Discerning the Impact of Powder Feedstock Variability on Structure, Property, and Performance of Selective Laser Melted Alloy 718: A Principal Component Analysis (PCA) of Feedstock Variability

Extensive mechanical, chemical and microstructural analyses were conducted on additively manufactured Alloy 718 to characterize powders from multiple vendors to determine the effects of variations observed in the powders had on the consolidated material. With over 190 variables examined, it was necessary to reduce the number of variables and identify the variables and classes of variables that had the greatest effect. Principle Component Analysis (PCA) was used to reduce the number of variable to effectively 12 while identifying several classes of variables as most important

Direct observations of nucleation in a nondilute multicomponent alloy

The chemical pathways leading to gamma-prime(L1_2)-nucleation from nondilute Ni-5.2 Al-14.2 Cr at.%, gamma(f.c.c.), at 873 K are followed with radial distribution functions and isoconcentration surface analyses of direct-space atom-probe tomographic images. Although Cr atoms initially are randomly distributed, a distribution of congruent Ni3Al short-range order domains (SRO), =0.6 nm, results from Al diffusion during quenching. Domain site occupancy develops as their number density increases leading to Al-rich phase separation by gamma-prime-nucleation, =0.75 nm, after SRO occurs.Comment: 5 pages, 4 figure

Revealing Interfaces and Nanostructure: The Application of Atom Probe Tomography to Nickel Based Superalloys

No abstract availabl

Best-Fit Ellipsoids of Atom-Probe Tomographic Data to Study Coalescence of Gamma Prime (L1_2) Precipitates in Ni-Al-Cr

An algorithm is presented to fit precipitates in atom probe tomographic data sets as equivalent ellipsoids. Unlike previous techniques, which measure only the radius of gyration, these ellipsoids retain the moments of inertia and principle axes of the original precipitate, preserving crystallographic orientational information. The algorithm is applied to study interconnected gamma prime precipitates (L1_2) in the Gamma-matrix (FCC) of a Ni-Al-Cr alloy. The precipitates are found to coagulate along -type directions.Comment: Accepted for publication in Scripta Materialia, added information about local magnification effect

Effects of solute concentrations on kinetic pathways in Ni-Al-Cr alloys

The kinetic pathways resulting from the formation of coherent L12-ordered y'-precipitates in the g-matrix (f.c.c.) of Ni-7.5 Al-8.5 Cr at.% and Ni-5.2 Al-14.2 Cr at.% alloys, aged at 873 K, are investigated by atom-probe tomography (APT) over a range of aging times from 1/6 to 1024 hours; these alloys have approximately the same volume fraction of the y'-precipitate phase. Quantification of the phase decomposition within the framework of classical nucleation theory reveals that the y-matrix solid-solution solute supersaturations of both alloys provide the chemical driving force, which acts as the primary determinant of the nucleation behavior. In the coarsening regime, the temporal evolution of the y'-precipitate average radii and the y-matrix supersaturations follow the predictions of classical coarsening models, while the temporal evolution of the y'-precipitate number densities of both alloys do not. APT results are compared to equilibrium calculations of the pertinent solvus lines determined by employing both Thermo-Calc and Grand-Canonical Monte Carlo simulation.Comment: Submitted to Acta Materialia, June, 200

TEMPORAL EVOLUTION OF SUB-NANOMETER COMPOSITIONAL PROFILES ACROSS THE GAMMA/GAMMA' INTERFACE IN A MODEL Ni-Al-Cr SUPERALLOY

Early-stage phase separation in a Ni-5.2 Al-14.2 Cr at.% superalloy, isothermally decomposing at 873 K, is investigated with atom-probe tomography. Sub-nanometer scale compositional profiles across the gamma/gamma'(L12) interfaces demonstrate that both the gamma-matrix and the gamma'-precipitate compositions evolve with time. Observed chemical gradients of Al depletion and Cr enrichment adjacent to the gamma'-precipitates are transient, consistent with well-established model predictions for diffusion-limited growth, and mark the first detailed observation of this phenomenon. Furthermore, it is shown that Cr atoms are kinetically trapped in the growing precipitates

Compositional Pathways and Capillary Effects during Early-stage Isothermal Precipitation in a Nondilute Ni-Al-Cr Alloy

For a Ni-5.2 Al-14.2 Cr at.% alloy with moderate solute supersaturations, the compositional pathways, as measured with atom-probe tomography, during early to later stage y'(LI2)-precipitation (R = 0.45-10 nm), aged at 873 K, are discussed in light of a multi-component coarsening model. Employing nondilute thermodynamics, detailed model analyses during quasistationary coarsening of the experimental data establish that the y/y' interfacial free-energy is 22- 23+/-7 mJ/sq m. Additionally, solute diffusivities are significantly slower than model estimates. Strong quantitative evidence indicates that an observed y'-supersaturation of Al results from the Gibbs-Thomson effect, providing the first experimental verification of this phenomenon. The Gibbs-Thomson relationship, for a ternary system, as well as differences in measured phase equilibria with CALPHAD assessments, are considered in great detail

LCF Life of NiCr-Y Coated Disk Alloys After Shot Peening, Oxidation and Hot Corrosion

In a prior companion study (Ref. 1), three different Ni-Cr coating compositions (29, 35.5, 45 wt% Cr) were applied at two thicknesses by Plasma Enhanced Magnetron Sputtering (PEMS) to two similar Ni-based disk alloys. One coating also received a thin ZrO2 overcoat. The low cycle fatigue (LCF) life of each coating was determined at 760 C and was less than that of the uncoated specimens. In this followon effort, shot peening was examined as a means to improve the as-deposited coating morphology as well as impart a residual compressive stress in the near-surface region. After evaluating the effect of the shot peening on the LCF life, the effectiveness of the shot-peened coating in protecting the disk alloy from oxidation and hot corrosion attack was evaluated. This evaluation was accomplished by exposing coated and shot-peened specimens to 500 h of oxidation followed by 50 h of hot corrosion, both at 760 C in air. These exposed specimens were then tested in fatigue and compared to similarly treated and exposed uncoated specimens. For all cases, shot peening improved the LCF life of the coated specimens. More specifically, the highest Cr coating showed the best LCF life of the coated specimens after shot peening, as well as after the environmental exposures. Characterization of the coatings after shot peening, oxidation, hot corrosion and LCF testing is presented and discussed

Location-Specific Microstructure and the Effect of Heat Treatment on Electron-Beam Melted Ni-Based Superalloy LSHR

Examine set-to-set variability with 4 build sets with the same layout Establish the effect of solutioning heat treatments (HT) on as-fabricated grain structure and microstructure (gamma)-solvus= 1156 C (1) As-fabricated (2) 2 hour subsolvus HT(-15 C) + conventional 2-step aging (3, 4, 5) 0.5 hour, 2 hours, 4 hours supersolvus HT (+18 C) + 2 step aging Gain insight into location-specific microstructure to correlate later to mechanical properties (tensile, creep, LCF behavior