## LSU Historical Dissertations and Theses

1998

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Mechanical Engineering

Aravamudhan Raman

#### Abstract

Three major aspects of the superalloy IN738LC are studied in this dissertation: (i) precipitate microstructure evolution, (ii) tensile mechanical properties, and (iii) thermal expansion characteristics. IN738LC is a Ni-base, $\gamma\sp\prime$ Ni$\sb3$Al(Ti) precipitate-strengthened alloy. This alloy includes $\sim$20-43 volume % $\gamma\sp\prime$ and also some (Ta,Ti)C at the grain boundaries and in the matrix. Different heat treatments were carried out to study the precipitate evolution mechanisms and kinetics in this superalloy. It was found that the proper solution treatment producing the single-phase supersaturated solid solution condition is 1235$\sp\circ$C/4h/WQ. Solution treatment at 1200$\sp\circ$C/4h/WQ, however, produces a refined precipitate microstructure. Aging treatments after both of these solution treatments give similar microstructures. The precipitates grow in cuboidal form after agings at 1120-1130$\sp\circ$C/24h. However, longer aging times ($>$48h) yield a duplex-size precipitate morphology in this temperature range. Likewise, the duplex-size morphology develops at 1140$\sp\circ$C, but in a much shorter time (5 min.), and it is stable in the range 1140-1150$\sp\circ$C. Activation energy calculations showed that the precipitate microstructure becomes more unstable and it dissolves into the matrix when the temperature increases above 1150$\sp\circ$C, and in the range 1160-1225$\sp\circ$C, a unimodal stable, fine-size precipitate microstructure develops. XRD studies showed that different precipitate microstructures possess different preferred orientations. Precipitates tend to have the $\{$220$\}$ or $\{$200$\}$ orientation. Whereas most of the precipitate microstructures have single texture, the duplex-size precipitate microstructure has the $\{$111$\}$ and $\{$200$\}$ preferred orientations for the matrix and the precipitates, respectively. Tensile mechanical and thermal expansion tests proved that the size and morphology of the precipitate phase is very effective in determining the magnitude of these properties. Generally, microstructures having fine-size precipitates show higher yield strength, and elasticity modulus; in contrast, microstructures with coarser precipitates exhibit more elongation, strain-harden more, and acquire higher tensile strength. Tensile fracture is usually of the cleavage type with fine precipitates and ductile with the coarse ones. Thermal expansivity of the microstructures with fine precipitates is in general less than that of the microstructures with coarse precipitates.

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