Studies on the Measurement and Modeling of Lattice Strains in Rolled Zircaloy-2
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Neutron diffraction is a widely used technique for measuring internal stresses inside polycrystalline materials. By examining the diffraction patterns collected during in situ uniaxial deformation, the lattice strains along various crystallographic directions can be calculated. These lattice strains give insight into the active deformation mechanisms active within the material during plastic deformation. This is most commonly done by fitting model results to the experimentally measured lattice strains through an iterative process of refining the model parameters. A numerical optimization technique was successfully applied to the problem of refining the input parameters of an elastoplastic self-consistent (EPSC) model. The results were found to be comparable to those obtained by a past researcher manually refining the model parameters and subjectively judging the fit to the experimental data. The numerical optimization method was able to reach an acceptable result much faster than is possible by a human being (days as opposed to weeks or months), meaning that it has the potential to reduce the turn-around time from data collection to interpretation/publication significantly. At the same time, common experimental techniques for conducting diffraction experiments during uniaxial deformation tests were examined. It is common to use an interrupted loading scheme where the sample is brought to a certain loading condition and then held steady while the neutron data is collected, a processes that often takes several minutes. This interrupted loading may be done such that the sample is held at constant stress, strain, or simply by having the load frame stay in a constant position. Each of these different loading modes results in a particular type of relaxation within the sample as it is being held, so a series of experiments were conducted to investigate any impact these different relaxation types may have on the measured values of the lattice strains. Overall it was found that both qualitative and quantitative differences in the recorded data can arise as a result of the different loading modes, and that such differences tend to manifest themselves at or near the point at which the material begins to yield macroscopically.