Selecting Au-Pt test mass candidate in a narrow composition window: Experiments and first-principles

High-performance Au-Pt alloys are indispensable for precision applications, particularly as test-mass (TM) materials in space-based gravitational-wave observatories. However, the composition-dependent mechanisms governing their properties remain unclear, hindering the design of alloys that simultaneously achieve ultra-low magnetic susceptibility and high hardness. Here, five Au-Pt alloys with 25–29 at% Pt were fabricated under identical processing conditions. Microstructural characterization shows that all solution-treated alloys are single-phase face-centered cubic (FCC) with abundant annealing twins. In Au71Pt29, stacking faults and dislocation tangles are additionally observed in certain regions. Increasing Pt content leads to a marked grain coarsening without pronounced texture. Nanoindentation and magnetic susceptibility measurements reveal a linear increase in hardness from 1.48 to 1.91 GPa, while susceptibility changes modestly from −26.94 to −22.36 ppm. These trends are attributed to strengthened Au-Pt bonding, as shown by the significant increase in -IpCOHP values (from 0.3410 to 0.3533), reflecting stronger bonding due to higher Pt content and increased Pt D-orbital electron density of states near the Fermi level. First-principles calculations of elastic properties and electronic structure show good agreement with the experimental measurements. Our study provides a comprehensive understanding of the composition–structure–property relationships in Au-Pt alloys and offers a solid theoretical foundation for the design of TM materials.