Influence of boron and carbon on the shrinkage defects of TiAl alloys

Alloys whose nominal compositions were Ti-47Al-2Cr-2Nb, Ti-47Al-2Cr-2Nb-0.8B and Ti-47Al-2Cr-2Nb-0.1C were vacuum induction skull melted. Ingots were produced by pouring the molten alloys into a room-temperature graphite mold. Chemical compositions of ingots obtained were analyzed respectively. Microstructure as well as shrinkage cavity and porosity were also investigated with stereo microscope and scanning electron microscope (SEM). The results indicate that Ti-47Al-2Cr-2Nb alloy shows an intense columnar-growth tendency, and shrinkage porosity distributes dispersedly in the center zone around axis of ingot. Microstructure is refined and the columnar-growth tendency is weaken with the addition of 0.8% boron (atomic fraction). Shrinkage porosity becomes more concentrated to form a cavity in the center zone of ingots, and the density and size of micro porosity also decrease. However, the addition of 0.1% carbon (atomic fraction) doesn't result in any apparent effects in terms of shrinkage porosity or microstructure compared to ingots without any additive.