT with different n.•Shear bands in HPT sample was detected by SEM.•SEM allowed to study the accumulation of SB under an increased strain.
Conventional methods for producing wires for high-speed railways include continuous casting, rolling and/or subsequent drawing and do not always allow achieving the required level of operation ...properties. In the presented work, as an alternative, under the conditions of continuous processing, a combined method is proposed, including radial swaging (two cycles) and subsequent deformation by the ECAP-Conform method combined with forming the shaped profile of a low-alloy bronze wire of the Cu-Cr system. The stress-strain state and thermal conditions of plastic processing operations were investigated using computer simulation. It is established that the combined treatment by radial swaging with subsequent ECAP-Conform and the forming operation of the shaped profile allows achieving a total true deformation up to the value e ~ 7. A comparative physical experiment showed that the relative ductility of bronze after rotational reduction is 1.5 times greater than after rolling, and is 23% and 15%, respectively.
In this study, a novel severe plastic deformation technique referred to as rotational constrained bending (RCB) is introduced. A special constrained bending die, which imposes bending deformation to ...the billet during the first pass, was developed for repetitive processing of pre-extruded commercially pure Ti. Strain-induced microstructural changes were investigated by a special technique of automated crystal orientation mapping in TEM simultaneously with advanced X-ray line profile analysis. Plastic deformation distribution, imposed to the billets after a selected number of passes, was followed by precise microhardness mapping. Exceptional microstructure refinement was attained by the application of repetitive bending deformation. Average grain size decreased down to 400 nm, and the dislocation density increased by about 35 pct after ten passes. X-ray macrotexture measurements revealed the formation of a basal slip texture component commonly observed in HCP materials processed by equal channel angular pressing; however, exceeding four passes, a strong
11
2
¯
0
fiber texture started to form. Mechanical testing in tension showed a significant increase in strength in the RCB-processed samples. The proof-stress and tensile strength increased by 30 and 15 pct after four passes, respectively. At a higher number of passes, the proof stress slightly decreased because of the texture softening.
Graphic Abstract
An effective approach to increasing the fatigue resistance of metal products is to create residual compressive stresses on the surface of the product using surface plastic deformation (SPD) ...processing SPD. In the present study, with the help of the finite element analysis, one of the effective SPD methods, the process of abrasive-free ultrasonic finishing (AFUF), is studied. Another well-known approach to improving mechanical characteristics, including the fatigue resistance, is the creation of an ultrafine-grained (UFG) structural state in the product. This study is devoted to investigation of the stress–strain state of a UFG workpiece subjected to SPD by the AFUF method using the finite element analysis. Commercially pure Grade 4 titanium in the UFG state obtained by the equal channel angular pressing “conform” method (ECAP-C) is chosen as the workpiece material. In the course of the study, the stress–strain state of the deformation zone after a single impact of an indenter with subsequent unloading is analyzed in the elastoplastic formulation of the problem. The effect of the oscillation amplitude and geometrical characteristics of the indenter on residual radial stresses, including their depth of occurrence, average normal stress, and the accumulated effective strain, has been analyzed. It has been established that, with an increase in the indenter radius, the value of the accumulated effective strain (
e
) decreases. The behavior of distribution of the
e
parameter shows a gradient character with its values decreasing from the surface to the center of the workpiece. An analysis of the simulation results shows that the residual radial stresses in the region of the deformation zone are predominantly compressive stresses and, accordingly, allow increasing the fatigue resistance of the final product. It has been established that, with an increase in the indenter oscillation amplitude, the values of residual radial stresses also rise, with their maximum achieving 540 MPa at the amplitude of 75 µm and the depth of occurrence of these stresses reaching 0.3 mm. Increasing the indenter radius, or, in other words, in fact, the contact surface area, leads to an increase in the residual radial compressive stresses, which turns out to be an almost linear increase.
This paper investigates the features of deformation processing by continuous free bending in rollers, aimed at enhancing the mechanical properties of long-length metallic billets without any change ...in their cross section. As the material for the investigation, we selected a chromium-zirconium bronze for electrical engineering applications, used, for example, for the production of contact wires that require high mechanical characteristics. Computer simulation reveals that during the processing by continuous free bending, a gradient field of accumulated strain is formed, with maximum values in the peripheral region and with minimum values in the center of the billet. It is found that during bending, by the 6th cycle the peripheral layers of the billet accumulate a strain of e~2, and the damage reaches a value of ~1. In the process of verification by means of a physical experiment, cracks emerge on the billet surface during the 6th cycle, confirming the simulation data. Structural analysis shows that during multi-cycle processing by this method, a gradient type of structure is formed. An important advantage of the processing by continuous free bending is the enhancement of the mechanical properties of long-length samples without any reduction in the cross-sectional area: e.g., after 4 processing cycles the ultimate tensile strength increased from 250±20 to 380±20 MPa, and the variation of the cross-sectional area was ~5-7%. The obtained results are in agreement with some principles of severe plastic deformation processing.
The choice of rate conditions of deformation is one of the key factors that determine the mode of thermomechanical processing of metallic materials. The aim of this work is to study the rheological ...behavior as well as the structural response of the Cu-0.6Cr alloy under conditions of large deformations. For this, physical modeling of upsetting of specimens on a Gleeble 3500 installation at deformation rates of 3, 30, and 300 mm/s, structural characterization of the obtained specimens and computer simulation of the equal-channel angular pressing (ECAP) process in the Deform 3D software package were carried out. It was found that with an increase in the rate of deformation, the value of deformation heating increased. If at the rate of ECAP 30 mm/s and at the initial temperature of 20°C, the temperature of the main volume of a round billet with a diameter of 20 mm reached 80-90°C, at 300 mm/s increased up to 150°C. It is shown that with an increase in the deformation rate up to 300 mm/s the necessary pressing forces increase by 1.5 times, and, accordingly, the pressures during pressing and the load on the tool increase.
The development of high-speed railways (HSR) is an important task aimed at improving the transport system of today's global community. The most heavily-loaded element of HSR, governing their ...reliability and lifetime, is a contact wire. In this connection, the present study was focused on designing a new set of physico-mechanical properties of a wire made from the heat-treatable Cu-0.65Cr alloy with the use of a new procedure of continuous plastic processing, based on the principles of severe plastic deformation. Thus, an important difference from the common technical solutions used in the production of wires was the use of a combination of SPD processes - radial forging and ECAP-Conform, joined with the shape-forming of shaped sections of wire. In our study, we performed finite-element computer and physical modeling of the processes of plastic and heat treatment. Using computer modeling, we demonstrated that as a result of the implementation of the new procedure of continuous processing, a rather homogeneous strained state is formed in the workpiece, and the accumulated strain is in a range of e = 6-7. At all stages of plastic processing, compressive stresses prevail in the deformation site. As a result of physical modeling, we produced laboratory samples of contact wire from the heat-treatable Cu-0.65Cr alloy with a cross-section area of 150 mm2. Metallographic studies reveal that a banded structure of a grain-subgrain type with a fragment size below 1 micron is formed in the laboratory samples of contact wire. The ultimate tensile strength of these samples after heat treatment is 550-560 MPa, the electrical conductility is 72-75% IACS, the ductility is 16-20%.
This paper investigates the stress-strain state and the features of structural transformation in materials with different types of crystalline lattices in the process of deformation by free bending ...of axis-symmetrical billets. As the materials for the study, we selected oxygen-free M00B Cu and Grade 4 CP Ti. Using mathematical modeling, we study the stress-strain state of the samples from the selected materials in the process of bending via different routes. We study the structure in the mesoscale and determine the features of structural transformation. It is found that in the process of bending of Cu, a refined surface layer with a thickness of 20-30 μm and a cell size of ≈1 μm forms already after 1 processing cycle. In the process of bending of Ti, twinning occurs in peripheral regions, and the efficiency of twinning via route C is higher than that via route A. The most efficient, in terms of strengthening, processing routes of bending are determined.
The results of the studying the transformation of structural parameters in the samples of the Cu-0.6Cr alloy during high-temperature free upsetting and subsequent cooling at different rates with the ...use of different media: liquid nitrogen, water and air are presented. Using EBSD and X-ray diffraction (XRD) analysis, we demonstrate that a decrease in the cooling rate of the samples of the Cu-0.6Cr alloy after a large deformation of e~2 at a temperature of 800-850 °C promotes the formation of smaller structural fragments, an increase in the dislocation density and a reduction in the fraction of high-angle boundaries.
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