Papers by Pedro D Peralta
Encyclopedia of Materials: Science and Technology, 2001
High-Power Laser Ablation VII, 2008
ABSTRACT We present two laser driven shock wave loading techniques utilizing long pulse lasers, l... more ABSTRACT We present two laser driven shock wave loading techniques utilizing long pulse lasers, laser-launched flyer plate and confined laser ablation, and their applications to shock physics. The full width at half maximum of the drive laser pulse ranges from 100 ns to 10 µs, and its energy, from 10 J to 1000 J. The drive pulse is smoothed with a holographic optical element to achieve spatial homogeneity in loading. We characterize the flyer plate during flight and dynamically loaded target with temporally and spatially resolved diagnostics. The long duration and high energy of the drive pulse allow for shockless acceleration of thick flyer plates with 8 mm diameter and 0.1–2 mm thickness. With transient imaging displacement interferometry and line-imaging velocimetry, we demonstrate that the planarity (bow and tilt) of the loading is within 2−7 mrad (with an average of 4 ± 1 mrad), similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible in particular when a plasma shield is adopted. For flyer plate loading, supported shock waves can be achieved. Temporal shaping of the drive pulse in confined laser ablation enables flexible loading, e.g., quasi-isentropic, Taylor-wave, and off-Hugoniot loading. These dynamic loading techniques using long pulse lasers (0.1–10 µs) along with short pulse lasers (1–10 ns) can be an accurate, versatile and efficient complement to conventional shock wave loading for investigating such dynamic responses of materials as Hugoniot elastic limit, plasticity, spall, shock roughness, equation of state, phase transition, and metallurgical characteristics of shock-recovered samples, in a wide range of strain rates and pressures at meso-and macroscopic scales.
arXiv: Materials Science, 2016
Microstructure-property relationships of resistance spot welded 6061-T6 aluminum alloy lap joints... more Microstructure-property relationships of resistance spot welded 6061-T6 aluminum alloy lap joints were investigated via mechanical testing and microscopy techniques. Quasi-static tensile and novel shear punch tests were employed to measure the mechanical properties of the distinct weld regions. Quasi-static tensile and shear punch tests revealed constantly decreasing strength and ductility as the weld center was approached. For instance, the ultimate tensile strength of the fusion zone decreased by ~52% from the parent material (341 MPa to 162 MPa) while the yield strength decreased by ~62% (312 MPa to 120 MPa). The process-induced microstructures were analyzed with scanning electron microscopy and optical microscopy to elucidate the underlying cause of the reduced mechanical properties. Fractography reveals void growth from particles being the dominant damage mechanism in the parent material as compared to void nucleation in the fusion zone. Overall, significant changes in the mech...
Materials Science and Engineering: A, 2018
The mechanical properties of the weld regions of a 6061-T6 resistance spot welded lap joint are d... more The mechanical properties of the weld regions of a 6061-T6 resistance spot welded lap joint are determined. The change in mechanical properties resulting from RSW are linked to the changes observed in the microstructure. Processing currents and strain rates are varied to probe the effects of processing temperature at strain rates from 10 −3 to 10 3 s −1. Results show that material strength decreases within the heat affected zone (HAZ) and fusion zone due to precipitate dispersion. Further, decreased ductility results at quasi-static strain rates from accelerated crack growth arising near voids formed during weld formation, but the short time scale at higher strain rates limits the ability for crack growth from these voids allowing the material to exhibit higher ductility. Overall, significant changes in the mechanical behavior across the weld resulting from a change in microstructure congruent with precipitate dispersion are apparent for all processing conditions.
Nature Communications, 2018
Fundamentally, material flow stress increases exponentially at deformation rates exceeding, typic... more Fundamentally, material flow stress increases exponentially at deformation rates exceeding, typically,~10 3 s −1 , resulting in brittle failure. The origin of such behavior derives from the dislocation motion causing non-Arrhenius deformation at higher strain rates due to drag forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate-insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/ delaying dislocation interaction with phonons until higher strain rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high-temperature strength maintained up to 80% of the melting point (~1356 K).
Journal of Physics: Conference Series, 2016
While numerous continuum material strength and phase transformation models have been proposed to ... more While numerous continuum material strength and phase transformation models have been proposed to capture their complex dependences on intensive properties and deformation history, few experimental methods are available to validate these models particularly in the large pressure and strain rate regime typical of strong shock and ramp dynamic loading. In the experiments and simulations we present, a rippled shock is created by laser-ablation of a periodic surface perturbation on a metal target. The strength of the shock can be tuned to access phase transitions in metals such as iron or simply to study high-pressure strength in isomorphic materials such as copper. Simulations, with models calibrated and validated to the experiments, show that the evolution of the amplitude of imprinted perturbations on the back surface by the rippled shock is strongly affected by strength and phase transformation kinetics. Increased strength has a smoothing effect on the perturbed shock front profile resulting in smaller perturbations on the free surface. In iron, faster phase transformations kinetics had a similar effect as increased strength, leading to smoother pressure contours inside the samples and smaller amplitudes of free surface perturbations in our simulations.
Volume 9: Mechanics of Solids, Structures and Fluids, 2013
Global and local microstructural weak links for spall damage were investigated using 3-D characte... more Global and local microstructural weak links for spall damage were investigated using 3-D characterization in polycrystalline (PC) and multicrystalline (MC) copper samples, respectively. All samples were shocked via flyer-target plate experiments using a laser drive at low pressures (2–6 GPa). The flyer plates measured approximately 500 μm thick and 8 mm in diameter and the target plates measured approximately 1000 μm thick and 10 mm in diameter. Electron Backscattering Diffraction (EBSD) and optical microscopy were used to determine to presence of voids and relate them to the surrounding microstructure. Statistics on the strength of grain boundaries (GBs) was conducted by analyzing PC samples and collecting the misorientation across GBs with damage present, and it was found that a misorientation range of 25–50° is favorable for damage. Statistics were also taken of copper PC samples that had undergone different heat treatments and it was found that although the 25–50° range is less ...
MRS …, 1998
... F. Chu, DJ Thoma, K. J. McClellan, P. Peralta, FX Li, and E. Fodran Materials Science and Tec... more ... F. Chu, DJ Thoma, K. J. McClellan, P. Peralta, FX Li, and E. Fodran Materials Science and Technology Division, Mail Stop G755, Los Alamos ... melting point (2453 K) and also has a solubility range of 2 - 3 atomic percent, which can aid in processing and alloy design strategies. ...
JOM, 2010
Digital image correlation (DIC) is a powerful tool for quantifying local stresses and strains. Th... more Digital image correlation (DIC) is a powerful tool for quantifying local stresses and strains. The demand for environmentally benign Pb-free solders and the push toward smaller portable electronics will make it more likely for solder interconnects to en-counter mechanical shock through dropping or mishandling. Thus, quantifying the strain rate behavior of Pb-free solders from the quasi-static to the shock regime
Oriented bicrystals of pure C11 " MoSi 2 have been grown in a tri-arc furnace using the Czoc... more Oriented bicrystals of pure C11 " MoSi 2 have been grown in a tri-arc furnace using the Czochralski technique. Two single-crystal seeds were used to initiate the growth. Each seed had the orientation intended for one of the grains of the bicrystals, which resulted in a 603 twist boundary on the (1 1 0) plane. Seeds were attached to a water-cooled seed rod, which was pulled at 120 mm/h with the seed rod rotating at 45 rpm. The water-cooled copper hearth was counter-rotated at 160 rpm. Asymmetric growth ridges associated with each seed crystal were observed during growth and con"rmed the existence of a bicrystal. It was also found that careful alignment of the seeds was needed to keep the grain boundary from growing out of the boule. The resulting boundary was characterized by imaging and crystallographic techniques in a scanning electron microscope. The boundary was found to be fairly sharp and the misorientation between the grains remained within 23 from the misorientation...
Mechanisms relating plasticity and dislocation arrangements in metals and their alloys during cyc... more Mechanisms relating plasticity and dislocation arrangements in metals and their alloys during cyclic hardening and saturation at room temperature are reviewed. Emphasis is placed on face centered cubic (fcc) metals with wavy slip, but comparisons are made with planar slip alloys and examples of their behavior are given. Dislocation arrangements and their relationship to cyclic plasticity are first discussed for fcc single crystals, where they are well understood, in terms of plastic strain amplitudes, crystallography, and testing mode. Then, cyclic deformation for non-fcc metals is discussed, with emphasis on pure iron, magnesium, and titanium due to their engineering applications. Behavior of fatigued polycrystals is reviewed and compared to that in single crystals. Cyclic deformation and dislocation structures in alloys are reviewed next, with emphasis on materials with planar slip. Finally, effects of microstructural length scales on dislocation structures are briefly discussed.
Scripta Materialia, 2020
Gestational diabetes mellitus (GDM) not only increases perinatal complications, but also increase... more Gestational diabetes mellitus (GDM) not only increases perinatal complications, but also increases lifetime risk of type 2 diabetes. Recently, many studies have shown that women with GDM also have an increased risk of cardiovascular disease (CVD). The risk of preeclampsia, hypertension, dyslipidemia, metabolic syndrome, atherosclerosis, and CVDs were elevated in women with GDM compared with in those without GDM. Although it was not confirmed whether the management of GDM could reduce the development of CVDs, it is important to identify and manage CVDs in women with GDM.
JOM, 2020
Uranium dioxide is the standard fuel of the current nuclear reactor fleet and a potential fuel fo... more Uranium dioxide is the standard fuel of the current nuclear reactor fleet and a potential fuel for future generation IV reactors. Enhancing the performance of uranium dioxide fuels necessitates a broader understanding of the fuel pellet/cladding mechanical interaction; this can be achieved through further study of the mechanical behavior of UO2. Small-scale mechanical testing can be used to evaluate the mechanical properties of materials while reducing the total volume of material required for testing. This is beneficial when studying nuclear materials because small test samples reduce the dose to the researcher and can reduce costs by permitting mechanical testing to be performed outside of hot cells. In this work in situ transmission electron microscopy microcantilever and nanoindentation testing was used to evaluate the deformation of UO2 at room temperature. The in situ testing was also used along with an innovative helium implantation technique to evaluate the effect of helium bubbles on the low temperature deformation of UO2. It was found that, under similar testing conditions, the unimplanted material formed a single critical crack while material containing helium bubbles at a dose of 1018 ions/cm2 responded with a large number of nanocracks, enhancing the toughness of the material.
JOM, 2016
Three-dimensional x-ray tomography (XRT) provides a nondestructive technique to characterize the ... more Three-dimensional x-ray tomography (XRT) provides a nondestructive technique to characterize the size, shape, and location of damage in dynamically loaded metals. A shape-fitting method comprising the inertia tensors of individual damage sites was applied to study differences of spall damage development in face-centered-cubic (FCC) and hexagonal-closed-packed (HCP) multicrystals and for a suite of experiments on high-purity copper to examine the influence of loading kinetics on the spall damage process. Applying a volume-weighted average to the best-fit ellipsoidal aspect-ratios allows a quantitative assessment for determining the extent of damage coalescence present in a shocked metal. It was found that incipient transgranular HCP spall damage nucleates in a lenticular shape and is heavily oriented along particular crystallographic slip directions. In polycrystalline materials, shape distributions indicate that a decrease in the tensile loading rate leads to a transition to coalesced damage dominance and that the plastic processes driving void growth are time dependent.
TMS/Characterization, 2016
Three-dimensional X-ray tomography (XRT) provides a non-destructive technique to determine the lo... more Three-dimensional X-ray tomography (XRT) provides a non-destructive technique to determine the location, size, and shape of spall damage within shock loaded metals. Polycrystalline copper samples of varying thermomechanical histories were shocked via plate impacts at low pressures to ensure incipient spall conditions. Additionally, samples of similar heat-treated microstructures were impacted at various loading rates. All 3D XRT volumetric void data underwent smoothing, thresholding, and volumetric sieves. The full inertia tensor was found for each void, which was used to create best fit ellipsoids correlating shape to damage modes. Density distributions were plotted for the best-fit ellipsoid semi-axes aspect ratios alc and blc, where, a≤b≤c. It was found that >60% of voids in heat-treated samples resembled transgranular damage, whereas >70% of voids in the rolled sample resembled intergranular damage. Preliminary analysis also clearly indicates an increase of void coalescence with decreasing tensile loading stress rates for impacted samples of similar microstructures.
Scripta Metallurgica et Materialia, 1995
Scripta Materialia, 2010
Correlations between spall damage and local microstructure were investigated using polycrystallin... more Correlations between spall damage and local microstructure were investigated using polycrystalline copper samples via laser-driven plate impacts at low pressures. Electron backscattering diffraction was used to relate the presence of porosity to microstructural features such as grain boundaries and triple points. Preferred void-nucleation sites were identified in terms of their crystallography via statistical sampling in serial sectioned specimens. Results indicate that terminated twins and grain boundaries with misorientations between 25°and 50°are the preferred locations for intergranular damage localization.
Acta Materialia
Abstract This work investigates macroscopic plastic behavior in polycrystalline Ti with varying o... more Abstract This work investigates macroscopic plastic behavior in polycrystalline Ti with varying oxygen concentrations using both experiments and a crystal plasticity finite element framework applied to a representative volume element (RVE) of the microstructure. The proposed multiscale framework makes use of parameters such as critical resolved shear stress (CRSS) ratios obtained from lower length scale first-principle calculations and strain hardening rates obtained from experiments. Generalized stacking fault energy calculations (GSFE) in combination with the Peierls-Nabarro model and a temperature dependent phenomenological description of CRSS were utilized to compute the CRSS ratios for different slip systems in Ti with and without oxygen. Experimentally measured stress-strain responses along the rolling direction for polycrystalline Ti with different oxygen concentrations were used to obtain the strain hardening rates. The crystal plasticity framework was then calibrated using the computed CRSS ratios and the strain-hardening rate for Ti with various oxygen concentrations. The calibrated model was then used to predict the macroscopic response of Ti under different loading conditions and orientations with different oxygen concentrations. Without tuning the fitting parameters for the crystal plasticity framework, we show that the model is able to simulate the experimental responses within the experimental uncertainty. Thus, a systematic calibration procedure is presented to capture the macroscopic homogeneous responses of Ti with various oxygen concentrations.
Philosophical Magazine A
Calculations of the elastic stresses in the region adjacent to a twin boundary (TB) subjected to ... more Calculations of the elastic stresses in the region adjacent to a twin boundary (TB) subjected to uniaxial tension are carried out for different orientations of the tensile axis. It is found that the principal stresses reach a maximum when the tensile axis is a ⟨111⟩ direction and that their value strongly depends on orientation. In general, the presence of the
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Papers by Pedro D Peralta