Microporosity reduction in modified RR2072 single crystal superalloys

The effect of the addition of grain boundary strengthening elements (carbon, hafnium) on the microporosity of an experimental single crystal superalloy RR2072 has been investigated. Quantitative examination showed that the volume fraction of microporosity decreased in the alloys modified by these additions, and the reduction was associated with MC phase/carbon distribution and/or high hafnium content. Excessive carbon (i.e. >0.05 wt-%), however, did not reduce microporosity as efficiently as expected. Detailed analysis has revealed that carbon atoms, whether they stay in MC carbides or in the matrix as interstitial atoms, cause lattice expansion and thus reduce volume contraction during solidification. This mechanism is responsible for microporosity reduction in the carbon bearing alloys. Based on this mechanism of interstitial induced lattice expansion, theoretical calculation is consistent with experimental results. In the carbon and hafnium bearing material, however, high hafnium is another important reason for the reduction of microporosity.