Control of nitrogen content in SPS-consolidated Nb-matrix composites via pressure-assisted mechanical alloying

Niobium-matrix composites reinforced with nitrides exhibiting superior mechanical properties have been developed. Nb powder was mechanically activated by high-energy ball milling under 3 bar N 2 . Longer milling times (2–40 h) increased nitrogen uptake and enabled the control of a Nb–nitride amount after spark plasma sintering (1500 °C, 10 min). The microstructure comprises a niobium matrix with β -Nb 4.62 N 2.14 , which gradually transforms into γ –Nb 4 N 3 as milling time increases. Moreover, as the β -phase content rises from 3 vol% to 26 vol%, precipitates evolve from thin platelets along grain boundaries to spheroidal particles. Among the tested conditions, the composite milled for 2 h showed the best balance of strength and ductility, achieving a plastic strain of 17.6%, a yield strength of 1294 MPa, and a compressive strength of 2170 MPa. Prolonged milling (above 5 h) maintains a high compressive strength (2185 MPa) but reduces plasticity, consistent with the higher fraction of brittle nitride phases. Moreover, composites milled for 2–5 h and then SPS-ed revealed superior performance at 800 °C, resulting from an optimized two-phase microstructure, in which a continuous and ductile α -Nb matrix cooperates with a moderate fraction of hard β -Nb 4.62 N 2.14 nitride reinforcement.