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

Superconducting quantum interference device (SQUID) microscopes may serve a s useful nondestructive evaluation (NDE) tools since they can precisely mea sure the local magnetic field variation that can be related to the characte ristics of ferromagnetic materials. To demonstrate this, we have studied ma gnetic functionally graded materials (FGMs) in the Fe-Cr-Ni alloy system us ing a high-transition-temperature (HTc) SQUID microscope. The FGMs were eit her 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 defor mation or the temperature gradients. The results from this technique and th e microstructural properties from optical imaging are discussed in conjunct ion with the magnetic field images obtained from the SQUID microscope. By e xploring the results, the feasibility and benefit of utilizing SQUID micros copy as a NDE tool are discussed. (C) 2001 American Institute of Physics.