Marko Knezevic

High-Pressure Double Torsion as a Severe Plastic Deformation Process: Experimental Procedure and Finite Element Modeling

In the present study, a severe plastic deformation process of high-pressure torsion (HPT) has been modified. The new process is called high-pressure double torsion (HPDT) as both anvils of the conventional HPT process rotate in opposite directions. We manufactured sets of aluminum and pure copper samples using both the HPT process and the newly developed HPDT process to compare between microstructures and microhardness values. Our investigations showed that the copper samples processed by HPDT exhibited larger gradients in microstructure and higher values of hardness. Subsequently, we carried out a set of finite element simulations in ABAQUS/explicit to better understand the differences between the HPT process and the HPDT process. A comparison of the strain distributions of the HPT and HPDT samples revealed a decreasing trend in strain values as the radius increased at the middle surface of the samples. Analysis of the equivalent stress values revealed that stress values for the HPDT samples were higher than those of the HPT samples. Finally, the comparison of the max principal stress values indicated that in the HPDT sample, the extent of the compressive stresses was larger than those in the HPT sample.

Enhancement of orientation gradients during simple shear deformation by application of simple compression

We use a multi-scale, polycrystal plasticity micromechanics model to study the development of orientation gradients within crystals deforming by slip. At the largest scale, the model is a full-field crystal plasticity finite element model with explicit 3D grain structures created by DREAM.3D, and at the finest scale, at each integration point, slip is governed by a dislocation density based hardening law. For deformed polycrystals, the model predicts intra-granular misorientation distributions that follow well the scaling law seen experimentally by Hughes et al., Acta Mater. 45(1), 105–112 (1997), independent of strain level and deformation mode. We reveal that the application of a simple compression step prior to simple shearing significantly enhances the development of intra-granular misorientations compared to simple shearing alone for the same amount of total strain. We rationalize that the changes in crystallographic orientation and shape evolution when going from simple compression to simple shearing increase the local heterogeneity in slip, leading to the boost in intra-granular misorientation development. In addition, the analysis finds that simple compression introduces additional crystal orientations that are prone to developing intra-granular misorientations, which also help to increase intra-granular misorientations. Many metal working techniques for refining grain sizes involve a preliminary or concurrent application of compression with severe simple shearing. Our finding reveals that a pre-compression deformation step can, in fact, serve as another processing variable for improving the rate of grain refinement during the simple shearing of polycrystalline metals.

Delineation of First-Order Elastic Property Closures for Hexagonal Metals Using Fast Fourier Transforms

Property closures are envelopes representing the complete set of theoretically feasible macroscopic property combinations for a given material system. In this paper, we present a computational procedure based on fast Fourier transforms (FFTs) for delineation of elastic property closures for hexagonal close packed (HCP) metals. The procedure consists of building a database of non-zero Fourier transforms for each component of the elastic stiffness tensor, calculating the Fourier transforms of orientation distribution functions (ODFs), and calculating the ODF-to-elastic property bounds in the Fourier space. In earlier studies, HCP closures were computed using the generalized spherical harmonics (GSH) representation and an assumption of orthotropic sample symmetry; here, the FFT approach allowed us to successfully calculate the closures for a range of HCP metals without invoking any sample symmetry assumption. The methodology presented here facilitates for the first time computation of property closures involving normal-shear coupling stiffness coefficients. We found that the representation of these property linkages using FFTs need more terms compared to GSH representations. However, the use of FFT representations reduces the computational time involved in producing the property closures due to the use of fast FFT algorithms. Moreover, FFT algorithms are readily available as opposed to GSH codes.

A multi-scale model for texture development in Zr/Nb nanolayered composites processed by accumulative roll bonding

Recently it has been demonstrated that nanolayered hcp/bcc Zr/Nb composites can be fabricated with a severe plastic deformation technique called accumulative roll bonding (ARB) [1]. The final layer thickness averaged to approximately 90 nm for both phases. Interestingly, the texture measurements show that the textures in each phase correspond to those of rolled single-phase rolled Zr and Nb for a wide range of layer thickness from the micron to the nanoscales. This is in remarkable contrast to fcc/bcc Cu/Nb layered composites made by the same ARB technique, which developed textures that strongly deviated from theoretical rolling textures of Cu or Nb alone when the layers were refined to submicron and nanoscale dimensions. To model texture evolution and reveal the underlying deformation mechanisms, we developed a 3D multiscale model that combines crystal plasticity finite element with a thermally activated dislocation density based hardening law [2]. For systematic study, the model is applied to a two-phase Zr/Nb polycrystalline laminate and to the same polycrystalline Zr and polycrystalline Nb as single-phase metals. Consistent with the measurement, the model predicts that texture evolution in the phases in the composite and the relative activities of the hcp slip modes are very similar to those in the phases in monolithic form. In addition, the two-phase model also finds that no through-thickness texture gradient develops. This result suggests that neither the nanoscale grain sizes nor the bimetal Zr/Nb interfaces induce deformation mechanisms different from those at the coarse-grain scale.

Residual Ductility and Microstructural Evolution in Continuous-Bending-under-Tension of AA-6022-T4

A ubiquitous experiment to characterize the formability of sheet metal is the simple tension test. Past research has shown that if the material is repeatedly bent and unbent during this test (i.e., Continuous-Bending-under-Tension, CBT), the percent elongation at failure can significantly increase. In this paper, this phenomenon is evaluated in detail for AA-6022-T4 sheets using a custom-built CBT device. In particular, the residual ductility of specimens that are subjected to CBT processing is investigated. This is achieved by subjecting a specimen to CBT processing and then creating subsize tensile test and microstructural samples from the specimens after varying numbers of CBT cycles. Interestingly, the engineering stress initially increases after CBT processing to a certain number of cycles, but then decreases with less elongation achieved for increasing numbers of CBT cycles. Additionally, a detailed microstructure and texture characterization are performed using standard scanning electron microscopy and electron backscattered diffraction imaging. The results show that the material under CBT preserves high integrity to large plastic strains due to a uniform distribution of damage formation and evolution in the material. The ability to delay ductile fracture during the CBT process to large plastic strains, results in formation of a strong <111> fiber texture throughout the material.

Towards Computationally Tractable Simulations of Metal Forming Processes With Evolving Microstructures

Performing microstructure sensitive metal-forming simulations is widely recognized as a computational challenge because of the need to store large sets of state variables related to microstructure data. We present a rigorous methodology for the compaction of microstructural data associated with a material point and show that the statistical distributions of microstructure of any size can be compacted to several hundred grains. The methodology is based on the spectral representation of microstructure distribution functions through the use of generalizes spherical harmonics. Subsequently, we present a computational framework aimed at dramatically reducing time needed for microstructure sensitive simulations of metal forming processes. The framework is based on a combination of the recently developed numerical implementations of crystal plasticity models in the spectral representation for obtaining the response of single crystals and specialized computer hardware that integrates a graphics-processing unit. We apply these two methodologies on a plane strain compression case study and obtain speedup factors exceeding three orders of magnitude.Copyright

Modeling Tensile, Compressive, and Cyclic Response of Inconel 718 Using a Crystal Plasticity Model Incorporating the Effects of Precipitates

A comprehensive elasto-plastic polycrystal plasticity model is developed for Ni-based superalloys. To demonstrate the microstructure sensitive predictive characteristics, the model is applied to an Inconel 718 (IN718) superalloy that was produced by additive manufacturing (AM). The model with the same set of material and physical parameters is compared against a suite of compression, tension, and large strain cyclic mechanical test data applied in different AM build directions. The model embeds the contributions of solid solution, precipitates shearing, and grain size and shape effects into the initial slip resistance. The hardening law is based on the evolution of dislocation density. It is demonstrated that the model is capable of predicting the particularities of both monotonic and cyclic deformation to large strains of the alloy including decreasing hardening rate during monotonic loading, the non-linear unloading upon the load reversal, the Bauschinger effect, the hardening rate change during loading in the reverse direction as well as anisotropy and concomitant microstructure evolution. The microstructure constituents and behavior of IN718 under these conditions is similar to other Ni-based superalloys, and therefore, it is anticipated that the general model developed here can be applied to other superalloys fabricated using AM and other approaches. Additionally, the material is tested in fatigue and results are presented and discussed.

OpenMP and MPI implementations of an elasto-viscoplastic fast Fourier transform-based micromechanical solver for fast crystal plasticity modeling

Abstract We explore several parallel implementations of an elasto-viscoplastic fast Fourier transform (EVPFFT) model using Message Passing Interface (MPI), OpenMP, and a hybrid of MPI and OpenMP to efficiently predict micromechanical response of polycrystals. Performance studies using EVPFFT are performed based on domain decomposition over voxels of a periodic cell, which is a representative volume element (RVE) of polycrystalline copper. We begin by parallelizing the computationally intensive Newton–Raphson (NR) single crystal solver within EVPFFT. Next, we compare the performance of the serial and parallel FFTW (Fastest Fourier Transform in the West) using OpenMP (OpenMP-FFTW) and MPI (MPI-FFTW) with the original Numerical Recipes-based FOURN routine within EVPFFT. In the parallel environment, we find that the FFT calculations are best performed using the MPI version of FFTW. Finally, the remainder of the code, except read/write subroutines, is parallelized. Significant speedups of the original EVPFFT model are achieved using MPI on shared memory multicore workstations. Furthermore, results achieved on a distributed memory Cray supercomputer show promising strong and weak scalability and in some cases even super scalability for the single crystal NR solver in EVPFFT. MPI-FFTW also scales perfectly for microstructure RVEs larger than 643 FFT voxels. For example, the MPI-EVPFFT parallel version of the code accelerates the simulations for approximately two orders of magnitude using 64 cores over the old serial code for an RVE size of 1283. The parallel EVPFFT code developed in this work can run massive voxel-based microstructural RVEs taking the advantages of thousands of logical cores provided by more advanced clusters.

Formability of Magnesium Alloy AZ31B from Room Temperature to 125 °C Under Biaxial Tension

Magnesium AZ31B sheets of 2 mm thickness were stretch formed using a 101.6 mm diameter punch at room temperature and subsequent increments from 25 to 125 °C. Surface strains were measured using a digital image correlation method in order to ensure that biaxial stretching was achieved. The punch height versus load curve was found to be the same for temperatures of 25 and for 50 °C, while at 75 °C the load for a given punch height was less. This difference seems to indicate a change in deformation mechanism between 50 and 75 °C. Electron Backscatter Diffraction (EBSD) was used to quantify features of the microstructure in the as-received and the strained specimens. Rather than a sudden transition from twinning to slip at low temperatures, it appears that twinning gradually decreases and slip activity increases as temperatures rise across the range from 25 to 125 °C. This confirms recent predictions found in the literature. The twin activity predominantly involves the formation of compression twins which rapidly transform further to create secondary twins for easier strain accommodation.