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School of Materials Science and Engineering

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Effects of strain rate on mechanical properties and fracture mechanisms in dual phase steels

(Georgia Institute of Technology, 2019-07-31) Sharma, Maruwada Sukanya

Dual Phase (DP) steels are a class of Advanced High Strength Sheet (AHSS) steels which are used as structural components of an automobile body. They possess a good combination of strength and formability coupled with crashworthiness. The microstructure of DP steels consists largely of ferrite and martensite. These commercial grade steels may also contain a coating layer to protect the steel against atmospheric corrosion. These steels are exposed to strain rates of the order of 10-10^2/s during sheet metal forming operations, and strain rates of the order of 10^2−10^4/s can be reached under an automotive crash condition. The fracture mechanisms of these DP steels at slow strain rates are well understood; however, these may not be representative of the material’s fracture response under dynamic or high strain rate loading conditions. The mechanical behavior of DP steels under dynamic rates (10^2−10^4/s) have been studied in the past but there are no conclusive results on the operative fracture mechanisms. Another important aspect currently lacking is the effect of the protective coating under dynamic rates. Hence, to address these critical gaps, an understanding of the role of all microstructural features (substrate and coating) on the fracture response of DP steels under varying strain rates is required. Thus, the objective of this work is to investigate and quantitatively characterize the fracture surfaces of DP steels generated under a wide range of strain rates and gain an understanding on the microstructure-based fracture mechanisms. Four DP steels, of two nominal strength levels (590 MPa and 980 MPa) are subjected to strain rates spanning twelve orders of magnitude (10^(-6)/s to 10^6/s). Three kinds of 980 MPa DP steels with and without protective coatings are investigated. While DP 590 and DP 980 contained different amounts, all three DP 980 steels contained similar volume fractions of martensite. The differences in the volume fractions and connectivity of the martensite in the different DP steels are estimated using quantitative characterization of microstructures. The mechanical properties are measured for strain rates spanning twelve orders of magnitude from 10^(-6)/s (quasi-static strain rate) to 10^6/s (dynamic strain rates). Servo-hydraulic machines, Hopkinson bar and plate impact gas gun experiments are used to generate the different magnitudes of stresses and strain rates. An important aspect of the work performed in this study is that all of the quasi-static and intermediate strain rate experiments on the four DP steels are conducted with the same specimen geometry to eliminate the effects of post-uniform elongation and allow valid comparisons of ductility across different magnitudes of strain rates. The effect of the volume fraction of martensite is discussed both in terms of its effect on the mechanical properties and on the fracture response. Discussions on the effects of adding protective coating layers and the resulting microstructure of the three DP 980 steels are provided to understand the differences in the mechanical properties and fracture response of these steels at both quasi-static and dynamic strain rates. The strain rate sensitivity of both the mechanical properties and fracture response as a function of the underlying microstructure is also explored. The main thrust of the current work is to employ quantitative fractography, a stereological technique, to understand the effects of the quantity, distribution and morphology of the various microstructural constituents of the substrate and coating on the operative fracture mechanisms of DP steels under varying strain rates. For this purpose, the area fractions of various features observed on the fracture surfaces are estimated. Ultimately, hypotheses for the fracture mechanisms of these steels as a function of strain rate are presented. The significance of this study is to help gain a deeper understanding on the differences in microstructure obtained in DP steels of similar nominal strength levels when processed to contain protective coatings, and the effects of these differences on the mechanical properties and fracture response under strain rates representative of automotive crash. The results of the current work will help design better grades for improved forming and higher crash-resistant automobile parts.
Shock compression induced phase changes in cerium-based metallic glass

(Georgia Institute of Technology, 2018-05-29) Bryant, Alex W.

The research performed in this work was aimed at investigating pressure-induced phase changes in a Ce-based metallic glass (MG) through the use of laser-driven shock experiments and atomic resolution structural characterization. MGs exhibit very high strength, have intrinsically low density, and plastically deform by shear banding. MGs are also metastable and can undergo phase changes upon heating and/or application of high pressure into higher density configurations. The atomic structure changes concomitant with these phase transitions occurring during high pressure shock compression are not well understood, which provides the motivation for the present work. Thermal analysis of Ce3Al MG melt-spun ribbons was first performed to characterize the crystallization response and structure. Ce3Al MG was found to strongly resist growth of crystallites but easily nucleate. Thermal crystallization occurs via a two-stage primary path wherein a metastable phase forms and converts fully into the hexagonal-intermetallic α-Ce3Al. The Avrami number and dimensionality constants indicate the crystallization occurs via plate-like growth, resulting in thermally crystallized grains on the order of 6 nm and a density ~4% greater than the reference α-Ce3Al. Shock compression experiments performed using the Nd:YAG 3 J laser and velocity interferometry allowed for in operando measurements of particle velocity coupled with sample recovery for structural analysis. The results provide a clear indication of the Hugoniot Elastic Limit (at ~1.8 GPa) as evidenced by the presence of a two wave structure in the velocity profile. At shock pressures exceeding the elastic limit, plastic deformation of the Ce3Al MG occurs via structural transformation to the crystalline state forming α-Ce3Al with nanocrystalline grain sizes, higher densities, and plate-like growth. The trends suggest that shock compression causes break-up of grains, higher densities due to Ce 4f delocalization, and increased preferred orientation. Shock compression experiments were also performed using the 50 J Omega laser facility at the Laboratory for Laser Energetics. A stack of samples was shock-compressed with pressures progressively decreasing across the stack thickness, resulting in two regimes of recovered samples. Highly deformed and partly damaged samples close to the shock front showed varying degrees of long-range order, medium-range order, and short-range order with distance away from the shock front. Visually undeformed samples showed decreased bond lengths for the nearest-neighbors, second nearest-neighbors, and fourth nearest-neighbors but increased bond lengths for the third nearest-neighbors, with associated densification of ~2-6% in all layers. These changes in the undeformed samples are indicative of polyamorphism. The visually undeformed samples also reveal an increase in magnitude of structural change with increased distance away from the shock-front, up to a maximum beyond which increasing distance decreases the magnitude of the bond length shifts. This trend is indicative of competing effects for densification and dilation, associated with the extreme and complex states generated. The mechanism and characteristics of the shock induced crystallized Ce3Al MG are different from the hydrostatic pressure-induced crystallization of Ce3Al MG (which occurs via a coordinated and instantaneous rearrangement of all atoms into the FCC-Ce3Al phase) and thermal crystallization into α-Ce3Al (which occurs via diffusional nucleation and growth). Shock-induced crystallization during shock compression occurs in a nucleation-like collective rearrangement with limited kinetic allowance for growth, resulting in larger needlelike crystallites than could nucleate through thermal processes. The dilatory effects and increased driving forces caused by shear bands and shock-induced heating result in larger grain sizes and longer lattice parameters. Increases in shock pressures appear to create larger driving forces for the formation of lower energy plate-like morphologies and higher densities while simultaneously breaking crystallites into smaller sizes.
Multilayer optical structures for time-resolved meso-scale sensing of shock-compression in heterogeneous materials

(Georgia Institute of Technology, 2018-04-10) Scripka, David

Heterogeneous materials play important roles in many different applications across a wide range of industries. Examples include engineered composites, particulate systems, and energetic materials, which all display complex meso-scale features and behaviors. This complexity leads to significant gaps in the understanding of heterogeneous materials, especially under extreme conditions such as shock-compression. A fundamental challenge in this area of research is a lack of experimental diagnostics that can provide spatially-resolved information under the demanding temporal and environmental conditions of shock loading. Multilayer optical structures, due to their unique spectral responses that can be correlated to externally induced loads, have the potential to serve as a new class of sensor for these complex materials and conditions. This work presents the theory, development, and evaluation of novel multilayer optical structures as time-resolved pressure sensors with meso-scale spatial sensitivity. Time-resolved spectroscopy of laser-driven shock-compression experiments on the multilayers demonstrated spectral shifts of the characteristic spectral peaks to shorter wavelengths (blueshifts), and simultaneous velocimetry established that these spectral shifts are unambiguously correlated to the laser-driven shock pressure. An optomechanical model was developed and used to predict the spectral response of the multilayers as a function of pressure, and when informed with quality empirical data, quantitatively matches the experimentally observed blueshift. Experiments and simulations of spatially heterogeneous shock loading demonstrate the ability of the multilayers to resolve not only multiple pressures but also to capture the subtle features present in shock-compressed heterogeneous materials, all while maintaining nano-second level temporal resolution. Overall, multilayer-based sensing is a fundamentally new time-resolved diagnostic method in the fields of high-strain-rate material behavior and shock physics. This work has provided the theoretical and empirical foundation for broad classes of different multilayer structures, and demonstrated their unique potential utility for capturing the complex meso-scale pressure histories needed to enable new insights into the dynamic response of heterogeneous materials.
Physical mechanisms of laser-activated nanoparticles for intracellular drug delivery

(Georgia Institute of Technology, 2017-05-18) Holguin, Stefany Yvette

Novel intracellular drug delivery techniques are needed to overcome the barrier of the cell’s plasma membrane. In this study, we leveraged a novel, laser-mediated technique known as transient nanoparticle energy transduction (TNET), in which carbon black (CB) nanoparticles in suspension with DU145 cells and small molecules irradiated by nanosecond-pulsed near infrared (NIR) laser energy leads to efficacious delivery and high cell viability. To gain mechanistic insight into TNET, we studied various aspects of this in vitro system, including cellular mechanics, total energy input, and the role of photoacoustics. First, we studied the role of cellular mechanics in TNET by way of the cytoskeleton and plasma membrane fluidity. From these studies, we concluded that cytoskeletal mechanics are integral to resulting bioeffects achieved with TNET, whereas the fluidity of the plasma membrane is not. Next, we studied the effect of energy input into the system, which was increased by increasing laser fluence, CB nanoparticle concentration and number of laser pulses. We found that total energy input strongly correlated with resulting bioeffects. Lastly, we studied the effects of three different carbon-based nanoparticles – CB, multi-walled carbon nanotubes (MWCNT) and single-walled (SWCNT) carbon nanotubes – on cellular bioeffects. In addition to the different bioeffect profiles, CB, MWCNT, and SWCNT also exhibited differences in the intensity of photoacoustic output in the form of a single, mostly positive-pressure pulse of ~100 ns duration. Lack of a universal correlation between peak pressure and cellular bioeffects, suggested that total energy input rather than pressure output was more mechanistically relevant to TNET. Overall, this work provides functional characterization and mechanistic understanding the cellular bioeffects cause by TNET. These studies will contribute to a necessary understanding of TNET that will enable rational design of TNET systems for future applications and possible translation into the clinic.
Dynamic deformation of titanium-based bulk metallic glass composites

(Georgia Institute of Technology, 2016-11-10) Diaz, Rene Orlando

This work sought to understand the role of the microstructure of titanium-based bulk metallic glass (BMG) and bulk metallic glass matrix composites (BMG-MCs) under dynamic deformation. BMG-MCs provide enhanced toughness and ductility in contrast to monolithic BMGs through in-situ formed crystalline dendrites. The BMG and BMG-MC system investigated in this work is the titanium-based "DVX" system consisting of Ti-Zr-V-Cu-Be with varying size, morphology, and distribution of the second phase dendrites. The effect of processing and the subsequent effect on dynamic properties is also addressed with the DV1 BMG-MC processed by two different methods -- semi-solid forging (DV1-SSF) and suction casting (DV1-SC) -- yielding different microstructures with the same composition. The focus of this work was to determine the influence of the glass-composite structure of titanium-based bulk metallic glass matrix composites with in-situ precipitated dendrites of varying composition, crystallinity, and morphology in the dynamic deformation response compared to monolithic titanium-based bulk metallic glasses. Precipitated second phase crystallites complicates the deformation and fracture mechanisms of the bulk material in contrast to that for monolithic bulk metallic glasses. The present study sought to provide a comprehensive assessment of the microstructural response on the dynamic yielding and spall response through controlled plate impact experiments. The experiments consisted of simultaneous impact of two samples with one being probed using VISAR interferometry and the other being recovered for post-mortem fractography and characterization. The dynamic properties observed focused primarily on the dynamic compressive yielding, referred to as the "Hugoniot Elastic Limit", and the dynamic tensile strength referred to as the "spall strength", were determined using VISAR interferometry from experiments performed at impact pressures from 6.0 -- 17.3 GPa. The spall strength and HEL were also determined as a function of strain rate from decompression, peak pressure, and subsequent recompression states after spallation. The decompressive strain rate sensitivity provides insight on the resistance to spall fracture and showed the DV1-SSF alloy, to have the highest resistance to spall fracture. The recompression characteristics after spallation were indicative of the role of microstructure on dynamic fracture characteristics. The recompressive strain rate sensitivity showed that the DV1-SSF results in the most ductile fracture response compared to the other DVX alloys. Post-mortem microstructural characterization done on the recovered samples provided a good correlation with the observed dynamic fracture characteristics seen during recompression. The dynamic fracture of the titanium-based bulk metallic glass was found to have the same macroscopic, microscopic, and nanoscale deformation mechanisms seen in zirconium-based BMGs in the form of simultaneous maximum in-plane shear stress failures from uninterrupted shear band formation, "cup"-"cone" fracture facets formed in the uniaxial-strain region, and presence of veiny shear band patterns consisting of alternating shear-transformation zones (STZs) and tension-transformation zones (TTZs). The dynamic fracture was seen to be directly dependent on the glass content, dendrite size measured using the mean linear intercept through the dendrite, interdendritic spacing measured through the mean free path through the matrix, interfacial surface area, two-dimensional matrix connectivity, and microhardness of the dendrite. The experimental microstructural parameters of glass volume fraction, surface area per unit volume, and mean linear intercept through the dendrite were in utilized to develop a stereologically driven empirical model to project the spall strength performance across varying strain rates. The stereological projection of spall strength performance revealed potential maximized spall strength performance for a metallic glass composite to be ~70-90% glass content with the surface area per unit volume and mean linear intercept through the dendrite peaking in the mid-range values.
The mechanochemistry in heterogeneous reactive powder mixtures under high-strain-rate loading and shock compression

(Georgia Institute of Technology, 2015-11-16) Gonzales, Manny

This work presents a systematic study of the mechanochemical processes leading to chemical reactions occurring due to effects of high-strain-rate deformation associated with uniaxial strain and uniaxial stress impact loading in highly heterogeneous metal powder-based reactive materials, specifically compacted mixtures of Ti/Al/B powders. This system was selected because of the large exothermic heat of reaction in the Ti+2B reaction, which can support the subsequent Al-combustion reaction. The unique deformation state achievable by such high-pressure loading methods can drive chemical reactions, mediated by microstructure-dependent meso-scale phenomena. Design of the next generation of multifunctional energetic structural materials (MESMs) consisting of metal-metal mixtures requires an understanding of the mechanochemical processes leading to chemical reactions under dynamic loading to properly engineer the materials. The highly heterogeneous and hierarchical microstructures inherent in compacted powder mixtures further complicate understanding of the mechanochemical origins of shock-induced reaction events due to the disparate length and time scales involved. A two-pronged approach is taken where impact experiments in both the uniaxial stress (rod-on-anvil Taylor impact experiments) and uniaxial strain (instrumented parallel-plate gas-gun experiments) load configurations are performed in conjunction with highly-resolved microstructure-based simulations replicating the experimental setup. The simulations capture the bulk response of the powder to the loading, and provide a look at the meso-scale deformation features observed under conditions of uniaxial stress or strain. Experiments under uniaxial stress loading reveal an optimal stoichiometry for Ti+2B mixtures containing up to 50% Al by volume, based on a reduced impact velocity threshold required for impact-induced reaction initiation as evidenced by observation of light emission. Uniaxial strain experiments on the Ti+2B binary mixture show possible expanded states in the powder at pressures greater than 6 GPa, consistent with the Ballotechnic hypothesis for shock-induced chemical reactions. Rise-time dispersive signatures are consistently observed under uniaxial strain loading, indicating complex compaction phenomena, which are reproducible by the meso-scale simulations. The simulations show the prevalence of shear banding and particle agglomeration in the uniaxial stress case, providing a possible rationale for the lower observed reaction threshold. Bulk shock response is captured by the uniaxial strain meso-scale simulations and is compared with PVDF stress gauge and VISAR traces to validate the simulation scheme. The simulations also reveal the meso-mechanical origins of the wave dispersion experimentally recorded by PVDF stress gauges.
Impact-initiated combustion of aluminum

(Georgia Institute of Technology, 2015-11-11) Breidenich, Jennifer L.

This work focuses on understanding the impact-initiated combustion of aluminum powder compacts. Aluminum is typically one of the components of intermetallic-forming structural energetic materials (SEMs), which have the desirable combination of rapid release of thermal energy and high yield strength. Aluminum powders of various sizes and different levels of mechanical pre-activation are investigated to determine their reactivity under uniaxial stress rod-on-anvil impact conditions, using a 7.62 mm gas gun. The compacts reveal light emission due to combustion upon impact at velocities greater than 170 m/s. Particle size and mechanical pre-activation influence the initiation of aluminum combustion reaction through particle-level processes such as localized friction, strain, and heating, as well as continuum-scale effects controlling the amount of energy required for compaction and deformation of the powder compact during uniaxial stress loading. Compacts composed of larger diameter aluminum particles (~70µm) are more sensitive to impact initiated combustion than those composed of smaller diameter particles. Additionally, mechanical pre-activation by high energy ball milling (HEBM) increases the propensity for reaction initiation. Direct imaging using high-speed framing and IR cameras reveals light emission and temperature rise during the compaction and deformation processes. Correlations of these images to meso-scale CTH simulations reveal that initiation of combustion reactions in aluminum powder compacts is closely tied to mesoscale processes, such as particle-particle interactions, pore collapse, and particle-level deformation. These particle level processes cannot be measured directly because traditional pressure and velocity sensors provide spatially averaged responses. In order to address this issue, quantum dots (QDs) are investigated as possible meso-scale pressure sensors for probing the shock response of heterogeneous materials directly. Impact experiments were conducted on a QD-polymer film using a laser driven flyer setup at the University of Illinois Urbana-Champaign (UIUC). Time-resolved spectroscopy was used to monitor the energy shift and intensity loss as a function of pressure over nanosecond time scales. Shock compression of a QD-PVA film results in an upward shift in energy (or a blueshift in the emission spectra) and a decrease in emission intensity. The magnitude of the shift in energy and the drop in intensity are a function of the shock pressure and can be used to track the particle scale differences in the shock pressure. The encouraging results illustrate the possible use of quantum dots as mesoscale diagnostics to probe the mechanisms involved in the impact initiation of combustion or intermetallic reactions.
Effects of microstructure on the spall behavior of aluminum-magnesium alloys

(Georgia Institute of Technology, 2014-04-03) Whelchel, Ricky L.

This research focuses on the spall properties of aluminum-magnesium (Al-Mg)alloys.Aluminum alloy 5083 (Al 5083) was used as a model alloy for the work performed in this study. Al-Mg alloys represent a light-weight and corrosion resistant alloy system often used in armor plating. It is desirable to process armor plate material to yield a microstructure that provides maximum resistance to spall failure due to blast and projectile impacts. The blast and impact resistance has often been quantified based on the measurement of the spall strength and the Hugoniot elastic limit (HEL). The spall properties of Al-Mg alloys were measured for four different microstructural states resultant from varying processing conditions. The four microstructures include: (a) textured grain structure from a rolled Al 5083-H116 plate, (b) sub-micron grain structure produced using equi-channel angular pressing (ECAP),(c) equiaxed grain structure, and (d) precipitation hardened microstucture from an Al-9wt.% Mg alloy. The overall results show that grain size is not the most dominant microstructural feature affecting spall strength in aluminum alloys, when the impact conditions are the same. Texture, especially if brittle inclusions align along the grains, appears to have the most dominant effect resulting in decreased spall strength. Furthermore, one-dimensional modeling shows that the inclusion size and distribution is the controlling factor for void formation during spalling. Grain size does affect the decompression rate dependence of each microstructure, whereby smaller grain sizes result in a larger power law exponent for fits of spall strength versus decompression rate. Unlike the spall strength, the HEL shows an increasing trend with decreased grain size, as would be expected from a Hall-Petch type effect, indicating that a smaller grain size is best for penetration resistance. Samples processed using ECAP alone provide the best combination of spall strength and HEL and therefore the most promise for improved blast and penetration resistance of aluminum-magnesium alloy armor plates.
Shock-compression of Ni-Al nanolayered foils using controlled laser-accelerated thin foil impact

(Georgia Institute of Technology, 2013-08-30) Kelly, Sean Christopher

A laser-driven flyer impact system was constructed, characterized, and validated for performing uniaxial-strain experiments to investigate the shock equation-of-state (EOS) and processes leading to reaction initiation in thin, fully-dense Ni-Al nanolayered foils. Additionally, various fully-dense Ni-Al mixtures with highly heterogeneous microstructures and widely varying length scales were investigated to understand influence of meso-scale features on the shock compression and reaction response. Ni-Al composites are a class of reactive materials also called Structural Energetic Materials (SEMs), which aim to combine stiffness and strength with the ability to release large amounts of energy through highly exothermic reactions when the constituents are intimately mixed during shock loading. While porous reactive materials have been studied extensively, the processes leading to reaction initiation in fully-dense mixtures consisting of phases with disparate mechanical properties is more ambiguous. A table-top, small-scale laser system was developed for studying shock-induced effects in extremely thin reactive materials. Laser accelerated thin foil impact experiments utilizing time-resolved interferometry allowed for measuring the Hugoniot of the nanolayered Ni-Al foil over a range of particle velocities/pressures. Separate recovery experiments were performed by shock-loading Ni-Al foils slightly below the reaction initiation threshold and performing post-mortem TEM/STEM analysis to identify the constituent mixing processes leading to reaction. Direct-shock experiments were performed on the different fully-dense Ni-Al mixtures and hydrodynamic simulations using real microstructures allowed direct correlations with the experiment results, which yielded an improved understanding of the effect of phase arrangement on the shock propagation and reaction initiation response. The EOS experiments performed at particle velocities > 200 m/s showed a deviation from the predicted inert trend and recovered targets showed complete reaction to the B2-NiAl intermetallic phase. The measured deviation from inert behavior and state of recovered material suggests the occurrence of a shock-induced chemical reaction. The shocked (but unreacted) Ni-Al materials contained distinct constituent mixing features (layer jets and intermixed zones), where significant elemental penetration occurred and are likely sources of reaction initiation. The observed results provide the first clear evidence of shock-induced reactions in fully-dense nanolayered Ni-Al foils.
Fragmentation and reaction of structural energetic materials

(Georgia Institute of Technology, 2013-08-20) Aydelotte, Brady Barrus

Structural energetic materials (SEM) are a class of multicomponent materials which may react under various conditions to release energy. Fragmentation and impact induced reaction are not well characterized phenomena in SEMs. The structural energetic systems under consideration here combine aluminum with one or more of the following: nickel, tantalum, tungsten, and/or zirconium. These metal+Al systems were formulated with powders and consolidated using explosive compaction or the gas dynamic cold spray process. Fragment size distributions of the indicated metal+Al systems were explored; mean fragment sizes were found to be smaller than those from homogeneous ductile metals at comparable strain rates, posing a reduced risk to innocent bystanders if used in munitions. Extensive interface failure was observed which suggested that the interface density of these systems was an important parameter in their fragmentation. Existing fragmentation models for ductile materials did not adequately capture the fragmentation behavior of the structural energetic materials in question. A correction was suggested to modify an existing fragmentation model to expand its applicability to structural energetic materials. Fragment data demonstrated that the structural energetic materials in question provided a significant mass of combustible fragments. The potential combustion enthalpy of these fragments was shown to be significant. Impact experiments were utilized to study impact induced reaction in the indicated metal+Al SEM systems. Mesoscale parametric simulations of these experiments indicated that the topology of the microstructure constituents, particularly the stronger phase(s), played a significant role in regulating impact induced reactions. Materials in which the hard phase was topologically connected were more likely to react at a lower impact velocity due to plastic deformation induced temperature increases. When a compliant matrix surrounded stronger, simply connected particles, the compliant matrix accommodated nearly all of the deformation, which limited plastic deformation induced temperature increases in the stronger particles and reduced reactivity. Decreased difference between the strength of the constituents in the material also increased reactivity. The results presented here demonstrate that the fragmentation and reaction of metal+Al structural energetic materials are influenced by composition, microstructure topology, interface density, and constituent mechanical properties.