Dina Goldbaum

Nanoindentation studies to separate thermal and optical effects in photo-softening of azo polymers

Mechanical characterization of an azobenzene dye-containing polymer (p4VP(DY7)0.50) by nanoindentation shows a significant photo-softening effect under visible irradiation at 532 nm. Both strong rate-dependent plastic softening, as well as a rate-independent elastic modulus decrease are observed. Indentation at elevated, sub-glass transition temperatures results in only a rate-independent decrease in hardness however. These findings indicate, from a mechanics standpoint, for the first time a distinct mechanism for the photomechanical softening in azo-materials that is different from a simple thermal effect. The main hallmark of the photosoftening effect was significant viscoplastic flow with strong rate dependence. The presence of the viscoplastic softening in azo-polymers was accounted for in analysis of nanoindentation data to obtain an accurate elastic modulus unaffected by the presence of creep in the unloading curves. These results provide considerable insight into the long-observed and long-debated mechanism of all-optical surface patterning below Tg.

Microstructure and Mechanical Properties of Ti Cold-Spray Splats Determined by Electron Channeling Contrast Imaging and Nanoindentation Mapping

Cold spray is a thermo-mechanical process where the velocity of the sprayed particles affects the deformation, bonding, and mechanical properties of the deposited material, in the form of splats or coatings. At high strain rates, the impact stresses are converted into heat, a phenomenon known as adiabatic shear, which leads to grain re-crystallization. Grain re-crystallization and growth are shown to have a direct impact on the mechanical properties of the cold-sprayed material. The present study ties the microstructural features within the cold-sprayed Ti splats and the substrate to the bonding mechanism and mechanical properties. High-resolution electron channeling contrast imaging, electron backscatter diffraction mapping, and nanoindentation were used to correlate the microstructure to the mechanical properties distribution within the titanium cold-spray splats. The formation of nanograins was observed at the titanium splat/substrate interface and contributed to metallurgical bonding. An increase in grain re-crystallization within the splat and substrate materials was observed with pre-heating of the substrate. In the substrate material, the predominant mechanism of deformation was twinning. A good relationship was found between the hardness and distribution of the twins within the substrate and the size distribution of the re-crystallized grains within the splats.

Cold-spray processing of titanium and titanium alloys

The knowledge base for cold spray has grown significantly in recent years and advances in the technology have shown viability for the process for titanium-based materials. For titanium and its alloys, the capability to rapidly produce thick, unoxidized deposits in open air makes cold spray a particularly attractive process. Here, selected results from the available literature are discussed to highlight key practical and fundamental aspects in cold-spray processing of titanium-base powders. An overview is presented for the general process and its underlying particle deformation and bonding mechanisms. The properties of cold-sprayed titanium material are characterized and process–microstructure–property relationships reviewed. Coating and additive manufacturing applications are then briefly outlined with a discussion of practical considerations. In the concluding remarks, views are offered on the current status and possible future directions of development for the technology.

Microstructural Characterization of Mg-0.3Al-0.2Ca Alloy Using Ion Milling Surface Preparation Technique

The Vickers microindentation and spherical nanoindentation tests were carried out on a polycrystalline Mg–0.3Al–0.2Ca alloy specimen. Specimens were prepared with chemical polishing and ion milling techniques. The electron channeling contrast imaging (ECCI) and electron backscattered diffraction (EBSD) were used to study the plastic deformation inside and around the indents with a cold-field emission scanning electron microscope. The distribution of strain fields, slip lines, and twins was imaged on an indented surface. A crystallographic orientation mapping was performed to map the local crystallographic misorientation associated with channeling contrast variations. The chemical polishing technique was used to remove the material from the surface and allow the study of deformed microstructure at a maximum depth of the indent. The combination of ECCI and EBSD with chemical polishing and ion milling surface preparation techniques proved to be beneficial to study the indentation-induced plastic deformation in a polycrystalline Mg–0.3Al–0.2Ca alloy specimen.

Electron Channeling Contrast Imaging of Plastic Deformation Induced by Indentation in Polycrystalline Nickel

Vickers microindentation and Berkovich nanoindentation tests were carried out on a polycrystalline nickel (Ni) bulk specimen. Electron channeling contrast imaging (ECCI) in conjunction with electron backscattered diffraction was used to image and characterize plastic deformation inside and around the indents using a field emission scanning electron microscope. The ECCI was performed with a 5 keV beam energy and 0° tilt specimen position. The strain field distribution, slip lines, and Taylor lattices were imaged on an indented surface. Orientation mapping was used to investigate the local crystallographic misorientation and identify specific ⟨110⟩ slip systems. An ion milling surface preparation technique was used to remove materials from the surface which permitted the study of deformed microstructure below the indent. A dislocation density of 1011 cm-2 was calculated based on the curvature of bend contours observed in the ECCI micrographs obtained from the Vickers indents. A yield strength of 500 MPa was calculated based on the size of the strain field measured from the ECCI micrographs of the nanoindents. The combination of ion milling, ECCI, and electron backscattered diffraction was shown to be beneficial to investigate the indentation-induced plastic deformation in a polycrystalline Ni bulk specimen.

Nanostructure, osteopontin, and mechanical properties of calcitic avian eggshell

The biomineral phase of avian eggshell is characterized at the nanostructure scale and correlated with functional properties. Avian (and formerly dinosaur) eggshells form a hard, protective biomineralized chamber for embryonic growth—an evolutionary strategy that has existed for hundreds of millions of years. We show in the calcitic chicken eggshell how the mineral and organic phases organize hierarchically across different length scales and how variation in nanostructure across the shell thickness modifies its hardness, elastic modulus, and dissolution properties. We also show that the nanostructure changes during egg incubation, weakening the shell for chick hatching. Nanostructure and increased hardness were reproduced in synthetic calcite crystals grown in the presence of the prominent eggshell protein osteopontin. These results demonstrate the contribution of nanostructure to avian eggshell formation, mechanical properties, and dissolution.

Reference Specimen for Nondestructive Evaluation: Characterization of the Oxide Layer of a Cold Shot in Inconel 600

The presence of a cold shot in an aircraft turbine blade can lead to the catastrophic failure of the blade and ultimately to the failure of the power plant. Currently, no nondestructive evaluation (NDE) method exists to detect this kind of defect. This deficiency is primarily due to the fact that the only known cold shot defects in existence are those found in failed blades. Therefore, in order to develop effective NDE methods, reference specimens are needed which mimic the embedded oxide layer that is a primary distinguishing feature of a cold shot. Here, we present a procedure to synthetically reproduce the features of a real cold shot in Inconel 600 and the precise characterization of this oxide layer as a reference specimen suitable for NDE evaluation. As a first step to develop a suitable NDE technique, high-frequency ultrasound simulations are considered. A theoretical 1-D model is developed in order to quantify the multiple reflection-transmission trajectory of the acoustic wave in the reference specimen. This paper also presents an experimental determination of the density and the Young’s modulus of the Inconel 600 oxide, which are required as inputs to calculate the acoustic impedance used in the theoretical model.