Brandon McWilliams

Fully coupled thermal–electric-sintering simulation of electric field assisted sintering of net-shape compacts

A fully coupled thermal–electric-sintering finite element model was developed and implemented to predict heterogeneous densification in net-shape compacts using electric field assisted sintering techniques (FAST). FAST is a single-step processing operation for producing bulk materials from powders, in which the powder is heated by the application of electric current under pressure. Previous modeling efforts on FAST have mostly considered the thermal–electric aspect of the problem and have largely neglected the sintering aspect of the problem. A new model was developed by integrating a phenomenological sintering model into a previously established thermal–electric finite element framework to predict the densification kinetics of the sample. The model was used to quantify the effect of specimen geometry on the evolution of thermoelectric gradients and resulting heterogeneous sintering kinetics during FAST processing of a conductive powder. It is shown that the new model which considers sintering kinetics and density-dependent properties provides a substantial increase in accuracy compared to thermal–electric only models. It is also shown that small changes in local resistance due to densification can greatly impact the distribution of thermoelectric gradients during the process, which are exacerbated by heterogeneous stress states induced by sample geometry. Experimental characterization of sintered specimens is used to provide qualitative validation of the model predictions.

Extremely high strength and work hardening ability in a metastable high entropy alloy

Design of multi-phase high entropy alloys uses metastability of phases to tune the strain accommodation by favoring transformation and/or twinning during deformation. Inspired by this, here we present Si containing dual phase Fe42Mn28Co10Cr15Si5 high entropy alloy (DP-5Si-HEA) exhibiting very high strength (1.15 GPa) and work hardening (WH) ability. The addition of Si in DP-5Si-HEA decreased the stability of f.c.c. (γ) matrix thereby promoting pronounced transformation induced plastic deformation in both as-cast and grain refined DP-5Si-HEAs. Higher yet sustained WH ability in fine grained DP-5Si-HEA is associated with the uniform strain partitioning among the metastable γ phase and resultant h.c.p. (ε) phase thereby resulting in total elongation of 12%. Hence, design of dual phase HEAs for improved strength and work hardenability can be attained by tuning the metastability of γ matrix through proper choice of alloy chemistry from the abundant compositional space of HEAs.

Dynamic micromechanical modeling of textile composites with cohesive interface failure

Micromechanical finite element modeling has been employed to investigate the failure of several compositionally varied textile composite materials under dynamic loading. A previously developed cohesive element failure model for interface strength is employed at the phase boundary between the fiber tows and the interstitial matrix to determine the effects of interface properties on the failure behavior of a 2D plain weave and 3D orthogonal weave S2 glass/BMI composite. Thus, tow pullout and separation have been included in addition to more classical micro-level failure modes such as fiber breakage and matrix microcracking. The dynamic response of a representative volume element (RVE) is determined at strain rates of 1000 and 10,000 strain/s in an explicit finite element formulation. A parametric study has investigated compositional effects on impact strengths of two weave geometries with a relatively ‘strong’ versus ‘weak’ interface property at 10,000 and 1000 strain/s in tension and compression. The successful implementation of the cohesive failure scheme into the textile RVE framework is shown, and fundamental macro-level failure cases are investigated to relate micromechanical parametric variation to consequent strength effects.

Comparison of SPS Processing Behavior between As Atomized and Cryomilled Aluminum Alloy 5083 Powder

Aluminum 5083 powder, both as atomized and cryomilled, was consolidated via spark plasma sintering (SPS). This study quantified and compared the effects of heating an aluminum alloy powder directly through Joule heating vs indirectly through thermal conduction from the die during SPS processing. When consolidated under the same processing conditions, the cryomilled powders showed faster heating rates and densification than the as atomized powder. It was also possible to process the cryomilled powder in a non-conductive die but not the as atomized powder. This could be ascribed to an improvement in electrical conductivity of the powder due to the break up and redistribution of surface oxides after cryomilling. The changes in behavior as a result of cryomilling and/or changing die material led to samples with different fracture morphologies and increased hardness values.

Dynamic Failure Mechanisms in Woven Ceramic Fabric Reinforced Metal Matrix Composites During Ballistic Impact

The complex interaction of dynamic stress waves during ballistic impact provides the opportunity to simultaneously observe the high strain rate loading response under various triaxialities including tension, compression, and shear. In this work the dynamic failure mechanisms of woven ceramic fabric reinforced aluminum metal matrix composites (MMC) during ballistic impact are experimentally investigated. In addition to experimental characterization, an orthotropic elastic-plastic constitutive model with hydrostatic pressure dependent yield is implemented in an explicit finite element code to quantify the stress states present during the progressive damage and failure of the MMC during the penetration and perforation process.

Simulation of High Rate Failure Mechanisms in Composites During Quasi-static Testing

High Rate testing is often difficult to perform, requires specialized equipment and often had results that are difficult to interpret. Being able to simulate the same failure mechanisms at high rates would enable rapid material selection. In this experimental investigation, the mechanical response is determined of different hybrid composites materials. A modified through-the-thickness tests has been used to force a high order of failure mode that is similar to those seen in high speed impact. Failure is observed in the samples and stresses on the surface are determine to help with failure envelope measurements. The details of a comparison between 2D fabrics vs. 3D woven fabrics has been under taken and a comparison will be presented showing the similarities and differences in response. A discussion of the comparison between the high rate testing and the low-rate testing will be investigated and discussion on the applications of the testing will be examined for failure envelope prediction.

Effect of Particle Size Distribution on the Response of Metal-Matrix Composites

An enhanced continuum model for ceramic particle reinforced metal matrix composites (MMCs) is used to explore the effect of particle size distribution on the variability in deformation response of heterogeneous microstructures. The model incorporates particle size dependent strengthening through a “punched” zone around the particles that is the result of an increase in dislocation density due to geometrically necessary dislocations generated by the mismatch in coefficients of thermal expansion of the particle and matrix. In this work, these zones are explicitly accounted for in mesoscale finite element simulations of representative heterogeneous composite microstructures consisting of randomly distributed particles in a metal matrix. Additionally, particle-matrix interface decohesion is incorporated through the use of cohesive zones. The results demonstrate that in the absence of material failure, the mean particle size of a distribution is sufficient to predict the elastic–plastic response with nominal variance in the response of the composite. The effect of interface strength on particle stresses is quantified and shown to reduce particle fracture in distributions containing large particles.

Processing method and process modeling of large aperture transparent magnesium aluminate spinel domes

Polycrystalline spinel serves as an alternative to materials such as sapphire and magnesium fluoride that are currently being used in electromagnetic window applications such as missile domes, where high strength, high hardness and high transmittance in the visible and infrared spectra are required. The cubic crystal lattice of spinel imparts an isotropy to the bulk optical property, which eliminates optical distortion due to birefringence that occurs in sapphire and other non-cubic materials. The current study is to find a reliable manufacturing process to produce large magnesium aluminate spinel domes from powder consolidation efficiently. A binder-less dry ball milling process was used to deflocculate the spinel powder to increase its fluidity in an effort to ease the shape-forming. Dry ball milling time trials were conducted at several intervals to determine the appropriate level of time required to break up both the hard and soft agglomerates associated with the virgin spinel powder. The common problems encountered in dry powder shape-forming are crack growth and delamination of the green body during cold isostatic pressing (CIPing). The cracking and the delamination are due to the buildup of stress gradients on the green body that are created by the frictional force between the powder and the die wall or mold wall. To understand the stresses during the CIPing process, a finite element analysis of stresses on the green body was conducted. The simulation was used to evaluate the effect of die tooling and process characteristics on the development of stress gradients in the green body dome. Additionally, the effect of friction between the die wall and powder was examined by the simulation. It was found that by mitigating the frictional forces, cracking and delamination on the green body could be eliminated. A stepped-pressure CIPing technique was developed to reduce stress gradient build-up during CIPing. Also, oleic acid lubricant was applied to the die wall to reduce the wall friction between the powder and the die itself. As a result of these two above-mentioned methods, it was demonstrated that it is possible to consolidate a binder-free powder into large defect-free domes.