Hall-Petch strengthening of an Fe-C-Cr-Ni eutectic alloy under additive manufacturing conditions

This paper deals with Hall-Petch strengthening of an Fe-based eutectic alloy that is applicable to the additive manufacturing. This alloy consists of a dual-phase structure of a γ-Fe (Ni) matrix and ~20 vol.% of eutectic chromium carbide rods. The distance between the eutectic carbide rods or rod spacing was varied over a wide range from ~10 1 to ~10 -2 μm by varying cooling rate during solidification from ~10 -1 to ~10 7 o C/s. This wide variety of the cooling rate was provided by a variety of cooling methods of the liquid alloy, including slow furnace cooling of the liquid in an alumina crucible, arc melting and fast cooling of a button-like small ingot in a graphite crucible, rapid cooling of small melt pools formed on the surface of the ingot samples using a TIG welder, and extremely rapid cooling of fine melt pools formed on the surface of the ingot samples using a laser scanner. As the rod spacing decreased from ~10 1 to ~10 -2 μm, Vickers hardness of the as-solidified alloy increased from ~300 to ~600 HV. This considerable increase in hardness was due to the Hall-Petch effect, and the relation between the Vickers hardness, HV , and the rod spacing, λ , was described as a regression equation of HV = 67.4 λ -1/2 + 305 with a high correlation coefficient of R 2 = 0.9009. When this alloy was applied to the laser powder bed fusion, the as-built prototype exhibited a high Vickers hardness of ~720 HV, more than twice that of Inconel 718 Ni-based superalloy produced using the same process. • An Fe-based alloy containing 2.0% C, 33.1% Cr, 6.4% Ni and 1.0% Si (mass%) was found to have an almost full eutectic structure. • The eutectic structure consisted of ~80 vol.% of γ-Fe and ~20 vol.% of rod-like chromium carbide of M 7 C 3 type. • As cooling rate during solidification of this alloy increased from ~10 -1 to ~10 7 o C/s, eutectic carbide spacing decreased from ~10 to ~10 -1 μm, and Vickers hardness of the as-solidified alloy increased from ~300 to ~600 HV. • When this alloy was applied to the Laser Powder Bed Fusion process (LPBF process), the LPBF product exhibited ~720 HV, which was more than twice that of a LPBF product of Inconel 718 Ni-based superalloy.