We studied the microstructure formation mechanisms of spinodal Fe–10%Cu alloys (mass%) fabricated using electron-beam powder bed fusion with various scanning speeds. Cross-correlation electron backscattered diffraction analysis was utilized to investigate the crack initiation and propagation mechanisms related to dislocation density and residual stress in the as-built Fe–10%Cu alloys. The as-built alloys with low scanning speeds have equiaxed microstructures with coarse grains, including Cu particles. As the scanning speed increased, the grain size and Cu particle size decreased, and micro-cracks initiated at the edge of lack-of-fusion defects and then grew along the grain boundary parallel to the built direction (BD). In addition, coarse Fe3O4 particles formed on the boundary caused a decrease in thermal conductivity and tensile strength. A strong compressive residual stress parallel to the BD acts as a driving force for micro-crack propagation. The rapid cooling rate enhances local dislocation density, and lattice rotation also causes micro-crack growth, thereby deteriorating mechanical and thermal properties. Therefore, the scanning speeds should be controlled below 2000 mm/s for good strength and superior conductivity of the spinodal Fe–Cu alloy.
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