Grain refinement of a gamma-based titanium aluminide using microalloy additions
The influence on the as-cast grain size of a range of alloying additions to some TiAI-based
alloys has been assessed using optical microscopy, analytical scanning and analytical
transmission electron microscopy. It has been found that additions of boron, either as TiB2
or AlB 12,resulted in significant grain refinement, but of the other additives (yttrium, yttria,
silicon and sulphur) only sulphur had any significant effect. Measurements of tensile
properties of the grain refined alloys showed a significant improvement, and creep rates
were not degraded. Failure of the fully lamellar structure during tests was found to occur by
the splitting of grains, in the hard orientation, parallel to the lamellae.
Determination of the nature of B-containing precipitates in the alloys, formed at high cooling
rates, showed that they were a eutectic structure of either TiB and B2 or TiB2 and B2
depending upon alloy composition. At lower cooling rates TiB2 usually formed, unless Ta
was present in the alloy in which case a complex structure of TiB and TaB formed. In all
cases, irrespective of the alloy or boron addition method, a minimum concentration of about
0.6 at.% B was required to achieve an equiaxed grain structure in plasma-melted ingots. At
lower concentrations the dendrite arm spacing and columnar grain width are reduced. In Ta
and W-containing alloys there is a marked tendency for the borides to form large (400J.1m)
clusters. The size and frequency of these clusters can be influenced by the sequence and the
form in which the various alloying additions are made.
The mechanism by which grain refinement occurs is the rejection of boron from the liquid
during solidification, leading to local high concentrations of boron at the roots of the
secondary dendrite arms, which under normal casting conditions then break off and act as
nuclei for equiaxed grains. The boron is then precipitated, as eutectic phase, in the
interdendritic regions. Under quiescent casting conditions grain refinement does not occur
with concentrations of boron sufficient to refine plasma-melted material, although again the
boron precipitates interdendritically.
Grain refined samples forge with very little cracking under conditions which result in
extensive cracking in non grain refined samples. Zener pinning of the grains by the borides
ensures that grain growth in the forged and forged plus heat treated material is very slow.
These results are discussed in terms of the importance of grain refinement in TiAI-based