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

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Thakur, Anurag
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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 $\gamma\prime$ particles respectively. The volume-fraction of primary $\gamma\prime$ 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 $\gamma\prime$ 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 $\gamma\prime$ particles, degeneration and dissociation of primary carbide particles to form secondary $\gamma\prime$ 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 $\gamma\prime$ particles. The overaged microstructure for the 1175$\sp\circ$C and 1225$\sp\circ$C consisted of a unimodal distribution of degenerate $\gamma\prime$ particles. The activation energy for the coarsening of secondary $\gamma\prime$ 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.)