List of Abstracts 2018: SESSION A - NANOMATERIALS
Date: November 14, 2018
Continuous equal channel angular rolling method a severe plastic deformation process with ultrasonic energy is suitable for the shear deforming a long and thin sheet continuously. An interesting issue is that thickness of a sheet is not reduced during ECAR. Although the shear texture and fine grain structure are easily obtained by ECAR, yet the ECAR process’s difficulties in terms of technical control, such as surface defect, low ductility and low processing speed, still remain. The surface defects and processing speed are partially improved by applying a series deformation of rolling and ECAR. The article presents unique construction, methodological and analytical procedures for processes of a SPD-representative which are represented by the method ECAR. They specify in detail the technical requirements whose new design solutions should be implemented.
In this paper, the processes of grain refinement and phase transformations in the metastable β-Ti alloys Ti-15Mo and TNTZ during severe plastic deformation by high-pressure torsion (HPT) at room temperature are investigated. Using transmission electron microscopy and X-ray phase analysis, the non-monotonic character of the variation of the ω-phase volume fraction has been established for an increasing number of turns. The role of the ω-phase in the formation of the strength and elastic properties of alloys is discussed. Using the example of the Ti-15To alloy, it was shown that nanostructure formation in the alloy with the minimum content of the ω-phase allows for achievement of significant strengthening in the alloy, while preserving a relatively low elastic modulus.
Acknowledgements
The work was supported by the RFBR grant No. 18-32-00585.
In this study, Ti15Mo, a metastable beta titanium alloy was subjected to severe plastic deformation (SPD) by high pressure torsion (HPT) and equal channel angular pressing (ECAP). A subsequent annealing of these materials results in heterogeneous precipitation of the alpha phase. The morphology and the heterogeneity of the precipitation of the alpha phase has been studied using a combination of scanning and transmission electron microscopy. It was revealed that a high density of dislocations, grain boundaries and lattice defects promote the diffusion of the atoms, hence, the nucleation and growth of the alpha phase particles.
Commercially pure titanium and Ti15Mo alloy were prepared by a powder metallurgy route consisting of gas atomization, cryogenic milling and spark plasma sintering (SPS). Due to ductility of Ti at cryogenic temperature, powder particles were not significantly refined by milling, but each particle was repetitively severely plastically deformed. This process resulted in refined microstructure with grain size below 100 nm. Parameters of spark plasma sintering were optimized in order to achieve fully dense material and limit recrystallization and grain growth. Prepared bulk materials were thoroughly studied by scanning electron microscopy and microhardness measurement. In the metastable Ti15Mo alloy, milling and sintering conditions significantly affected final phase composition of the alloy due to inevitable precipitation of alpha phase particles.
Date: November 14, 2018
Due to low yield stress and ductility of pure zinc, the main scientific efforts are focused on improving the mechanical properties of zinc alloys. Grain refinement is one of the best known strengthening methods. However, a decrease in grain size changes the major deformation mechanism that occurs in zinc and zinc alloys.
In this research, the effect of Ag, Cu and Mn additions (0.5 atomic percent) on zinc processed via equal channel angular pressing (ECAP) was investigated. No improvement in mechanical properties was observed after grain refinement to the mean diameter of approx. 20 μm for zinc and below 3.2 μm for the alloys. Furthermore, all investigated materials are characterized by lower tensile properties than those processed by conventional hot indirect extrusion. Low tensile properties of the investigated alloys are followed by very high elongation values measured to be approx. 38%, 150%, 350% and 96% in Zn, Zn-Ag, Zn-Cu and Zn-Mn, respectively.
ECAP allows to obtain fine-grained zinc and zinc alloys. The softening of the investigated alloys could be explained based on many phenomena. Firstly, grain refinement blocks the deformation by twinning that results in lower strain and grain refinement hardening. Secondly, the texture obtained after the ECAP process via route Bc promotes dislocation slip through the basal slip system and hinders twinning. Last but not least, grain refinement below the critical grain size could activate deformation by viscous glide and grain boundary sliding. The presented results show that grain refinement strengthening obtained via ECAP is ineffective in low alloyed zinc alloys.
This research is focused on a study of the relationship between microstructure, mechanical behavior and impact toughness of the two-phase Ti alloy VT8M-1 (Ti-5.7Al-3.8Mo-1.2Zr-1.3Sn) in ultrafine-grained (UFG) conditions, which was produced by two different techniques - by equal-channel angular pressing (ECAP) and by rotary forging (RF). The UFG structures formed in the billets are characterized by different shapes and sizes of the α- and β-phases. It is shown that the formation of a predominantly equiaxed UFG structure in the alloy by ECAP leads to high strength, but lower impact toughness, as compared to the initial coarse-grained state. After processing by RF, the formation in the alloy of a bimodal UFG structure, consisting of elongated weakly-deformed grains of the primary α-phase and heavily fragmented α+β regions, produces an ultimate tensile strength (UTS) not lower than the UTS of the same alloy after ECAP processing, but higher values of impact toughness. It is discussed that in order to increase the impact toughness of UFG Ti alloys, one can use approaches realizing ductility enhancement associated with an increase of strain hardening capacity.
Acknowledgements
The work was supported by the Russian Science Foundation (grant No. 18-79-10158).
Zr is, like Cr, one of the most promising alloying elements. Microalloying with these elements enables increasing significantly not only the mechanical characteristics of copper alloys by cold deformation methods, while preserving a high electrical conductivity, but also imparting hot-strength properties. At room temperature, zirconium bronze is slightly inferior to chromium bronze in terms of strength characteristics, but it has a higher electrical conductivity and ductility. This set of properties determines the operational efficiency of wires. Another important parameter is the wear resistance of wires. Taking into account the operational restrictions related to wear, amounting to 20% of the wire's cross-sectional area, wires can be strengthened not across the whole section, but gradiently, i.e. when surface layers are strengthened more intensively and the core is strengthened less intensively. This will enable providing the enhanced wear resistance and strength of a wire, and further increasing its ductility and bending fatigue. One of the methods to produce such properties is the method of rotary active alternating bending, which enables, in the conditions of multi-cycle processing, accumulating a high level of strain predominantly in the surface layers of wires. In this work, we present the study of this method with respect to the formation of a gradient-type structure in the samples of a Cu-Zr alloy, with the use of computer and full-scale modeling.
ACKNOWLEDGEMENTS: This research was supported by the Ministry of Science and Higher Education of the Russian Federation under grant agreement No. 14.586.21.0059 (unique project identifier RFMEFI58618X0059).
Electrolytic Tough Pitch (ETP) copper has an advantageous conductivity yet its insufficient mechanical strength limits the possibilities of implementation as structural components. It is possible to overcome this drawback by refining microstructure through SPD techniques, which leads to significant growth of mechanical parameters and only slight decrease of conductivity. ETP copper rods were processed up to 8 times using route Bc by Incremental Equal Channel Angular Pressing resulting in acquisition of ultrafine grained microstructure. The Incremental ECAP processing allowed to process billet of significant dimensions of 10 x 10 mm cross section and 200 mm length. Rods were examined in terms of mechanical strength, uniformity of microstructure throughout the processed rods and electrical conductivity. Furthermore, the course of the manufacturing process was examined to investigate its regularity and influence on the properties of the rods.
Due to the lack of slip systems in the crystal structure, magnesium and its alloys can hardly be deformed at room temperature. Indeed, deformation temperature should be raised in order to achieve the maximum workability. By increasing deformation temperature, restoration mechanisms activate which can result in extensive recovery and recrystallization and degrading the mechanical properties after severe plastic deformation (SPD). Prediction of fracture during SPD processing can help to propose methods to reduce the deformation temperature without fracture. For this purpose, Cockroft-Latham fracture criterion was implemented into finite element simulation using Deform 3D commercial package software was used to predict fracture during equal channel angular pressing (ECAP) of AZ31 alloy. The model was initially used for prediction of fracture at room temperature. Then, the simulation was run at higher temperatures for the same purpose and eventually, with utilization of copper casing tube (CCT) on the Mg sample during ECAP. Fracture was predicted to start at the top surface and deeply diffused into the middle of the sample which was in perfect agreement with experimental results. The model was then used to predict the minimum working temperature of the alloy and the optimum CCT thickness. It was found that with application of CCT, the deformation temperature can be reduced to 175 C and more significant strengthening can occur in this alloy by SPD processing.