Microstructural responses of a nickel-base cast IN-738 superalloy to a variety of pre-weld heat-treatments

This dissertation reports an investigation involving a thorough characterization of the microstructural response of cast IN-738 to a variety of pre-weld heat-treatment schemes. The as-received material showed cored dendritic microstructure, containing coarse grains, about 0.5% casting micropores, 0.7% primary carbide particles, and 42% primary γ\prime particles respectively. The volume-fraction of primary γ\prime particles decreased continuously from 20.1% to about 5% for solution-treatment temperatures (STT) in the range of 1120$\sp\circ$C-1225$\sp\circ$C. The solvus of primary γ\prime particles was higher than the 1160$\sp\circ$C-1175$\sp\circ$C range suggested by Steven and Flewitt (27). The brine-quenched samples from the STT manifested extensive intergranular cracking and aging contraction stresses. Aging resulted in coarsening of primary and secondary γ\prime particles, degeneration and dissociation of primary carbide particles to form secondary γ\prime particles andfine $\rm M\sb{23}C\sb6$ carbide particles on the grain-boundaries. Only long-term aging showed the presence of these secondary carbide particles in the grain interiors. Aging at 845$\sp\circ$C, after solution-treating at 1120$\sp\circ$C showed a distinct bimodal distribution of γ\prime particles. The overaged microstructure for the 1175$\sp\circ$C and 1225$\sp\circ$C consisted of a unimodal distribution of degenerate γ\prime particles. The activation energy for the coarsening of secondary γ\prime particles using the LSEM theory was 241 KJ/molK, in reasonable agreement with the 259KJ/molK calculated by Henderson et. al. (45). Grain-boundary $\rm M\sb{23}C\sb6$ carbide particles have a solvus of about 1025$\sp\circ$C and liquate/dissolve at higher temperatures. The liquation of low-melting continuously linked fine carbide particles on the boundaries was another sole cause for intergranular quench cracks. The coherency strains are usually responsible for heat-affected zone microfissuring in this material and were found to be always lower for the overaged material as compared to the peak-aged and the underaged material. (Abstract shortened by UMI.)

Author dc:creator

• Thakur, Anurag

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1993/932

OAI identifier oai:identifier

oai:mspace.lib.umanitoba.ca:1993/932