Neutron diffraction texture measurements provide bulk averaged textures with excellent grain orientation statistics, even for large-grained materials, owing to the probed volume being of the order of 1 cm³. Furthermore, crystallographic parameters and other valuable microstructure information such as phase fraction, coherent crystallite size, root-mean-square microstrain, macroscopic or intergranular strain and stress, etc. can be derived from neutron diffractograms. A procedure for combined high stereographic resolution texture and residual stress evaluation was established on the pulsed-neutron-source-based engineering materials diffractometer TAKUMI at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Research Center, through division of the neutron detector panel regions. Pole figure evaluation of a limestone standard sample with a well known texture suggested that the precision obtained for texture measurement is comparable to that of the established neutron beamlines utilized for texture measurement, such as the HIPPO diffractometer at the Los Alamos Neutron Science Center (New Mexico, USA) and the D20 angle-dispersive neutron diffractometer at the Institut Laue–Langevin (Grenoble, France). A high-strength martensite–austenite multilayered steel was employed for further verification of the reliability of simultaneous Rietveld analysis of multiphase textures and macro stress tensors. By using a texture-weighted geometric mean micromechanical (BulkPathGEO) model, a macro stress tensor analysis with a plane stress assumption showed a rolling direction–transverse direction (RD–TD) in-plane compressive stress (about −330 MPa) in the martensite layers and an RD–TD in-plane tensile stress (about 320 MPa) in the austenite layers. The phase stress partitioning was ascribed mainly to the additive effect of the volume expansion during martensite transformation and the linear contraction misfit between austenite layers and newly transformed martensite layers during the water quenching process.

Applications of neutron diffraction to quantitative microstructure evaluation of steel investigated by a project commissioned by the Innovative Structural Materials Association (ISMA)/New Energy and Industrial Technology Development Organization (NEDO) are summarized. First, the volume fraction of metastable austenite (γ) for a 1.5Mn-1.5Si-0.2C steel was measured by various techniques including backscatter electron diffraction (EBSD) and X-ray diffraction. It is recommended to measure both of volume fraction and texture simultaneously using neutron diffraction. Second, the γ reverse transformation behavior was in situ monitored using dilatometry, EBSD, X-ray diffraction and neutron diffraction. The γ reversion kinetics showed excellent agreements between dilatometry and neutron diffraction, whereas the γ formation started at much higher temperatures in the cases of EBSD and X-ray diffraction measurements. Such discrepancy in transformation temperature is attributed to the change in chemical compositions at the specimen surface by heating to a high temperature either in vacuum or in helium gas atmosphere; Mn and C concentrations were found to decrease with heating. Phase transformation behavior from γ upon cooling was also in situ monitored, which enabled us to elucidate the changes in lattice parameters of ferrite (α) and γ affected by not only thermal contraction but also transformation strains, thermal misfit strains and carbon enrichment in γ in the above hypoeutectoid steel. Pearlitic transformation started after carbon enrichment into γ reached approximately 0.76 mass% and contributed to diffraction line broadening. Finally, martensitic transformation behavior with or without ausforming at 700 ℃ was monitored for a medium carbon low alloyed steel. Dislocation density after ausforming was successfully determined using the convolutional multiple whole profile fitting method for 10 s time-sliced data. The changes in γ and martensite lattice parameters upon quenching were tracked every 2 s and new insights on internal stresses and the axial ratio of body centered tetragonal martensite were obtained.

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