Detrimental effect of cellular precipitation on the creep strength of Inconel740H

Cellular precipitation (also known as discontinuous precipitation) has been observed at the grain boundaries of a newly developed nickel-based Inconel740H alloy designed for use at 700?°C in advanced ultrasupercritical coal-fired power plants. By means of element mapping and selected area diffraction, the cellular precipitates were identified as Cr-rich M₂₃C₆ carbides. The onset of cellular precipitation was found to follow a pucker mechanism in Inconel740H. The cellular precipitates at the grain boundaries, even at low volume fractions, were severely detrimental to the creep strength at 750?°C. The creep rupture life of Inconel740H containing cellular precipitates at grain boundaries was only one-tenth of that for the alloy without cellular precipitates. The reason for the drastically decreased creep rupture life is attributed to the poor resistance of cellular precipitates to crack propagation during creep.     

Nano-size solidification microconstituents in electro-spark deposited Ni-base superalloy

The microstructure of electro-spark deposited IN 718 was investigated by analytical transmission electron microscopy. The analysis shows in contrast to general assumption, extreme rapid cooling rates involved in electro-spark deposition (ESD) did not produce partitionless solidification, devoid of second phase microconstituents in the material. Nanosized Laves and MC carbide particles were formed in the interdendritic regions of IN 718 electro-spark weld deposit, which indicate the occurrence of segregation-induced phase formation during solidification of the weld. The formation of Laves phase particles, which is known to be deleterious to the mechanical properties of IN 718, requires adequate consideration in the application of IN 718, and possibly other Nb-bearing nickel-base superalloys, processed by ESD.