Kevin J. Hemker

Domain wall motion and its contribution to the dielectric and piezoelectric properties of lead zirconate titanate films

In this article, domain wall motion and the extrinsic contributions to the dielectric and piezoelectric responses in sol–gel derived lead zirconate titanate (PZT) films with compositions near the morphotropic phase boundary were investigated. It was found that although the films had different thicknesses, grain sizes, and preferred orientations, similar intrinsic dielectric constants were obtained for all films between 0.5 and 3.4 μm thick. It was estimated that about 25%–50% of the dielectric response at room temperature was from extrinsic sources. The extrinsic contribution to the dielectric constant of PZT films was mainly attributed to 180° domain wall motion, which increased with both film thickness and grain size. In studies on the direct and converse longitudinal piezoelectric coefficients of PZT films as a function of either stress or electric driving field, it was found that the ferroelastic non-180° domain wall motion was limited. Thus extrinsic contributions to the piezoelectric response were s...

Structure and Mechanical Behavior of Bulk Nanocrystalline Materials

The reduction of grain size to the nanometer range (˜2-100 nm) has led to many interesting materials properties, including those involving mechanical behavior. In the case of metals, the Hall-Petch equation, which relates the yield stress to the inverse square root of the grain size, predicts great increases in strength with grain refinement. On the other hand, theory indicates that the high volume fraction of interfacial regions leads to increased deformation by grain-boundary sliding in metals with grain size in the low end of the nanocrystalline range. Nanocrystalline ceramics also have desirable properties. Chief among these are lower sintering temperatures and enhanced strain to failure. These two properties acting in combination allow for some unique applications, such as low-temperature diffusion bonding (the direct joining of ceramics to each other using moderate temperatures and pressures). Mechanical properties sometimes are affected by the fact that ceramics in a fine-grained form are stable in a different (usually higher pressure) phase than that which is considered “normal” for the ceramic. To the extent that the mechanical properties of a ceramic are dependent on its crystal-lographic structure, these differences will become evident at the smaller size scales. It is uncertain how deformation takes place in very fine-grained nanocrystalline materials. It has been recognized for some time that the Hall-Petch relationship, which usually is explained on the basis of dislocation pileups at grain boundaries, must break down at grain sizes such that a grain cannot support a pileup. Even some of the basic assumptions of dislocation theory may no longer be appropriate in this size regime. Recently considerable progress has been made in simulating the behavior of extremely fine-grained metals under stress using molecular-dynamics techniques. Molecular-dynamics (MD) simulations of deformation in nanophase Ni and Cu were carried out in the temperature range of 300–500 K, at constant applied uniaxial tensile stresses between 0.05 GPa and 1.5 GPa, on samples with average grain sizes ranging from 3.4 nm to 12 nm.

Experimental Observations of Stress-Driven Grain Boundary Migration

Moving Boundaries Classical models of fine-grained metals view grain boundaries as static objects, but this view has been challenged by recent experimental observations. Drawing on techniques used by the fracture mechanics community, Rupert et al. (p. 1686) present experiments on freestanding aluminum films that show specific geometries cause either stress or strain concentrations on deformation. Confirming recent simulations, shear stresses were found to be a key driver of grain boundary motion. Shear stresses drive grain boundaries to move in a manner consistent with predictions of coupled grain boundary migration. In crystalline materials, plastic deformation occurs by the motion of dislocations, and the regions between individual crystallites, called grain boundaries, act as obstacles to dislocation motion. Grain boundaries are widely envisaged to be mechanically static structures, but this report outlines an experimental investigation of stress-driven grain boundary migration manifested as grain growth in nanocrystalline aluminum thin films. Specimens fabricated with specially designed stress and strain concentrators are used to uncover the relative importance of these parameters on grain growth. In contrast to traditional descriptions of grain boundaries as stationary obstacles to dislocation-based plasticity, the results of this study indicate that shear stresses drive grain boundaries to move in a manner consistent with recent molecular dynamics simulations and theoretical predictions of coupled grain boundary migration.

Microsample tensile testing of nanocrystalline metals

Abstract A novel non-contact strain measurement technique has been employed to measure the tensile properties of extremely small ‘microsamples’ of pure high-density ultrafine-grained Al (ufg-Al) nanocrystalline Cu (n-Cu) and nanocrystalline Ni (n-Ni). These microsample tests confirmed the absence of Youngs modulus variations for metals with grain sizes approaching 25 nm. Significant strength enhancements were associated with the nanocrystalline specimens; the tensile stresses achieved in these microsample tests were measured to be an appreciable fraction of the theoretical shear strength for these metals. The ufg-Al samples (diameter, 250 nm) exhibited extensive plasticity while deformation in the n-Ni (diameter, 28 nm) remained almost entirely elastic up to failure at 1500MPa. The n-Cu samples were found to have a multiscale grain structure that produced an attractive balance of strength and ductility. Transmission electron microscopy investigations of deformed n-Ni failed to produce any evidence of dislocation activity. In the absence of dislocation motion, the tensile strength of truly nanocrystalline metals is remarkably high but currently dominated by intrinsic porosity and mesoscale microcrack coalescence.

Microsample tensile testing of nanocrystalline copper

The tensile properties of nanocrystalline copper with grain sizes <100 nm produced by surface mechanical attrition treatment have been characterized using a microsample testing technique. The nanocrystalline copper exhibits a yield strength as high as 760 MPa, with a small elongation to failure. Factors leading to the high strength and low ductility are discussed

Effect of specimen size on Young's modulus and fracture strength of polysilicon

The microstructure of polysilicon specimens of varying size was examined and tensile tests were conducted to determine if the measured modulus and strength depend on the size of the specimen. All specimens were from the same MUMPs 25 run at MCNC, and the thicknesses were 1.5, 2.0, and 3.5 /spl mu/m. Microstructure was examined in specimens as narrow as 2 /spl mu/m and ranging up to 20 /spl mu/m in width. The tensile specimens tested were 6, 20, or 600 /spl mu/m wide and 250, 1000, or 4000 /spl mu/m long. Nothing in the transmission electron microscopy (TEM) observations indicates any effect of specimen size on the microstructure; the columnar grains are fine (0.2-0.5 /spl mu/m) and uniformly distributed. The widths of all specimens were found to differ from the specified mask values, and a more pronounced variation was measured for the smaller specimens. Three different approaches are used to measure Youngs modulus, and they all give a value of 158/spl plusmn/10 GPa with no evidence of substantial effects of specimen size. However, the strength does increase somewhat as the total surface area of the test section decreases-from 1.21 GPa/spl plusmn/0.08 GPa to 1.65/spl plusmn/0.28 GPa-reflecting the fact that the larger specimens have more surface flaws. Test techniques and procedures are briefly presented along with detailed analyses of the results.

Evolution of a diffusion aluminide bond coat for thermal barrier coatings during thermal cycling

Abstract The thermal cyclic durability of a TBC is thought to be strongly dependent on the physical and mechanical properties of the bond coat layer. A novel high temperature microsample tensile testing technique has been employed to characterize the mechanical behavior of a platinum modified nickel aluminide bond coat at 0% and 28% of cyclic life in the temperature range of 25 to 1150 °C. Values for the coefficient of thermal expansion and the Young’s modulus are reported. The bond coat exhibits a ductile to brittle transition temperature at approximately 600 °C, and above this temperature the yield and creep strength decreases rapidly with temperature. A power law description of elevated temperature stress relaxation is developed. The intermediate temperature strength was found to increase with thermal cycling, while the high temperature strength remained the same. This evolution in properties has been related to the development of a martensitic transformation that occurs during each thermal cycle.

Modelling the flow stress anomaly in γ-TiAl I. Experimental observations of dislocation mechanisms

Abstract Mechanical tests on a polycrystalline γ Ti47A151,Mn2 alloy between ‐ 150 and 1000°C and subsequent microstructure analysis clearly distinguish between three temperature domains which correspond to different deformation mechanisms. At low temperatures, the motion of superdislocations dragging faulted dipoles is rate controlling. At intermediate temperatures where a stress anomaly is observed, screw simple dislocations are observed with cusps, the density of which increases with increasing temperature. A description of the simple dislocation motion based on these observations is developed. The glide of a screw simple dislocation in two planes through a Peierls mechanism is believed to be the intrinsic source for the formation of pinning points. It is proposed that these pinning points can be erased by lateral motion of superkinks: an unzipping process. A model for this pinning-unzipping mechanism will be fully developed in part 11. At high temperatures, the climb of simple dislocations appears to c...

Characterization and modeling of a martensitic transformation in a platinum modified diffusion aluminide bond coat for thermal barrier coatings

Abstract Phase transformations in a platinum modified nickel aluminide bond coat were investigated by in situ high temperature X-ray diffraction analysis. Three phases, L1 0 martensite, B2 (β-(Ni,Pt)Al) and L1 2 (γ′-Ni 3 Al), were identified at different temperature ranges. The martensite is stable at temperatures below 620 °C, and the β-phase is stable at elevated temperatures. The reversible transformation, M↔β, is the principal reaction occurring throughout the bond coat layer during thermal cycling. Quantitative measurements indicate that the molar volume of the β-phase is approximately 2% larger than that of the martensite. Finite element simulations incorporating the volume change associated with this transformation indicate that the transformation significantly influences the distribution of stresses and strains in TBC systems. The effect of the martensite on TBC life is sensitive to the transformation temperatures relative to the creep strength of the bond coat.

Measurements of antiphase boundary and complex stacking fault energies in binary and B-doped Ni3Al using TEM

Abstract The fourfold dissociation of superdislocations in Ni3Al has been recorded on images that were formed using second-order reflections, and the dissociation distances of the Shockley partial dislocations that bound a complex stacking fault (d CSF) have been measured. Comparisons with computer simulated (2g · 5g) images highlight the presence of supplementary intensity peaks when d CSF is less than 3.0 nm. These simulations also indicate that the distance between the weaker pair of intensity peaks can be used to measure d CSF if the experimental observations are corrected for the image shift that occurs during the microscopy. Experimental observations confirm the presence of the supplementary peaks, and measured values of d CSF were found to be larger in binary Ni3Al than in the boron-modified alloy. These measured values have been corrected through comparison to image simulations, and the corresponding complex stacking fault and antiphase boundary energies have been calculated using anisotropic elas...