Tomoko Sano

High Strain Rate Performance of Pressureless Sintered Boron Carbide

The processing technique used to consolidate ceramics powders can have a large effect on the microstructure, and hence the performance of the material. In this research, microstructure, mechanical properties, and the high strain rate compressive behavior of pressureless sintered boron carbide (B4C) samples were examined and compared to those of conventional hot pressed B4C. Penetration velocity tests were conducted on identical targets made with the pressureless sintered B4C samples and hot pressed B4C. Microstructural and post mortem characterization showed that test results of the pressureless sintered B4C were affected by significant porosity in the samples. The effects of the processing technique on the microstructure, properties, and the high rate behavior of the pressureless sintered B4C will be discussed.

The Effect of Friction Stir Welding on the Microstructure and Tensile Properties of Al 2139-T8 Alloys

The welding process introduces heat, plastic deformation, and chemical variation into the weld joints and alters the microstructure, strength, and ductility of the welded region. In this research, two plates of Al 2139-T8 alloys were welded together by friction stir welding (FSW). An evaluation of the crystallographic texture, grain size, and morphology of the grains in the FSW region, in comparison to those of the grains outside of the weld region, was made by scanning electron microscopy (SEM) and electron backscattered diffraction. In addition, the quasi-static tensile behavior of samples from the FSW region and those from outside of the FSW region was characterized by in-situ tensile experiments in the SEM with digital image correlation. It was found that the ultimate tensile strength was greater in the samples from outside FSW region and the elongation was greater for samples from the FSW region. The full results of the effect of the FSW on the Al2139 microstructure and tensile behavior will be discussed in detail.

Femtosecond Laser Machining of Micro-tensile Specimens for High Throughput Mechanical Testing

Recently, a variety of techniques for fabricating microscale specimens for mechanical testing have been developed. However, many of these techniques are limited by material compatibility, specimen sizes, low throughput, or microstructural damage. In contrast, the advent of femtosecond laser systems offer opportunities to circumvent many of these challenges. Here we demonstrate the use of a femtosecond laser milling system to rapidly fabricate micro-tensile specimens across a range of critical dimensions, from 10’s to 100’s of microns. By incorporating the laser milling system into a custom micromechanical testing apparatus, we are able to achieve high throughput investigations of mechanical properties at the microscale. These capabilities are demonstrated on metal and ceramic materials.

Dynamic Failure and Fragmentation of a Hot-Pressed Boron Carbide

This study investigates the failure and fragmentation of a hot-pressed boron carbide during high rate impact experiments. Four impact experiments are performed using a composite-backed target configuration at similar velocities, where two of the impact experiments resulted in complete target penetration and two resulted in partial penetration. This paper seeks to evaluate and understand the dynamic behavior of the ceramic that led to either the complete or partial penetration cases, focusing on: (1) surface and internal failure features of fragments using optical, scanning electron, and transmission electron microscopy, and (2) fragment size analysis using state-of-the-art particle-sizing technology that informs about the consequences of failure. Detailed characterization of the mechanical properties and the microstructure is also performed. Results indicate that transgranular fracture was the primary mode of failure in this boron carbide material, and no stress-induced amorphization features were observed. Analysis of the fragment sizes for the partial and completely penetrated experiments revealed a possible correlation between larger fragment sizes and impact performance. The results will add insight into designing improved advanced ceramics for impact protection applications.

High Throughput Femtosecond-Laser Machining of Micro-Tension Specimens

Femtosecond laser-based machining shows great promise for micro-scale specimen fabrication as this technique combines fast and precise milling while minimizing surface damage. Whereas focused ion beam (FIB) based micromachining techniques achieve milling rates smaller than ~1 µm3/s, femtosecond lasers can achieve milling rates up on the order of 103 µm3/s, allowing for high throughput specimen fabrication and experimentation. Utilizing only a femtosecond laser and a precison three-axis positioning stage, microtensile specimens were created with gauge widths less than 50 µm from bulk metals.

Correlation of Microstructure to Mechanical Properties in Two Grades of Alumina

In this research, two grades of alumina, one at nominally 85% composition and the other at 99.5% were characterized. Microstructural and phase characterization was conducted using Scanning Electron Microscopy, Energy Dispersive Spectroscopy, X-ray Diffraction, and micro-computed X-ray tomography. It was determined that the Knoop hardness values were influenced by the porosity in the 85% composition. Quasi-static compressive tests and high strain rate compression experiments were conducted to determine the influence of the microstructure to the compressive properties. It was observed that the overall compressive strengths increased with strain rate. Although the Knoop hardness values were much lower in the 85% alumina due to the porosity, the compressive strength at both quasi-static and dynamic strain rates were not significantly lower than those of the 99.5% composition.

Analysis of the Microstructure and Properties of Friction stir Weld Zones in the Al 2139-T8 Alloy

The welding process introduces heat, plastic deformation, and chemical variation into the weld joints and modifies the microstructure, strength, and ductility of the welded region. Samples from two plates of Al 2139-T8 alloys welded together by friction stir welding (FSW) were examined. The microstructure of the stir zone (SZ), thermomechanical affected zone (TMAZ), and the unaffected base alloy were evaluated by scanning electron microscopy (SEM) and electron backscattered diffraction. The deformation and quasi-static tensile behavior of the samples from the SZ, TMAZ, and unaffected base alloy were examined by in-situ tensile experiments in the SEM, at room temperature and at 300°C. The SZ samples showed higher elongation to failure and the unaffected base alloy exhibited the least amount of elongation. The ultimate tensile strength was determined to be lower in the TMAZ, and the yield strength increased away from the FSW region. The higher temperature tensile experiments showed a significant reduction in strength. The full results of the effect of the FSW on the A12139-T8 microstructure and tensile behavior will be discussed in detail.

In-Situ Scanning Electron Microscopy Comparison of Microstructure and Deformation Between WE43-F and WE43-T5 Magnesium Alloys

In-situ tensile testing in the scanning electron microscope was used to investigate the quasi-static deformation behavior and fracture mechanism of WE43 magnesium alloys. The in-situ tensile experiments were conducted at room temperature at a constant crosshead speed of 0.5 mm / min. One set of samples was a rolled and quenched F temper alloy and the other set was an artificially aged T5 temper alloy. The objective of this research was to determine the effect of tempering on precipitates chemistries, microstructure, and mechanical properties. The sample orientation is known to affect the tensile properties. Hence tensile specimens with different sample orientation were tested. The crystallographic orientations were characterized by electron backscattered diffraction. Strong textures were observed with rolling plane crystals indicating a basal plane orientation.

The Effect of the Variation of Microstructure in the Friction Stir Welded Zone on the Strain and Tensile Properties of Al2139

The friction stir welding process creates three distinct zones; the weld nugget, themomechanically affected zone, and the heat affected zone. These zones have varying microstructure, texture, amounts of plastic deformation and dynamic recrystallization. The elevated local stresses and non-uniform strains under tension are attributed to the inhomogeneity in the weld. The ultimate tensile strength (UTS), yield stress, and elastic modulus were determined by tensile tests of samples in three orthogonal orientations from inside and outside the weld. The samples from inside the weld showed lower UTS and yield strength, but longer elongation than those from outside the weld. Digital image correlation was used to map the strain variations of the samples during the tensile tests. In addition, electron backscattered diffraction was used to determine the grain size and texture variation of the grains in the weld zones. The correlation of the microstructure variation on the tensile properties will be discussed.