David R. Clarke

The influence of particle size and particle fracture on the elastic/plastic deformation of metal matrix composites

A methodology is introduced for calculating the deformation response of particulate reinforced metal matrix composites in terms of an effective medium approach combined with the essential features of dislocation plasticity. By comparing the simulated and experimental stress-strain curves for two sets of SiC-Al composites, the effects of particle size and size distribution and volume fraction on the deformation response are given. A feature of the methodology is that the effects of particle cracking during deformation can be incorporated quantitatively. When included, the simulations lead to excellent agreement with experiments on the effects of particle cracking on both the overall deformation response and the tension-compression deformation asymmetry.

SPALLING FAILURE OF A THERMAL BARRIER COATING ASSOCIATED WITH ALUMINUM DEPLETION IN THE BOND-COAT

Abstract A plasma-sprayed thermal barrier coating is observed to spall after oxidation at 1121°C from a CoNiCrAlY bond-coated superalloy at the interface between the thermally grown oxide (TGO) and the zirconia thermal barrier coating (TBC). Phase characterization by photostimulated luminescence and X-ray diffraction, as well as microstructural characterization by scanning electron microscopy, indicates that the spalling is associated with the conversion of the initially formed α -alumina thermally grown oxide to α -chromia and a (Co,Ni)(Cr,Al) spinel. It is proposed that the phase conversion occurs after the alumina TGO has cracked on thermal cycling and the underlying bond-coat alloy is depleted of aluminum with concurrent enrichment of the oxide by Cr, Co and Ni. The observations suggest that monitoring the luminescence intensity as a function of oxidation time might form the basis of a nondestructive tool for detecting the onset of failure based on the disappearance of α -alumina.

Measurement of the stress in oxide scales formed by oxidation of alumina-forming alloys

The alumina scales on a variety of high-temperature alloys are found to fluoresce when illuminated with light having a frequency greater than 18,000 cm−1. The fluorescence exhibits two narrow lines characteristic of chromium-doped alpha-aluminum oxide. The frequency shift of the two lines from the room-temperature, stress-free values of 14,402 cm−1 (1.786 eV) and 14432 cm−1 (1.789 eV) provides a noncontact measure of the stress in the alumina scales using the piezospectroscopic effect. In addition, the broadening of the lines is a measure of the stress gradient in the scale. The physical basis for the fluorescence technique is described together with its implementation for highspatial-resolution (∼2 μm) measurements. As illustration, room-temperature measurements of the residual stress in scales formed at 1100°C on single-crystal NiAl, polycrystalline Ni3Al, two Fe−Ni−Cr−Al alloys, and two Ni−Al base superalloys are presented.

The mechanical behavior of silicon during small-scale indentation

The mechanical behavior of crystalline silicon during small-scale indentation has been studied using a Nanoindenter. Tests were performed on bothp-type andn-type materials in the (100), (110), and (111) orientations at peak loads ranging from 0.5 to 120 mN. The indentation load-displacement curves exhibit two features which appear to be unique to silicon. First, at large peak loads, a sharp discontinuity in displacement is observed as the indenter is unloaded. Second, at small peak loads, a large, non-degenerative hysteresis is exhibited. Possible mechanistic origins for the discontinuity and hysteresis are discussed.

Stress development in alumina scales formed upon oxidation of (111) NiAl single crystals

Abstract Stress evolution in the α phase of the alumina scale formed on single crystals of (111) NiAl as a function of oxidation time at 1100 °C is reported. The measurements are made at room temperature using the fluorescence from trace Cr 3+ impurities in the NiAl that are incorporated into the oxide scale during oxidation. After a short transient period, the room-temperature stress is essentially independent of oxidation time. At all oxidation times, there exist large stress gradients through the scale thickness, indicating that either fresh oxide is formed non-uniformly through the thickness of the growing scale or stress relaxation occurs non-uniformly.

Electrical resistance of metallic contacts on silicon and germanium during indentation

The effects of indentation on the electrical resistance of rectifying gold-chromium contacts on silicon and germanium have been studied using nanoindentation techniques. The DC resistance of circuits consisting of positively and negatively biased contacts with silicon and germanium in the intervening gap was measured while indenting either directly in the gap or on the contacts. Previous experiments showed that a large decrease in resistance occurs when an indentation bridges a gap, which was used to support the notion that a transformation from the semiconducting to the metallic state occurs beneath the indenter. The experimental results reported here, however, show that a large portion of the resistance drop is due to decreases in the resistance of the metal-to-semiconductor interface rather than the bulk semiconductor. Experimental evidence supporting this is presented, and a simple explanation for the physical processes involved is developed which still relies on the concept of an indentation-induced, semiconducting-to-metallic phase transformation.

On the Measurement of Strain in Coatings Formed on a Wrinkled Elastic Substrate

Highly stressed coatings, such as those formedby oxidation can, on occasion, wrinkle. Such wrinklinghas been suggested as a mode of deformation by which theoverall strain energy in a compressively stressed coating can be reduced. One of the consequencesof wrinkling is that the strain in the coating does notremain independent of position, but rather varies overthe wavelength of the wrinkling. The strain variation caused by sinusoidal wrinkling iscalculated using finite-element methods and the effectson both photostimulated Cr3+ luminescencepiezospectroscopy measurements and X-ray measurementscalculated. Wrinkling is shown to decrease theelastic-strain-energy density in the coating. A directmeasure of the decrease is the shift in the R2Cr3+ luminescence line and the X-raydiffraction peaks. Wrinkling of a compressive coating also causes stressesto be created perpendicular to the coating-substrateinterface, tensile at the crests in the wrinkles, andcompressive stress at the troughs.

Hysteresis and discontinuity in the indentation load-displacement behavior of silicon

Observation de deux caracteristiques: une forte discontinuite dans le deplacement lors du dechargement lorsque les charges sont elevees, et un fort hysteresis charge-deplacement lorsque les charges sont faibles. Lhysteresis est coherent avec lhypothese du deplacement de volume du a une transformation de phase. La discontinuite du deplacement hors charge semble due a la formation de fissures laterales

Growth and characterization of (111) and (001) oriented MgO films on (001) GaAs

The effects of substrate preparation on the structure and orientation of MgO films grown on (001) GaAs using pulsed laser deposition has been investigated. Textured MgO films displaying a (111)MgO∥(001)GaAs orientation relation with x‐ray rocking curve full width at half maximum (FWHM) values as low as 1.8° were obtained in cases where the native GaAs surface oxide was only partially desorbed prior to growth. Reflection high‐energy electron diffraction, transmission electron microscopy (TEM), and x‐ray pole figure analysis of these films reveals a preferential orientation within the plane of the substrate: [110]MgO∥[110]GaAs and [112]MgO∥[110]GaAs. An interfacial layer (∼5 nm thick) was observed in high resolution TEM analysis, and was attributed to a remnant native GaAs oxide layer. Complete desorption of the native GaAs oxide at ∼600u2009°C in vacuum prior to MgO growth led to significant surface roughening due to Langmuir evaporation, and resulted in randomly oriented polycrystalline MgO films. Growth o...

Measurement of residual stresses in sapphire fiber composites using optical fluorescence

Abstract The residual stresses in c-axis sapphire fibers in a γ-TiAl matrix and in a polycrystalline Al2O3 matrix as a function of distance below a surface are determined. They are obtained from the shift in frequency of the characteristic R2 fluorescence line of chromium in sapphire obtained by focusing an optical probe at different depths in a sapphire fiber intersecting the surface of the composite. The method is described together with its calibration. Both the axial and radial components of the residual stress in the fiber are observed to vary over a depth of approximately the fiber diameter and are then almost independent of depth.