Origins of heterogeneities in plasma melted ingots of γ-TiAl

Ingots of a TiAl-based alloy Ti–48Al–2Mn–2Nb (at.%) have been produced by cold hearth plasma melting using single, double and triple melting in order to compare the heterogeneity in finished ingots and to determine the origins of any observed heterogeneities. The feedstock for the single–melted ingots was compacts made up of Ti sponge, Al powder and master alloys of Mn and Nb. Glow discharge spectrometry (GDS) and energy dispersive X-ray (EDX) analysis have been used as the main tools to assess the elemental distribution in ingots. The average standard deviation in the Al concentration was found to be 1.05 at.% for a single melted ingot along its length and diameter. The standard deviation of the concentration in ingots produced by double or triple melting was below detectability level of the glow discharge spectrometer which for Al varied from 0.05 to 0.40 at.%. It has been found that the macrosegregation patterns observed in the single melted ingots were principally due to heterogeneities in the original feedstock being carried over into the crucible, i.e. due to melting-induced segregation. The patterns of heterogeneities were not consistent with segregation occurring during solidification. Double melting or triple melting reduces the extent of heterogeneity. It is argued that the key to producing homogenous ingots is to have homogeneous liquid metal before pouring to the crucible.