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Massive nanoprecipitation in an Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition

In this project we study the development of a maraging steel alloy consisting of Fe, Ni and Al, that shows pronounced response to the intrinsic heat treatment imposed during Laser Additive Manufacturing (LAM). Without any further heat treatment, it was possible to produce a maraging steel that is intrinsically precipitation strengthened by an extremely high number density of 1.2x1025 m-3 NiAl nanoparticles of 2‑4 nm size. The high number density is related to the low lattice mismatch between the martensitic matrix and the NiAl phase.

Due to the layer-by-layer build-up of additively manufactured parts, the deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3-5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 1025 NiAl precipitates per m3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment.

(a). Precipitates visualized by drawing an isoconcentration surface at 15 at% Al. (b) Corresponding proximity histogram plotting the chemical composition as a function of the distance to the isoconcentration surface. Expected Al concentrations for NiAl and Ni3Al are shown.

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