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UNDERSTANDING THE FUNDAMENTAL MECHANISMS OF MESO-SCALE ENGINEERING OF GRAIN BOUNDARIES IN NI-BASE SUPERALLOYS
Title
UNDERSTANDING THE FUNDAMENTAL MECHANISMS OF MESO-SCALE ENGINEERING OF GRAIN BOUNDARIES IN NI-BASE SUPERALLOYS
Creator
Alabbad, Bader
Date
2019
Description
Polycrystalline nickel base superalloys are known to possess high temperature strength, good crack growth resistance and corrosion resistance...
Show morePolycrystalline nickel base superalloys are known to possess high temperature strength, good crack growth resistance and corrosion resistance at high temperatures. Engineering the morphology of the grain boundaries can yield to extent the high temperature resistance for polycrystalline nickel-based superalloys. Meso-scale engineering for grain boundary structures can be utilized through the development of serrated grain boundaries to improve the damage tolerance of polycrystalline nickel- based superalloys. Grain boundary precipitates induce the formation of the grain boundary serrations as precipitates protrude the grain boundary. Understanding and controlling the cooling processes from above the solvus temperature plays an important role in controlling the size and population of γ′ precipitates to modify specific microstructural features that can impact the resulting properties. Two stepped cooling process with a transition just below the γ′ solvus has the ability to form a unique microstructure as heterogeneous nucleation and growth of large γ′ precipitates along the grain boundaries during the initially slow cooling rate. The eventual transition to a standard or conventional cooling rate will then govern the formation of the secondary, intragranular γ′ precipitates. The stepped cooling process was compared to the conventional fast continuous cooling process. The effect of grain boundary misorientation on the precipitation behavior of grain boundary γ′ precipitates revealed coarse grain boundary γ′ precipitates reside along high angle grain boundaries. Mechanical testing was carried out to investigate their mechanical properties to compare and benchmark to those produced via conventional continuous cooling to quantify the extent by which serrated boundaries can be used to improve the properties where the modified microstructures led to great high temperature properties.Nickel-base superalloy 718Plus is a recently developed alloy that is being considered for use in a number of advanced gas turbine engine applications. A constructed heat treatment was proposed to drive the precipitation of fine η precipitates along grain boundaries. Lamellar η phase precipitates were able to induce the formation of serrated grain boundaries. Discontinuous cellular precipitation reaction led the formation of the lamellar η phase precipitates. The characteristic growth of lamellar η phase precipitates was influence by the grain boundary structure. Creep tests were carried out to provide a better understanding of how serrated grain boundaries formed using η phase precipitates can impact the creep properties of ATI 718Plus.
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