A new method for determining the defocusing correction for crystallographic texture measurement
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A New Method for Determining the Defocusing Correction for Crystallographic Texture Measurement C.K. CHOW Sample
The most popular method for determining crystallographic texture is the reflection method, initially suggested by Schulz. ~ This method uses an X-ray diffractometer to measure the pole density of a selected crystallographic plane at all points in space. Figure 1 illustrates this method. A flat sample is placed in the diffractometer with the surface normal initially in the plane of the X-ray path. During measurement, the sample is rotated and tilted simultaneously about its normal and AB axes. The reflected X-ray intensity is an indication of the number of poles pointing in the a and - 05 directions, where a is the inclination angle and 05 is the rotation angle. A plot of the normalized pole intensities in stereographic projection is the direct-pole figure. The Schulz reflection method has adequate resolution, and data acquisition and analysis can be performed automatically by computer. 2'3 However, there is a problem with intensity drop due to defocusing at high tilt angles, which limits its use to a < 70 deg. When the sample is tilted during texture measurement, the beam strikes areas on the sample that are not on the focusing sphere. This broadens the reflected beam and fewer X-rays pass through the receiving s l i t / T h e absorption of X-rays, due to the type of material 5 and its metallurgical condition,6 also contributes to the defocusing effect. Thus, to determine the pole density, especially at high tilting angles, intensity reduction due to defocusing must be considered. The defocusing effect has been examined theoretically;7 however, the problem is very complex and probably not all variables have been taken into consideration. 8 One must therefore measure the reduction in X-ray intensity experimentally. The most common method for determining the defocusing correction is to use a specimen that is known to have a random texture. The normalized reflection intensity of the random sample at various tilt angles provides the required correction factors. A popular method for making a randomtexture sample is by using powder, produced by filing the material for which the texture is to be measured. The powder is then heat-treated to simulate the metallurgical conditions of the original material. There are problems in producing a truly random powder sample. It is difficult to produce powder that is spherical. Therefore, the sample may consist of compact nonspherical particles and may not be textureless. Furthermore, it is not always possible to heat-treat a powder to produce the same metallurgical condition as the original textured material. The flatness of the powder sample 9 and the roughC.K. CHOW is Research Scientist with Materials and Mechanics Branch, Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, MB ROE 1L0, Canada. Manuscript submitted August 6. 1984. METALLURGICAL TRANSACTIONS A
Fig. l - - T h e Schulz reflection method for crystallographic textu
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