Interplay of intermetallic precipitation and nano Y2O3 dispersion on microstructure evolution and strengthening in hydrogen sintered Mo-Ni-Si-Co alloys

Abstract

This work investigates liquid-phase-sintered Molybdenum (Mo) alloys, with and without Y2O3 dispersion, that integrate enhanced hardness and significant strength. Six alloys with compositions of Mo80Ni10Si10 (S1), Mo80Ni10Co10 (S2), Mo80Ni10Si5Co5 (S3), Mo79Ni10Si10(Y2O3)(1) (S4), Mo79Ni10Co10(Y2O3)(1) (S5), and Mo79Ni10Si5Co5(Y2O3)(1) (S6) (wt%) were consolidated by pressureless multistage H-2 sintering with maximum sintering temperature of 1500 degrees C for 90 min. Y2O3 persisted as a stable nanometric dispersion (<50 nm) refining grains in S6, with Mo3Si intermetallics (<90 nm) in S3. All alloys showed a Mo-rich bcc phase along with MoNi, Ni2Si, Mo3Si, and Mo3Co2. When paired with appropriate Si/Co partitioning, Y2O3 addition increased relative density to 96.15% in S6 compared to non-Y2O3 dispersed S1 (77.72%). The distortion parameter reduced from 2.41 (S1) to 0.09 (S6), Mo grain contiguity from 0.62 (S1) to 0.42 (S6), and dihedral angle from 71.28 degrees (S1) to 52.45 degrees (S6), illustrating superior densification and nominal geometric instability during sintering. HRTEM revealed a lattice misfit of -25.04% between the Mo matrix (110) and Y2O3 (213) in S6, indicating an incoherent interface. S2 exhibits outstanding hardness (11.78 +/- 0.58 GPa), maximum indentation fracture toughness (4.35 MPa root m), and S6 achieved the lowest pore vol% (6.68%), maximum compressive strength (1083.8 MPa), compressive strain (14.6%), and outstanding product of strength and elongation (PSE) (15.02 GPa %). These enhancements result from the synergistic interaction of regulated silicide precipitation, solid-solution hardening in the Ni-Co matrix, Y2O3 particle strengthening, Hall-Petch grain refinement. Intragranular dispersion strengthening of Y2O3 supersedes other strengthening contributions in S6, thereby achieving a significant strength/ductility trade-off.