Wallace D. Porter

Role of yttrium in glass formation of Fe-based bulk metallic glasses

In this study, we discovered that a small addition of Y is very effective in improving glass-forming ability of Fe-based alloys. As-cast bulk amorphous alloys containing 2 at. % Y showed large thermal stability, with glass transition temperatures above 900 K and supercooled liquid regions above 55 K, and high strength, with Vickers hardnesses larger than HV 1200. The beneficial effect of Y on glass formation is twofold: (1) Y adjusted the compositions closer to the eutectic and thus lowered their liquidus temperatures, and (2) Y improved the manufacturability of these alloys by scavenging the oxygen impurity from it via the formation of innocuous yttrium oxides.

The effect of grain size, porosity and yttria content on the thermal conductivity of nanocrystalline zirconia

In order to accommodate the ever increasing inlet temperatures of gas turbines, air plasma sprayed (APS) or electron beam physically vapor deposited (EB-PVD) yttria stabilized zirconia thermal barrier coatings (TBC`s) are used to insulate the metallic surfaces. Because of its historic use as a TBC, the thermal diffusivity and conductivity of single crystal and polycrystalline stabilized zirconia have been the subject of numerous experimental investigations. However, to the knowledge of the authors, the thermal conductivity of nanocrystalline (gain size < 100 nm) zirconia has not yet been determined. To ascertain whether or when grain boundary effects begin to dominate thermal conductivity, k, values for a variety of nanocrystalline zirconias of different densities (60--100%), grain sizes (30--400 nm), and purities (0--15wt.% yttria) are compared in this work. Finally the measured values are compared with the thermal conductivities of commercially available air plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) coatings.

Comparison of thermal expansion and oxidation behavior of various high-temperature coating materials and superalloys

Abstract The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is a primary cause of spallation of protective oxide scales. This study compares thermal expansion behavior and cyclic oxidation performance of the two major composition classes of high-temperature commercial coatings for protection of single-crystal superalloys. The thermal expansion of cast MCrAlY (M = Ni and/or Co) alloys and cast aluminides (NiAl, (Ni,Pt)Al and Ni3Al) was measured at temperatures up to 1300°C and compared to that of a single-crystal Ni-base superalloy. The tendency for scale spallation from each alloy was evaluated by cyclic oxidation testing at 1150°C. The coefficients of thermal expansion for the aluminides were lower than those of the MCrAlY-based alloys at all temperatures and scale adherence to the Hf-doped aluminides was generally superior. Scale adherence to the various compositions of MCrAlY-type alloys did not directly correlate to their thermal expansion behavior or substrate strength. For both types of materials, the presence of a reactive element (Y,Hf, etc.) had no detectable effect on thermal expansion but a major effect on scale adherence. There was no obvious influence of Al content on the thermal expansion of β phase Ni–Al compositions. The addition of Pt resulted in a lower average thermal expansion for hyperstoichiometric (Ni,Pt)Al at temperatures above 930°C, but this effect was not observed in hypostoichiometric (Ni,Pt)Al.

Oxidation and degradation of a plasma-sprayed thermal barrier coating system

The isothermal oxidation behavior of thermal barrier coating (TBC) specimens consisting of single-crystal superalloy substrates, vacuum plasma-sprayed Ni-22Cr-10Al-1Y bond coatings and air plasma-sprayed 7.5 wt.% yttria stabilized zirconia top coatings was evaluated by thermogravimetric analysis at 1150{degrees}C for up to 200 hours. Coating durability was assessed by furnace cycling at 1150{degrees}C. Coatings and reaction products were identified by x-ray diffraction, field-emission scanning electron microscopy and energy dispersive spectroscopy.

Size effects in PbTiO3 nanocrystals : Effect of particle size on spontaneous polarization and strains

The spontaneous polarization (Ps) and spontaneous strains (xi) in mechanically unclamped and surface charge compensated PbTiO3 nanocrystals were determined as a function of particle size in the range <150nm by differential scanning calorimetry and x-ray powder diffraction, respectively. Significant deviations from bulk order parameters (P,xi) have been observed as the particle size decreased below ∼100nm. The critical size (rc) below which the ferroelectric tetragonal phase transforms to the paraelectric cubic phase was determined as ∼15nm. The depression in transition temperature with particle size is 14 °C at 28 nm. No change in the order of m3m→4mm ferrodistortive phase transition is observed. A simple analysis showed that ΔHtr∕(kBT)∼103 at 25 °C for r=16nm, indicating that the stabilization of the cubic phase at rc cannot be linked to an instability in dipolar ordering due to thermal agitations. Comparison of the spontaneous volumetric strains with the strain induced by surface stress indicated that t...

Thermal properties of Ti4AlN3

In this article we report on the atomic displacement parameters, lattice expansions, heat capacity, and thermal conductivity of samples of Ti4AlN3 in the 298–1370 K temperature range. Rietveld refinement of high temperature neutron diffraction data shows that the nitrogen is substoichiometric and the formula is Ti4AlN2.9. In this structure, the atomic displacement parameters of the Al atoms are higher than those of either the Ti or N atoms. The Ti–N bonds adjacent to the Al planes are about 2.5% shorter than the Ti–N bonds in the inner layers. The thermal expansion coefficients along the a and c axes are, respectively, (9.6±0.1)×10−6 and (8.8±0.1)×10−6 K−1. The unit cell expansivity, (9.4±0.1)×10−6 K−1, is in agreement with the dilatometric bulk thermal expansivity (9.7±0.2)×10−6 K−1. The heat capacity, cp, is 150 J/mol K at ambient temperatures and extrapolates to ≈220 J/mol K at 1300 K. At all temperatures cp equals four times the molar heat capacity of TiN. The room temperature thermal conductivity is ...

Thermodynamics of the tetragonal-to-monoclinic phase transformation in fine and nanocrystalline yttria-stabilized zirconia powders

The current study uses high-temperature differential scanning calorimetry to document the shift in phase-transformation temperature with particle size throughout a series of alloys in the zirconia–yttria system (0–1.5 mol% yttria). The tetragonal-to-monoclinic (T→M) phase-transformation temperature is seen to vary inversely with particle size. It is shown that a simple thermodynamic approach first proposed by Garvie predicts this inverse linear relationship. Subsequent determination of the key thermodynamic parameters therein (e.g., the surface and volume free energy, enthalpy, and entropy changes involved in the phase transformation) allows a complete predictive equation for the T→M phase transformation in the yttria–zirconia system to be developed as a function of particle size and yttria dopant level. The yttria–zirconia phase diagram is then redrawn with grain size as a third variable. It should be stressed that the current analysis is valid for particulate systems only; a parallel paper tackles the problem for fine-grained yttria–zirconia solids, where the approach is similar, but additional strain energy terms come into play.

Thermal properties of Nb2SnC

In this article we report on the thermal properties of the ternary carbide Nb2SnC. Its heat capacity, cp, is 75 J/mol K at ambient temperatures and increases to ≈115 J/mol K at 1300 K. Analysis of cp measurements in the 4–15 K temperature range yields a Debye temperature of 324 K and a density of states at the Fermi level of 2.66 (eV unit cell)−1. At room temperature, the thermal conductivity is 17 W/m K and it increases to ≈31 W/m K at 1500 K. At temperatures greater than 600 K, roughly 85% of the heat is transported by electronic defects (i.e., electrons and/or holes). The thermal properties of Nb2SnC are quite comparable to those of near-stoichiometric NbCx.

Phonon anharmonicity and negative thermal expansion in SnSe

In this paper, the anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy, in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. Finally, the origin of the anharmonic phonon thermodynamics is linked to the electronic structure.

Thermal expansion data on several iron- and nickel-aluminide alloys

Iron and nickel aluminides are ordered intermetallic alloys that are being developed for high-temperature structural applications because they have good oxidation resistance at > 1,000 C due to the formation of adherent alumina firms. More complex aluminide alloys have recently been developed to improve their mechanical properties behavior and to improve oxidation/corrosion resistance. The Fe[sub 3]Al alloy composition has a B2-ordered phase structure at about 550 to 900 C but transforms to the D0[sub 3] ordered phase at lower temperatures. While a considerable amount of mechanical property, structural microstructural, and corrosion data has been generated in the development of the iron- and nickel-aluminide alloys, there have been correspondingly little physical properties data. Physical properties like thermal expansion are basic material parameters used by engineers to select materials and by designers to perform stress analyses. The purpose of this paper is to present the preliminary thermal expansion data measured on leading or representative new alloys being developed from each of these three iron- or nickel-aluminide alloy types.