Synthesis mechanism of Ni-Co alloy uniting ultrahigh strength with tensile ductility via abrasive-assisted electroforming

Persistent efforts are being made to combine strength and ductility in metallic nanomaterials. We successfully prepared a homogeneous nanocrystalline Ni-27%Co alloy exhibiting ultrahigh strength and sufficient ductility using abrasive-assisted electroforming (AAE) with free ceramic beads. The AAE process permits superior control of the microstructure of nanocrystalline Ni-Co alloy. Our results demonstrated that the structure of the Ni-27% Co alloy (AAE) consists of ultrafine equiaxed nanocrystals with an average grain size of 88 nm and a twin boundary fraction of 24%. The alloy structure results from the combined effect of the addition of Co as an alloying element and the electrochemical and physical action of the moving ceramic beads on the nucleation and growth. The Ni-27%Co alloy (AAE) exhibited superior yield and tensile strength of 1103 +/- 47 and 1639 +/- 19 MPa, respectively, while having excellent ductility with tensile elongation to failure of 8.5 +/- 0.5%. The yield strength of the Ni-27%Co alloy (AAE) was approximately 1.6 times that of the Ni-26%Co alloy (Direct current electroforming, DCE). This result can be attributed to the contribution of strengthening mechanisms that involve frictional stress, dislocation, grain boundary, solid solution, and twin boundary strengthening. Among them, the grain boundary (Hall-Petch) strengthening, arising from grain boundary-dislocation interactions, and twin boundary strengthening, arising from the intersection of the twin-dislocation, account for approximately 70% of the total yield strength. The tensile ductility of the Ni-27%Co alloy (AAE) at ultrahigh flow stress can be pre-served by simultaneous strain and strain-rate hardening using the AAE method. This study provides a novel strategy for synthesizing ductile ultrahigh strength materials for applications in microelectromechanical systems, surface coatings, and X-ray telescopes.

Automated summary

Persistent efforts are being made to combine strength and ductility in metallic nanomaterials. Homogeneous nanocrystalline Ni-27%Co alloy with ultrahigh strength and sufficient ductility was successfully prepared using abrasive-assisted electroforming (AAE) with free ceramic beads.