Neutron Larmor Diffraction for the Determination of Absolute Lattice Spacing

The Neutron Larmor diffraction method is applied for the first time to determine absolute lattice spacing of a Silicon powder and polycrystalline IN718 samples. The absolute value of lattice spacing can be measured to an accuracy of 7· 10A. However, absolute determination of the lattice parameter is limited by the quality of the reference specimen used. Absolute lattice spacing determined by Larmor diffraction measurements of IN718 samples are shown and compared to the results obtained via conventional neutron diffraction. INTRODUCTION Residual stress analysis using neutron diffraction relies on the comparison of the lattice spacing d of a stressed component with the lattice spacing do of a macroscopically stress free reference sample. In neutron Larmor diffraction the lattice spacing d is encoded in the spin precession of a polarized monochromatic neutron beam scattered at the sample. In contrast to conventional neutron diffraction methods, Lannor diffraction is independent of the divergence of the incident beam, the spread of the wavelength and the scattering angle 28 [I]. Based on this, the Larmor technique is insensitive to alignment errors of the sample, which may cause large shifts in measured 20 values in conventional diffractometer techniques [2]. Hence, the method enables the measurement of relative lattice spacing changes with a resolution of 6d/d IO--{> hitherto not possible using classical diffractometers [3]. In this paper we present for the first time results of the determination of absolute values for lattice spacing using neutron Larmor diffraction. The accuracy of the method is demonstrated for an NHS-Silicion powder sample with a well known lattice constant and is then applied to determine absolute lattice spacing of the industrially used Ni-base superalloy Inconel 718. The latter one is compared to results from conventional diffraction experiments. 201 Copyright ©JCPDS-International Centre for Diffraction Data 2009 ISSN 1097-0002