Concurrent toughening and hardening in AgMgNi alloys by internal oxidation

Abstract

Enhancing the ductility of internally oxidized AgMg alloys has posed a longstanding challenge. A new method to achieve simultaneous hardening and toughening of AgMgNi alloys is presented by means of internal oxidation. The influence of Ni content on the internal oxidation process and the mechanical behavior of AgMgNi alloys is systematically investigated. It is found that Ni addition induces grain refinement by forming nanoscale Ni particles, which act as heterogeneous nucleation sites and inhibit grain growth during internal oxidation. This enhances the plasticity and toughness of the alloys via the Hall–Petch effect. The alloys exhibit a conductivity of ~ 42 MS·m⁻¹ and surface hardness of ~ HV 125, which are insensitive to the variation of Ni content within 0 wt%–2 wt%. The optimal range of Ni content for achieving the best combination of hardness, strength and toughness is 0.15 wt%–0.3 wt%, corresponding to alloys with a tensile strength above 300 MPa and a toughness surpassing 3300 MJ·m⁻³. Higher Ni contents reduce the internal oxidation depth (from about 340.6 to about 238.4 μm) and the tensile strength (from about 342.1 to about 230.1 MPa) of the alloys by generating micrometer-sized Ni-rich particles in the matrix, which consume oxygen, obstruct some of the oxygen diffusion channels and impede the oxidation front advancement. The non-oxidized region, which does not benefit from oxidation strengthening, diminishes the overall strength of the alloy. These results reveal the crucial role of Ni in regulating the internal oxidation dynamics and microstructure evolution of AgMgNi alloys, and suggest a novel approach for designing high-performance alloys with concurrent hardening and toughening.

Graphical abstract