Richard K. Everett

Intermetallic phase formation during annealing of Al/Ni multilayers

The phase evolution during annealing of Al/Ni multilayer samples prepared by ion‐beam sputtering with composition modulation wavelengths Λ between 10 and 400 nm was determined using x‐ray diffraction and differential scanning calorimeter measurements. Samples with average compositions of Al0.40Ni0.60 and Al0.75Ni0.25 were investigated. For the Al0.40Ni0.60 samples the following results were obtained. A measure of the degree of periodicity and the sharpness of the interfaces in a sample with Λ=80 nm was the large number (over 20) of peaks observed in small‐angle x‐ray scattering measurements. A sample with Λ=10 nm was transformed by heat treatment directly to the AlNi phase. In the Λ=80 nm sample, the first phase formed after annealing was the metastable η phase. The η phase was identified as Al9Ni2. In the 400 nm wavelength sample, both the metastable η phase and the stable Al3Ni formed after the first exothermic reaction. For the Al0.75Ni0.25 samples two results were obtained. A Λ=11.4 nm sample transfor...

Barnacle cement: a polymerization model based on evolutionary concepts

SUMMARY Enzymes and biochemical mechanisms essential to survival are under extreme selective pressure and are highly conserved through evolutionary time. We applied this evolutionary concept to barnacle cement polymerization, a process critical to barnacle fitness that involves aggregation and cross-linking of proteins. The biochemical mechanisms of cement polymerization remain largely unknown. We hypothesized that this process is biochemically similar to blood clotting, a critical physiological response that is also based on aggregation and cross-linking of proteins. Like key elements of vertebrate and invertebrate blood clotting, barnacle cement polymerization was shown to involve proteolytic activation of enzymes and structural precursors, transglutaminase cross-linking and assembly of fibrous proteins. Proteolytic activation of structural proteins maximizes the potential for bonding interactions with other proteins and with the surface. Transglutaminase cross-linking reinforces cement integrity. Remarkably, epitopes and sequences homologous to bovine trypsin and human transglutaminase were identified in barnacle cement with tandem mass spectrometry and/or western blotting. Akin to blood clotting, the peptides generated during proteolytic activation functioned as signal molecules, linking a molecular level event (protein aggregation) to a behavioral response (barnacle larval settlement). Our results draw attention to a highly conserved protein polymerization mechanism and shed light on a long-standing biochemical puzzle. We suggest that barnacle cement polymerization is a specialized form of wound healing. The polymerization mechanism common between barnacle cement and blood may be a theme for many marine animal glues.

Diffusional reactions in composite synthesis

Abstract The thermal stability of advanced composites is dominated by the behavior of internal interfaces. In order to develop effective processing strategies and stable composite designs, it is essential to consider the relevant phase diagrams which are of ternary order or higher. In addition to phase diagram information, kinetic data such as the interdiffusion pathway and reaction rates are required to understand and control the possible interfacial chemical reactions. With this information, it may be possible to bias the reactions and to alter pathways. Often the initial nucleation stage of interfacial reactions has been neglected, but recent results indicate new kinetic behavior can develop during intermediate phase nucleation in a large concentration gradient. Multilayer thin film samples are well suited for probing the initial kinetic path and structural evolution during interdiffusion reaction and phase nucleation. In Al/Ni multilayer samples with compositional modulation wavelengths between 10 and 400 nm, thermal signal onsets due to phase nucleation have been examined to monitor the reaction kinetics and to probe the interdiffusion that precedes phase nucleation. The analysis of both bulk diffusion couple and multilayer sample behavior offers the basis for phase compatibility control that can be applied in developing stable composite structures by in situ reaction processing.

Reaction kinetics and biasing in Al/Ni multilayers

Abstract The phase evolution from annealing ion-beam-sputtered Al/Ni multilayers was studied using X-ray diffraction and differential scanning calorimetry measurements. The sequence of phase formation depends on the modulation wavelength Λ and the average composition 〈 c 〉. The initial phase formed may also depend on the composition gradient. Annealing short Λ samples with a composition of either Al 0.40 Ni 0.60 or Al 0.75 Ni 0.25 produced only AlNi or Al 3 Ni, i.e. the stable phase with the same 〈 c 〉. At intermediate Λ, Al 9 Ni 2 was often the initial phase formed. Experiments were performed on multilayer samples in which 20 nM AlNi layers, denoted as biasing layers, were placed between each Al and Ni layer. The AlNi modified the kinetics, increasing the nucleation temperatures and changing the amounts of the product phases. The AlNi layer appears to dissociate in the later stages of annealing. Composition gradients and biasing layers may be used to control the phases present in technological materials.

Barnacle Balanus amphitrite adheres by a stepwise cementing process.

Barnacles adhere permanently to surfaces by secreting and curing a thin interfacial adhesive underwater. Here, we show that the acorn barnacle Balanus amphitrite adheres by a two-step fluid secretion process, both contributing to adhesion. We found that, as barnacles grow, the first barnacle cement secretion (BCS1) is released at the periphery of the expanding base plate. Subsequently, a second, autofluorescent fluid (BCS2) is released. We show that secretion of BCS2 into the interface results, on average, in a 2-fold increase in adhesive strength over adhesion by BCS1 alone. The two secretions are distinguishable both spatially and temporally, and differ in morphology, protein conformation, and chemical functionality. The short time window for BCS2 secretion relative to the overall area increase demonstrates that it has a disproportionate, surprisingly powerful, impact on adhesion. The dramatic change in adhesion occurs without measurable changes in interface thickness and total protein content. A fracture mechanics analysis suggests the interfacial materials modulus or work of adhesion, or both, were substantially increased after BCS2 secretion. Addition of BCS2 into the interface generates highly networked amyloid-like fibrils and enhanced phenolic content. Both intertwined fibers and phenolic chemistries may contribute to mechanical stability of the interface through physically or chemically anchoring interface proteins to the substrate and intermolecular interactions. Our experiments point to the need to reexamine the role of phenolic components in barnacle adhesion, long discounted despite their prevalence in structural membranes of arthropods and crustaceans, as they may contribute to chemical processes that strengthen adhesion through intermolecular cross-linking.

Spatial distribution of MnS inclusions in HY-100 steel

High strength steels have been shown to fail by a ductile fracture process which includes void nucleation, growth, and linking by coalescence and void sheeting. Since the voids are thought to nucleate at MnS inclusions and initial void nucleating strains are considered small, some relationship between the inclusion spatial distribution and the initial void spatial distribution appears reasonable. The initial void spatial distribution is desired for improved models of void growth and coalescence behavior. This paper reports on the density, and size and spatial distributions of MnS inclusions in an HY-100 steel.

Growth and development of the barnacle Amphibalanus amphitrite: time and spatially resolved structure and chemistry of the base plate

The radial growth and advancement of the adhesive interface to the substratum of many species of acorn barnacles occurs underwater and beneath an opaque, calcified shell. Here, the time-dependent growth processes involving various autofluorescent materials within the interface of live barnacles are imaged for the first time using 3D time-lapse confocal microscopy. Key features of the interface development in the striped barnacle, Amphibalanus (= Balanus) amphitrite were resolved in situ and include advancement of the barnacle/substratum interface, epicuticle membrane development, protein secretion, and calcification. Microscopic and spectroscopic techniques provide ex situ material identification of regions imaged by confocal microscopy. In situ and ex situ analysis of the interface support the hypothesis that barnacle interface development is a complex process coupling sequential, timed secretory events and morphological changes. This results in a multi-layered interface that concomitantly fulfills the roles of strongly adhering to a substratum while permitting continuous molting and radial growth at the periphery.

Barnacles resist removal by crack trapping

We study the mechanics of pull-off of a barnacle adhering to a thin elastic layer which is bonded to a rigid substrate. We address the case of barnacles having acorn shell geometry and hard, calcarious base plates. Pull-off is initiated by the propagation of an interface edge crack between the base plate and the layer. We compute the energy release rate of this crack as it grows along the interface using a finite element method. We also develop an approximate analytical model to interpret our numerical results and to give a closed-form expression for the energy release rate. Our result shows that the resistance of barnacles to interfacial failure arises from a crack-trapping mechanism.

Uniformity and interfaces in ion-beam deposited Al/Ni multilayers

The uniformity and reaction kinetics of ion-beam deposited Al/Ni multilayer samples with the same composition, Al 81.8 Ni 18.2 , and modulation wavelength, Λ = 20 nm, but with different total film thicknesses were investigated by x-ray diffraction and differential scanning calorimetry measurements. The total film thicknesses varied between approximately 0.5 and 2.0 μm. It was found that the interface widths were approximately 1 nm and the Ni layers are much more disordered than the Al layers. The thicker samples show an increase in disorder on a length scale comparable to Λ. In other experiments, a change was observed with increasing modulation wavelength from semicoherent interfaces with a low density of misfit dislocations to semicoherent interfaces with a high density of misfit dislocations. The reaction kinetics for forming the Al 9 Ni 2 phase is independent of the sample thickness.

A mesoscale computer simulation of multiaxial yield in gasar porous copper

Abstract The mesoscale biaxial plastic flow of a porous copper material produced by the ‘gasar’ gas-eutectic solidification process is examined in this study. The pore morphology is characterized by elongated, oriented and partially ordered pores. Tensile and compressive testing suggested a very anisotropic yield response, weak coupling between stresses generated by deformations applied in the directions parallel to and transverse to the longitudinal pore axes, and bulk density decreases associated with compression in the longitudinal direction. A two dimensional explicit finite element model, based on an image of the microstructure, was subjected to combinations of tensile, zero and compressive displacement loads applied to the model boundaries. The simulations provided insight into the evolution of local micro yielding, plastic connectivity across the microstructures and buckling of the pores to explain the experimental observations.