Characterization of Gradient Microstructures in Complex Materials by High Energy Small Angle and Wide-Angle X-ray Scattering
Andrew Allen (NIST)
Major Acknowledgments:
Jan Ilavsky, Jon Almer and Dean Haeffner (APS XOR)
Anand Kulkarni (SUNY, Stony Brook)
The advent of 3 rd generation synchrotron radiation sources has generated a renaissance in small-angle x-ray scattering (SAXS) research. Current APS SAXS instruments encompass transmission and grazing-incidence geometries, pinhole and Bonse-Hart USAXS configurations, time-dependent speckle analysis, and high-energy SAXS (HE-SAXS). At XOR sector 1-ID, HE-SAXS exploits the appreciable high-energy x-ray brilliance from APS Undulator A to provide an incident beam of energy up to 120 keV [1]. The advantages of conducting SAXS studies at these energies are: (i) absorption effects are greatly reduced, permitting the use of thick (mm) samples, comparable to those used in neutron scattering; and (ii) HE-SAXS measurements can be combined with high-energy wide-angle x-ray scattering (HE-WAXS) to provide complementary x-ray diffraction patterns for the same sample geometry. Due to the high energy, HE-WAXS measurements are themselves made at small scattering angles. Thus HE-SAXS and HE-WAXS data can be associated with identical sample volumes and almost identical experiment geometries, allowing physical and chemical microstructures to be correlated as a function of position. Materials science issues explored to date include phase transformation behaviors in bulk metallic glass [2], and the relationship between texture, composition and microstructure gradients that govern the properties of advanced coatings [3] and solid oxide fuel cell (SOFC) layers [4].
Figure 1. Comparison of void surface area variation across SOFC interfaces (from HE-SAXS) and corresponding x-ray diffraction data (from HE-WAXS).
Despite an impressive current capability, new areas of materials science will open up with the next-generation of HE-SAXS / HE-WAXS instruments. The most “cutting-edge” studies will benefit from significant improvements over the current instrument resolution in beam size and energy while maintaining high flux density. The smallest possible incident beam is needed to achieve the required spatial resolution for characterizing strain, texture, phase / composition and microstructure gradients encountered in many advanced materials systems. Presently, the beam size without focussing can be reduced to 5 mm x 50 mm; with focussing this has recently been improved to 2 mm x 16 mm. However, current proposals to improve the APS beam characteristics and develop new undulator designs should enable further gains in x-ray flux, with beam resolutions < 1 mm x 3 mm. This would present new opportunities for investigating stress, texture and composition variations around precipitates in alloys or within nanocomposites, plastic deformation during fatigue crack growth, nucleation and growth phenomena, kinetics of first order phase transitions, etc.
Improvements in energy resolution should open up further research areas. The present fractional energy resolution is about 10 -3 but, recently, resolution to 10 -4 has been demonstrated [1]. Coupled with the ability to pass the beam through significant sample thicknesses, this would open up anomalous SAXS and WAXS studies of systems containing heavy elements such as actinides - potentially important for the nuclear industry. Higher resolution should also enable more comprehensive studies of phenomena such as dislocation interactions in bulk solids.
High X-ray energies are well-suited for complicated sample environments due to the high penetrating power. Thus, development of in situ environments such as accommodating an operating SOFC will further expand the possible research opportunities. This and several other challenges must be addressed. For example, sub- mm sample positioning and orientation reproducibility are critical for ensuring registry of SAXS and WAXS measurement positions. Also, since many applications require small beams and sampling volumes, rapid X-ray diffraction and fluorescence measurements will be needed for mapping the beam position within a sample. With the challenges met, and with the advances envisaged, HE-SAXS / HE-WAXS studies should play a uniquely powerful role in several of the new materials research areas outlined above.
REFERENCES:
[1] S.D. Shastri; "Combining flat crystals, bent crystals and compound refractive lenses for high-energy X-ray optics (50-200 keV),” J. Synchrotron Rad., 11, 150-156 (2004).
[2] X-L. Wang, J. Almer, C.T. Liu, Y.D Wang, J.K. Zhao, A.D. Stoica, D.R. Haeffner and W.H. Wang, “In situ synchrotron study of phase transformation behaviors in bulk metallic glass by simultaneous diffraction and small angle scattering,” Phys. Rev. Lett., 91, [26] p.265501 (2003).
[3] A. Kulkarni, H. Herman, J. Almer, U. Lienert, D. Haeffner, J. Ilavsky, S Fang, and P. Lawton; “Depth-resolved porosity investigation of EB-PVD thermal barrier coatings using high-energy X-rays,” J. Am. Ceram. Soc., 87, 268–274 (2004).
[4] A.J. Allen, T.A. Dobbins, J. Ilavsky, F. Zhao, A. Virkar, J. Almer and F. DeCarlo; “Characterization of solid oxide fuel cell layers by computed X-ray microtomography and small-angle scattering,” Proc. 28th Int. Cocoa Beach Conf., in press (2004).
