Kevin P. Trumble

Low-cost manufacturing process for nanostructured metals and alloys

In spite of their interesting properties, nanostructured materials have found limited uses because of the cost of preparation and the limited range of materials that can be synthesized. It has been shown that most of these limitations can be overcome by subjecting a material to large-scale deformation, as occurs during common machining operations. The chips produced during lathe machining of a variety of pure metals, steels, and other alloys are shown to be nanostructured with grain (crystal) sizes between 100 and 800 nm. The hardness of the chips is found to be significantly greater than that of the bulk material.

Spontaneous infiltration of non-cylindrical porosity: Close-packed spheres

Abstract Previous experiments indicated that the contact angles required for spontaneous infiltration of molten metal alloys into particulate ceramic preforms are significantly less than 90°. Analysis of capillarity in pores between packed particles shows the origin of this effect to be in the non-cylindrical curvature of these pore surfaces. A simple geometrical approach for predicting critical contact angles, θ c , for spontaneous infiltration of non-cylindrical pores is presented and applied to the pore structure between closepacked spheres. Although the critical contact angle for entry of the first layer of pores varies with the direction of infiltration, complete spontaneous infiltration of the structure is limited by penetration of the tetrahedral pores, for which θ c = 50.7°.

Thermal residual stresses in co-continuous composites

The thermal residual stresses in two types of co-continuous composites copper/aluminum oxide (Cu/Al2O3) and aluminum/aluminum oxide (Al/Al2O3) were measured by neutron diffraction experiments. These stresses were generated during the cooling after high processing temperature. The coefficient of thermal expansion (CTE) mismatch of metal and ceramic phases led to significant amount of thermal stresses. In both the composites, the metallic phase was found to be under tension and aluminum-oxide phase under compression. Even though the magnitude of compressive stress in both the composites was similar; the two metal-phases had very different magnitude of tensile stresses. The difference in volume fraction, CTE, elastic stiffness and plastic flow properties led to this difference. The hydrostatic stresses were found to be predominant in both the phases. Finite element simulations were used to predict the stress distributions inside each phase and at the interfaces. A representative unit cell approach was considered to represent the composite. Concept of effective ΔT was utilized to simulate the thermal stress distribution inside the two phases in the unit cell. This model utilized the neutron diffraction measurements to predict the stress distribution inside each phase and at the interface. The simulations showed that significant amount of tensile stresses develop at the metal–ceramic interfaces.

R-curve behavior in alumina-zirconia composites with repeating graded layers

Abstract The single-edge-V-notched-beam testing geometry was used to measure the crack growth resistance (R-curve) behavior of multilayer graded alumina–zirconia composites for crack extensions parallel to the graded direction. Fracture mechanics weight function analysis was applied to explain the R-curve behavior of a compositional and grain-size graded microstructure. The results were then used to differentiate the influence of residual stress from other closure stresses, attributed to crack bridging, on the measured R-curve behavior.

Fracture resistance curve behavior of multilayered alumina–zirconia composites produced by centrifugation

Abstract The single-edge-V-notched-beam (SEVNB) testing method was used to measure the crack growth resistance (R-curve) behavior of multilayered alumina–zirconia composites. Crack initiation and extension from the V-notch tip were observed via in situ optical microscopy. The resulting R-curves were compared with an R-curve measured from a monolithic composite having a similar composition and a homogeneous microstructure, where the influence of layer–layer interfaces, gradient microstructures, and the direction of crack propagation on the resulting R-curves were observed. Additionally, the stress intensity factor for crack initiation from the V-notch tip was ∼0.2 MPa·m1/2 higher than the stress intensity factor to further extend this crack.

The influence of CuAlO2 on the strength of eutectically bonded Cu/Al2O3 interfaces

Abstract This work investigates the effect of continuous CuAlO2 interfacial reaction product layers on the strength of eutectic bonded copper–alumina interfaces. The fracture resistance was characterized for interfaces produced by eutectic bonding with and without continuous CuAlO2 interlayers. Results show that continuous CuAlO2 layers can weaken the overall Cu/Al2O3 bond.

Preferred orientation of BSCCO via centrifugal slip casting

Due to the highly anisotropic properties of BSCCO superconductors, the bulk properties of these materials can be greatly affected by preferential orientation. Substantial [ital c]-axis orientation normal to the desired direction of current flow has been demonstrated by centrifugally slip casting lead-doped BSCCO-2223. The strong preferred orientation developed in the centrifugally slip-cast material demonstrates high critical current potential.

Geometric flow control of shear bands by suppression of viscous sliding.

Shear banding is a plastic flow instability with highly undesirable consequences for metals processing. While band characteristics have been well studied, general methods to control shear bands are presently lacking. Here, we use high-speed imaging and micro-marker analysis of flow in cutting to reveal the common fundamental mechanism underlying shear banding in metals. The flow unfolds in two distinct phases: an initiation phase followed by a viscous sliding phase in which most of the straining occurs. We show that the second sliding phase is well described by a simple model of two identical fluids being sheared across their interface. The equivalent shear band viscosity computed by fitting the model to experimental displacement profiles is very close in value to typical liquid metal viscosities. The observation of similar displacement profiles across different metals shows that specific microstructure details do not affect the second phase. This also suggests that the principal role of the initiation phase is to generate a weak interface that is susceptible to localized deformation. Importantly, by constraining the sliding phase, we demonstrate a material-agnostic method—passive geometric flow control—that effects complete band suppression in systems which otherwise fail via shear banding.

THERMAL INSTABILITY OF (BI, PB)2SR2CA2CU3OX IN CONTACT WITH SILVER

The chemical and microstructural evolution of particulate (Bi, Pb)2223 in contact with pure bulk silver at 830 to 905h{degree}C under flowing air and Ar{endash}5{percent} O{sub 2} atmospheres has been studied. Specimens annealed under P{sub O{sub 2}} and temperature conditions for which the (Bi, Pb)2223 phase itself is stable, based on a critical assessment of the literature, revealed a silver/(Bi, Pb)2223 interface-limited reaction, producing both solid and liquid decomposition phases. The (Bi, Pb)2223 not in direct contact with silver exhibited no reaction under the same conditions. Solid silver dissolves into the (Bi, Pb)2223 incongruent liquid, facilitating the incongruent melting reaction. On cooling, equiaxed metallic silver particles precipitated uniformly throughout bulk specimens partially melted on a silver substrate. {copyright} {ital 1999 Materials Research Society.}

TEXTURE AND MICROSTRUCTURE DEVELOPMENT IN AI O3-PLATELET REINFORCED Ce-ZrO /Al O3 LAMINATES PRODUCED BY CENTRIFUGAL CONSOLIDATION

complexity and anisotropy of the microstructure warrants a quantitative analysis of the microstructural evolution prior to any property evaluation. Quantitative image analysis was used to examine changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets with sintering time. In all cases, special attention was given to the effects of texture during microstructural evolution. Platelet alignment enhanced densification via anisotropic shrinkage, overcoming constraint that otherwise inhibits densification in platelet-containing materials. Also, platelet alignment and microstructural design were used to initiate and control anisotropic grain growth. Platelet growth (at the expense of smaller particles of the same phase) during annealing promoted further phase segregation and produced higher platelet content composites consisting of larger platelets, without having to consolidate high contents of large platelets.