Kyu Cho

Fracture toughness of advanced ceramics at room temperature

This report presents the results obtained by the five U.S. participating laboratories in the Versailles Advanced Materials and Standards (VAMAS) round-robin for fracture toughness of advanced ceramics. Three test methods were used: indentation fracture, indentation strength, and single-edge pre-cracked beam. Two materials were tested: a gas-pressure sintered silicon nitride and a zirconia toughened alumina. Consistent results were obtained with the latter two test methods. Interpretation of fracture toughness in the zirconia alumina composite was complicated by R-curve and environmentally-assisted crack growth phenomena.

Prediction models for the yield strength of particle-reinforced unimodal pure magnesium (Mg) metal matrix nanocomposites (MMNCs)

Particle-reinforced metal matrix nanocomposites (MMNCs) have been lauded for their potentially superior mechanical properties such as modulus, yield strength, and ultimate tensile strength. Though these materials have been synthesized using several modern solid- or liquid-phase processes, the relationships between material types, contents, processing conditions, and the resultant mechanical properties are not well understood. In this paper, we examine the yield strength of particle-reinforced MMNCs by considering individual strengthening mechanism candidates and yield strength prediction models. We first introduce several strengthening mechanisms that can account for increase in the yield strength in MMNC materials, and address the features of currently available yield strength superposition methods. We then apply these prediction models to the existing dataset of magnesium MMNCs. Through a series of quantitative analyses, it is demonstrated that grain refinement plays a significant role in determining the overall yield strength of most of the MMNCs developed to date. Also, it is found that the incorporation of the coefficient of thermal expansion mismatch and modulus mismatch strengthening mechanisms will considerably overestimate the experimental yield strength. Finally, it is shown that work-hardening during post-processing of MMNCs employed by many researchers is in part responsible for improvement to the yield strength of these materials.

Sintering Aids in the Consolidation of Boron Carbide (B4C) by the Plasma Pressure Compaction (P2C) Method

Abstract Boron carbide (B4C) powder has been densified by a novel method of powder consolidation known as Plasma Pressure Compaction (P2C). The P2C technique allows for rapid consolidation of powder by Joule heating of the powder bed. Powder is placed in graphite dies, and uniaxial pressure and low-voltage, high-amperage (10 V, 5000 amps maximum) direct current are applied to achieve densification. Pure B4C powder was consolidated at lower temperature and hold time to densities equal to those achieved by conventional hot pressing. With the addition of a small amount of alumina (Al2O3) as a sintering aid, densities as high as 97% theoretical were attained.

Plasma Pressure Compaction of Tungsten Powders

Abstract Compacts of tungsten powders were consolidated by Plasma Pressure Compaction (P2C), an electric discharge technique. The powders were a variety of commercially available grades ranging in average particle size from submicron to 12 microns. Following consolidation, the density of the compacts was measured, and the microstructure examined. Results revealed the effect of powder size, pulsed current treatment, final hold temperature, and applied pressure on final part density and microstructure development. Most important to the purpose of the study, it was found that the short cycle time of P2C did not suppress grain growth in the compacts of submicron powder. Thus, grain growth remained a consequence of full densification. Implications of these results for the development of ultra fine-grained microstructures using P2C are discussed.

Effect of crystallographic alignment on the magnetocaloric effect in alloys near the Ni2MnGa stoichiometry

Prior to the development of commercial applications of magnetic refrigerator technology, a large magnetocaloric effect (MCE) in polycrystalline materials must be realized for relatively low magnetic field changes. To increase the MCE, a crystallographic alignment technique, consisting of thermal cycling about the martensite phase transition temperature under a compressive stress, was applied to Heusler alloys with nominal composition Ni2+xMn1−xGa (x = 0.14, 0.16). Magnetic measurements prior to grain alignment show that the maximum entropy changes of −16 J kg−1K−1 and −24 J kg−1K−1 for samples with x = 0.14 and 0.16, respectively, occurred for a magnetic field change of 7 T. After grain alignment, there was a 56%–79% enhancement of the maximum magnetic entropy change for the same magnetic field change of 7 T. This suggests that thermal cycling under compressive stress may either increase grain alignment (e.g., texture) along the magnetic easy (001) axis, and/or enhance the ease with which a magnetic field...

On the superposition of strengthening mechanisms in dispersion strengthened alloys and metal-matrix nanocomposites: Considerations of stress and energy

Yield strength improvement in dispersion strengthened alloys and nano particle-reinforced composites by well-known strengthening mechanisms such as solid solution, grain refinement, coherent and incoherent dispersed particles, and increased dislocation density resulting from work-hardening can all be described individually. However, there is no agreed upon description of how these mechanisms combine to determine the yield strength. In this work, we propose an analytical yield strength prediction model combining arithmetic and quadratic addition approaches based on the consideration of two types of yielding mechanisms; stress-activated and energy-activated. Using data available in the literature for materials of differing grain sizes, we consider the cases of solid solutions and coherent precipitates to show that they follow stress-activated behavior. Then, we applied our model with some empirical parameters to precipitationhardenable materials of various grain sizes in both coherent and incoherent precipitate conditions, which demonstrated that grain boundary and Orowan-strengthening can be treated as energy-activated mechanisms.

Mechanical performance of particulate-reinforced Al metal-matrix composites (MMCs) and Al metal-matrix nano-composites (MMNCs)

The metal-matrix composites/nano-composites (MMCs/MMNCs) reinforced with hard ceramic particulates have received a tremendous attention due to their potential improvements in physical and mechanical performances. In the present work, we have comprehensively collected currently available experimental data sets of Al-based MMCs/MMNCs and have carried out thorough analyses to quantitatively address the impacts of the reinforcement volume fractions, reinforcement particle sizes, and metal-matrix grain sizes on their mechanical properties including the yield strength, ultimate strength, and strain to failure of composites. We also performed a quantitative analysis on the strengthening mechanisms of Al MMNCs to reveal that the grain refinement can play a major role in increasing the strength of composites. Al-based MMC or MMNC materials generally exhibited an indirect relationship between the strength increase and strain-to-failure increase. The results include a critical comparison for the mechanical performance of particulate-reinforced composites for both pure and alloyed Al matrices to elucidate the contemporary status of Al MMC and MMNC materials.

Impact of Volume Fraction and Size of Reinforcement Particles on the Grain Size in Metal–Matrix Micro and Nanocomposites

In metal–matrix micro and nanocomposites (MMCs and MMNCs), the presence and interactions of various strengthening mechanisms are not well understood, but grain boundary strengthening is considered as one of the primary means of improving the yield strength of composites. Owing to the importance of grain size on mechanical properties, it is necessary to be able to describe how incorporation of nanoparticles (NPs) in both powder metallurgy (PM) and solidification processing (SP) affects this critical property. In the present work, we provide a basis for an empirical equation that relates particle fraction and particle size to MMNC grain size for both PM and SP synthesis methods. The model suggests that NPs retard grain coarsening in PM MMNCs and also seems to describe the effect of reinforcement concentration on grain size in SP MMCs and MMNCs.

Brownian Motion Effects on Particle Pushing and Engulfment During Solidification in Metal-Matrix Composites

Particle pushing and/or engulfment by the moving solidification front (SF) is important for the uniform distribution of reinforcement particles in metal-matrix composites (MMCs) synthesized from solidification processing, which can lead to a substantial increase in the strength of the composite materials. Previous theoretical models describing the interactions between particle and moving SF predict that large particles will be engulfed by SF while smaller particles including nanoparticles (NPs) will be pushed by it. However, there is evidence from metal-matrix nanocomposites (MMNCs) that NPs can sometimes be engulfed and distributed throughout the material rather than pushed and concentrated in the last regions to solidify. To address this disparity, in this work, an analytical model has been developed to account for Brownian motion effects. Computer simulations employing this model over a range of the SF geometries and time steps demonstrate that NPs are often engulfed rather than pushed. Based on our results, two distinct capture mechanisms were identified: (i) when a high random velocity is imparted to the particle by Brownian motion, large jumps allow the particle to overcome the repulsion of the SF, and (ii) when the net force acting on the particle is insufficient, the particle is not accelerated to a velocity high enough to outrun the advancing SF. This manuscript will quantitatively show the effect of particle size on the steady state or critical velocity of the SF when Brownian motion are taken into consideration. The statistical results incorporating the effects of Brownian motion based on the Al/Al2O3 MMNC system clearly show that ultrafine particles can be captured by the moving SF, which cannot be predicted by any of classical deterministic treatments.

Data characterizing flexural properties of Al/Al2O3 syntactic foam core metal matrix sandwich

Microstructural observations and flexural property datasets are provided for aluminum alloy matrix syntactic foam core sandwich composites. The tests are conducted in three-point bending configuration. The data supplied includes methods used for conducting microscopy and mechanical testing. Raw load–displacement data, which is used to plot stress–strain graphs, obtained during the flexural test is also included. Images from a DSLR camera are stitched together to form a detailed failure sequencing video. Failure of specimens is captured in sequential images using a digital camera. These images are stitched together to develop a video for visualization of failure mechanisms. Calculations are also included for a theoretical model that is used to estimate the flexural properties of the syntactic foam core sandwich.