Mechanical and microstructural investigations of the biaxial Bauschinger effect in sheet metals

众所周知的包辛格效应指的是与载荷反转相关的屈服应力的变化。关于单轴加载下的包辛格效应及其相应的微观结构机制，已有大量的研究报道。然而，在多轴加载条件下，特别是在板材变形过程中，可能会出现类似的影响，这种影响很可能与成形技术高度相关。在本研究项目中，首次研究了载荷从双轴拉伸到双轴压缩的板材的双轴面内变形，重点研究了双轴Bauschinger效应。开姆尼茨工业大学开发了特殊的新实验装置，一方面允许进行单轴拉伸和压缩测试，另一方面允许进行板材的双向压缩-压缩测试。这些新的力学测试方法现在将被用来解决材料科学中有趣的挑战，即记录双轴包辛格效应，并确定其微观结构来源。作为模型板材，将使用技术上纯铝(EN-AW 1050)和DC06钢。在详细的力学研究中，十字形试件将(与Erlangen的合作伙伴合作)系统地变形到不同的双向拉伸应变水平，然后从这些试件中切出较小的试件并承受双向压缩载荷。补充有限元模拟将被用来分析相应的应力和应变状态，特别关注板材中的织构对各向异性材料行为的影响。此外，模拟将允许单独考虑强度差异影响。在这个项目中，将特别注意通过广泛使用电子显微镜(扫描电子显微镜、EBSD、STEM、TEM、透射菊池衍射)和X射线衍射方法来识别和量化在不同变形阶段对双轴包辛格效应有强烈影响的微观结构机制(最重要的是位错胞的形成和湮灭)。此外，将详细考虑加载历史过程中残余应力的产生和演变，并与使用X射线衍射进行的微观结构观察有关。将对不同变形量的样品(载荷反转前后)进行非原位测量，并在双向变形期间进行现场测量，后者与来自瑞士维利根的同事合作，在专门的Poldi中子衍射装置上进行。

The well-known Bauschinger effect refers to changes of the yield stress associated with load reversal. Numerous studies have been reported on the Bauschinger effect under uniaxial loading and on the corresponding microstructural mechanisms. However, similar effects can be expected to occur under multi-axial loading conditions, particularly during deformation of sheet metals, where such effects may well be highly relevant for forming technology. In this research project, biaxial in-plane deformation of sheet metals with a load reversal from biaxial tension to biaxial compression is studied for the first time with a focus on biaxial Bauschinger effects. Special new experimental setups were developed at Technische Universität Chemnitz, allowing for uniaxial tension and compression testing, on the one hand, and for biaxial compression-compression testing, of sheet materials, on the other hand. These new mechanical testing methods will now be used to tackle the interesting materials science challenge of documenting biaxial Bauschinger effects, and to identify their microstructural origins. As model sheet materials, technologically pure aluminum (EN-AW 1050) and DC06 steel will be used. In detailed mechanical investigations, cruciform samples will be deformed (in collaboration with partners in Erlangen) systematically to different biaxial tensile strain levels, and then smaller samples will be cut from these samples and subjected to biaxial compressive loading. Complementary Finite Element simulations will be used to analyze the corresponding stress and strain states, with a special focus on the effect of textures in the sheet materials on anisotropic material behavior. Moreover, the simulations will allow for a separate consideration of strength differential effects. Special attention in this project will be paid to the identification and quantification of the microstructural mechanisms (most importantly, the formation and annihilation of dislocation cells) that strongly affect the biaxial Bauschinger effects during the various deformation stages, by extensively using electron microscopy (SEM, EBSD, STEM, TEM, transmission Kikuchi diffraction) and X-ray diffraction methods. Furthermore, the generation and evolution of residual stresses during the loading history will be considered in detail and related to microstructural observations using X-ray diffraction. Both ex situ measurements of samples after different amounts of deformation (prior to and after the load reversal), and in situ measurements during biaxial deformation will be performed, the latter in collaboration with colleagues from Villigen, Switzerland, at the dedicated POLDI setup for neutron diffraction.