Enrique Lavernia

• University of California, Davis

The present work investigates porosity formation in spray deposition processes. The emphasis is on one possible mechanism of micro-pore formation during droplet spreading and solidification: liquid-jet overflow. To this end, the Navier-Stokes equations are solved numerically using finite differences and the free surface is tracked using the Volume...

Ti-23Nb-0.7Ta-2Zr-O gum metal (GM) is an attractive candidate material for applications that require superior mechanical properties. In our earlier investigation of the GM [1], geometrical softening and the generation of adiabatic shear bands (ASBs) were proposed as primary reasons for the documented anisotropic impact response. In the present stud...

Ultrafine grained (UFG) metals and alloys typically exhibit mechanical and thermal instabilities, partially due to the high density of lattice defects, which limits their engineering applications. Annealing represents a simple and effective way to regain strain hardening, ductility and thermal stability, and stabilize the UFG structures. In this st...

To identify the critical issues that affect the evolution of microstructure during additive manufacturing, we investigated the influence of process parameters on the evolution of the dimensional and surface quality, microstructure, internal defects, and mechanical properties in 316L stainless steel (SS) components fabricated using laser engineered...

It has been hypothesized that the superimposition of an applied pressure enhances mass transfer during pressure-assisted sintering; however, a pressure-related coefficient D T , representing the contribution of a diffusion flux has never been established heretofore. Here, we aim to establish a theoretical framework to determine the D T by combining...

The well-established enhanced mechanical performance of ultrafine-grained (UFG) materials is often accompanied by poor thermal stability which precludes their use in some applications. To provide fundamental insight into the problem of thermal stability, we studied the microstructure of two different UFG microstructures in Cu: one with lamellar gra...

During spark plasma sintering (SPS), it is widely argued that the densification mechanism of powder particles can be correlated with some specific physical quantities; however, an atomic diffusion coefficient D has yet to be derived. In this work, we report on the establishment of a framework, which can be used to derive the value of the D and furt...

We report on the formulation of a factor, f, that when applied together with the activation energy for viscous flow (Q), can be used to provide important insight into the densification mechanism that are active during powder sintering. We ascertain the validity of this formulation by comparing the densification behaviour of atomized and milled powd...

The thermal stability of multilayered NiFe specimens was investigated by using scanning differential calorimetry. Grain growth was found occurred at similar temperature, around 400 °C. Four different heating rates were selected and the activation energy was determined by using modified Kissinger analysis. The activation energy for grain growth is s...

In spark plasma sintering (SPS) it is widely acknowledged that there are specific physicochemical mechanisms associated with atomic diffusion; however, a reaction diffusion rate coefficient K has yet to be determined. In this work, we derive the K of titanium-copper diffusion couples using isothermal heat treatment of pulsed electric current (PEC),...

In this work, a bulk gum metal (GM) was fabricated via arc melting from high purity powders. The ingots were first extruded using a conventional route followed by equal channel angular pressing (ECAP). The mechanical behavior of the extruded GM and ECAP-processed GM was studied under both quasi-static and high strain rate compression conditions to...

We report on a novel approach to synthesize (Ti100-x-yFexCoy)82Nb12.2Al5.8 (at.%) bimodal alloys and provide fundamental insight into their underlying microstructural evolution and mechanical behavior. In our work, a bimodal microstructure is attained via selection of phases and composition in a eutectic reaction followed by semi-solid sintering. S...

Particulate reinforced aluminum-based metal matrix composites (Al MMCs) continue to be of interest, partly due to their low density, but also because of their ability to provide tailored property combinations, such as high specific stiffness, specific strength and creep resistance. This article provides a review on the progress that has been made i...

Dislocation mechanisms for the formation of twins in b.c.c., f.c.c., and h.c.p. crystals have been briefly reviewed. An attempt has been made to understand in terms of these models the influence of temperature, composition, pre-strain, and microstructure on deformation twinning. Furthermore, accommodation processes occurring at twins terminating wi...

We investigate the process-structure-property relationships for 316L stainless steel prototyping utilizing 3-D laser engineered net shaping (LENS), a commercial direct energy deposition additive manufacturing process. The study concluded that the resultant physical metallurgy of 3-D LENS 316L prototypes is dictated by the interactive metallurgical...

In this study we report novel results obtained with an extruded fine-grained Mg-2.5 at.% Y alloy (FG Mg-2.5Y) exhibiting tension/compression yield symmetry and reduced strength differential, in addition to well-balanced strength and ductility. On the basis of detailed post-mortem transmission electron microscopy studies, atom probe tomography, and...

In this work, the authors report on the formation of toughened TiB2-based ceramic composites using Ti60.8Al25Nb10V3Mo1B0.2 (at%) metallic glass powder as sintering aid fabricated by spark plasma sintering. The TiB2 ceramic powder blended with 5 wt% metallic glass sintering aid exhibits an interesting sigmoidal shrinkage behavior which is argued to...

We report on a novel phenomenon, that is a high-strain-induced reverse martensitic transformation in an ultrafine-grained Ti–36Nb–2Ta–3Zr (wt.%) alloy processed by equal channel angular pressing (ECAP) at room temperature. Our results show that a martensitic transformation from body-centred cubic β matrix to orthorhombic α″ martensite occurs under...

This work studies the microstructural evolution of nanocrystalline (<1 µm) barium titanate (BaTiO3), and presents high pressure in field-assisted sintering (FAST) as a robust methodology to obtain >100 nm BaTiO3 compacts. Using FAST, two commercial ~50 nm powders were consolidated into compacts of varying densities and grain sizes. Microstructural...

The mechanical response of heterophase interfaces has attracted substantial attention in recent years. Here, we utilized an in situ transmission electron microscopy (TEM) technique to isolate an individual nanoscale ceramic/metal interface and characterize its nanomechanical response. The interface, at which there was a Mg-rich segregation nanolaye...

The inverse polarizing effect of Sub-Wavelength Metallic Gratings (SWMGs) is employed to improve the lithography performance by controlling the polarization. The SWMGs are intentionally created on the top surface of mask. Its polarization selectivity is deliberately designed according to the bottom mask patterns. A series of simulations and optimiz...

The addition of rare elements to Mg enhances mechanical behavior via solution and precipitation strengthening mechanisms. To provide fundamental insight into the underlying mechanisms, we apply density-functional theory (DFT) calculations to systematically study the generalized planar fault energy (GPFE) for pure Mg and its alloys with Gd, Y, and G...

The accurate determination of stacking fault energies (SFE) and associated restoring forces is important for understanding plastic deformation, especially the dislocation emission and motion in metals. In this work, we use density-functional theory (DFT) calculations to, systematically study the all-dimension relaxed atomic models of Mg crystal sli...

We report on a study of the design, phase formation, microstructure, mechanical behavior and strengthening mechanisms of a novel single-phase Co25Ni25Fe25Al7.5Cu17.5 (at.%) high-entropy alloy (HEA). In this investigation, a bulk nanocrystalline (nc) Co25Ni25Fe25Al7.5Cu17.5 HEA with the facecentered cubic (FCC) crystal structure was fabricated by me...

We report on a study of the microstructure and mechanical behavior of a bulk nanostructured (NS) / ultra-fine grained (UFG) aluminum fabricated by cryomilling and high-pressure spark plasma sintering (SPS). The compressive yield stress of the consolidated material is determined to be 380 MPa, which is significantly higher than that of commercial st...

Bending tests of submicrometer Al pillars were performed in-situ in a transmission electron microscope (TEM). The Al crystal in the bent region experienced substantial lattice distortion, as well as grain refinement resulting in a disordered dislocation configuration arising from a random distribution of low angle grain boundaries (LAGBs). This obs...

The influence of Cu-rich precipitates (CRPs) and reverted austenite (RA) on the strength and impact toughness of a Cu-containing 3.5 wt pct Ni high-strength low-alloy (HSLA) steel after various heat treatments involving quenching (Q), lamellarization (L), and tempering (T) is studied using electron back-scatter diffraction, transmission electron mi...

We report on a study of the influence of microstructure on the mechanical behavior of bulk nanoporous Cu fabricated by chemical de-alloying of Cu50Al50, Cu40Al60, Cu33Al67 and Cu30Al70 (at.%) alloys. The precursor Cu-Al alloys were fabricated using arc melting and bulk nanoporous Cu was obtained by subsequent de-alloying of Cu-Al alloys in 20 wt.%...

To enhance the toughness of metal matrix nanocomposites, we demonstrate a strategy that involves the introduction of spatial arrays of nanoparticles. Specifically, we describe an approach to synthesize a microstructure characterized by arrays of fiber-like nanoparticle-rich (NPR) zones that contain spherical nanoparticles of B4C (sn-B4C) embedded i...

Intragranular coupling of dislocations and precipitates is accomplished in an ultrafine grained aluminum 7000 series alloy through a unique thermo-mechanical processing route that involves high strain rate extrusion at ambient temperature as the last step. The as-extruded materials also exhibited a unique bimodal microstructure consisting of: (1) e...

We describe a study of the temperature dependent deformation behavior of a multilayered NiFe-60 wt%Fe alloy with a layer thickness of 5 μm fabricated by electrodeposition. The structure of adjacent layers alternates between a nanocrystalline and a coarse grained. Uniaxial tensile tests at temperature between 20 °C and 400 °C and strain rate of 10−4...

Bulk Al materials with average grain sizes of 0.47 and 2.4 µm, were fabricated by quasi-isostatic forging consolidation of two types of Al powders with average particle sizes of 1.3 and 8.9 μm, respectively. By utilizing the native amorphous Al2O3 (am-Al2O3) film on the Al powders surfaces, a continuous, ~7 nm thick, am-Al2O3 network was formed in...

The synthesis of γ-TiAl from elemental metals via solid-state reactive diffusion processing routes involves multiple reaction steps with the formation of various intermediate intermetallic compounds, starting with TiAl3 because this phase is favored kinetically. To understand the processes by which the TiAl3 intermediate is eliminated during synthe...

The mechanical behavior, with particular emphasis on the damage mechanisms, of SiCp/Al composites was studied by both experiments and finite element analysis in this paper. A 3D microstructure-based finite element model was developed to predict the elasto-plastic response and fracture behavior of a 7vol.% SiCp/Al composite. The 3D microstructure of...

Recent interest in the study of stacking faults and non-basal slip in Mg alloys is partly based on the argument that these phenomena positively influence mechanical behaviour. Inspection of the published literature, however, reveals that there is a lack of fundamental information on the mechanisms that govern the formation of stacking faults, espec...

We report on an investigation of the quasi-static and dynamic mechanical behavior of an ultrafine-grained (UFG) aluminum composite reinforced with nanoscale boron carbide under uniaxial compression. Aluminum composites reinforced with either nanometric or half-micron sized B4C exhibit high strength~900 MPa and considerable strain-to-failure under c...

The effect of Co on the microstructure and mechanical behavior of a high-entropy alloy (HEA), Al0.6Ni- FeCrCo, consolidated via spark plasma sintering (SPS) or hot pressing (HP) from powders, was studied in detail. The microstructure of the mechanically alloyed (MA'ed) Al0.6NiFeCrCo HEA consisted of a primary body-centered cubic (BCC) solid-solutio...

Multilayered (ML) NiFe thick foils have been studied in order to provide insight in synthesizing structure-modulated sheets with superior mechanical properties. The specific electrodeposition parameters were selected to produce ML samples with modulated grain size distribution and layer thicknesses varied from 30. nm up to 5. μm. The influence of l...

An ultrafine-grained (UFG) aluminum nano-composite was fabricated using two severe plastic deformation steps: cryomilling of powders (and subsequent consolidation of blended powders by forging) followed by high-pressure torsion (HPT). The forged bulk composite featured a trimodal structure comprised of UFG, coarse grain (CG) regions, and ceramic pa...

The preparation of transmission electron microcopy (TEM) samples from powders with particle sizes larger than ~100 nm poses a challenge. The existing methods are complicated and expensive, or have a low probability of success. Herein, we report a modified methodology for preparation of TEM samples from powders, which is efficient, cost-effective, a...

Trimodal Al alloy (AA) matrix composites consisting of ultrafine-grained (UFG) and coarse-grained (CG) Al phases and micron-sized B4C ceramic reinforcement particles exhibit combinations of strength and ductility that render them useful for potential applications in the aerospace, defense and automotive industries. Tailoring of microstructures with...

Metal/ceramic Interface Structures and Segregation Behavior in Aluminum-based Composites - Volume 21 Issue S3 - Xinming Zhang, Tao Hu, Jorgen F. Rufner, Thomas LaGrange, Geoffrey H. Campbell, Enrique J. Lavernia, Julie M. Schoenung, Klaus van Benthem

To provide insight into the mechanical behavior and microstructural evolution of bulk ultrafinegrained (UFG) Ti-6Al-4V alloys, we produced Ti-6Al-4V alloy sheets with grain size smaller than 300 nm through severe warm rolling of three different starting microstructures (i.e., lamellar, equiaxed, and hybrid, that is half equiaxed plus half lamellar...

It is well known that micrometer-sized and/or submicrometer-sized metallic crystals exhibit “smaller is stronger” size effect: the yield strength σ varies with sample dimension D roughly as a power-law σ ∼ D−m. For some materials, near-theoretical strength values can be attained by reducing the dimensions of crystals to sub-micrometric or nanometri...

Bulk nanostructured and ultrafine-grained binary Al–Fe alloys have been studied in the past for their remarkable strength, hardness, and thermal stability. These properties have been attributed to a combination of solid solution strengthening, precipitate strengthening, and grain boundary strengthening. However, to date, no systematic investigation...

The effect of Co on the microstructure and mechanical behavior of a high-entropy alloy (HEA), Al0.6NiFeCrCo, consolidated via spark plasma sintering (SPS) or hot pressing (HP) from powders, was studied in detail. The microstructure of the mechanically alloyed (MA'ed) Al0.6NiFeCrCo HEA consisted of a primary body-centered cubic (BCC) solid-solution...

To provide fundamental insight into the influence of length scales on the spatial distribution and characteristics of ceramic/metal interfaces in nanostructured metal matrix composites, we studied an Al alloy reinforced with a broad size distribution of B4C particles, ranging from several nanometers to submicrometers. The B4C was incorporated into...

The full-width at half-maximum (FWHM), integral width (IntW), and Scherrer methodologies were used jointly with X-ray diffraction (XRD) peak broadening and transmission electron microscopy (TEM) data to provide insight into the mechanisms that govern grain growth behavior in an Al 5083-B4C nanocomposite. Grain growth kinetics were studied by fittin...

We describe a novel approach to synthesize gradient microstructures, defined hereafter as containing a broad but continuous distribution of grain sizes. These microstructures extend the concept of a bimodal grain size distribution and the ability to design with multiple length scales. We demonstrate the proposed approach via experiments involving c...

We report on a study of the nanocrystalline structure in Ti, which was produced by cryogenic milling followed by subsequent consolidation via severe plastic deformation using high pressure torsion. The mechanisms that are believed to be responsible for the formation of grains smaller than 40 nm are discussed and the influence of structural characte...

A trimodal metal matrix composite (MMC) based on AA (Al alloy) 5083 (Al-4.4Mg-0.7Mn-0.15Cr wt pct) was synthesized by cryomilling powders followed by compaction of blended powders and ceramic particles using two successive dual mode dynamic forgings. The microstructure consisted of 66.5 vol pct ultrafine grain (UFG) region, 30 vol pct coarse grain...

Palladium and its alloys are model systems for studying the solid-state storage of hydrogen. Mechanical milling is commonly used to process complex powder systems for solid-state hydrogen storage; however, milling can also be used to evolve nanostructured powder to modify hydrogen sorption characteristics. In the present study, cryomilling (mechani...

A theoretical framework that incorporates the influence of second-phase particles and solute segregation at grain boundaries (GBs) on stress-induced GB migration and grain rotation is formulated in the present paper. In our work, we modified the well-established Cahn–Taylor model to account for the drag stresses generated by second-phase particles...

Additions of Sc to an Al-Mg matrix were investigated, paying particular attention to the influence of Al3Sc precipitates and other dispersoids, as well as grain size, on mechanical behavior. Prior studies have shown that Sc significantly increases the strength of coarse-grained Al-Mg alloys. Prompted by these findings, we hypothesized that it would...

The emergence of Al-Li alloys as potential light metal, for safe use in a spectrum of aircraft structures and related aerospace applications has in recent years engendered an unprecedented widespread interest aimed at studying, understanding and improving their mechanical properties. In this chapter, we present and discuss some of the key aspects r...

To make possible the ability to produce a bulk nanostructured Ti–Nb–Ta–Zr alloy from elemental powders using the synergistic influence of the techniques of cryomilling, spark plasma sintering (SPS) and high-pressure torsion (HPT) and to concurrently provide an insight into the role of each process on aspects related to alloying of the elemental pow...

In spark plasma sintering (SPS), thermal and electric fields are applied simultaneously as a material is densified under pressure. The interactions between these two types of physical fields influence the densification behavior during SPS. Moreover, the uniformity and spatial distribution of these fields are also influenced by sample size. In the c...

To gain fundamental insight into the relationship between length scales and mechanical behaviour, Ni-Fe multilayered materials with a 5-μm-layer thickness and a modulated grain size distribution have been synthesized by pulsed electrodeposition. Microstructural studies by SEM and TEM reveal the alternating growth of well-defined layers with either...

In this study, we report on the influence of high pressure on the microstructure evolution of cryomilled nanostructured Al alloy powders during spark plasma sintering (SPS). Our experimental results suggest that the particular mechanism that governs grain growth during SPS depends on the magnitude of the applied pressure. In the case of material co...

In situ nano-TiB2 reinforced ultrafine-grained (UFG) Al composites were prepared via combined processes of flux-assisted synthesis (FAS) and asymmetrical rolling (ASR). The UFG Al composite with an ASR reduction ratio of 97% exhibits an average matrix grain size of 380 nm and an average TiB2 particulate size of 50 nm. Dislocation density in the com...

This issue is the fifth in a series of special issues on ultrafine-grained (UFG) materials to be published in the Journal of Materials Science. The scientific studies collected here represent selected papers from the Eighth International Symposium on Ultrafine-Grained Materials (UFG VIII), which was held in San Diego, CA, USA from February 16–20, 2...

To provide insight into the microstructural evolution and mechanical behavior of bulk nanostructured Ti, we used cold gas dynamic spraying of Ti particles to synthesize thick coatings (e.g., >10 mm in thickness). Accordingly, the grain size, lattice parameter, lattice strain, residual stress, porosity, microhardness, tensile, and compressive behavi...

Mg65Cu25Gd10 bulk metallic glass (BMG), containing a high density of intersecting extended shear bands (SBs), was fabricated from densification of amorphous powder via high-pressure torsion (HPT). The extended SBs, up to 400 nm in width and containing nanocrystalline structures, were studied by electron microscopy. The mechanisms responsible for th...

Carbon nanotubes (CNTs) were dispersed in gas atomized Cu47.5Zr47.5Al5 (CZA) and Cu50Zr50 (CZ) amorphous powders, in an effort to elucidate the mechanisms of adhesion of CNTs onto amorphous metallic powders. CNTs were homogenously dispersed in water using a zwitterionic (ZW) surfactant. Then CZA and CZ powders were submersed in the ZW-CNT suspensio...

The age-hardening response for ultrafine-grained, powder-metallurgy- consolidated aluminum 7091 was investigated for the first time. Peak hardening occurred after aging at 353 K (80 °C) for only 4 hours; further aging for up to 26 hours resulted in only slight fluctuations in hardness values. After the 4-hour aging, the precipitate population consi...

A system for coating a surface comprises providing a source of amorphous metal, providing ceramic particles, and applying the amorphous metal and the ceramic particles to the surface by a spray. The coating comprises a composite material made of amorphous metal that contains one or more of the following elements in the specified range of compositio...

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composi...

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was es...

To provide insight into the influence of an electric field on the kinetics of diffusion, fully lamellar γ-TiAl was processed by a rapid, two-stage, solid-state reactive sintering via spark plasma sintering (SPS) of a cryomilled Ti, Al powder blend. Cryomilling was implemented in the current study to attain a nanostructured grain size in the Ti and...

The current study shows the dramatic effect of an electric field (EF) and use of nanosized cryomilled grains on accelerating sintering kinetics during spark plasma sintering of blended elemental powder compacts of Ti53Al47 targeted to produce γ-TiAl intermetallic compounds. The EF had the dominating effect since it reduced the activation barrier fo...

Ultrafine-grained (UFG) Ni and multi-walled carbon nanotube (CNT) composite powders were prepared using non-covalent functionalization of CNTs to promote cohesion between the metal powders and CNTs. Following consolidation using spark plasma sintering, the resultant Ni–CNT composites had densities >97 % with well-dispersed CNT reinforcements. Tensi...

In this study we analyze the phenomenon of stress-induced grain growth in nanostructured Al containing a high volume fraction of nanoscale oxide particles during high-temperature extrusion. Our results show that in the absence of an externally applied stress, grain growth was essentially inhibited during annealing at 600 °C. In contrast, when the s...

A high-strength, ultrafine-grained Al alloy with a lamellar structure was fabricated via cryomilling and consolidation (hot isostatic pressing), followed by a two-stage thermomechanical processing approach (e.g. high temperature rotary swaging and room temperature high strain rate extrusion). Each lamellar band consisted of multiple ultrafine grain...

We report on the novel application of nanoscratch characterization to provide insight into the plasticity mechanisms responsible for the behaviour of composites. Accordingly, we conduct deformation characterization with nanoscratch testing (DCNT) to study the deformation behaviour of two B4C reinforced ultrafine grained Al alloy tri-modal composite...

Using transmission electron microscopy, we document, for the first time, the presence of stacking faults on the (10-10) prism planes in a fine-grained Mg–Y alloy. These prism stacking faults were found to be exclusively contiguous to a {10-12} deformation twin. In addition, the {10-12} twin contained a high density of basal plane stacking faults. A...

Trimodal composites, consisting of nanocrystalline or ultrafine grains (UFGs), coarse grains (CGs), and ceramic particles, were originally formulated to achieve combinations of physical and mechanical properties that are unattainable with the individual phases, such as strength, ductility, and high-strain-rate deformation. The concept of a trimodal...

Ultrafine-grained (UFG) aluminium, processed under fast cooling rate conditions (12Ks(-1)) following hot rolling (water quenched) exhibits enhanced thermal stability due to an increase in concentration of solid solution atoms, relative to the furnace cooled material. The influence of fraction recrystallized on yield stress and uniform elongation is...

To provide insight into the influence of the length scale on the kinetics of phase evolution during severe plastic deformation, we studied the microstructure evolution of cryomilled Al and Ti mixture, which is further subjected to high-pressure torsion (HPT). The cryomilled microstructure consisted of elemental Al and Ti, and the subsequent HPT def...

To provide insight into the relationships between precipitation phenomena, grain size and mechanical behavior in a complex precip-itation-strengthened alloy system, Al 7075 alloy, a commonly used aluminum alloy, was selected as a model system in the present study. Ultrafine-grained (UFG) bulk materials were fabricated through cryomilling, degassing...

Carbon nanotubes have emerged as potential reinforcements that can be used to engineer composites with tailored combinations of physical and mechanical properties. In the present study, an Al alloy-based nanostructured composite containing multiwalled carbon nanotubes (MWCNT) as a reinforcement phase has been synthesized by a combination of two nov...