Primary α phase and its effect on the impact ductility of a high Cr content cast Ni-base superalloy

摘要

A high Cr content cast Ni-base superalloy, K4648, which contains 32-35 wt.% chromium, was prepared by vacuum induction melting, and poured into testing bars or heavy section specimens. The as-cast, heat treated and fractured specimens were investigated by optical metallography, quantitative metallography, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The isothermal solidification followed quenching (ISQ) technique and differential scanning calorimeter (DSC) experiments were also carried out. The result indicated that the primary a phase precipitated from the residual liquid in interdendritic region at 1190 °C near the solidus temperature. This phase can be represented as α-(Cr,Ni) in K4648 alloy because it solid solutionized by about 30 at.% of element Ni. The Vickers microhardness value of primary a phase is 6.3 GPa relative to 1.9 GPa of the 7 matrix at the load of 0.2 N. This phase is brittle and tends to crack due to stress concentration during the solidification process. The primary a phase solutionized or transformed into M_(23)C_6 carbides during solid solution treatment in the range of 1180-1220 °C However, small amounts of a phase remained even at the obvious incipient melting temperature of 1200-1220 °C. Therefore, primary a phase must be controlled during solidification process. The faster cooling during solidification process can decrease the amount of primary a phase in both as-cast and heat-treated K4648 alloys. Specimens cut from slow cooling heavy section castings with large amount of primary a phase possessed an impact ductility of 13.9 J/cm~2, which is much lower than 35.4 J/cm~2 of samples from faster cooling thin section testing bars. The extensive cracked α phase or transformed M_(23)C_6 carbides can be observed at the fracture surfaces and longitudinal sections of impact specimens. Large blocky primary a phase with a network-like distribution, forming as a result of slow cooling, has detrimental effects on the impact ductility of K4648 alloy and it is difficult to completely remove by heat treatment. Proper parameter to obtain faster cooling in the solidification of the alloy must be considered to avoid the detrimental primary a phase precipitation.