Polycrystalline Ni-base superalloys are used as turbine disks in the hot section in jet engines, placing them in a high temperature and stress environment. As operating temperatures increase in search of better fuel efficiency, it becomes important to understand how these higher temperatures are affecting mechanical behavior and active deformation mechanisms in the substructure. Not only are operating temperatures increasing, but there is a drive to design next generation alloys in shorter time periods using predictive modeling capabilities. This dissertation focuses on mechanical behavior and active deformation mechanisms found in two different advanced polycrystalline alloy systems, information which will then be used to build advanced predictive models to design the next generation of alloys.
The first part of this dissertation discusses the creep behavior and identifying active deformation mechanisms in an advanced polycrystalline Ni-based superalloy (ME3) that is currently in operation, but at higher temperatures and stresses than are experienced in current engines. Monotonic creep tests were run at 700°C and between 655-793MPa at 34MPa increments, on microstructures (M1 and M2) produced by different heat treatments. All tests were crept to 0.5% plastic strain. Transient temperature and transient stress tests were used determine activation energy and stress exponents of the M1 microstructure. Constant strain rate tests (at 10-4s-1) were performed on both microstructures as well.
Following creep testing, both microstructures were fully characterized using Scanning Electron Microscopy (SEM) for basic microstructure information, and Scanning Transmission Electron Microscopy (STEM) to determine active deformation mechanism. It was found that in the M1 microstructure, reorder mediated activity (such as discontinuous faulting and microtwinning) is dominant at low stresses (655-724 MPa). Dislocations in the ¿ matrix, and overall planar dislocation activity were also present. At higher stresses (758-793MPa), there is still planar activity present, but now non-planar “wavy” slip appears. Wavy slip was also present in the constant strain rate sample of the M1 microstructure. M2 similar activity as M1, but wavy slip was not present in either the high stress creep conditions or the constant strain rate conditions.
The second part of this dissertation focused on Alloy X, which is a next generation alloy system. Three variants were examined (decreased levels of Cr and Co, decreased levels of Cr and Co with added Hf, and decreased levels of Cr and Co with added Hf and a faster cooling rate). The variants were all tested at 700°C/690MPa, 760°C/490MPa, and 815°C/345MPa, to various strains. In Alloy X, the dominant mechanisms that appear throughout all strains, stresses, and temperatures are unpaired dislocations and discontinuous stacking faults. In the Hf bearing variants, unpaired dislocations appear to start at grain boundaries and move in to the grains with increasing strain and temperature. Through 3D stereo imaging using STEM, it appears that the dislocations are able to climb over precipitates at larger strains. It is postulated that the decreasing levels of Cr and Co have changed the stacking fault energy so it is unfavorable to form microtwins anymore. This was discussed in terms of the Dislocation Activity Diagram (DAD).