Doctor of Philosophy, The Ohio State University, 2020, Materials Science and Engineering

The realization of advanced alloy compositions in service relies on a thorough understanding of metallurgical processing variables. Within this work, the gamma prime precipitation of an advanced powder metallurgy nickel-base superalloy during controlled cooling from supersolvus temperatures is compared to prior alloy generations using a complement of characterization and modeling approaches. The on-cooling precipitation of the alloy is studied and characterized to calibrate a multi-scale precipitation model. The proposed framework incorporates a computationally efficient addition to the mean-field modeling approach that increases its ability to model dynamic, multi-modal gamma prime burst events. The gamma prime size predicted by the model shows good agreement with experimental results. The precipitation calculation is applied to the element integration points of a continuum Finite Element heat conduction simulation, where the latent heat generated from the precipitation is accounted for. The results are compared to experimental findings and indicate potential use of the model for evaluating precipitation effects at multiple length scales. The lattice misfit evolution of two commercial PM nickel superalloys during cooling from supersolvus temperatures is also characterized, using in-situ synchrotron X-Ray Diffraction (XRD). The diffraction pattern deconvolution necessary for quantifying misfit was accomplished by combining observation of the superlattice peak intensities with thermodynamic modeling to quantify the intensity relationship between the overlapping phases. The misfit from the XRD measurements was compared to the Scanning Electron Microscopy observations of gamma prime particle shapes for a subset of the experimental conditions. The trend of measured misfit agreed with the microstructural characterization. Time-resolved observations of the on-cooling lattice parameter suggest that lower-temperature changes to the peak intensity characteristics coinc (open full item for complete abstract)

Committee: Michael Mills (Advisor); Wei Zhang (Advisor); Yunzhi Wang (Committee Member); Stephen Niezgoda (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science

Doctor of Philosophy, Case Western Reserve University, 2018, Materials Science and Engineering

The fabrication of the bulk powder a'-phase nitrogen martensite in (Fe0.99Cr0.01)Nx system using ball milling at cryogenic temperature was investigated in this thesis. The nitrogen austenite powder particles were sealed in a grinding chamber in a nitrogen atmosphere and ball milled at liquid nitrogen temperature using a cryomill. The amount of martensitic phase transformation was greatly enhanced and the yield of the a'-phase was increased from 28 vol.%, obtained from quench, to 93 vol.% due to the deformation. The X-ray diffractometry and vibrating sample magnetometry measurements were used to determine the martensite phase fraction and the saturation magnetization of the samples. The effect of (1) milling frequency, f, (2) grinding ball size, D, (3) number of grinding balls, (4) total milling time, tT, (5) Cr alloying content, and (6) particle size of cryomilled powder on the amount of deformation-induced martensitic phase transformation was studied from a kinetic energy perspective. A model was developed and used to calculate the kinetic energy applied to powder particles during the ball milling process:Ek=4mb·(L-D)2·f3·tT, where mb is the mass of the grinding ball and L the length of the grinding chamber. This was correlated to the martensite start temperature with deformation Md, Md (oC)=(68±8)×10-3·Ek (J). The martensite start temperature can then be expressed as MST'=MST [N]+Md [Ek]. The relation between the deformation-induced martensite start temperature MST', which is a function of nitrogen content, and the kinetic energy applied by ball milling was determined to be MST' (oC)=(68±8)×10-3·Ek (J)-(169±12) for nitrogen martensite with (10.0±0.1) at.%. nitrogen content. This equation was determined to apply until Ek=3000J, at which time no further transformation occurred. At 3000J< Ek<10000J, no additional transformation occurred beyond the 93 vol.% maximum. Further increasing the kinetic energy leads to decomposition of the martensite to the equilibrium alpha (open full item for complete abstract)

Committee: David Matthiesen (Advisor); Matthew Willard (Committee Member); John Lewandowski (Committee Member); Kathleen Kash (Committee Member) Subjects: Materials Science

Master of Sciences (Engineering), Case Western Reserve University, 2015, Materials Science and Engineering

The lattice parameters of the gamma iron as a function of chromium alloy content were determined using high temperature X-ray diffraction measurements for three Fe-rich alloys. The three concentrations were: (1) pure iron powder, (2) 1 at.% Cr, and (3) 1.8 at.% Cr, and the temperature range was between 800℃ to 1300℃. Linear relationships between lattice parameter and temperature were observed and determined in all three samples. The lattice parameters of the γ-phase of the three samples at room temperature were determined by extrapolating the high temperature data to 20℃. The values are (0.3572±0.0005) nm, (0.3604±0.0003) nm, and (0.3609±0.0003) nm for pure iron powder and iron-chromium powders with 1 at.% Cr and 1.8 at.% Cr, respectively. A linear least squares regression analysis yielded: a=0.0021(nm/(at.%))×CCr+0.3575(nm).

Committee: David Matthiesen PhD (Advisor); Frank Ernst PhD (Committee Member); Matthew Willard PhD (Committee Member) Subjects: Materials Science