Abstract
The purities of melting stocks and solidification conditions control the soundness of single crystal castings of a γ'-strengthened Ni-base superalloy. In this study, the effects of impurities (O, N, C and S) and solidification conditions on the defects such as nonmetallic inclusions, freckle and microporosity in NASAIR 100 single crystal cast through the liquid metal cooling (LMC) method were investigated. The following results were obtained.(1) The impurities suppressed under the values as shown below will not impare the castability of the alloy into single crystals, because they hardly enlarge the melting range for the puriest one.O≤12ppm, N≤33ppm, C≤150ppm, S≤60ppm(2) The above LMC single crystals do not contain any freckle.(3) The melting stocks contaminated by the impurities as shown below transfer nonmetallic inclusions to the above LMC single crystals, such as Al2O3, (Ti, Ta) N, (Ti, Ta) C or (Ti, Ta)xS.O≥12ppm, N≥10ppm, C≥50ppm, S≥20ppm(4) The LMC single crystals embody a very few and small microporosity, totally around 0.1% in areal fraction and each less than 80μm in dia. The impurities in melting stocks listed at (1) will not affect the porosity in castings, while the γ/γ' eutectics originated from the constitutional supercooling under the large product of G and R subdue the evolution of it.
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