Crystal plasticity modeling of transformation plasticity and adiabatic heating effects of metastable austenitic stainless steels

Strain induced phase transformation in metastable 301LN stainless steel generates a heterogeneous multiphase microstructure with a capability to achieve excellent strain hardening. The microstructural deformation mechanisms, prior deformation history and their dependency on strain rate and temperature determine much of the desired dynamically evolving strength of the material. To analyze microscale deformation of the material and obtain suitable computational tools to aid material development, this work formulates a crystal plasticity model involving a phase transformation mechanism together with dislocation slip in parent austenite and child martensite. The model is used to investigate microstructural deformation with computational polycrystalline aggregates. In this context, material's strain hardening and phase transformation characteristics are analyzed in a range of quasi-static and dynamic strain rates. Adiabatic heating effects are accounted for in the model framework to elucidate the role of grain level heating under the assumption of fully adiabatic conditions. The model's temperature dependency is analyzed. The modeling results show good agreement with experimental findings.publishedVersionPeer reviewe

Reply to Comment on Circular Dichroism in the Angle-Resolved Photoemission Spectrum of the High-Temperature Bi2Sr2CaCu2O8 Superconductor http://arxiv.org/abs/1004.1648

We conclude that arguments of Norman et al. in their Comment do not provide a significant basis for their claim that the geometric mechanism for explaining the observations reported by them is not viable. More generally, our study highlights the importance of assessing structural issues before invoking exotic mechanisms for explaining unusual spectroscopic observations, especially in complex materia

Experimental and Numerical Studies on the Abrasive and Impact Behavior of Wear Resistant Steels

The demand for more wear resistant materials originates from modern applications of many industries, such as mining, automotive, aerospace and civil structures. The motivation to develop more efficient engineering structures and components can be seen beneficial in both economically and environmentally. Lighter, higher strength and more wear resistant solutions can be attractive, for example because of savings in energy consumption (e.g., petrol and running costs), higher load bearing capability per material thickness/volume, and increased component lifespan. Steels remain still today very competitive materials for various wear applications because of their relatively good wear resistance in many conditions arising from their excellet mechanical properties, and because of the reasonable cost of manufacturing and processing of the components.The steels exposed to high stress abrasive and impact wear conditions, for example in the equipment used in mining, are required to withstand heavy static and dynamic loadings for long periods of time. The evaluation of the performance of different steels in these type of conditions is often performed with experimental setups imitating the real loading conditions and material characterizations done afterwards, giving an insight into the material’s wear behavior in a particular tribosystem.This work concentrates on the characterization of the mechanical behavior of wear resistant steels subjected to abrasive and impact loadings by hard particles. The mechanical behavior of the steels was first characterized at a wide range of strain rates from 10−3 to 4000 s−1 . Although the increase in the flow stress with the increasing the strain rate is well established, limited information is available of the behavior of these steels in the dynamic range. For example, the localization phenomena, such as adiabatic shear banding, have an important role in the failure behavior of the martensitic steels. On the other hand, the strain hardening behavior of austenitic manganese steels that evolves with strain and strain rate is affected largely by the twinning phenomenon. Two in-service cases including sample materials from a jaw crusher and from a cutting edge of a bucket loader were also characterized and analyzed. The observations made on the deformed microstructures of the laboratory and in-service samples formed the basis for the simulation approaches developed in this work.High stress abrasion experiments were performed and further developed for the testing of wear resistant steels to study their capabilities to surface harden and to withstand wear. The results show that the surface hardening of the steels has a substantial effect on their wear rates. The common single scratch experiments, however, were shown to be insufficient to reveal all important aspects related for example to the surface hardening of the studied materials, and therefore different types of multi-scratch experiments were also applied. The characterization also showed that the martensitic steels generate two types of tribolayers depending on the prevailing contact conditions.High velocity impact testing was conducted with a novel high velocity particle impactor device. The steels showed dependence on several external factors and conditions, such as impact energy, impact angle, and incident impulse. It was shown that the wear characteristics depended on the deformation mechanisms such as ploughing or cutting in addition to some more special mechanisms such as shear banding, which becomes active only at higher impact energies and/or higher strain rates. The strain hardening had both positive and negative effects on the material’s resistance against impacts depending on the loading conditions.Two numerical crystal plasticity models were implemented to assist the development of the understanding of the deformation behavior at micro-scale. First a phenomenological model including dislocation slip and twinning was formulated to describe the micromechanical phenomena occurring in austenitic manganese steels. The model was found capable of representing the material behavior with a satisfactory accuracy in the studied deformation conditions, starting from the single crystal behavior and extending to the polycrystal level. A multi-scale method linking the application and microstructural scales was also demonstrated using a jaw crusher as an example. Implementation of a crystal plasticity method for BCC microstructure in the large deformation framework was also carried out. The model was extended to include a phenomenological description of the shear banding phenomenon in the microscale. The extension was demonstrated with simulations on single crystals with four different initial orientations. The results indicated that shear banding is a heavily orientation dependent phenomenon, but its relevance for the performance of polycrystalline microstructures still requires further examinations

Crystal Plasticity Modeling of Grey Cast Irons under Tension, Compression and Fatigue Loadings

The study of the micromechanical performance of materials is important in explaining their macrostructural behavior, such as fracture and fatigue. This paper is aimed, among other things, at reducing the deficiency of microstructural models of grey cast irons in the literature. For this purpose, a numerical modeling approach based on the crystal plasticity (CP) theory is used. Both synthetic models and models based on scanning electron microscope (SEM) electron backscatter diffraction (EBSD) imaging finite element are utilized. For the metal phase, a CP model for body-centered cubic (BCC) crystals is adopted. A cleavage damage model is introduced as a strain-like variable; it accounts for crack closure in a smeared manner as the load reverses, which is especially important for fatigue modeling. A temperature dependence is included in some material parameters. The graphite phase is modeled using the CP model for hexagonal close-packed (HCP) crystal and has a significant difference in tensile and compressive behavior, which determines a similar macro-level behavior for cast iron. The numerical simulation results are compared with experimental tensile and compression tests at different temperatures, as well as with fatigue experiments. The comparison revealed a good performance of the modeling approach

Crystal plasticity with micromorphic regularization in assessing scale dependent deformation of polycrystalline doped copper alloys

It is planned that doped copper overpacks will be utilized in the spent nuclear fuel repositories in Finland and in Sweden. The assessment of long-term integrity of the material is a matter of importance. Grain structure variations, segregation and any possible manufacturing defects in microstructure are relevant in terms of susceptibility to creep and damage from the loading evolution imposed by its operating environment. This work focuses on studying the microstructure level length-scale dependent deformation behavior of the material, of particular significance with respect to accumulation of plasticity over the extensive operational period of the overpacks. The reduced micromorphic crystal plasticity model, which is similar to strain gradient models, is used in this investigation. Firstly, the model’s size dependent plasticity effects are evaluated. Secondly, different microstructural aggregates presenting overpack sections are analyzed. Grain size dependent hardening responses, i.e., Hall-Petch like behavior, can be achieved with the enhanced hardening associated with the micromorphic model at polycrystalline level. It was found that the nominally large grain size in the base material of the overpack shows lower strain hardening potential than the fine grained region of the welded microstructure with stronger strain gradient related hardening effects. Size dependent regularization of strain localization networks is indicated as a desired characteristic of the model. The findings can be utilized to provide an improved basis for modeling the viscoplastic deformation behavior of the studied copper alloy and to assess the microstructural origins of any integrity concerns explicitly by way of full field modeling

Dislocation density in cellular rapid solidification using phase field modeling and crystal plasticity

International audienceA coupled phase field and crystal plasticity model is established to analyze formation of dislocation structures and residual stresses during rapid solidification of additively manufactured 316L stainless steel. The work focuses on investigating the role of microsegregation related to the intragrain cellular microstructure of 316L. Effect of solidification shrinkage is considered along with dislocation mediated plastic flow of the material during solidification. Different cellular microstructures are analyzed and the characteristics of the cell core, boundary and segregation pools are discussed with respect to heterogeneity of dislocation density distributions and residual stresses. Quantitative comparison with experimental data is given to evaluate the feasibility of the modeling approach

Dislocation density in cellular rapid solidification using phase field modeling and crystal plasticity

A coupled phase field and crystal plasticity model is established to analyze formation of dislocation structures and residual stresses during rapid solidification of additively manufactured 316L stainless steel. The work focuses on investigating the role of microsegregation related to the intra-grain cellular microstructure of 316L. Effect of solidification shrinkage is considered along with dislocation mediated plastic flow of the material during solidification. Different cellular microstructures are analyzed and the characteristics of the cell core, boundary and segregation pools are discussed with respect to heterogeneity of dislocation density distributions and residual stresses. Quantitative comparison with experimental data is given to evaluate the feasibility of the modeling approach

Existence, character and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe_{2-x}Co_{x}As_{2}

Low energy electron diffraction (LEED) experiments, LEED simulations and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe_{2-x}Co_{x}As_{2} (x = 0.1, 0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe_2-As block. As a result, surface related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe_{2-x}Co_{x}As_{2} system for accurate determination of self-energies using ARPES.Comment: 4 pages, 5 figures includes supplementary materia