Powder metallurgy processing and deformation characteristics of bulk multimodal nickel

• Bulk multi-modal Ni was processed by SPS from a powder blend.
• Ultrafine-grained matrix or rim observed around spherical microcrystalline entities
• Yield strength (470 MPa) and ductility (2.8% plastic strain) were measured.
• Debonding was found at the matrix/microcrystalline entity interfaces.
• In-situ TEM showed twinning, dislocation emission and annihilation at grain boundaries.

Spark plasma sintering was used to process bulk nickel samples from a blend of three powder types. The resulting multimodal microstructure was made of coarse (average size ~ 135 μm) spherical microcrystalline entities (the core) surrounded by a fine-grained matrix (average grain size ~ 1.5 μm) or a thick rim (the shell) distinguishable from the matrix. Tensile tests revealed yield strength of ~ 470 MPa that was accompanied by limited ductility (~ 2.8% plastic strain). Microstructure observation after testing showed debonding at interfaces between the matrix and the coarse entities, but in many instances, shallow dimples within the rim were observed indicating local ductile events in the shell. Dislocation emission and annihilation at grain boundaries and twinning at crack tip were the main deformation mechanisms taking place within the fine-grained matrix as revealed by in-situ transmission electron microscopy. Estimation of the stress from loop's curvature and dislocation pile-up indicates that dislocation emission from grain boundaries and grain boundary overcoming largely contributes to the flow stress.