The Minor Precipitation at the Final Stage of U720Li Solidification

The minor precipitations caused by B and Zr which are the normal constituents of U720Li alloy have been studied by analyzing the solidification process and the composition evolution. The present study aims to supply the elementary information about the existing form of B and Zr in the as-cast microstructure, which is helpful for the subsequent processing, such as homogenization treatment. The M 3B2 and Ni5Zr phases were observed in the U720Li alloy in as-cast state, which were usually accompanying with each other together with η-Ni3Ti phase at the edge of eutectic (γ + γ′). Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following: γ matrix, eutectic (γ + γ′), η-Ni3Ti, M 3B2 and Ni5Zr. The in situ composition analysis by EDS and EPMA revealed that the precipitation and microstructure were governed by the composition evolution in the liquids. The solidification of γ matrix increased the Ti concentration in the residual liquids and resulted in the eutectic (γ + γ′) formation; the (γ + γ′) formation increased the Ti/Al radio in the liquids and the η-Ni3Ti was formed in front of the eutectic (γ + γ′); the η-Ni3Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M 3B2 boride is formed; the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni5Zr is formed finally. The melting points are in the ranges of 1130–1140 °C for Ni5Zr phase, 1180–1190 °C for M 3B2 boride and 1190–1200 °C for η-Ni3Ti phase.