Solute-induced grain refinement and defect suppression in boron-modified molybdenum manufactured via laser powder-bed fusion

Molybdenum manufactured with laser powder bed fusion (LPBF) has an undesirable coarse-grained, columnar microstructure interspersed with intergranular cracks, high porosity, and poor mechanical strength. These defects result from a combination of the harsh LPBF process conditions and the disadvantageous properties of molybdenum, such as its high brittle-ductile transition temperature and low tolerance for oxygen impurities. In order to suppress these defect-forming mechanisms and improve the suitability for LPBF, alloy-side material adjustments with simultaneous process optimization are necessary. In this work, the effect of adjusting Mo by adding 3.5 at.% B is investigated experimentally. Mo-3.5 at.% B specimens can be produced entirely free of cracks, with a density of 99.8%. The specimens have a microstructure of fine, equiaxed grains with an average grain size of 31 μm and an aspect ratio of 1.3, thus achieving substantial refinement of the otherwise typically coarse-grained columnar, anisotropic microstructure of pure Mo in LPBF. Furthermore, the grains possess a honeycomb-like cellular subgrain structure. This structure is formed through the solute rejection effect of B during the solidification and consists of initially solidified pure α-Mo cells with a cell size <1 μm and a honeycomb-like network of an ∼100 nm thick intercellular Mo₂B phase completely covering the α-Mo cells. In addition, the formation of boron oxide inclusions, presumably B₂O₃, with a size of <50 nm within the Mo₂B phase, provides an effective mechanism for scavenging oxygen impurities, thus ensuring segregation-free grain boundaries in Mo-3.5 at.% B. The microstructural modifications substantially improve the mechanical properties. Under appropriate process conditions, with the substrate plate preheating temperature playing a crucial role, a bending strength of 1120 ± 172 MPa and a hardness of 379 ± 24 HV10 at room temperature can be achieved. At a test temperature of 600 °C, an increase in the bending strength to 2265 MPa is observed, and the bending angle simultaneously increases from 2° at room temperature to 35° at 600 °C. These findings indicate that the strength of Mo-3.5 at.% B is limited by the brittle behavior of the material at lower temperatures, at which residual defects are likely to initiate fracture.