An X-ray diffraction method of mapping grain structures in a polycrystalline material sample (3), where an X-ray detector detects (12) spots or substantially line-shaped segments (13a-f) from beams diffracted from at least some of the grains. A processing device analyses values received from the X-ray detector (12) and identifies at least the position of spots or the line-shaped segments to generate a 3D model of the polycrystalline material. The line-shaped segments are paired as originating from diffractions from the same grain and the positions of the paired line-shaped segments are used in determining the crystallographic grain position of this grain within in the polycrystalline material sample. The length of the paired line-shaped segments is used in determining a width of this grain.
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