Directional coalescence of ' precipitates during long time aging of CoNiAlW superalloy

The abnormal directional growth or coalescence of the strengthening gamma' phase in CoNiAlW superalloy during aging at 900 degrees C in the absence of applied stress have been investigated in this research. The microstructural evaluations depicted that gamma' precipitates which were close enough together coalesced during long time aging. This abnormal directional growth has not been observed, previously. The elastic strain energy was recognized as the governing driving force for gamma' precipitates coalescence. This was verified by adding Cr atoms to the superalloy that caused a reduction in the lattice misfit between gamma and gamma' phases. The mechanism of directional coalescence of gamma' precipitates is proposed in this research. Furthermore, the critical gamma channel width (h*), that gamma' precipitates coalescence occurs for values smaller than h*, in terms of the lattice misfit (delta), the interfacial energy (S gamma/gamma '), and the volumetric free energy for the formation of gamma' precipitate (Delta Gv) was described. The results show that in this superalloy with positive misfit, the extent of coalescence of gamma' precipitates decreases with decreasing lattice misfit.