Microstructure and Phase Composition of a Gradient Material “Stainless Steel/Cr-Ni Alloy” Produced by Electron-Beam Additive Manufacturing

Using the methods of scanning and transmission electron microscopy, the microstructure of a gradient material “Fe-18Cr-10Ni-Ti-0.08C steel / 80Ni-20Cr alloy” (wt.%) produced by the double-wire electron-beam additive manufacturing is studied. The chemical composition of the material continuously changes from the one corresponding to the Fe-18Cr-10Ni-Ti-0.08C stainless steel to that of the 80Ni-20Cr alloy, with the intermediate gradient layer formed by deposition of two wires in different ratios. The microstructure of the part of the samples, which corresponds to pure steel without any addition of the NiCr alloy, is characterized by large columnar austenitic grains (γ-phase) with a small fraction of dendritic δ-ferrite (about 20%). An addition of the NiCr alloy suppresses the formation of the δ-phase during the steel additive growth, thus the main part of the gradient sample has an austenitic coarse-grained structure. In the part of the material, containing a high fraction of the NiCr alloy, particles of intermetallic phases are formed. Stabilization of the austenitic structure of the stainless steel with an addition of the NiCr alloy is discussed in terms of changing the chromium-nickel equivalent and the mechanism of crystallization in the melting pool during the additive manufacture.