Isothermal Aging Behaviors of Copper–Titanium–Magnesium Supersaturated Solid-Solution Alloys

Abstract

Herein, the isothermal aging behavior of copper–titanium–magnesium (Cu–Ti–Mg) supersaturated solid-solution alloys, with different compositions, under test conditions of 450°C for 100 h, has been thoroughly investigated in a comparative study using various electron microscopy and microanalytical techniques. The Vickers hardness and electrical conductivity of the ternary alloys were recorded at slightly elevated (during aging) and reduced levels than their binary counterparts without Mg doping. Hence, it is proposed that the hardness and conductivity values are approximated from the superposition effect of precipitation hardening stimulated by Ti solutes and solution hardening by both Ti and Mg solutes. Furthermore, the tensile tests for these ternary specimens have demonstrated that Mg doping has a substantial effect on the improvement of the tensile strength and fracture elongation properties of binary Cu–Ti alloys. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy imaging combined with atomic-resolution energy-dispersive X-ray spectroscopy mapping analysis confirmed that the same metastable precipitate phase is responsible for peak hardening in ternary and Cu–Ti binary alloys. In addition, a large part of the Mg solutes is homogeneously distributed over the matrix regions, while there is also a smaller part of those present in the precipitates. The potential effects of Mg doping on the microstructures of Cu–Ti alloys were elucidated and the structural environment, which may yield relatively high mechanical properties, was discussed using the aforementioned observations.

Fig. 10 (a) Atomic-scale HAADF-STEM image of a single precipitate particle of the β′ crystal present in the Cu–2Ti–2Mg PA specimen, which was taken in the [001]_α incidence of the α-Cu FCC matrix, together with (b) the digital Fourier diffractogram of the lattice image in the inset.