Kenneth Vecchio
University of California, San Diego, NanoEngineering, Faculty Member
In-situ experiments conducted within a transmission electron microscope provide the operator a unique opportunity to directly observe microstructural phenomena, such as phase transformations and dislocation-precipitate interactions, “as... more
In-situ experiments conducted within a transmission electron microscope provide the operator a unique opportunity to directly observe microstructural phenomena, such as phase transformations and dislocation-precipitate interactions, “as they happen”. However, in-situ experiments usually require a tremendous amount of experimental preparation beforehand, as well as, during the actual experiment. In most cases the researcher must operate and control several pieces of equipment simultaneously. For example, in in-situ deformation experiments, the researcher may have to not only operate the TEM, but also control the straining holder and possibly some recording system such as a video tape machine. When it comes to in-situ fatigue deformation, the experiments became even more complicated with having to control numerous loading cycles while following the slow crack growth. In this paper we will describe a new method for conducting in-situ fatigue experiments using a camputer-controlled tensile straining holder.The tensile straining holder used with computer-control system was manufactured by Philips for the Philips 300 series microscopes. It was necessary to modify the specimen stage area of this holder to work in the Philips 400 series microscopes because the distance between the optic axis and holder airlock is different than in the Philips 300 series microscopes. However, the program and interfacing can easily be modified to work with any goniometer type straining holder which uses a penrmanent magnet motor.
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Since the discovery in 1984 by Shechtman et al. of crystals which display apparent five-fold symmetry, extensive effort has been given to establishing a theoretical basis for the existence of icosahedral phases (eg.2.). Several other... more
Since the discovery in 1984 by Shechtman et al. of crystals which display apparent five-fold symmetry, extensive effort has been given to establishing a theoretical basis for the existence of icosahedral phases (eg.2.). Several other investigations have been centered on explaining these observations based on twinning of cubic crystals (eg.3.). Recently, the existence of a stable, equilibrium phase T2Al6 Li3Cu) possessing an icosahedral structure has been reported in the Al-Li-Cu system(4-6).In the present study an Al-2.6wt.%Li-l.5wt.%Cu-0.lwt.%Zr alloy was heat treated at 300°C for 100hrs. to produce large T2 precipitates. Convergent Beam Electron Diffraction (CBED) patterns were obtained from two-fold, three-fold, and apparent five-fold axes of T2 particles. Figure 1 shows the five-fold symmetric zero layer CBED pattern obtained from T2 particles.
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Shock-induced reactions (or shock synthesis) have been studied since the 1960’s but are still poorly understood, partly due to the fact that the reaction kinetics are very fast making experimental analysis of the reaction difficult. Shock... more
Shock-induced reactions (or shock synthesis) have been studied since the 1960’s but are still poorly understood, partly due to the fact that the reaction kinetics are very fast making experimental analysis of the reaction difficult. Shock synthesis is closely related to combustion synthesis, and occurs in the same systems that undergo exothermic gasless combustion reactions. The thermite reaction (Fe2O3 + 2Al -> 2Fe + Al2O3) is prototypical of this class of reactions. The effects of shock-wave passage through porous (powder) materials are complex, because intense and non-uniform plastic deformation is coupled with the shock-wave effects. Thus, the particle interiors experience primarily the effects of shock waves, while the surfaces undergo intense plastic deformation which can often result in interfacial melting. Shock synthesis of compounds from powders is triggered by the extraordinarily high energy deposition rate at the surfaces of the powders, forcing them in close contact, activating them by introducing defects, and heating them close to or even above their melting temperatures.
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Research Interests: Engineering, Materials Science, Strain Rate, Polymer, Mathematical Sciences, and 11 moreHigh strain rate testing, Physical sciences, Composite Material, Split Hopkinson Pressure Bar, Yield stress, Experimental Data, Pressure Effect, Temperature Effect, Hydrostatic Pressure, Strain Rate Effect, and Solids and Structures
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Although it has been well established that microvoid coalescence occurs during static or quasi-static fracture in ductile materials, the exact mechanism for microvoid formation is still unclear. It has been argued that microvoids initiate... more
Although it has been well established that microvoid coalescence occurs during static or quasi-static fracture in ductile materials, the exact mechanism for microvoid formation is still unclear. It has been argued that microvoids initiate and grow from second phase particles. However this argument cannot be used to explain the existence of microvoids on the fracture surfaces of "pure" materials. An alternative mechanism for their formation in "pure" materials is that they initiate and grow along dislocation cell walls. If this premise is true; then the nature and extent of microvoid coalescence should be related to the stacking fault energy (SFE) of the material since the latter is a controlling parameter in the formation of dislocation cells. The relationship between microvoid coalescence and stacking fault energy may have some basis since absolute cell dimensions are of the same magnitude as the observed dimple sizes. The present study examines the effect of dislocation cell structures on the formation of microvoids as a function of the stacking fault energy of a given material through direct observation of the void formation and growth process within the TEM. The fundamental aspects of the work is to correlate the dislocation substructures, void initiation, growth, and coalescence to the resulting fracture surfaces.
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Optical pseudogap energy in multicomponent carbonitrides is shown to be tunable and predictable based on composition. Carbonitride hue depends on pseudogap energy, and chroma can be predicted based on the electronic density of states of... more
Optical pseudogap energy in multicomponent carbonitrides is shown to be tunable and predictable based on composition. Carbonitride hue depends on pseudogap energy, and chroma can be predicted based on the electronic density of states of precursor materials. A model is developed for the prediction of color appearance of any combination of group 4/5 B1-carbides and nitrides. Novel hues previously inaccessible in mononitrides and monocarbides, such as pink and purple, are synthesized in multicomponent carbonitrides.
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Abstract FeAl/FeAl2 eutectoid metallic-intermetallic laminate (MIL) composites were synthesized using a “multiple-thin-foil” configuration and a “two-stage reaction” strategy. Microstructure analysis via scanning electron microscope... more
Abstract FeAl/FeAl2 eutectoid metallic-intermetallic laminate (MIL) composites were synthesized using a “multiple-thin-foil” configuration and a “two-stage reaction” strategy. Microstructure analysis via scanning electron microscope (SEM), energy-dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD) confirms the formation of a two-intermetallic eutectoid structure, which decomposes from the high-temperature Fe5Al8 phase. The metal layers of eutectoid-MIL composites are fabricated with either pure iron or two different stainless steels without altering the intermetallic regions. The volume fraction of eutectoid layers is adjusted for optimizing the performance. The off-eutectoid phase is switched between FeAl and FeAl2 to investigate the effect on strength and ductility. The microstructure of the interfacial regions is fine-tuned, further demonstrating the ability to independently control the constituents of MIL composites. Additionally, a hybrid MIL composite of FeAl and the eutectoid structure are synthesized as a ‘proof-of-concept’. Finite element analysis (FEA) simulation is utilized to study the internal stress in the MIL composites from a macroscopic point of view. Incremental compression tests were conducted to track the fracture evolution from a microscopic point of view.
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Abstract Novel Ceramic-Fiber-Reinforced-Metal-Intermetallic-Laminate (CFR-MIL) composites, Ti–Al3Ti–Al2O3–Al, were synthesized by reactive foil sintering in air. Microstructure controlled material architectures were achieved with... more
Abstract Novel Ceramic-Fiber-Reinforced-Metal-Intermetallic-Laminate (CFR-MIL) composites, Ti–Al3Ti–Al2O3–Al, were synthesized by reactive foil sintering in air. Microstructure controlled material architectures were achieved with continuous Al2O3 fibers oriented in 0° and 90° layers to form fully dense composites in which the volume fractions of all four component phases can be tailored. Bend fracture specimens were cut from the laminate plates in divider orientation, and bend tests were performed to study the fracture behavior of CFR-MIL composites under three-point and four-point bending loading conditions. The microstructures and fractured surfaces of the CFR-MIL composites were examined using optical microscopy and scanning electron microscopy to establish a correlation between the fracture toughness, fracture surface morphology and microstructures of CFR-MIL composites. The fracture and toughening mechanisms of the CFR-MIL composites are also addressed. The present experimental results indicate that the fracture toughness of CFR-MIL composites determined by three- and four-point bend loading configurations are quite similar, and increased significantly compared to MIL composites without ceramic fiber reinforcement. The interface cracking behavior is related to the volume fraction of the brittle Al3Ti phase and residual ductile Al, but the fracture toughness values appear to be insensitive to the ratio of these two phases. The toughness appears to be dominated by the ductility/strength of the Ti layers and the strength and crack bridging effect of the ceramic fibers.
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Five single-phase W- and Mo-containing high-entropy borides (HEBs) have been made via reactive spark plasma sintering of elemental boron and metals. A large reactive driving force enables the full dissolution of 10-20 mol. % WB2 to form... more
Five single-phase W- and Mo-containing high-entropy borides (HEBs) have been made via reactive spark plasma sintering of elemental boron and metals. A large reactive driving force enables the full dissolution of 10-20 mol. % WB2 to form dense, single-phase HEBs, including (Ti0.2Zr0.2Hf0.2Mo0.2W0.2)B2, (Ti0.2Ta0.2Cr0.2Mo0.2W0.2)B2, (Zr0.2Hf0.2Nb0.2Ta0.2W0.2)B2, and (Zr0.225Hf0.225Ta0.225Mo0.225W0.1)B2, for the first time; in contrast, such single-phase W-containing HEBs cannot be made via conventional fabrication routes. In the processing science, this result serves perhaps the best example demonstrating that the phase formation in high-entropy ceramics can strongly depend on the kinetic route. A scientifically interesting finding is that HEBs containing softer WB2 and/or MoB2 components are significantly harder than (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2. This exemplifies that high-entropy ceramics can enable unexpected/improved properties beyond the rule of mixture.
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The introduction of Al-Li based alloys into commercial applications has not occurred without some major difficulties in alloy development. Poor ductility and low fracture toughness in these alloys has hindered their use in structures... more
The introduction of Al-Li based alloys into commercial applications has not occurred without some major difficulties in alloy development. Poor ductility and low fracture toughness in these alloys has hindered their use in structures where they must serve as integral load-carrying members. On the other hand, superior fatigue and stiffness properties of these alloys has provided great incentive to pursue further development. The reason for the low ductility and poor fracture toughness has been attributed to several possible mechanisms including: i) shear band formation due to planar slip., and ii) strain localization resulting from large grain boundary precipitates. The rather good fatigue crack propagation characteristics have been attributed to significant macroscopic out-of-plane crack growth. However, little work has aimed at understanding the micromechanisms of this out-of-plane crack growth during cyclic loading. In addition, the role of the numerous precipitates that can exist in these complex Al-Li-X alloys to the fatigue crack growth resistance has not been examined closely.In this research we have examined the microscopic evolution of fatigue deformation in two Al-Li-X alloys using in-situ fatigue experiments conducted within an intermediate voltage transmission electron microscope.
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Single crystals of aluminum and copper were cyclically deformed, in single slip, to presaturation at 77 and 298 K, respectively. The dislocation substructures were carefully analyzed using conventional BF and DF TEM with particular... more
Single crystals of aluminum and copper were cyclically deformed, in single slip, to presaturation at 77 and 298 K, respectively. The dislocation substructures were carefully analyzed using conventional BF and DF TEM with particular attention directed towards the dislocation dipole spacing. It was found that, in both metals, the dipole spacing was independent of the location in the heterogeneous substructure, which consisted of dense dipole bundles (or veins), and the relatively low dislocation-density channels. Furthermore, the stress to separate the largest spacing dipoles was nearly equal to the applied stress. The stress necessary to pass dislocations through the dense veins was also about equal to the applied stress. The observations and calculations suggest a uniform state of stress throughout the heterogeneous dislocation substructure, without the presence of significant internal stresses. Convergent beam electron diffraction (CBED) experiments were also undertaken. These results will be discussed in terms of dislocation dynamics and their consistency with the absence of long-range internal stresses.
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Under high strain rates, plastic deformation can be assumed to be adiabatic, and a significant temperature increase can occur at large strains. In this study, shock-hardened polycrystalline copper was subjected to high strains (γ ~ 5) at... more
Under high strain rates, plastic deformation can be assumed to be adiabatic, and a significant temperature increase can occur at large strains. In this study, shock-hardened polycrystalline copper was subjected to high strains (γ ~ 5) at high strain rates (γ˙ ~ 104 s−1) using a stepped specimen in a Hopkinson bar. Microstructural analysis by transmission electron microscopy revealed that the highly deformed shear-band region consisted of a gradual decrease in grain size with increasing strain. The center of the shear band is characterized by very small grains (~ 0.1μm) with a relatively low dislocation density. An analysis is developed, based on dynamic recrystallization (enabled by the adiabatic temperatures rise associated with plastic deformation), that predicts a transient grain size during plastic deformation in which grain size varies inversely with strain and strain rate. This analysis has been used to predict that nanocrystalline grain sizes are achievable. This dramatic microstructural refinement (from > 50μ.m to < 0.1μm) enables a thermomechanical response that may lead to extensive, stable plastic deformation tension.
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In the past decade, the concept of high-entropy alloys (HEAs) or multi-principal element alloys (MPEAs), which are composed of at least four principal elements, significantly expands the compositional space for alloy design. This concept... more
In the past decade, the concept of high-entropy alloys (HEAs) or multi-principal element alloys (MPEAs), which are composed of at least four principal elements, significantly expands the compositional space for alloy design. This concept can also be employed in the design of superelastic alloys to promote the development of this functional material field. Here, we report the orientation-dependent superelasticity of a metastable Fe-27.5Ni-16.5Co-10Al-2.2Ta-0.04B (at.%) HEA through in situ micropillar compression tests along ⟨001⟩, ⟨011⟩, and ⟨111⟩ orientations. Our results show that considerable superelastic strains can be achieved along the three orientations in the metastable HEA via a reversible martensitic transformation. Thermoelastic martensite with thin-plate morphology was observed under cryogenic conditions. This work demonstrates that the maximum superelastic strains vary with different orientations, and the ⟨001⟩-oriented specimen shows the largest superelastic strain. The...
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The Al-Li alloy system can provide a unique opportunity to study the thermal history of shear-band formation by following the thermal dissolution of the precipitate phase δ’ (Al3Li) as a function of Li concentration and δ’ solvus... more
The Al-Li alloy system can provide a unique opportunity to study the thermal history of shear-band formation by following the thermal dissolution of the precipitate phase δ’ (Al3Li) as a function of Li concentration and δ’ solvus temperature. The Al-Li system was chosen primarily for the rather rapid precipitation and dissolution kinetics resulting from the high diffusivity of Li in the Al lattice. δ’ is a spherical, coherent and metastable precipitate which is the main strengthening phase in dilute Al-Li alloys. Although δ’ is metastable, the location of the δ’ solvus in the Al-Li phase diagram has been well documented for dilute Li additions (<5 wt.% Li).e.g.1 The metastable δ’ solvus increases in temperature with increasing Li concentration. As such, the dissolution of δ’ within the shear bands, as a function of Li alloy concentration, can provide an internal temperature probe to study the thermal history of the shear band.
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The high modulus and limited plasticity of most ceramic materials inhibits the systematic study of deformation and fracture mechanisms. However, the use of repeatedly applied transient stress pulses allows incremental damage to be... more
The high modulus and limited plasticity of most ceramic materials inhibits the systematic study of deformation and fracture mechanisms. However, the use of repeatedly applied transient stress pulses allows incremental damage to be introduced without necessarily fracturing the specimen. In this study two types of hot pressed silicon nitrides, one having an amorphous boundary phase (6% yttria, 3% alumina), and the other having a crystalline boundary phase (8% yttria, 1% alumina) were tested using a novel split Hopkinson pressure bar technique with a momentum trap.
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It has been well documented that when a large difference in the coefficients of thermal expansion (CTE) exist between the matrix and reinforcement in metal-matrix composites (MMCs) internal stresses can develop which are sufficiently high... more
It has been well documented that when a large difference in the coefficients of thermal expansion (CTE) exist between the matrix and reinforcement in metal-matrix composites (MMCs) internal stresses can develop which are sufficiently high to generate dislocations at the reinforcement/matrix interface. Numerous observations have been made of this phenomenon via TEM which have shown a variety of different dislocation substructures and dislocation punching mechanisms. An important consequence of this phenomenon is that the metal matrix becomes strain hardened as the dislocation density increases, thereby reducing subsequent plastic flow of the matrix. One notable feature of the dislocation punching mechanism is that prismatic dislocation loops are commonly observed emanating from the interface. In two recent studies it was found that dislocations were not emitted immediately upon cooling, but rather at some lower critical temperature. A number of microstructural and processing paramete...
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In-situ composites of thermodynamically compatible phases have been the subject of recent investigations and in particular, those comprising 2-phase Nb/Nb5Si3 structures. Chemical reactions initiated by high velocity shock waves can be... more
In-situ composites of thermodynamically compatible phases have been the subject of recent investigations and in particular, those comprising 2-phase Nb/Nb5Si3 structures. Chemical reactions initiated by high velocity shock waves can be extremely rapid and (unlike more conventional techniques) are capable of forming composite compounds with carefully controlled, fine-scale microstructures. The current investigation involves the use of SEM, TEM and EDX to fully characterize the microstructure of niobium silicide specimens (of initial bulk compositions Si 40 at.% Nb and Nb 33 at.% Si) formed by shock-synthesis of nominally 5 μm size particles in a powder gas gun at an impact velocity of 1.3 km/s.Recovered specimens were in the form of 34 mm diameter × 7 mm thick disks. Metallographic sectioning and examination by SEM in the backscattered imaging mode showed that both disks had undergone a chemical reaction with the exception of the peripheral regions which were merely compacted. Examin...
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Abstract FeAl-based Metallic-Intermetallic Laminate (MIL) composites exhibit enhanced strength and ductility compared to previously studied MIL composites. The deformation and fracture evolution of the FeAl-based MIL composites are... more
Abstract FeAl-based Metallic-Intermetallic Laminate (MIL) composites exhibit enhanced strength and ductility compared to previously studied MIL composites. The deformation and fracture evolution of the FeAl-based MIL composites are investigated here via incremental compression testing. Microstructure assessment via electron backscatter diffraction suggests that deformation proceeds in a fairly homogeneous manner across gradients in the microstructure. Eventual failure is mainly induced by normal stresses, whereas other MIL composites typically fail by shear induced localizations. Geometrically necessary dislocation analysis indicates the FeAl regions deform in similar manners for the three MIL composites (Fe-FeAl-MIL, 430SS-FeAl-MIL, and 304SS-FeAl-MIL), and each fails in a similar mode. While the FeAl phase is the majority constituent of the composites, the mechanical properties are significantly influenced by the softer metal layers. The transition layer formed between the Fe-based metal layers and FeAl regions is the most critical constituent of the composites. Although the volume fraction of the transition layer is only ∼15%, a stronger transition layer can improve the work hardening behavior of the FeAl phase, increasing MIL composite strength by as much as 1 GPa. The findings can guide the design of the MIL composites to achieve even better mechanical properties.
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Abstract Thermomechanical-cycling processes were applied to a metastable Ti-23Nb-0.7Ta-2Zr-1.0O (at%) alloy to control a volume fraction of mechanical twins, resulting from activation of twinning-induced plasticity effects. Analysis of... more
Abstract Thermomechanical-cycling processes were applied to a metastable Ti-23Nb-0.7Ta-2Zr-1.0O (at%) alloy to control a volume fraction of mechanical twins, resulting from activation of twinning-induced plasticity effects. Analysis of the microstructure features designed using electron backscattering and X-ray diffraction revealed the deformation bands induced by the cycling process were characterized as only {332} β twinning. It was also observed that the fraction of twins was significantly increased with increasing the number of the cycles in the process, resulting in a pronounced softening effect. To shed a light on the relevant micro-scale features (e.g., geometric orientation and stress concentrations) responsible for the microstructure evolution and mechanical response, the local Schmid factor of grains and geometrically-necessary dislocation densities were evaluated from experimentally observed results, and correlated with deformation twinning. A good correlation between the local geometric Schmid factor and softening behavior, due to texture evolution within twins was established, as well as the effect of local stress concentrations on microstructure evolution of this Ti-Nb Gum metal. Significant decrease in micro-hardness (approximately 18% from untwinned structure) was achieved by highly twinned structure, and a possible deformation mechanism was established to enhance ductility without evident elastic properties variation in metastable β Ti alloys.