Stresstech (Finland)

Abstract XRD residual stress analyses by means of the conventional sin2ψ-method in accordance with standards EN 15305 or ASTM E2860 require well-aligned and calibrated X-ray diffractometers. The qualification of an X-ray diffractometer is performed by measurements of a stress-free powder sample and a certified ILQ reference sample from round robin experiments. In order to obtain reliable qualification results, the traceability of residual stress data to SI units is optimal, which can be achieved via Standard Reference Material 660c (LaB6) from NIST. However, in many cases, such as the traditional residual stress analysis of ferritic steel samples using Cr Kα radiation, the peak positions and intensities of LaB6 are unfavorable for SI-traceability measurements. The technical difficulties arising from this condition can be overcome by the application of Fe Kα radiation providing a diffraction peak of LaB6 close to the ones of common materials. This work quantitatively investigates the usability of Fe Kα radiation for XRD residual stress analyses including SI-traceability of stress-free powder samples based on LaB6. Moreover, its potential for multi-phase residual stress determination in duplex steels is demonstrated as a laboratory application example.

We built a custom-made holder with a Hall-effect sensor to measure the single point magnetic flux density inside a transmission electron microscope (TEM, JEM-F200, JEOL). The measurement point is at the same place as the sample inside the TEM. We utilized information collected with the Hall-effect sensor holder to study magnetic domain wall (DW) dynamics by in-situ Lorentz microscopy. We generated an external magnetic field to the sample using the objective lens (OL) of the TEM. Based on our measurements with the Hall-effect sensor holder, the OL has nearly linear response, and when it is switched off, the strength of the magnetic field in the sample region is very close to 0 mT.A ferritic-pearlitic sample studied has globular and lamellar cementite (Fe3C) carbides in the ferrite matrix. Based on the in-situ Lorentz microscopy experiments, DWs in the ferritic matrix perpendicular to the lamellar carbides start to move first at ∼10 mT. At 160 mT, DWs inside the globular carbide start to disappear, and the saturation occurs at ∼210 mT. At 288 mT, the DWs parallel to the lamellar carbides still exist. Thus, these lamellar carbides are very strong pinning sites for DWs. We also run dynamical micromagnetic simulations to reproduce the DW disappearance in the globular carbide. As in the in-situ experiments, the DWs stay stable until the external field reaches the magnitude of 160 mT, and the DWs disappear before the field is 214 mT. In general, the micromagnetic simulations supported very well the interpretation of the experimental findings.