Effects of texture gradients on yield loci and forming limit diagrams in various aluminum-lithium sheet alloys
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I.
INTRODUCTION
THE recently developed aluminum-lithium (A1-Li) alloys are particularly attractive to the aerospace industry because of their lower density and higher stiffness compared to conventional alloys. Usually, commercially produced A1-Li alloys have highly developed crystallographic textures, which lead to pronounced anisotropy of mechanical properties such as yield stress, II,21 elastic modulus, t3'41 plastic flow, [4,51and formability, t2,6,71These pronounced directional properties are one of the main barriers that impede the replacement of conventional alloys by A1-Li alloys. Therefore, it is necessary to improve the understanding of the influence of crystallographic texture on the mechanical properties of A1-Li alloys. In addition, the prediction of mechanical properties is a useful tool for material, process, and product design. This theoretical study requires knowledge of the influence of crystallographic texture on the shape of the yield locus calculated with polycrystal models. The yield-locus shape strongly influences the plastic properties of materials. However, a strong throughthickness texture gradient has been observed in A1-Li alloys. 14,8~91Therefore, it is important to know how this texture gradient influences the plastic properties. In previous articles the effect of a texture gradient on plastic anisotropy and yield strength for AI-Li 8090 and 2090 alloys was studiedJ 2,j~ The purpose of the present work
was to study the effects of a texture gradient on the yieldlocus shape and the forming limit diagram (FLD) of AILi alloy sheets. Yield loci can be assessed directly by mechanical measurements or can be predicted by phenomenological theories or polycrystai models. Experiments are expensive, time consuming, and difficult to perform and interpret. An interesting advantage of polycrystal models is that they only require simple and relatively standard experimental data as an input and the orientation distribution function (ODF) to compute the yield locus of a real material. Moreover, this theoretical approach concerning the plastic deformation of alloys can specifically address the influence of crystallographic texture on the shape of the yield locus. Finally, the yield locus can be used in conjunction with strain-hardening parameters to predict the level and shape of the FLD. In the present work, through-thickness texture gradients have been determined systematically by X-ray texture analyses at various thickness locations in the sheets. An average texture was measured by neutron diffraction. This quantitative texture data was utilized for the prediction of yield loci. Subsequently, FLDs were calculated using these predicted yield loci as a constitutive description of the plastic behavior of the sheets m,121 and criteria for local necking analysis.
II. XIAO-HU ZENG, Research Assistant, is with the Division of Engineering Materials, Department of Mechanical Engineering, Linkrping University, S-581 83 Linkrping, Sweden. FREDERIC BARLAT, Scientific Associate, is with the Alloy Technology Div
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