D. A. Korzekwa

Bio: D. A. Korzekwa is an academic researcher from Los Alamos National Laboratory. The author has contributed to research in topics: Yield (engineering) & Strain rate. The author has an hindex of 8, co-authored 9 publications receiving 410 citations.

Dislocation substructure as a function of strain in a dual-phase steel

Abstract: Dislocation structures in the ferrite of a C-Mn-Si dual-phase steel intercritically annealed at 810°C were characterized at various tensile strains by transmission electron microscopy At strains which corresponded to the second stage on a Jaoul-Crussard plot of strain hardening behavior, the dislocation density in the ferrite is inhomogeneous, with a higher density near the martensite. The third stage on a Jaoul-Crussard plot corresponds to the presence of a well-developed dislocation cell structure in the ferrite. The average cell size during this stage is smaller than the minimum size reported for deformed iron, and the cell size was inhomogeneous, with a smaller cell size near the martensite.

Analysis of ridging in aluminum auto body sheet metal

Abstract: A finite-element model is presented for the analysis of surface roughening of aluminum sheet metal. A hybrid finite-element model is developed which accounts for the elasto-viscoplastic constitutive response of a single crystallographic orientation. Initialization of a finite-element mesh representing several grains is performed using data gathered through automated collection of backscattered Kikuchi diffraction data. To handle a region that contains sufficient variation (contains numerous distinct grains), the implementation is carried out using distributed computation strategies. Application is made to 6111-T4 sheet metal intended for auto body applications. The numerical simulations are complemented with mechanical testing in plane strain and biaxial stretch. Based on the simulation results, there are two conclusions that can be drawn concerning the action of surface grains deforming through crystallographic slip. One is that grains can act collectively to form localized regions of thinning. The other is that grain interactions can lead to behavior which is different from that expected if grains deform with the average (macroscale) strain. Neighbor interactions can after the deformation from that computed using the macroscale deformation rate.

Impression creep behavior of SiC particle-MoSi2 composites

Abstract: Using a cylindrical indenter (or punch), the impression creep behavior of MoSi2-SiC composites containing 0–40% SiC by volume, was characterized at 1000–1200 °C, 258–362 MPa punch pressure. Through finite element modeling, an equation that depends on the material stress exponent was derived that converts the stress distribution beneath the punch to an effective compressive stress. Using this relationship, direct comparisons were made between impression and compressive creep studies. Under certain conditions, compressive creep and impression creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the punch. However, rate-controlling mechanisms may be quite different under the two stressing conditions, in which case impression creep data should not be used to predict compressive creep behavior. The addition of SiC affects the impression creep behavior of MoSi2 in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi2 grains and by obstructing or altering both dislocation motion and grain boundary sliding.

The influence of microstructure on the mechanical response of copper in shear

Abstract: Shear localization is often a failure mechanism in materials subjected to high strain rate loading. While the constitutive behavior of copper has been extensively studied, the influence of cold work, strain rate, and temperature on the microstructural development in Cu under shear loading conditions has received less systematic quantification. The purpose of this study is to quantify the mechanical response and the microstructural evolution of as-annealed and cryogenically rolled copper loaded dynamically in shear and to understand the mechanisms controlling shear deformation as well as the role of stored defects due to cryo-rolling on shear localization. It was found that localization is promoted in the cryo-rolled copper as compared to the as-annealed material and this instability is linked to stored defect structure specific to the cryo-rolled microstructure and its influence on the subsequent defect generation and storage in dynamically loaded Cu.

Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD

Abstract: We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD). We quantify orientation gradients, pattern quality and GND density variations at ferrite–ferrite and ferrite–martensite interfaces. Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods are demonstrated to calculate the GND density from the EBSD data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall GND density is shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite.

Theory of plastic deformation: - properties of low energy dislocation structures

Abstract: Work-hardening phenomena are based on the very fundamental principles (i) that at the position of every dislocation axis the respective resolved shear stress cannot exceed the friction stress, including the self-stress of bowing dislocations, and (ii) that always that structure forms which among those accessible by the dislocations minimizes stored energy per unit length of dislocation line. Such dislocation structures have been named LEDSs. The corresponding work-hardening theory, the mesh length theory, is applicable to all materials deforming via gliding dislocations and to all types of deformation. Results previously achieved with the mesh length theory are summarized, and a number of new developments are discussed. Depending on the dislocation structures formed, the work-hardening behavior differs. Easily intersecting glide causes dislocation cell structures with almost dislocation-free cell interiors delineated by dislocation rotation boundaries. Pronounced planar glide causes Taylor lattices characterized by local planar order parallel to the one or perhaps two most highly stressed glide plane(s), no systematic lattice rotations, and overall uniform dislocation density. The most widely observed basic features of work hardening are explained in general terms. Specific applications are indicated for layer-type crystals, h.c.p. single crystals, single-crystal and polycrystalline pure f.c.c. metals and α-brass-type alloys, precipitation-hardened materials and steels. Included are the different stages of work hardening, dynamical effects in low temperature plasticity, the general characteristics of grain boundary strengthening and the Hall-Petch relationship. In addition, proposed explanations for (i) glide system interactions in polyslip resulting in microbands and affecting texture formation, and (ii) creep without stress dependence of dislocation density, are discussed.

An Overview of Dual-Phase Steels: Advances in Microstructure-Oriented Processing and Micromechanically Guided Design

Abstract: Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) exp...

Texture control by thermomechanical processing of AA6xxx Al–Mg–Si sheet alloys for automotive applications—a review

Abstract: The properties of Al-alloys for car body applications are largely controlled by microstructure and crystallographic texture of the final sheets. In this paper, the impact of texture on formability and, in particular, on surface appearance of the sheets is reviewed. The paper summarizes the principles of microstructure and texture evolution during the main steps of the thermomechanical processing of age-hardenable Al–Mg–Si sheets (6 xxx series alloys). The most important parameters that may be used to modify the textures and hence to improve the resulting properties are outlined.