Advanced aviation gas turbine engines will require disk superalloys that can operate at higher temperatures and stresses than current conditions. Such applications will be limited by the tensile, creep, and fatigue mechanical properties of these alloys. These mechanical properties vary with the size, shape, and quantity of the g' precipitates that strengthen disk superalloys. It is therefore important to quantify these precipitate parameters and relate them to mechanical properties to improve disk superalloys. Favorable precipitate morphologies and practical processing approaches to achieve them can then be determined. A methodology has been developed at the NASA Lewis Research Center to allow the comprehensive quantification of the size, shape, and quantity of all types of g' precipitates.
Disk superalloys can contain micrometer, submicrometer, and fine aging g' precipitates, as in the preceding photo. Micrometer-size g' precipitates with a diameter greater than 1 mm can survive from the original solidification structure ("primary g'") or grow during low-temperature solution heat treatments. These precipitates were observed by optical and scanning electron microscopy of metallographically mounted, polished, and etched sections. Submicrometer-size g' precipitates between 0.1 and 1 mm in diameter often form during quenching from solution heat treatments. These precipitates often grow with their edges approximately aligned along preferred crystallographic planes to produce regularly aligned rounded cubes or connected rectangles. In general, transmission electron microscopy (TEM) of thin foils obtained consistently oriented, high-resolution images of the aligned morphology. Fine aging g' precipitates less than 0.1 mm in diameter formed later during solution quenching and subsequent lower temperature heat treatments. These very small, nearly spherical precipitates also had to be imaged by transmission electron microscopy to be accurately quantified.
The delicate balance between superior disk mechanical properties
and practical processing approaches requires unprecedented levels
of g' microstructural quantification.
Therefore, SigmaScan image analysis software was used to determine
the size, shape, and volume fraction of each type of g'
precipitate. A typical measured distribution of g'
feret diameter versus frequency is shown in the preceding bar
graph for a disk alloy specimen solution heat treated at 2124
°F for 1 hr and cooled in the furnace, then subsequently
heat treated at 1550 °F for 2 hr and at 1400 °F for
8 hr. Micrometer, submicrometer, and fine aging g'
precipitates were present in this specimen's microstructure. The
volume fractions of micrometer, submicrometer, and fine aging
g' precipitates for specimens of the
same disk alloy solution heat treated from 2025 to 2206 °F
are compared in the following figure. Increasing the solution
heat treatment temperature reduced micrometer
g' content and increased submicrometer g'
content. These changes can affect the strength and creep resistance
of this disk alloy. The accurate quantification of g'
morphology and associated mechanical properties for different
disk alloy microstructures can enable modeling of processing-microstructure-property
relationships in advanced disk alloys. This can aid in obtaining
improved mechanical properties in advanced disk alloys by using
practical processing to achieve favorable g'
morphologies.
Lewis contacts: Dr. Timothy P. Gabb, (216) 433-3272,
Timothy.P.Gabb@grc.nasa.gov;
Dr. John Gayda, (216) 433-3273,
John.Gayda@grc.nasa.gov;
David L. Ellis, (216) 433-8736,
David.L.Ellis@grc.nasa.gov;
and Dr. Anita Garg, (216) 433-8908,
Anita.Garg@grc.nasa.gov
Author: Dr. Timothy P. Gabb
Headquarters program office: OASTT
Programs/Projects: Propulsion Systems R&T, AST, EPM,
P&PM, HITEMP
Previous articleLast updated April 14, 1998, by Nancy.L.Obryan@nasa.gov
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