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.

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This study aims to investigate the primary carbides precipitation in H13 steel solidified at relatively high cooling rates, ranging from 300 to 6,000 °C•min -1 , based on in situ observations with a high temperature confocal laser scanning microscope. In the cooling rate range investigated, the solidification microstructure becomes more refined as cooling rate increases and the relationship between the secondary dendrite arm spacing (SDAS), λ 2 , and cooling rate, .T, can be expressed as λ 2 =128.45.. Regardless of cooling rates, two kinds of primary carbides, i.e., the Mo-Cr-rich and V-rich carbides, are precipitated along the interdendritic region and most of them are the Mo-Cr-rich carbides. The morphology of Mo-Cr-rich carbide is not obviously influenced by the cooling rate, but that of V-rich carbide is obviously affected. The increasing cooling rate markedly refines the primary carbides and reduces their volume fractions, but their precipitations cannot be inhibited even when the cooling rate is increased to 6,000 °C•min -1 . Besides, the segregation ratios (SRs) of the carbides forming elements are not obviously affected by the cooling rate. However, compared with the conventionally cast ingot, the SDAS and primary carbides in the steel solidified at the investigated cooling rates are much finer, morphologies of the carbides have changed significantly, and SRs of the carbides forming elements are markedly greater. The variation of primary carbide characteristics with cooling rate is mainly due to the change in SDAS.