Precipitation of metastable intermetallic phases and nanoindentation behaviors of a Ti-20Zr-9 Nb-4Sn (at%) alloy during aging treatment

Abstract

In this study, the microstructure evolution and nanoindentation behavior of a Ti-20Zr-9 Nb-4Sn (at%) alloy aged at 600 ºC for 1–60 h were investigated by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, Vickers microhardness test and nanoindentation technique to show the potential for precipitation strengthening effect of (Zr,Sn)-rich precipitates. The Vickers hardness in the 600 ºC aged specimens increased from 218 HV to the maximum of 242 HV with increasing the aging time from 1 to 13 h. With further prolonging the aging time to 60 h, the hardness decreased to 219 HV at 45 h aging and then almost keep constant at 45–60 h aging. The metastable intermetallic (Zr,Sn)-rich precipitates were formed in all aged specimens and their composition is highly dependent on the aging time as a result of elemental partitioning during aging. In the specimen aged for 24 h, the nanoindentation hardness (18.6 GPa) of the (Zr,Sn)-rich precipitates (Zr3Sn-type) was much higher than those of the α precipitate (9.3 GPa) and matrix (7.6 GPa), which suggests that the hard (Zr,Sn)-rich precipitate should be responsible for the high Vickers hardness in the specimen. These findings show that the precipitation hardening effect in Ti-Zr-Nb-Sn alloys is not just limited to the well-known isothermal ω and/or α precipitates and the (Zr,Sn)-rich precipitates also exhibit great potential for precipitation strengthening effect.