Investigation of residual strains by neutron diffraction in an AZ31 direct chill cast billet

Abstract

Neutron diffraction data was collected, showing the strain distributions along radial, axial and hoop directions in a direct chill cast billet of AZ31 magnesium alloy. Strain measurement by neutron diffraction is a non-destructive technique that uses the diffraction of a beam of thermal neurons to determine the atomic spacing within a small gauge volume inside polycrystalline materials. The benefits of using neutrons versus X-rays lie in the increased penetration depth achieved with neutrons, which can allow measurement of the internal strains several centimeters away from the material surface. A data-processing technique was proposed to assess and remove the irregular points, and the point-to-point fluctuations were evaluated. Residual strain measurements on the as-cast billet contribute direct observation of the stress/strain state in the billet. And the results also provide the data necessary to validate a thermo-mechanical model that predicts the evolution of stress/strain during the DC casting and subsequently to investigate the cracking defects in the billets.