Towards a Better Understanding of Phase Transformations in Uranium Alloys Through the Use of Scattering Techniques

This work represents a series of studies which utilize elastic scattering experiments with X-rays or neutrons to obtain information about microstructures and properties of materials. The studies presented in the first two chapters in this work detail efforts to develop and characterize the ability to measure crystallographic texture of materials at various beamlines within the two neutron scattering user facilities located at Oak Ridge National Laboratory, the Spallation Neutron Source (SNS) and the High Flux Isotope Reactor (HFIR). These beamlines included NOMAD (SNS), VULCAN (SNS), WAND2 (HFIR) and HIDRA (HFIR). The measurements made at WAND2 and HIDRA also contributed to an on-going round-robin study of crystallographic texture measurement at neutron scattering facilities around the world, including, LANSCE (Los Alamos National Laboratory), NCNR (National Institute Standards and Technology), ISIS (UK), FRM-II (Germany) and J-PARC (Japan).

The next three chapters detail efforts to further the understanding of phase transformations in two uranium alloys, U-6wt%Nb and U-10wt%Mo which have improved mechanical behavior, radiation swelling resistance, and corrosion resistance when compared to pure metallic uranium. These works utilized both in-situ and ex-situ scattering techniques to characterize the microstructural evolution during extended holds at high temperatures (~ 400-600°C). U-10wt%Mo, has been identified as a candidate replacement fuel for high performance research reactors in the United States, to reduce and ultimately eliminate the use of highly enriched uranium fuels at these facilities. In this work, the effects of replacing a small amount of Mo (0.2 wt%) with three different intentional or unintentional (impurity) solutes (Co, Cr, Ni) on the eutectoid phase transformation kinetics, compared to the binary U-10wt%Mo, were investigated using ex-situ neutron diffraction studies, conducted after prescribed heat treatments. Monotectoid phase transformation in U-6wt%Nb was also examined using a set of combined in-situ wide-angle (WAXS) and small-angle (SAXS) high energy X-ray scattering experiments at various temperatures to probe the time-temperature-transformation space. Synchrotron X-ray diffraction was required to obtain the necessary temporal resolution required to study relatively rapid phase transformations (which occur in seconds, not hours). At temperatures above ~350°C, U-6wt%Nb will begin to decompose towards the expected equilibrium microstructure of nearly pure α-U and γ2 (Nb-rich bcc phase) via two distinct mechanisms: a) continuous (intragranular) or b) discontinuous (lamellar, grain boundary) precipitation. The use of small-angle X-ray scattering provided a way of distinguishing between the two morphologies of α-U which precipitate via these two mechanisms. The quantitative analysis of the SAXS data required the input of the results obtained from the WAXS data, namely the bulk phase fractions and phase composition. This provided new insight into the phase transformation behavior in U-6wt%Nb, particularly during the early stages of the transformation.

Finally, preliminary results from an on-going study to estimate the single crystal elastic properties, Cij, of polycrystalline materials, including the monoclinic α’’ martensite phase of U-6wt%Nb. The finely twinned structure of α’’, which is only metastable at room temperatures, prevents the creation of a pristine single crystal of the material in the α’’ phase, which could be used for a direct measurement of the elastic properties. However, the extraction of single crystal elastic properties of materials with higher crystal symmetry has been demonstrated in prior studies by solving an inverse problem. The elastic strains for various (hkl) planes are measured from a polycrystal using diffraction while the sample is mechanically loaded, to obtain a set of diffraction elastic constants (DECs). Through the use of a micromechanical model (Kröner, self-consistent), the DECs can be calculated from a candidate set of Cij, which provides a framework for solving the inverse problem through global optimization techniques. The incorporation of uncertainty quantification techniques was also explored, and it has been found that inverse problem is insensitive to the values of certain Cij coefficients when only the data measured along the loading direction is used. However, when data from both the loading and transverse directions are used, the model becomes much better defined, enabling the recovery of the synthetic input nearly perfectly. Finally, the estimated Cij for α’’ appears to have a similar level of anisotropy to that which has been predicted by density functional theory at 0K.