Determination of thermal conductivity of eutectic Al-Cu compounds utilizing experiments, molecular dynamics simulations and machine learning

In this study, the thermal conductivity ( κ ) of Al-Cu eutectics were investigated by experimental and computational methods to shed light on the role of these compounds in thermal properties of Al-Cu connections in compound casting. Specifically, the nonequilibrium molecular dynamics (MD) method was utilized to simulate the lattice thermal conductivity ( κ l ) of six compositions of Al-Cu with 5-30 at.% Cu. To extend the results of the MD simulations to bulk materials, instead of using conventional linear extrapolation methods, a machine learning approach was developed for the dataset acquired from the MD simulations. The bootstrapping approach was utilized to find the most suitable method among the support vector machine (SVM) with polynomial and radial basis function (RBF) kernels and the random forest method. The results showed that the SVM model with RBF kernel performed the best, and thus was used to predict the bulk κ l . Subsequently, the chosen compositions were produced by induction casting and their electrical conductivities were measured via eddy current method for calculating the electronic contribution of κ using the Wiedemann-Franz law. Finally, the actual κ of the alloys were measured using the xenon flash method and the results were compared with the computational values. It was shown that the MD method is capable of successfully simulating the thermal conductivity of this system. In addition, the experimental results demonstrated that the κ of Al-Cu eutectics decreases almost linearly with formation of the Al2Cu phase due to increase in the Cu content. Overall, the current findings can be used to enhance the κ of cooling devices made via Al-Cu compound casting

A Microstructure-Sensitive Model for Simulating the Impact Response of a High-Manganese Austenitic Steel

Microstructurally informed macroscopic impact response of a high-manganese austenitic steel was modeled through incorporation of the viscoplastic self-consistent (VPSC) crystal plasticity model into the ANSYS LS-DYNA nonlinear explicit finite-element (FE) frame. Voce hardening flow rule, capable of modeling plastic anisotropy in microstructures, was utilized in the VPSC crystal plasticity model to predict the micromechanical response of the material, which was calibrated based on experimentally measured quasi-static uniaxial tensile deformation response and initially measured textures. Specifically, hiring calibrated Voce parameters in VPSC, a modified material response was predicted employing local velocity gradient tensors obtained from the initial FE analyses as a new boundary condition for loading state. The updated micromechanical response of the material was then integrated into the macroscale material model by calibrating the Johnson-Cook (JC) constitutive relationship and the corresponding damage parameters. Consequently, we demonstrate the role of geometrically necessary multi-axial stress state for proper modeling of the impact response of polycrystalline metals and validate the presented approach by experimentally and numerically analyzing the deformation response of the Hadfield steel (HS) under impact loading

Incorporation of Dynamic Strain Aging Into a Viscoplastic Self-Consistent Model for Predicting the Negative Strain Rate Sensitivity of Hadfield Steel

A new multiscale modeling approach is proposed to predict the contributions of dynamic strain aging (DSA) and the resulting negative strain rate sensitivity (NSRS) on the unusual strain-hardening response of Hadfield steel (HS). Mechanical response of HS was obtained from monotonic and strain rate jump experiments under uniaxial tensile loading within the 10 À4 to 10 À1 s À1 strain rate range. Specifically, a unique strain-hardening model was proposed that incorporates the atomic-level local instabilities imposed upon by the pinning of dislocations by diffusing carbon atoms to the classical Voce hardening. The novelty of the current approach is the computation of the shear stress contribution imposed on arrested dislocations leading to DSA at the atomic level, which is then implemented to the overall strain-hardening rule at the microscopic level. The new model not only successfully predicts the role of DSA and the resulting NSRS on the macroscopic deformation response of HS but also opens the venue for accurately predicting the deformation response of rate-sensitive metallic materials under any given loading condition

A Novel Approach for Monitoring Plastic Flow Localization during In-Situ Sem Testing of Small-Scale Samples

A novel method is proposed for monitoring the plastic flow localization during in-situ scanning electron microscopy (SEM) testing of small-scale AISI 316 L stainless steel. Stress-strain behavior of the material was obtained using a hybrid numerical-experimental (HNE) approach. By repeatedly illustrating each pair of sequentially taken SEM surface images throughout the deformation history in alternating order in form of a video, location of the material points which are not moving during the deformation can be detected. At the initial stages of deformation these points are located on the geometrical symmetry line of the test sample, however; when uniform straining limit of the material is reached, the locations of the stationary material points reveal the plastic localization regions. The current results clearly prove the feasibility of the presented method in monitoring primary plastic localization events through in-situ SEM tensile testing

On the mechanical response and microstructure evolution of NiCoCr single crystalline medium entropy alloys

Unusual strain hardening response and ductility of NiCoCr equiatomic alloy were investigated through microstructural analysis of [111], [110] and [123] single crystals deformed under tension. Nano-twinning prevailed at, as early as, 4% strain along the [110] orientation, providing a steady work hardening, and thereby a significant ductility. While single slip dominated in the [123] orientation at the early stages of deformation, multiple slip and nanotwinning was prominent in the [111] orientation. Significant dislocation storage capability and resistance to necking due to nanotwinning provided unprecedented ductility to NiCoCr medium entropy alloys, making it superior than quinary variants, and conventional low and medium stacking fault energy steels. A comparison of the current results on the ternary medium entropy alloy single crystals and those previously reported on the quinary and quaternary fcc equiatomic alloys demonstrates that a higher configurational entropy does not necessarily warrant improved mechanical properties

Improvement of the fatigue performance of an ultrafine-grained Nb–Zr alloy by nano-sized precipitates formed by internal oxidation

Due to copyright restrictions, the access to the full text of this article is only available via subscription.The formation of nano-sized precipitates in an ultrafine-grained Nb–Zr alloy was investigated. ZrO2 precipitates induced by internal oxidation during heat treatment at low homologous temperatures significantly intensify the hardness of the surface layer without deterioration of the surface quality, and thus significantly improve the fatigue performance.Deutsche Forschungsgemeinschaft ; National Science Foundatio

Improvement of the fatigue performance of an ultrafine-grained Nb–Zr alloy by nano-sized precipitates formed by internal oxidation

Due to copyright restrictions, the access to the full text of this article is only available via subscription.The formation of nano-sized precipitates in an ultrafine-grained Nb–Zr alloy was investigated. ZrO2 precipitates induced by internal oxidation during heat treatment at low homologous temperatures significantly intensify the hardness of the surface layer without deterioration of the surface quality, and thus significantly improve the fatigue performance.Deutsche Forschungsgemeinschaft ; National Science Foundatio

Microstructure-based modeling of the impact response of a biomedical niobium-zirconium alloy

This article presents a new multiscale modeling approach proposed to predict the impact response of a biomedical niobium-zirconium alloy by incorporating both geometric and microstructural aspects. Specifically, the roles of both anisotropy and geometry-based distribution of stresses and strains upon loading were successfully taken into account by incorporating a proper multiaxial material flow rule obtained from crystal plasticity simulations into the finite element (FE) analysis. The simulation results demonstrate that the current approach, which defines a hardening rule based on the location-dependent equivalent stresses and strains, yields more reliable results as compared with the classical FE approach, where the hardening rule is based on the experimental uniaxial deformation response of the material. This emphasizes the need for proper coupling of crystal plasticity and FE analysis for the sake of reliable predictions, and the approach presented herein constitutes an efficient guideline for the design process of dental and orthopedic implants that are subject to impact loading in service. Copyright © Materials Research Society 2014