Texture and Strain Experiments at OPAL

In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. The determination of global texture and the local variation of texture for example by inhomogeneous deformation are very important due to the coherence between the texture and the physical and mechanical properties of materials. Introduction Texture and stress analysis by measuring pole figures and strain with neutron diffraction techniques achieve world wide significance to researchers and industry. Due to the high penetration depth of neutron radiation, e.g. 20 mm into steel or 100 mm into aluminium, detailed stress and texture information within the bulk of components is accessible. The powder diffractometers at the OPAL reactor cover a wide range of non-destructive and phase specific texture and residual stress analysis. On the one hand the large beam size of the powder diffraction instruments affords the global texture measurement with the bulk information of large sample up to size of a few cm3. On the other hand, with beam line optics like focusing monochromators and slit systems small samples (< 1 mm3) or local resolved 3-d strain and/or texture mapping is feasible. Measurement of local variation of strain and texture is detailed described in [1] and calculation of physical and mechanical properties [2] is very important in response to develop new materials and the application of materials and components in new technologies. Permanent improvement of diffraction methods will offer deeper view inside materials and better understanding. Principle The method of strain and pole figure measurement by neutron diffraction is based on Bragg's law, eqn. 1: hkl hkl d θ λ sin 2 ⋅ = (1) An incident monochromatic neutron beam of wave length λ hits the sample and can be reflected under the Bragg angle 2θ. In a polycrystalline material reflection takes place in all those crystallites which are in reflection orientation. A typical diffraction diagram as shown in figure 1 covers all the information needed for texture and strain analysis. After analysing the diffraction diagram with peak fitting software (for example “2diffcalc” [3]) for every reflection the position 2θ and the integral intensity is noted. Materials Science Forum Vols. 638-642 (2010) pp 2823-2828 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.638-642.2823 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 137.157.8.253-23/11/09,22:02:25) Figure 1 Detector image and resulting diffraction diagram calculated by the software package "2diffcalc" On the one hand, the presence of stress leads to a rearrangement of the lattice spacings which results in different d-values. The strain of the specimen is defined as: