Marat I. Latypov

Cross Flow During Twist Extrusion: Theory, Experiment, and Application

Upon intensive investigation during the recent years, severe plastic deformation (SPD) has been commonly accepted as a strong tool for improving mechanical properties of metallic materials. The interest in commercial use of SPD materials with superior properties addresses the issue of scaling up the SPD methods. In this regard, methods that can provide SPD conditions in billets with large dimensions become of prime interest. Twist extrusion (TE) is such a process, whereby large strains are accumulated owing to repeated extrusion through a die that imposes shearing stresses. Despite a few studies of TE in the literature, many features of the processs nature remain unclear or even unknown. In the current article, we have studied an important effect of TE named “cross flow” that previously received scarce attention. By performing both experiments and simulations, we elucidated the mechanism of the cross flow as well as how it is affected by material properties and process conditions. Since practical significance of the cross flow became apparent, special attention was paid to the problem of control and reliable prediction of the cross flow. Finally, prospective applications of the investigated effect were suggested. Conclusions of the current study are anticipated to contribute to further research on simulation of other simple-shear-based SPD processes.

Simple shear model of twist extrusion and its deviations

Twist extrusion (TE) is a severe plastic deformation method with a potential for commercialization. Advancing TE toward industrial use requires in-depth understanding of deformation during the process and its dependence on processing factors. The helical flow model introduced with the concept of TE provides for a concise description of deformation in the process. To date, however, it was unclear under which conditions the helical flow model yields accurate predictions of deformation in TE. This paper presents a systematic finite-element study performed to identify effects of some key process and material factors on deformation in TE and its departure from the ideal deformation described by the helical flow model. It was found that high strain-hardening rate and friction lead to violations of the assumptions of the helical flow model and that these violations result in departure from the ideal deformation. Deviations from the ideal deformation tend to increase on decreasing the length of the twist channel. Friction effects appear especially critical to be considered for accurate prediction of deformation in TE. Finite-element simulations taking friction into account show good qualitative agreement with earlier marker-insert experiments. The results of the present finite-element study allowed for defining the simple shear model of TE.

Modeling and Characterization of Texture Evolution in Twist Extrusion

Twist extrusion (TE) is a severe plastic deformation method with a potential for commercialization. Deformation during the TE process is non-uniform and non-monotonic, which is expected to result in significant and non-trivial microstructural changes in metallic materials. In this study, texture evolution during TE of pre-textured copper was investigated. Experimental characterization of textures after various numbers of passes demonstrated that TE can be used for producing uniformly weak textures in pre-textured copper. Crystal plasticity simulations were found to run into the problem known as strain reversal texture. In particular, crystal plasticity simulations predicted the return of initial texture upon strain reversal in the first pass of TE, whereas the experimental texture was not reversed and had components related to simple shear. Grain refinement, imperfect strain reversal, and material asymmetry are proposed to be responsible for the occurrence of strain reversal texture in TE. Effects of the non-random initial texture on the microstructure and texture evolution are also discussed.

Data-driven reduced order models for effective yield strength and partitioning of strain in multiphase materials

Abstract There is a critical need for the development and verification of practically useful multiscale modeling strategies for simulating the mechanical response of multiphase metallic materials with heterogeneous microstructures. In this contribution, we present data-driven reduced order models for effective yield strength and strain partitioning in such microstructures. These models are built employing the recently developed framework of Materials Knowledge Systems that employ 2-point spatial correlations (or 2-point statistics) for the quantification of the heterostructures and principal component analyses for their low-dimensional representation. The models are calibrated to a large collection of finite element (FE) results obtained for a diverse range of microstructures with various sizes, shapes, and volume fractions of the phases. The performance of the models is evaluated by comparing the predictions of yield strength and strain partitioning in two-phase materials with the corresponding predictions from a classical self-consistent model as well as results of full-field FE simulations. The reduced-order models developed in this work show an excellent combination of accuracy and computational efficiency, and therefore present an important advance towards computationally efficient microstructure-sensitive multiscale modeling frameworks.

Continuum understanding of twin formation near grain boundaries of FCC metals with low stacking fault energy

Deformation twinning from grain boundaries is often observed in face-centered cubic metals with low stacking fault energy. One of the possible factors that contribute to twinning origination from grain boundaries is the intergranular interactions during deformation. Nonetheless, the influence of mechanical interaction among grains on twin evolution has not been fully understood. In spite of extensive experimental and modeling efforts on correlating microstructural features with their twinning behavior, a clear relation among the large aggregate of grains is still lacking. In this work, we characterize the micromechanics of grain-to-grain interactions that contribute to twin evolution by investigating the mechanical twins near grain boundaries using a full-field crystal plasticity simulation of a twinning-induced plasticity steel deformed in uniaxial tension at room temperature. Microstructures are first observed through electron backscatter diffraction technique to obtain data to reconstruct a statistically equivalent microstructure through synthetic microstructure building. Grain-to-grain micromechanical response is analyzed to assess the collective twinning behavior of the microstructural volume element under tensile deformation. Examination of the simulated results reveal that grain interactions are capable of changing the local mechanical behavior near grain boundaries by transferring strain across grain boundary or localizing strain near grain boundary.Metals: grain neighbours influence twin formation during deformationGrains that should not favour twin formation exhibit twinning as a result of surrounding grains acting on their boundaries. A team led by HyoungSeop Kim at the Pohang University of Science and Technology in the Republic of Korea simulated the deformation of synthetic metallic microstructures with many grains of different orientations, based on steels that deform by both dislocation slip and twinning mechanisms. Twinning first started near grain boundaries and depended on initial grain orientation but, with further deformation, strong twin activity on one side of a boundary triggered strong twin activity on the other side of that boundary. This happened even when the grain on the other side of the boundary was unfavourable to twinning. Taking into account grain neighbourhood may therefore help in optimising twin-forming alloys.

Off-axis twist extrusion for uniform processing of round bars

The present paper introduces a twist extrusion (TE) process capable of processing of round bars with uniform deformation and reports physical, analytical, and numerical modeling of the process. It is shown that the ability to treat round bars can be achieved by design of special off-axis TE dies in which the axis of the twist surface is displaced from the central axis of the bar being processed. Physical modeling conducted in the current study with plasticine demonstrates the feasibility of off-axis TE. A marker insert technique employed in the physical model reveals that tool-controlled flow (ideal helical flow) of the material is dominant in the process. Analytical model developed in the present study explains why using off-axis TE dies leads to uniform deformation and how this deformation uniformity depends on the die geometry. The main conclusions made upon analytical modeling are confirmed with complement finite element simulations. The simulations also show that the main deformation mode in off-axis TE is simple shear at the intersection planes between the twist and the straight channels of the die.

Toward architecturing of metal composites by twist extrusion

The paper presents a new route for realizing the concept of architecturing of composites by severe plastic deformation. The proposed route involves multi-pass twist extrusion of a composite with fibers. The potential of the method is first illustrated by mathematical modeling and then tested through pilot processing of a composite consisting of a copper matrix and a single aluminum fiber. Metallographic analysis revealed an unexpected shape of the fiber after processing. Finite element simulations were performed to understand the evolution of the fiber shape and to optimize the processing regime for achieving improved reinforcements in twist-extruded composites.

Investigation of thermal resistance and power consumption in Ga-doped indium oxide (In2O3) nanowire phase change random access memory

The resistance stability and thermal resistance of phase change memory devices using ∼40 nm diameter Ga-doped In2O3 nanowires (Ga:In2O3 NW) with different Ga-doping concentrations have been investigated. The estimated resistance stability (R(t)/R0 ratio) improves with higher Ga concentration and is dependent on annealing temperature. The extracted thermal resistance (Rth) increases with higher Ga-concentration and thus the power consumption can be reduced by ∼90% for the 11.5% Ga:In2O3 NW, compared to the 2.1% Ga:In2O3 NW. The excellent characteristics of Ga-doped In2O3 nanowire devices offer an avenue to develop low power and reliable phase change random access memory applications.

Design of Hierarchical Cellular Metals Using Accumulative Bundle Extrusion

This letter introduces a method for designing hierarchical cellular metals employing multipass accumulative bundle extrusion and selective dissolving. The method provides several degrees of freedom for manipulating both the cell-wall properties and architecture of cellular materials. Cellular copper was produced and analyzed as an example of implementing the proposed method. The material hierarchy that can be formed and controlled by means of multipass accumulative extrusion assures strength and enables the material to perform the prescribed functions.

Plastic Deformation Behavior and Microstructural Evolution of Al-Core/Cu-Sheath Composites in Multi-pass Caliber Rolling

Plastic deformation behavior and microstructural evolution of an Al-core/Cu-sheath composite during multi-pass caliber rolling are investigated using the finite element simulations and experimental analyses. The simulated equivalent plastic strains generated by 1 to 7 pass caliber rolling are correlated with the hardness values and microstructures measured in the longitudinal cross sections of the specimens. The average strains developed in the Al-core and Cu-sheath are almost identical, which satisfy the quasi-isostrain condition in composites with inner soft and outer hard materials. Both the Al-core and Cu-sheath exhibit increasing hardness, but decreasing hardening rates with an increase in the number of passes. The increasing hardness with an increase in the number of caliber rolling passes is attributable to the combined effect of increased dislocation density and decreased grain size. The simulated results for the hardness were shown to be in good agreement with the experimental data for Cu and Al. It was concluded that the finite element method is well placed as a tool for describing and predicting deformation behaviors during caliber rolling.