Effect of carbon addition on the microstructure and the mechanical properties of model single crystal nickel-base superalloy

In order to increase efficiency of land based industrial gas turbines, techniques are being developed to produce large single crystal Ni-base superalloys for turbine blades and vanes. During this development, grain defect formation during solidification has become an increasingly important problem. Typically, grain defects, such as freckles and misoriented grains are caused by the onset of thermosolutal convective instabilities during the dendritic solidification. In an effort to reduce grain defects in large single crystal alloys, carbon is intentionally added to the single crystal nickel alloys. A study was made on a model Ni based superalloy, LMSX1, with varying carbon contents. The results show that the alloy tendency to solidification defect formation decreases as the carbon content increases. A study of the as-cast microstructure shows that the amount of γ-γ′ eutectic phase decreases as the carbon content increases. A microprobe analysis using a line scan technique shows that the partitioning coefficient does not change with carbon addition. This means that the reduction of defect formation is not caused by a change in the segregation behavior of alloying elements. The carbides formed in these alloys are mostly of script type MC carbides which form in the interdendritic region. Longitudinal sections of the samples containing high carbon content show a continuous network of script carbides. The dense presence of these carbides in the interdendritic regions of the alloy prevents the thermosolutal fluid flow in these areas. As a result, one of the mechanisms by which defects form is suppressed.; Mechanical properties of the model alloys are greatly influenced by the carbon additions. Creep life was greatly decreased and minimum creep rate was increased with the addition of carbon to the model alloy. Addition of small amount of carbon (∼0.01 wt% carbon) improved fatigue life. However, higher carbon additions (>0.05 wt% carbon) resulted in a significant drop of fatigue life. At high carbon levels, fatigue life reaches a plateau and further carbon additions do not alter fatigue life significantly. Carbon additions resulted in a reduction of tensile elongation except at 0.05 wt% carbon level where elongation is increased. Generally, for all samples, the yield stress decreased with increasing carbon level in the alloy with a higher drop in samples with carbon levels of 0.05 wt%.; Stability of the model alloys were studied by long-term thermal exposure of the samples. Carbon additions have more detrimental effect on creep properties of exposed samples compared to non-exposed samples. Minimum creep rate increases at a higher rate for exposed samples compared to non-exposed samples. Exposure also affected fatigue properties and fatigue life was reduced at all carbon levels. A plateau of fatigue life versus carbon addition was reached at high carbon levels (>0.10 wt% carbon).