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 M₃B₂ 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, M₃B₂ 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 M₃B₂ 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 M₃B₂ boride and 1190-1200 °C for η-Ni₃Ti phase.