The determination of a stress-free lattice parameter within a stressed material using elastic anisotropy

A method for the calculation of a stress-free lattice parameter from the an alysis of diffraction data from stressed material is discussed, utilizing t he elastic anisotropy of the material. The technique is demonstrated using data obtained during a uniaxial tension test on untextured austenitic (face -centred cubic) steel. The uncertainty in the calculated lattice parameter for various choices of number of diffraction peaks and different number of stress levels available for the calculation is considered. It is shown that when all the data are within the elastic regime, an accurate evaluation of the reference lattice parameter can be made. When some data are in the pla stic regime, a more limited evaluation is possible. The use of plots of lat tice parameter against Gamma (hkl) [= (h(2)k(2) + h(2)l(2) + k(2)l(2))/(h(2 ) + k(2) + l(2))(2)] as a method for monitoring plasticity as well as freed om from deviatoric stress is demonstrated.