An inverse optimization strategy to determine single crystal mechanical behavior from polycrystal tests: Application to AZ31 Mg alloy

An inverse optimization strategy was developed to determine the single crystal properties from experimental results of the mechanical behavior of polycrystals. The polycrystal behavior was obtained by means of the finite element simulation of a representative volume element of the microstructure in which the dominant slip and twinning systems were included in the constitutive equation of each grain. The inverse problem was solved by means of the Levenberg-Marquardt method, which provided an excellent fit to the experimental results. The iterative optimization process followed a hierarchical scheme in which simple representative volume elements were initially used, followed by more realistic ones to reach the final optimum solution, leading to important reductions in computer time. The new strategy was applied to identify the initial and saturation critical resolved shear stresses and the hardening modulus of the active slip systems and extension twinning in a textured AZ31 Mg alloy. The results were in general agreement with the data in the literature but also showed some differences. They were partially explained because of the higher accuracy of the new optimization strategy but it was also shown that the number of independent experimental stress-strain curves used as input is critical to reach an accurate solution to the inverse optimization problem. It was concluded that at least three independent stress-strain curves are necessary to determine the single crystal behavior from polycrystal tests in the case of highly textured Mg alloys

Effect of rare earth additions on the critical resolved shear stresses of magnesium alloys

An inverse optimization strategy based on crystal plasticity finite element simulations of polycrystals was used to obtain the critical resolved shear stresses of two Mg?1%Mn alloys containing neodymium from macroscopic experimental data. It was found that, with respect to pure Mg, the presence of Nd increases the CRSSbasal, CRSStwinning, and the CRSSbasal/CRSStwinning ratio and decreases the CRSSnon-basal/CRSStwinning ratio. Additions of neodymium as high as 1 wt% result in similar CRSSs values for all deformation modes and, thus, in an isotropic yielding behavior

Microstructure-based modelling and Digital Image Correlation measurement of strain fields in austenitic stainless steel 316L during tension loading

The present work aims to compare the local strain fields on the microscopic scale of an austenitic stainless steel 316L sample during tension loading, obtained by experimental measurements and simulations. Experimentally, Digital Image Correlation (DIC) technique is used to track the strain maps during the specimen loading. Microstructure-based Finite Element (FE) models based on two different approaches for the material behaviour, Crystal Plasticity (CP) and Sistaninia-Niffenegger Fatigue (SNF) ones, provide the numerical results. The quality of the outcomes reinforces the use of a combination of these techniques to study the residual strain localization in the microstructure, as well as its evolution during the deformation process. Whereas the calculated and measured strain and stress fields showed good agreement in the elastic range, as expected, significant differences were found between CP and SNF approaches at large plastic strains

Origin of the reversed yield asymmetry in Mg-rare earth alloys at high temperature

© 2015 Acta Materialia Inc. The mechanical behaviour in tension and compression of an extruded Mg-1 wt.% Mn-1 wt.% Nd (MN11) alloy was studied along the extrusion direction in the temperature range -175 °C to 300 °C at both quasi-static and dynamic strain rates. Microstructural analysis revealed that the as-extruded bar presents a recrystallized microstructure and a weak texture that remain stable in the whole temperature range. A remarkable reversed yield stress asymmetry was observed above 150 °C, with the compressive yield stress being significantly higher than the tensile yield stress. The origin of this anomalous reversed yield stress asymmetry, which to date remains unknown, was investigated through the analysis of the macro and microtexture development during deformation, as well as by means of crystal plasticity finite element simulations of a representative volume element of the polycrystal. The critical resolved shear stresses of slip and twining for simulated single crystals were obtained as a function of the temperature by means of an inverse optimisation strategy. Experimental and simulation results suggest that the reversed yield asymmetry may be primarily attributed to the non-Schmid behaviour of pyramidal 〈c + 〉 slip, which is the dominant deformation mechanism at high temperatures. It is proposed, furthermore, that the asymmetry is enhanced at quasi-static strain rates by the stronger interaction of 〈c + a〉 dislocations with the diffusing solute atoms and particles in compression than in tension.Financial support from the PRI-PIBUS-2011-0917, PRI-PIBUS-2011-0990 and MAT2012-38962 projects funded by the Spanish Ministry of Economy and Competitiveness (MINECO) is gratefully acknowledged. The research leading to these results has also received funding from Madrid Regional Government under the S2013/MIT-2775 project.Peer Reviewe