Microstructure and properties of a novel Cu–3Ti–0.1Mg–0.05B–0.05 La alloy with high strength and conductivity

The Cu–Ti alloy has similar mechanical properties and electrical conductivity to the Cu–Be alloy. It also exhibits excellent high-temperature properties and stress relaxation resistance. Therefore, it has emerged as a promising material to replace the toxic Cu–Be alloy. With the technological advances, the new generation of connector materials put forward higher requirements for performance, such as strength over 1000 MPa and conductivity over 15%IACS. However, it is difficult to obtain Cu–Ti alloys with such high strength and conductivity. An effective way is to increase the aging temperature or prolong the holding time of the alloy. When the strength of the alloy is reduced, the increase in cost is inevitable. The refining of grains or the regulation of size and distribution of precipitates has proved more effective, which is also true for Cu–Ti alloys. Currently, the refined grain size is still 10–50 μm achieved through a series of common processing methods, including hot rolling, solid solution, and cold rolling. Therefore, the improvement of strength and conductivity is limited for the Cu–Ti alloy. This paper provides a preparation method for synchronously improving the strength and conductivity of the Cu–Ti alloy. The Cu–3Ti–0.1Mg–0.05B–0.05La alloy with an ultra-fine grain structure is obtained via the vacuum casting and cold billet opening. The secondary aging process is used to adjust the size and distribution of the second phase to obtain a Cu–Ti alloy strip with high strength and good conductivity. The results show that the Cu–3Ti–0.1Mg–0.05B–0.05La alloy displays the maximum microhardness of 356 HV and a conductivity of 14.5%IACS after aging at 400 ℃/2 h. The relationship between the second phase precipitation and properties of the Cu–3Ti–0.1Mg–0.05B–0.05La alloy was analyzed using TEM (Transmission electron microscope). The evolution of the second phase is the Ti-rich phase → the granular phase β′-Cu4Ti phase → the granular β′-Cu4Ti phase + lamellar β-Cu4Ti phase → the lamellar β-Cu4Ti phase. The granular β′-Cu4Ti phase is the most important strengthening phase; the lamellar β-Cu4Ti phase can decrease the strength of the alloy but increase the conductivity. The comprehensive properties of Cu–3Ti–0.1Mg–0.05B–0.05La alloy can be further optimized by the secondary aging process. The microhardness and electrical conductivity of the Cu–3Ti–0.1Mg–0.05B–0.05La alloy reach 341 HV and 20.5%IACS after the primary aging at 450 ℃/8 h + 50% cold rolling + secondary aging at 400 ℃/1 h.