During the fabrication of a FeCoMo alloy with laser powder bed fusion (LPBF), cracks are formed in the material. Coarse grains due to epitaxial growth, the brittle Fe-Co ordering phase, in-situ precipitation of the strengthening -phase and/or embrittlement by Si-oxides were identified as possible reasons for cracking. In this thesis, the nanostructure of the LPBF FeCoMo alloy was studied with atom probe tomography (APT). Two as-built conditions were investigated: one without in-process platform preheating and one with a preheating temperature of 500 �C. The results showed that Mo and impurity elements segregated to interdendritic spaces during solidication in LPBF and formed Mo-rich chain-like structures in the APT reconstruction. These structures were also observed with scanning electron microscopy SEM, but they could not be characterised due to the limited spatial resolution of a SEM. The Mo-segregations were partially dissolved by an intrinsic heat treatment (IHT) during LPBF. The precipitation behaviour during LPBF was investigated for the two conditions. When no preheating was applied, no precipitation occurred, however early stages of spinodal decomposition were observed. It is known that spinodal decomposition is the first stage of precipitation in the investigated FeCoMo alloy. With in-process platform preheating, various stages of precipitation were present in the as-built condition, from spinodal decomposition to early stages of particle growth. As a result, the hardness of as-built specimens increased with incremental distance from the top layer. The existence of precipitates in the preheated condition is also in agreement with preceding differential scanning calorimetry (DSC) measurements. Dilatometer measurements showed that no precipitation occurs from the austenite phase, hence the microstructure during LPBF was (at least partially) martensitic. In order to take into account the temperature increase at the tip apex during laser-APT and to depict the geometry of precipitates more accurately in the APT reconstruction, kf was adjusted based on a tip profile-based reconstruction using a scanning electron microscopy (SEM) image. As no precipitation occurred without in-process preheating during LPBF and the as-built specimen still cracked during the process, ?-phase precipitation is not the main reason for the cracking of the FeCoMo alloy during LPBF. With SEM, Si-oxides were observed to be homogeneously dispersed in the matrix. In APT, nm-sized Si-oxides were found, which exhibited a spherical geometry after kf was adjusted. These Si-oxides could be one reason for cracking.