The Minor Precipitation at the Final Stage of U720Li Solidification

Abstract

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 3 B 2 and Ni 5 Zr phases were observed in the U720Li alloy in as-cast state, which were usually accompanying with each other together with g-Ni 3 Ti phase at the edge of eutectic (c ? c 0 ). Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following: c matrix, eutectic (c ? c 0 ), g-Ni 3 Ti, M 3 B 2 and Ni 5 Zr. 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 c matrix increased the Ti concentration in the residual liquids and resulted in the eutectic (c ? c 0 ) formation; the (c ? c 0 ) formation increased the Ti/Al radio in the liquids and the g-Ni 3 Ti was formed in front of the eutectic (c ? c 0 ); the g-Ni 3 Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M 3 B 2 boride is formed; the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni 5 Zr is formed finally. The melting points are in the ranges of 1130-1140°C for Ni 5 Zr phase, 1180-1190°C for M 3 B 2 boride and 1190-1200°C for g-Ni 3 Ti phase.

Transformation Mechanism of (γ + γ′) and the Effect of Cooling Rate on the Final Solidification of U720Li Alloy

“…c 0 ) under SEM observation ( Fig. 2b), which is consistent with our previous study [4]. Interestingly, these three minor phases were usually close to each other and the borides were always attached to the g phases.…”

“…The in situ composition analysis by EDS (Table 1) suggests that the borides are mainly composed with Mo and Cr, which is in agreement with the element distribution determined by EPMA and the previous reports [12][13][14]. The Zr-bearing particles are mainly composed with Ni and Zr, which is consistent with the previous studies [4,15]. The g phase is highly enriched in Ni and Ti, and similar results have been reported in the literature [16].…”

“…The g phase is highly enriched in Ni and Ti, and similar results have been reported in the literature [16]. Besides, Zr is also dissolved in the g phase obviously, and similar results have been found in our previous study [4]. The coarse c 0 at the cap-like part of the (c ?…”

“…Ti) addition naturally causes the severe segregation and the heavy precipitation of eutectic (c ? c 0 ) in the interdendritic area [4]. It is well known that the eutectic (c ?…”

The role of boron in modifying the solidification and microstructure of nickel-base alloy U720Li

“…3b). According to our previous study, the h phase and Zr-rich phase are precipitated after the eutectic (g þ g 0 ) [37]. Since h phase and eutectic (g þ g 0 ) are both enriched in Ti, the formation of eutectic (g þ g 0 ) led to the decrease of Ti content in the residue liquid.…”