A neutron diffraction study of intergranular strains and plastic deformation

Neutron diffraction has been used for non-destructive residual strain measurements at-depth for the past twenty years. Traditionally, it is mainly used to determine macroscopic residual strains in samples, although it is sensitive to both macroscopic and microscopic strains. We have performed a series of experiments to study the generation of microscopic or intergranular strains in polycrystalline materials with different properties of their elastic and plastic deformation using neutron diffraction. The evolution of intergranular strains, parallel and transverse to the stress axis, have been made on uniaxial tensile samples of four different materials: Al7050, a high strength ferritic steel 350WT, Zircaloy-2 and 309 stainless steel. The angular variation of the strain states of the tensile samples have been measured to fully characterise the intergranular strain distribution. The experiments also provide the information on which reflection (hkl) is least affected by intergranular effects in different materials which is important in the selection of ( hkl) used for the macroscopic residual strain measurements.; Intergranular strains of opposite signs are observed for (111) and (002) reflections in the fcc alloy Al7050 because of the anisotropy of plastic deformation. In the case of the bcc ferritic steel, intergranular strains were found to be actively developed at small plastic deformation (<4.5%) and stay unchanged in subsequent loadings. Tensile strains of an average 7 x 10 --4 were observed for the (002) reflections transverse to the stress axis whereas intergranular effects are comparatively insignificant for other reflections. The hcp Zircaloy-2 is a thermally and mechanically anisotropic alloy. After 5% deformation, an in-plane biaxial stress state was found, with tensor components, s11 = 230 +/- 20MPa and s33 = --241 +/- 21MPa. 309 stainless steel is a fcc alloy with anisotropic elastic and plastic properties. Only (002) planes along the tensile axis display a tensile strain of 8 x 10--4 at plastic deformation of 7.2%. The intergranular strains for other reflections are within +/-2 x 10--4.; The experimental results indicate the magnitudes of the intergranular strains are large and easily measured, even though the materials are macroscopically stress free. The experimental results are compared directly with simulations based on the elasto-plastic self-consistent model of Turner and Tome. The model is in good agreements with the experiments for the ferritic steel and Zircaloy-2. In the comparison for the Al7050 experiments, qualitative agreement is found but there is a tendency of the model to overestimate the magnitudes of the intergranular strains. The model simulation for 309SS is not successful, possibly due to the inadequacy of the description of the hardening mechanisms.