Scanning Wide-Angle Neutron Diffraction and Its Application to Local Structures Investigations in Stainless Steel

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We applied a spatial scanning measurement technique to neutron diffraction on a powder diffractometer, and performed scanning wide-angle neutron diffraction (SWND) experiments to investigate local structures of materials. The SWND technique has an advantage that one can observe a diffraction pattern over a wide scattering angle range, so that the positional dependence of local crystal structures can be measured for practical durations. Moreover, because neutrons have a larger penetration depth, on the order of a few cm, than X-rays, the SWND technique provides structural information even in internal regions of materials. We succeeded in observing positional dependence of diffraction patterns from small areas (approximately $2 \times 5$ mm2) of a type-304 stainless steel plate with a dimension of $19.5 \times 50 \times 4.7$ mm3 subjected to a mechanical tensile strain of 90% of the 0.2% offset yield strength. In the SWND experiments, we confirmed that the crystallographic circumstance in the strain-loaded type-304 stainless steel is not homogeneous. For instance, we observed positional deviation in the grain direction of the master-alloy austenite phase, and local distribution of the stress induced martensite phase. The SWND technique is a particularly important probe for estimating the stress-induced phase, because it non-destructively observes even the inside of bulk materials.
2007-12-15