Abstract
The deformation behaviour of coarse-grained α-uranium is studied using crystal plasticity finite element simulations. The constitutive model includes 8 slip and 2 twin systems, based on dislocation densities as state variables. Polycrystal simulations are carried out to reproduce the manufacturing procedure: a quenching stage is followed by cutting out a sub volume to allow relaxation of the internal stresses. Room temperature tension and compression experiments are simulated for the textured samples that are generated by this process. Calibration of the model parameters is obtained by comparing simulated experiments against measured stress-strain curves and lattice strains from published in situ neutron diffraction studies. The comparison of the internal elastic strains allows the activity of slip and twin modes to be quantified, and to estimate their critical resolved shear stresses. The comparison between the present simulation results and previous models for fine-grained α-uranium provides information about the grain size dependence of the strength of slip and twin systems. The critical resolved shear stress of the most prominent twinning mode {130} is found to be similar to single crystal samples and much smaller than for fine-grained samples, showing the strong grain size dependence of this deformation mode.
Highlights
Understanding the deformation behaviour of -uranium, which is the stable phase of uranium up to 670° C [1], is important for the development of safer manufacturing procedures and to predict fracture.-uranium has a highly anisotropic crystal structure [2], elastic coefficients [3] and thermal expansion properties [4], which can give rise to significant thermal residual stresses [5]
The slip and twin systems were first studied by Cahn [12,7] using X-ray diffraction. 8 slip systems have been identified in -uranium, which have very different critical resolved shear stresses (CRSS) [13]. 9 different twin types have been observed experimentally [14], but only the 2 variants of {1 3 0} 3 1 ̄ 0 are frequently observed and occupy a significant volume fraction of the crystal [15]
We show that neither the set of parameters identified for single crystals [8] nor the ones for the fine-grained material [26] are suitable for coarse-grained -uranium
Summary
Understanding the deformation behaviour of -uranium, which is the stable phase of uranium up to 670° C [1], is important for the development of safer manufacturing procedures and to predict fracture. The crystal plasticity finite element method (CPFEM) takes into account the set of discrete slip and twin systems [23] and can account for large strains, elastic anisotropy, texture evolution during deformation and the lattice reorientation given by twinning [24] It can model the thermo-mechanical response of polycrystals using arbitrary boundary conditions and complex geometries. Simulations are compared to in situ neutron diffraction experiments [32] carried out at the ENGIN-X beamline of the ISIS Neutron Source, Rutherford Appleton Laboratory [33], and to tensile and compressive stress-strain curves obtained from the textured samples This comparison required the simulated elastic lattice strain in each grain to be extracted. The model developed is a first step to understand how fracture is affected by plastic deformation and texture in coarse-grained -uranium
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