Benjamin F. Schultz

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.

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.

Reactive stir mixing of Al–Mg/Al2O3np metal matrix nanocomposites: effects of Mg and reinforcement concentration and method of reinforcement incorporation

Metal matrix nanocomposites (MMNCs) synthesized by the inexpensive and scalable method of stir mixing have received relatively little attention due to the perceived difficulty of dispersing nanoparticles (NPs) in molten metal. However, matrix/reinforcement reactions may be useful in deagglomeration of the particles. A review of previous experimental studies shows that solid solution, Orowan, and grain boundary (GB) strengthening are most likely to influence the strength of reactive stir-mixed Al–Mg–Al2O3np MMNCs. Matrix/reinforcement reactions, porosity in stir-mixed MMNCs, and NP incorporation on grain size are also discussed. Analysis of variation of grain size with NP concentration shows that the MMNCs of this study do not follow the trend of MMNCs strengthened primarily by GB strengthening, and transmission electron microscope shows that individual NPs and agglomerates were present within the aluminum alloy matrix. This evidence coupled with strength beyond what would be expected from solid solution and GB strengthening indicate that Orowan strengthening is likely present—though significant gas porosity in low Mg concentration MMNCs is often detrimental.

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.

Synthesis and Quasi-Static Compressive Properties of Mg-AZ91D-Al2O3 Syntactic Foams

Magnesium alloys have considerably lower density than the aluminum alloy matrices that are typically used in syntactic foams, allowing for greater specific energy absorption. Despite the potential advantages, few studies have reported the properties of magnesium alloy matrix syntactic foams. In this work, Al2O3 hollow particles of three different size ranges, 0.106–0.212 mm, 0.212–0.425 mm, and 0.425–0.500 mm were encapsulated in Mg-AZ91D by a sub-atmospheric pressure infiltration technique. It is shown that the peak strength, plateau strength and toughness of the foam increases with increasing hollow sphere wall thickness to diameter (t/D) ratio. Since t/D was found to increase with decreasing hollow sphere diameter, the foams produced with smaller spheres showed improved performance—specifically, higher energy absorption per unit weight. These foams show better performance than other metallic foams on a specific property basis.

Synthesis and Properties of Metal Matrix Nanocomposites (MMNCS), Syntactic Foams, Self Lubricating and Self-Healing Metals

Recent work on particle reinforced metal-matrix nanocomposites (MMNCs), Syntactic Foams, and Self-Healing Metals by solidification synthesis will be reviewed. Particle-based MMNCs have been developed by several modern processing tools based on either solid- or liquid-phase synthesis techniques and are claimed to exhibit very exciting mechanical properties including significant improvements of modulus, yield strength, and ultimate tensile strength. The paper will describe work at UWM along with a critical review of work done elsewhere to identify the challenges and future opportunities in this area. Recent work will also be presented on synthesis and properties of energy-absorbing Al and Mg-based syntactic foams with a comparison to other related materials reported in the literature. In addition, advances in self-healing metal matrix composites will be discussed.

EXPERIMENT AND COMPUTATIONAL ANALYSIS OF SELF-HEALING IN AN ALUMINUM ALLOY

The development of self-healing metals is a novel idea that has not been explored in great detail yet. The concept of self-healing described in this paper consists of incorporating a low temperature melting alloy imbedded within a higher temperature alloy to create a self healing composite (SHC). When the SHC is damaged or cracked, heat may be applied to the affected area whereupon the low melting alloy will melt and flow into the crack healing the damage and sealing the crack. This study consists of theoretical analysis and design of self-healing in aluminum alloy matrix. The experimental and Computational Fluid Dynamics of a self-healing were designed by the authors, the design consists in an aluminum alloy matrix reinforced with microtubes of alumina (Al2O3) that are filled with a low melting point solder alloy. The objective of the study reported here was to find the influence and efficiency of a low melting solder alloy in healing an aluminum matrix. To check this effect a crack was created in the metal surface, piercing the microtube(s) filled with solder, and then the SHC was heated above the melting point of the solder alloy to melt and examine the flow of molten solder alloy into the crack. NOMENCLATURE

Impact of Brownian motion on the particle settling in molten metals

Understanding the settling behavior of nanoparticles in molten metals/alloys is important as it will aid in achieving uniform dispersions of reinforcement particles in metal matrix nanocomposites. Uniform dispersions are necessary to activate the Orowan strengthening mechanism, which can increase yield strength without significant diminishment of ductility. In this work, an analytical model of particle size effects on settling is described that takes into account both deterministic Stokes’ law and stochastic Brownian motion. The model shows a clear transitional behavior where settling velocity follows Stokes’ law for large particles and then drops to zero for small particles implying that Brownian motion predominates. It indicated that, in the Brownian motion regime, where the discrete nature of the liquid must be considered, the random motion imparted by unbalanced collisions can overwhelm the motions normally imposed by forces such as gravity, viscous drag, and thermal/concentration gradients.

Laser Engineered Net Shaping Process for 316L/15% Nickel Coated Titanium Carbide Metal Matrix Composite

This paper presents the investigation of the macrostructure, microstructure, and solidification structure of a 316L/15% nickel coated TiC metal matrix composite produced by the laser engineered net shaping (LENS™) process. The focus of this work was to (1) identify the solidification structure and to estimate growth/cooling rates at the solid/liquid interface, (2) identify and quantify discontinuities in the build structure, and (3) examine the effect of solidification and thermal history on the sample microstructure to further the understanding of the LENS process. A Numerical method was also developed to examine the influence of material type and LENS™ process parameters on the forming of the specific microstructures from thermodynamics and fluid dynamics point of view. Samples of 316L stainless steel were examined, microstructures of samples were used to estimate the corresponding cooling rate, and the cooling rate was compared with the results of numerical modeling. The computational results show reasonable agreement with experimentally determined cooling rate.

Design and Demonstration of Self-Healing Behavior in a Lead-Free Solder Alloy

A critical review of self-healing behavior in metals and metal composites with a focus on the opportunities and challenges inherent to their design and application will be presented. In this work, a self-healing composite (SHC) is designed, consisting of carbon fiber microtubes filled with a low melting temperature alloy imbedded within a lead-free solder alloy having a higher melting point than the filler alloy. In this concept, when the SHC is damaged or cracked, heat can be applied to the affected area whereupon the low melting alloy will melt and flow into the crack. Once heat is removed, the filler will solidify, sealing the crack to effectively heal the damaged alloy. This will lead to a decrease in the electrical resistivity of the healed composite compared to the damaged material. The research described in this paper explores the concept of self healing to fabricate self healing lead-free solder matrix composites and to probe the healing agent as sealing material, with the aid of Computational Fluid Dynamics (CFD) models developed by the authors for the manufacturing processes and self-healing behavior. The simulation consists of healing a cracked SHC lead-free solder alloy reinforced with carbon fiber microtubes filled with a low melting solder alloy. The objective of this study is to find the influence and efficiency of this concept of self-healing for lead-free solder alloys and their composites. Experimental validation of the CFD analysis will be presented. Micro-electric soldered joints are subjected to very large amounts of thermo-mechanical cycling, in which large amounts of stress are produced from serving as both an electrical and mechanical connection; thus a self healing technology could be extremely useful in variety of applications.