Microstructural features underpinning the mechanical behavior of powder metallurgy Cr-based alloys

Refractory materials such as Cr-based alloys offer the potential of enhanced elevated temperature performance but have been limited by their poor formability. However, powder metallurgy has been shown to be a viable pathway to fabricate these alloys. Nano phase separation sintering (NPSS) has been used in the literature to accelerate the densification of pressed and sintered Cr- and W-based alloys. Here, we explore microstructure evolution during NPSS in a binary Cr 85 Ni 15 alloy consolidated via (i) cold pressing & pressureless sintering and (ii) hot isostatic pressing followed by hot extrusion & hot upsetting, and the role of these different processing routes on resulting material properties. The alloy lacked room temperature tensile ductility regardless of consolidation process, with multi-length scale characterization, including scanning electron microscopy, transmission electron microscopy, atom probe tomography, X-ray diffraction, and uniaxial tensile testing, revealing that brittleness was due to intrinsically poor Cr grain boundary cohesion. Tensile testing conducted at 760 °C showed marked strength reduction for the extruded & upset (94%) condition compared to the pressed & sintered (18%). The formation of orthorhombic CrNi 2 intermetallics functioned as precipitate strengtheners and prevented elevated temperature softening in the pressed and sintered condition. The findings offer foundational insights into aiding the future development of Cr-based alloys. • Impurity embrittlement in Cr can be mitigated with a duplex microstructure. • Cr-alloy brittleness is due to poor BCC grain boundary cohesion. • CrNi 2 intermetallic laths reduce high-temperature softening in Cr grains. • A new pathway for CrNi 2 nucleation from BCC grains was identified. • CrNi 2 nucleation from BCC kinetically favorable compared to ordering from FCC.