Lattice strain and texture development in coarse-grained uranium – a neutron diffraction study

P Earp,T J Marrow,J Askew,S Kabra

doi:10.1088/1742-6596/1106/1/012012

Abstract

A neutron diffraction study of deformation in a cast uranium has been conducted for the first time. Lattice-scale plasticity in this coarse-grained material initiates at a lower stress than in a previous study of fine-grained material in the literature. This is attributed to a combination of larger thermal residual stresses in the coarse-grained material and the Hall-Petch effect making twinning easier in large grains. Asymmetry between the tensile and compressive response shows that twinning is the dominant plastic deformation mechanism at low strains. Axial texture changes for the cast uranium were calculated by post processing of the full diffraction spectra, which shows that lattice rotations associated with twinning occurred at yield. This lattice rotation was observed to disappear after unloading, which indicates that de-twinning can occur in uranium. ©British Crown Owned Copyright 2018/AWE

Highlights

Understanding fracture processes in materials such as cast alpha-uranium is a complex task; the combined effects of large grains [1], residual stresses and anisotropic mechanical properties creates unusual responses to deformation, and variable mechanical properties [2,3].Uranium metal is highly elastically anisotropic, both elastically and plastically
To fully understand local processes such as fracture, there is a need to develop models, such as the crystal plasticity finite element (CPFE) method, which can be used to predict the strength of engineering components whilst incorporating the internal microstructure of the material
A number of visco-plastic self-consistent (VPSC) modelling schemes have been developed for fine-grained wrought uranium grains [9] which has been incorporated into a finite element framework [10] which was used to model compressive and rolling deformation

Summary

Introduction

Understanding fracture processes in materials such as cast alpha-uranium is a complex task; the combined effects of large grains [1], residual stresses and anisotropic mechanical properties creates unusual responses to deformation, and variable mechanical properties [2,3].Uranium metal is highly elastically anisotropic, both elastically and plastically. In the room temperature alpha phase it has an orthorhombic crystal structure [4,5], an anisotropic coefficient of thermal expansion [6], and each grain has a limited number of slip modes; in particular, there are no active slip modes at room temperature to accommodate deformation perpendicular to the (001) plane [7]. To fully understand local processes such as fracture, there is a need to develop models, such as the crystal plasticity finite element (CPFE) method, which can be used to predict the strength of engineering components whilst incorporating the internal microstructure of the material.

Methods

Results

Discussion

Conclusion