Microstructure Evolution and Its Influence on Thermal Expansion and Tensile Properties of the Superalloy IN738LC at High Temperatures.

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\sb3Al(Ti) precipitate-strengthened alloy. This alloy includes $\sim$20-43 volume % \gamma\sp\prime and also some (Ta,Ti)C at the grain boundaries and in the matrix. Different heat treatments were carried out to study the precipitate evolution mechanisms and kinetics in this superalloy. It was found that the proper solution treatment producing the single-phase supersaturated solid solution condition is 1235\sp\circC/4h/WQ. Solution treatment at 1200\sp\circC/4h/WQ, however, produces a refined precipitate microstructure. Aging treatments after both of these solution treatments give similar microstructures. The precipitates grow in cuboidal form after agings at 1120-1130\sp\circC/24h. However, longer aging times ($\u3e$48h) yield a duplex-size precipitate morphology in this temperature range. Likewise, the duplex-size morphology develops at 1140\sp\circC, but in a much shorter time (5 min.), and it is stable in the range 1140-1150\sp\circC. Activation energy calculations showed that the precipitate microstructure becomes more unstable and it dissolves into the matrix when the temperature increases above 1150\sp\circC, and in the range 1160-1225\sp\circC, 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

Fine Precipitates in Nickel Base Superalloys

Presence of fine, secondary/tertiary precipitates in superalloys improves especially the creep-fatigue properties of these alloys. It is conveniently accepted that the fine precipitates form non-isothermally, for example, during cooling from an aging temperature or isothermally during a secondary, lower temperature aging. In the current study, several single-aging treatments were conducted to assess the formation of the fine precipitates in the polycrystalline, nickel-base superalloy IN738LC. The agings were carried out stress-free at 950oC, 1050oC, 1120oC, and 1140oC for various times. Stressed agings at 950oC and 1050oC were also conducted. A time-dependent isothermal formation of the fine precipitates was observed. The formation time decreased as the aging temperature increased. It is suggested that dissolution of some coarse precipitates, evolution of the precipitate-matrix interface toward a fully faceted one, and increased matrix channel width saturate the channels and control the formation of the fine precipitates

A Novel Multilevel Converter Used For Three Phase STATCOM under Unbalanced Load

In this paper, multi-level cascaded Static Synchronous Compensator (STATCOM) is proposed using a novel converter having reduced number of switches. STATCOM is analyzed by using DSP and Matlab, interactively. The level shifted carrier based SPWM method, instantaneous single phase active (P) and reactive (Q) power algorithms are applied to control this converters. The performance of STATCOM is investigated for unbalanced reactive load conditions. The simulation results of Matlab alone and Matlab interactively operated with DSP TMS320F28335 are presented