Observation of magnetic gradients in stainless steel with a high-Tc superconducting quantum interference device microscope

Superconducting quantum interference device (SQUID) microscopes may serve as useful nondestructive evaluation (NDE) tools since they can precisely measure the local magnetic field variation that can be related to the characteristics of ferromagnetic materials. To demonstrate this, we have studied magnetic functionally graded materials (FGMs) in the Fe–Cr–Ni alloy system using a high-transition-temperature (HTc) SQUID microscope. The FGMs were either fabricated by inhomogeneous mechanical deformation or by heat treatment in a temperature gradient. The magnetic properties of these materials were measured using the vibrating sample magnetometer technique along the deformation or the temperature gradients. The results from this technique and the microstructural properties from optical imaging are discussed in conjunction with the magnetic field images obtained from the SQUID microscope. By exploring the results, the feasibility and benefit of utilizing SQUID microscopy as a NDE tool are discussed.