Suppression of Discontinuous Precipitation in Cu-Ti Alloys by Aging in a Hydrogen Atmosphere

Abstract

Age-hardened Cu-Ti alloys are widely used as conductive materials in small electrical devices, so it is important for them to have suitable mechanical and electrical properties. One of the principal guidelines for improving the mechanical properties of Cu-Ti alloys is to suppress the formation of coarse discontinuous precipitates within them. Herein, we investigated the microstructure and properties of a Cu-4 at. pct Ti alloy aged under hydrogen pressures of 0 to 0.6 MPa, focusing on discontinuous precipitation at the grain boundaries. During the early stages of aging, fine needle-shaped metastable β′-Cu4Ti precipitates formed continuously in the matrix. With further aging, these β′-Cu4Ti precipitates continued to nucleate and grow, and simultaneously cellular components composing of the lamellae of Cu solid solution and the stable β-Cu4Ti discontinuous precipitates were formed at grain boundaries. Nevertheless, the formation of these cellular components was significantly suppressed when aging under hydrogen pressures of above 0.03 MPa. Upon aging under higher hydrogen pressures (> 0.10 MPa), some granular TiH2 precipitates formed, and the preformed β′-Cu4Ti precipitates decomposed. Upon aging under 0.03 MPa hydrogen pressure, fine metastable β′-Cu4Ti precipitates were formed dominantly, because the formation of stable β-Cu4Ti laminates was suppressed and the TiH2 precipitates was still not formed. Hence, the hardness was maintained under over-aging conditions.