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The starting alloy microstructure can be tailored to achieve varying degrees of grain refinement and enhance mechanical properties through severe plastic shear deformation during solid-phase processing. Crystal plasticity-based grain misorientation modeling, coupled with systematic pin-on-disk tribometry-based subsurface shear deformation experiments on as-cast Al-xSi alloys (x = 0, 1, 4 at %), was conducted. The post-deformation microstructural analysis, through a combined computational and experimental approach, conclusively shows that the initial volume fraction of the hard Si phase enhances the evolution of local lattice misorientation, leading to efficient grain refinement during severe plastic shear deformation. The shear-deformation–induced nanostructure resulted in more than double the nanoindentation hardness in the processed alloy.

Extreme shear deformation is used for several material processing methods and is unavoidable in many engineering applications in which two surfaces are in relative motion against each other while in physical contact. The mechanistic understanding of the microstructural evolution of multi-phase metallic alloys under extreme shear deformation is still in its infancy. Here, we highlight the influence of shear deformation on the microstructural hierarchy and mechanical properties of a binary as-cast Al-4 at.% Si alloy. Shear-deformation-induced grain refinement, multiscale fragmentation of the eutectic Si-lamellae, and metastable solute saturated phases with distinctive defect structures led to a two-fold increase in the flow stresses determined by micropillar compression testing. These results highlight that shear deformation can achieve non-equilibrium microstructures with enhanced mechanical properties in Al–Si alloys. The experimental and computational insights obtained here are especially crucial for developing predictive models for microstructural evolution of metals under extreme shear deformation. Extreme deformation of alloys is important for processing and applications. Here, extreme shear deformation of an Al–Si alloy induces grain refinement, multi-scale fragmentation of lamellae and generation of various defects, enhancing the mechanical properties of micropillars.

This work investigates a unique technique to join aluminum (Al) and magnesium (Mg) alloys to carbon fiber reinforced polymer (CFRP) composites without the use of adhesives or mechanical fasteners. The joints were made using an overcasting technique where the molten alloys were cast around the polymer composites using high pressure die casting method. Rapid cooling during casting allowed the composite to be embedded inside the cast metal without causing any gross damage to the former even though the molten metal temperature exceeded the melting point of the composite matrix by several 100’s of deg. C. The metal/composite interface was examined using optical and scanning electron microscopy and x-ray imaging, and the joint strength was examined through tensile testing. Although evidence of polymer melting was visible in the x-ray images and through microscopy, this melting was limited to the CFRP surface only while the bulk of the embedded section of the composite coupons retained their overall shape and integrity. Preliminary tension tests on Al/CFRP composites showed strength degradation of the CFRP and it failed a few mm outside the joint. This strength degradation in the CFRP suggests that the overcasting process needs further optimization to minimize thermal excursion in the CFRP section outside the joint. Nevertheless, the overcasting process shows great promise as a unique joining technique with application in fabricating light-weight structures for automotive and transportation industries.

Friction consolidation (FC) is a solid phase processing methodology that densifies a material through high-shear deformation and pressure at elevated temperature. The method has garnered interest in the scientific community because of its ability to produce extremely refined and homogeneous microstructures, off-axis texture development, and improved material properties. This manuscript presents an investigation of Cu and Ni material mixing via evaluation of morphological evolution, grain boundary characterization, and compositional analysis to provide insights on the operational alloying mechanisms occurring under high shear and elevated temperature. Using correlative microscopy techniques, we show alloying progresses via a combination of grain boundary diffusion and interfacial roughening at heterophase boundaries. Evidence supporting Cu infiltration along Ni-Ni grain boundaries along with asymmetric diffusion of Cu into Ni grains is highlighted. The resultant, consolidated microstructure was produced directly from a powder compact in ∼30 s and exhibited a submicrometer, equiaxed grain size.

Shear deformation at the nanoscale has practical applications due to the ability of shear strains to significantly change the microstructures and textures of the deforming materials, and for material removal processes at this scale. Here we demonstrate nanomechanical scratching with atomic force microscopy (AFM) as a tool to impose large shear strains on nanoscale material volumes in single-crystal copper. Nano-scratching, with the process parameters and AFM tip geometry used here, resulted in material removal through the cutting mode. This mode enabled the use of stress and strain models, developed for bulk machining, to be applied for AFM cutting as well. The models for bulk-scale were used to predict the strains in the chip (γ ≈ 3.9) and in the surface (γ ≈ 4.6) and the depth of deformed subsurface. Detailed characterization of the scratch region with transmission electron microscopy (TEM) showed microstructural refinement comprising 0.2 μm wide dislocation cells in the chips that resemble features during shear deformation of copper single-crystals at bulk-scale. The subsurface also contained dislocation networks and stacking faults up to ~1.1 μm depth. These results highlight the unique ability of AFM for imparting local shear deformation in materials and studying its effects on the microstructure.

Despite their excellent strength-to-weight ratio, wider use of magnesium (Mg) alloys for light-weight applications is limited by their poor corrosion resistance, especially in chloride-containing environments. The present study shows improved corrosion resistance imparted by laser surface processing (LSP) of a commercial AZ31 (Mg-3Al-1Zn) alloy. Nanosecond laser processing at three different power settings was carried out on the surface of a 1-mm-thick rolled AZ31 sheet. Electrochemical studies and salt spray testing (ASTM B117) indicate substantial enhancement of corrosion resistance in LSP-treated AZ31. The underlying reasons behind improved corrosion resistance of the LSP-AZ31 surface have been studied through detailed microstructural characterization and chemical analysis by SEM, TEM, and XPS. Formation of a ∼0.5 μm thick mixed metal (Mg, Al) oxide surface film, together with refinement in the size and number density of Al-Mn intermetallic particles, are shown to play a major role toward improved corrosion resistance after LSP treatment of the AZ31 alloy.

The starting alloy microstructure can be tailored to achieve varying degrees of grain refinement and enhance mechanical properties through severe plastic shear deformation during solid-phase processing. Crystal plasticity-based grain misorientation modeling, coupled with systematic pin-on-disk tribometry-based subsurface shear deformation experiments on as-cast Al-xSi alloys (x = 0, 1, 4 at%), was conducted. The post-deformation microstructural analysis, through a combined computational and experimental approach, conclusively shows that the initial volume fraction of the hard Si phase enhances the evolution of local lattice misorientation, leading to efficient grain refinement during severe plastic shear deformation. The shear-deformation–induced nanostructure resulted in more than double the nanoindentation hardness in the processed alloy.

Metastable orientation relationships (ORs) between Cu and Cr are stabilized via epitaxial thin film deposition, with the initial layer of Cu(001) or Cr(001) grown epitaxially on MgO(001). The Bain OR is observed by x-ray diffraction and scanning/transmission electron microscopy for Cu(001) / Cr(001). In contrast, three Cr/Cu ORs are found for Cr deposition on Cu(001): the Pitsch OR, and two previously unreported ORs related to the Bain and Pitsch ORs, respectively. Ab initio calculations predict the energetics of these metastable ORs, and reveal that the deformation resistance of Cr makes the Bain OR energetically unfavorable when Cr films are deposited on Cu(001). These results show that kinetic constraints imposed by controlling the substrate surface and deposition conditions can lead to metastable interfacial structures.

An improved understanding of grain growth kinetics in nanocrystalline materials, and in metals and alloys in general, is of continuing interest to the scientific community. In this study, Mg-Al thin films containing ~10 wt pct Al and with 14.5 nm average grain size were produced by magnetron sputtering and subjected to heat treatments. The grain growth evolution in the early stages of heat treatment at 423 K, 473 K, and 573 K (150 °C, 200 °C, and 300 °C) was observed with transmission electron microscopy and analyzed based upon the classical equation developed by Burke and Turnbull. The grain growth exponent was found to be 7 ± 2 and the activation energy for grain growth was 31.1 ± 13.4 kJ/mol, the latter being significantly lower than in bulk Mg-Al alloys. The observed grain growth kinetics are explained by the Al supersaturation in the matrix and the pinning effects of the rapidly forming beta precipitates and possibly shallow grain boundary grooves. The low activation energy is attributed to the rapid surface diffusion which is dominant in thin film systems.

The goal of this work is to improve our understanding of formability enhancement in aluminum (Al) sheet alloys that has generally been observed during high-strain-rate forming. In the work presented here, experiments and numerical modeling were used to investigate the room-temperature formability of AA5182-O Al alloy sheet (1 mm thick) at high strain-rates using the electro-hydraulic forming (EHF) technique. A finite element model, using Johnson–Cook constitutive equation, was developed to simulate the high-rate forming behavior of Al under EHF and test samples were designed to obtain different strain paths at the apex of the EHF domes. The deformation history of Al sheets, under free-forming conditions and inside a conical die, was experimentally determined and compared to the model predictions. Experimental data shows that the high-rate formability of AA5182-O Al at minor strains of ∼−0.1 and ∼0.05, relative to its corresponding quasi-static formability, was enhanced locally by ∼2.5× and ∼6.5× under free-forming and when forming inside the conical die, respectively. The in-plane peak engineering strain-rate associated with the enhanced formability during free-forming was measured to be ∼3900/s while the pre-impact strain-rate during conical-die forming was estimated to be ∼4230/s. The strain-path associated with enhanced formability was experimentally determined under a free-forming case and was found to be in good agreement with that predicted by the numerical model. To the authors’ knowledge, these results are the first to experimentally quantify the deformation history associated with enhanced formability that has often been reported in the literature.

Epsilon metal (ɛ-metal), an alloy of Mo, Pd, Rh, Ru, and Tc, is being developed as a waste form to treat and immobilize the undissolved solids and dissolved noble metals from aqueous reprocessing of commercial used nuclear fuel. Epsilon metal is an attractive waste form for several reasons: increased durability relative to borosilicate glass, it can be fabricated without additives (100% waste loading), and in addition it also benefits borosilicate glass waste loading by eliminating noble metals from the glass, thus the processing problems related to their insolubility in glass. This work focused on the processing aspects of the epsilon metal waste form development. Epsilon metal is comprised of refractory metals resulting in high alloying temperatures, expected to be 1500–2000 °C, making it a non-trivial phase to fabricate by traditional methods. Three commercially available advanced technologies were identified: spark-plasma sintering, microwave sintering, and hot isostatic pressing, and investigated as potential methods to fabricate this waste form. Results of these investigations are reported and compared in terms of bulk density, phase assemblage (X-ray diffraction and elemental analysis), and microstructure (scanning electron microscopy).

The macroscopic behavior of metallic materials is a complex function of microstructure. The size, morphology, volume fraction, crystallography, and distribution of a 2nd phase within a surrounding matrix all control the mechanical properties. Understanding the contributions of the individual microconstituents to the mechanical behavior of multiphase materials has proven difficult due to the inability to obtain accurate constitutive relationships of each individual constituent. In dual-phase steels, for example, the properties of martensite or ferrite in bulk form are not representative of their behavior at the microscale. In this study, micropillar compression was employed to determine the mechanical properties of individual microconstituents in metallic materials with “composite” microstructures, consisting of two distinct microconstituents: (I) a Mg–Al alloy with pure Mg dendrites and eutectic regions and (II) a powder metallurgy steel with ferrite and martensite constituents. The approach is first demonstrated in a Mg–Al directionally solidified alloy where the representative stress–strain behavior of the matrix and eutectic phases was obtained. The work is then extended to a dual-phase steel where the constitutive behavior of the ferrite and martensite were obtained. Here, the results were also incorporated into a modified rule-of-mixtures approach to predict the composite behavior of the steel. The constitutive behavior of the ferrite and martensite phases developed from micropillar compression was coupled with existing strength–porosity models from the literature to predict the ultimate tensile strength of the steel. Direct comparisons of the predictions with tensile tests of the bulk dual-phase steel were conducted and the correlations were quite good.

This paper describes the mechanical characterization and corrosion testing of X608 Al alloy that is being considered for A-pillar covers for heavy-duty truck applications. Recently, PNNL developed a thermo-mechanical process to stamp A-pillar covers at room temperature using this alloy, and the full-size prototype was successfully stamped by a tier-1 supplier. This study was conducted to obtain additional important information related to the newly developed forming process, and to further improve its mechanical properties. The solutionization temperature, pre-strain and paint-bake heat-treatment were found to influence the alloy’s fabricability and mechanical properties. Natural aging effect on the formability was investigated by limiting dome height (LDH) tests. Preliminary corrosion experiments showed that the employed thermo-mechanical treatments did not significantly affect the corrosion behavior of Al X608.

This work builds upon our recent advances in quantifying high-rate deformation behavior of sheet metals, during electro-hydraulic forming (EHF), using high-speed imaging and digital image correlation techniques. Aluminum alloy AA5182-O and DP600 steel sheets (1 mm thick, ∼152 mm diameter) were EHF deformed by high-energy (up to ∼34 kJ) pressure-pulse in an open die (free-forming) and inside a conical die. The deformation history (velocity, strain, strain-rate, and strain-path) at the apex of the formed domes was quantified and analyzed. The data shows that the use of a die in the EHF process resulted in an amplification, relative to free-forming conditions, of the out-of-plane normal velocity and in-plane strain-rate at the dome apex. This amplification is attributed to the focusing action of the die on account of its conical geometry. Further, while the strain-path at the dome apex was generally linear and proportional, the use of a die resulted in greater strain at the apex relative to the strain during free-forming. The sheet deformation profile in the EHF process was found to be different from that previously observed in electromagnetic forming (EMF) and, thus, the two processes are expected to result in different strain-paths and formability. It is anticipated that quantitative information of the sheet deformation history, made possible by the experimental technique developed in this work, will improve our understanding of the roles of strain-rate and sheet–die interactions in enhancing the sheet metal formability during high-rate forming.

A novel experimental technique, that combines high-speed imaging and digital image correlation techniques, has been developed and applied to investigate the high-rate deformation behavior of aluminum sheet during electro-hydraulic forming (EHF). Aluminum alloy AA5182-O sheets (1 mm thick and ∼152 mm diameter) were EHF deformed by high-energy (up to ∼21 kJ) pressure-pulse and the time-evolution of sheet-displacement, velocity, strain and strain-rate quantified. The data shows that different locations on the sheet undergo unique deformation history that is not apparent from the conventional post-mortem strain measurement (using etched circle/grid pattern) approach. Under the experimental conditions used in this work, the sheets were formed into domes and the maximum strain-rate observed was ∼664/s. Further, this maximum strain-rate was observed at an off-apex location and was ∼2.5 times greater than the maximum strain-rate at the dome apex. The maximum velocity observed was ∼100 m/s and the velocity–time data showed evidence of pressure-wave reverberations during the forming process. We believe that knowledge of such time-evolution of sheet deformation is necessary for a better understanding and accurate modeling of sheet formability that has often been reported to exceed quasi-static forming limits under high-rate forming conditions.

Diffusion of Si atom and vacancy in the A2-phase of α-Fe-Si alloys in the ferromagnetic state, with and without magnetic order and in various temperature ranges, are studied using AKSOME, an on-lattice self-learning KMC code. Diffusion of the Si atom and the vacancy are studied in the dilute limit and up to 12 at.% Si, respectively, in the temperature range 350-700 K. Local Si neighborhood dependent activation energies for vacancy hops were calculated on-the-fly using a broken-bond model based on pairwise interaction. The migration barrier and prefactor for the Si diffusion in the dilute limit were obtained and found to agree with published data within the limits of uncertainty. Simulations results show that the prefactor and the migration barrier for the Si diffusion are approximately an order of magnitude higher, and a tenth of an electron-volt higher, respectively, in the magnetic disordered state than in the fully ordered state. However, the net result is that magnetic disorder does not have a significant effect on Si diffusivity within the range of parameters studied in this work. Nevertheless, with increasing temperature, the magnetic disorder increases and its effect on the Si diffusivity also increases. In the case of vacancy diffusion, with increasing Si concentration, its diffusion prefactor decreases while the migration barrier more or less remained constant and the effect of magnetic disorder increases with Si concentration. Important vacancy-Si/Fe atom exchange processes and their activation barriers were identified, and the effect of energetics on ordered phase formation in Fe-Si alloys are discussed.

Monofilaments of Vectra A950, a thermotropic liquid crystalline polymer, with 0–10 wt.% of vapor-grown carbon nanofibers were extrusion-mixed and characterized by tensile testing, nanoindentation and fractography. A maximum increase in modulus (35%) and strength (18%) was observed at the 1–2% nanofiber level, and increases were observed with decreasing diameter for all nanofiber concentrations. Incomplete dispersion of nanofiber clumps and microvoiding are thought to be responsible for the observed decrease in property values at the higher nanofiber levels.

Aluminum alloy AA7075 sheets were deformed at room temperature at strain-rates exceeding 1000 /s using the electrohydraulic forming (EHF) technique. A method that combines high speed imaging and digital image correlation technique, developed at Pacific Northwest National Laboratory, was used to investigate high strain rate deformation behavior of AA7075. For strain-rate sensitive materials, the ability to accurately model their high-rate deformation behavior is dependent upon the ability to accurately quantify the strain-rate that the material is subjected to. This work investigates the objectivity of software-calculated strain and strain rate by varying different parameters within commonly used commercially available digital image correlation software. The results show that except for very close to the time of crack opening the calculated strain and strain rates are consistent and independent of the adjustable parameters of the software.

In this work, performed in close collaboration with PACCAR and Magna International (Stronach Centre for Innovation, SCFI), a 6xxx series aluminum alloy was used for the development of an A-pillar cover for the cab of a typical heavy-duty Class-8 truck. The use of Al alloy for the A-pillar cover represents an approximately 40% weight savings over its steel or molded fiberglass composite counterpart. For the selected Al alloy, a small amount of cold work (5% tensile strain), following prior hot-forming, was found to significantly improve the subsequent age-hardening response. The role of solutionizing temperature and rate of cooling on the age-hardening response after paint-bake treatment were investigated. For the temperature range selected in this work, higher solutionizing temperature correlated with greater subsequent age-hardening and vice-versa. However, the age-hardening response was insensitive to the mode of cooling (water quench vs. air cooling). Finally, a two-step forming process was developed where, in the first step, the blank was heated to solutionizing temperature, quenched, and then partially formed at room temperature. For the second step, the pre-form was reheated and quenched as in the first step, and the forming was completed at room temperature. The resulting A-pillars had sufficient residual ductility to be compatible with hemming and riveting operations that occur during downstream cab assembly.

The fracture toughness and resistance-curve (R-curve) behavior of Ti-Al3Ti metal-intermetallic laminate (MIL) composites have been studied in the crack-divider orientation, by examining the effect of ductile-laminate-layer thickness and volume fraction. The MIL composites were fabricated in open air by a novel one-step process, and the final structure consists of alternating layers of ductile Ti and brittle Al3Ti. Such a laminate architecture in conjunction with a relatively low volume fraction of tougher Ti (18 to 40 pct) was seen to augment the fracture toughness of the inherently brittle intermetallic by over an order of magnitude. Additionally, as a result of their low density, MIL composites exhibit a specific fracture toughness (K/ρ) on par with tougher but relatively denser ductile metals such as high-strength steel. Such vast improvements may be rationalized through the toughening provided by the ductile Ti layers. Specifically, toughening was obtained through plastically stretching the intact ductile Ti layers that formed bridging zones in the crack wake, thus reducing the crack driving force. Such toughening resulted in R-curve behavior, and the toughness values increased with an increase in the volume percentage of Ti. Weight-function methods were used to model the bridging behavior, and they indicated that large bridging zones (∼2 to 3 mm) were responsible for the observed increase in toughness. The role of large-scale bridging (LSB) conditions on the resistance curves is explored, and steady-state toughness (K SS ) values are estimated using small-scale bridging (SSB) approximations. A new approach to gage the potential of laminate composites in terms of their true fracture-toughness values as determined from a cyclic crack-growth fatigue test is proposed, wherein small-scale specimens can be utilized to obtain fracture-toughness values.

Fatigue crack propagation (FCP) has been studied in a new class of materials termed metal-intermetallic laminate (MIL) composites (Ti-Al3Ti). Due to ease of fabrication and control over layer makeup, these MIL composites can be tailored to optimize the constituent properties for structural and higher performance aerospace applications. Effects of ductile reinforcement (titanium alloy) type, thickness, and volume fraction were systematically investigated in both arrester and divider orientations. Stress intensity (Kmax) values as large as 40 MPa√m were observed in the higher crack growth regime, indicating that the fracture toughness of the MIL composites is comparable to common structural metals. In both divider and arrester orientations, the overall fatigue crack growth rate showed an improvement with increasing Ti volume fraction and with increasing Ti thickness (at constant ductile-phase volume fraction). It is noted that the fatigue resistance of monolithic Al3Ti was improved by an order of magnitude by incorporating just 20 vol pct ductile Ti. In the divider orientation, toughening is obtained through plastically stretching the intact ductile Ti ligaments that bridge the crack wake, thus reducing the crack driving force. By virtue of its morphology, the arrester orientation provides toughening by trapping the crack front entirely at the metallic-intermetallic interfaces, thus requiring the crack to renucleate at each interface. Results are compared with specific crack growth rates of conventional monolithic alloys and other composite systems such as TiNb/γ-TiAl and Nb/Nb3Al. Owing to their low density (∼3.8 g/cc), Ti-Al MIL composites exhibited specific crack growth rates (da/dN vs ΔK/ρ) on par with tougher, but relatively denser, ductile metals such as Ti alloys and high-strength steels.

The following response to the discussion of the above mentioned paper is divided into two parts: (1) a response to the issue of our comparison of the ‘apparent period of specimen oscillation’, τ, and the natural vibration period of a cracked specimen, T, and (2) a verification of the applicability of our model to ‘short time-to-fracture’ tests, which Rokach has questioned, due to a lack of knowledge of Hopkinson bar techniques.

Crack length calculations in bend specimens have been derived by analyzing the sample's load-line compliance. This compliance equation is based upon calculating stiffness that incorporates the effects of shear deformation, rotary inertia and crack length in a dynamic test. Three-point bend tests for a high-strength steel and an aluminum alloy were conducted under static and dynamic loading to investigate the validity of the equation derived. The present static test results show good agreement with those predicted from ASTM E399. Dynamic crack lengths determined by the formula proposed in this work are also in good agreement with other models in the literature, and with the experimental results presented here. The applicability of the present approach to effective crack length determination under small-scale yielding conditions is discussed.

The mechanical performance of Ti–Al3Ti metal–intermetallic laminate (MIL) composites synthesized by a reactive foil sintering technique was evaluated. The elastic properties and anisotropy of the laminates were calculated and successfully compared with resonant ultrasonic spectroscopy (RUS) measurements. The effect of internal stresses due to differences in the thermal expansion coefficient on fracture toughness was analyzed. The principal mechanisms of damage initiation and accumulation were identified experimentally. The compressive strength was modeled by FEM using the Johnson–Holmquist constitutive equation. The computed results were successfully compared with experiments.

In this paper, shear deformation and rotary inertia was introduced into the calculation of the dynamic stress intensity factor by means of solving the stiffness of a pre-cracked three-point bend specimen. A simple formula of dynamic stress intensity factor for a pre-cracked three-point bend specimen is derived using the vibration analysis method. Dynamic three-point bending tests were performed on a uniquely modified Hopkinson pressure bar, allowing the dynamic responses of the pre-cracked specimen, such as: the natural frequency, the period of apparent specimen oscillations, the dynamic loads, and the dynamic stress intensity factor to be analyzed experimentally and theoretically.

A split Hopkinson pressure bar (two-bar set-up) has been modified to perform dynamic three-point bend tests to measure dynamic fracture toughness, and to understand the influences of various experimental parameters, as well as inertial effects, on the dynamic material response. Modeling and analysis of the dynamic three-point bend test, as loaded by a modified split Hopkinson pressure bar, is conducted. The effects of support motion, crack propagation and plastic contact stiffness on total sample deflection are investigated. The effects of crack propagation and plastic contact stiffness on the contribution of support motion to the total sample deflection are also investigated theoretically and experimentally in this paper. Further, the effects of crack propagation and plastic contact stiffness on impactor and sample load are also addressed.

A novel method for measuring the dynamic fracture toughness, KId, using a Hopkinson pressure bar loaded instrumented Charpy impact test is presented in this paper. The stress intensity factor dynamic response curve (KI(t)−t) for a fatigue-precracked Charpy specimen is evaluated by means of an approximate formula. The onset time of crack initiation is experimentally detected using the strain gauge method. The value of KId is determined from the critical dynamic stress intensity factor at crack initiation. A KId value for a high-strength steel is obtained using this method at a stress-intensity-factor rate ( K Id ) greater than 106 MPa m / s .

The R-curve and fracture toughness behavior of single-edge notch beams of Ti–Al3Ti metallic–intermetallic laminate (MIL) composites has been investigated. Composites with 14, 20, and 35% volume fraction Ti, with a corresponding intermetallic layer thickness of ~540, ~440, and ~300 microns, respectively, were tested in crack arrester and crack divider orientations. In the arrester orientation, the R-curve could not be determined for the two highest Ti volume fraction compositions as the main crack could not be grown through the test samples. In the divider orientation, R-curves were determined for all three Ti volume fractions tested. The laminate composites were found to exhibit more than an order of magnitude improvement in fracture toughness over monolithic Al3Ti. Crack bridging and crack deflection by the Ti layers were primarily responsible for the large-scale bridging conditions leading to the R-curve behavior and enhanced fracture toughness. Estimates of steady-state toughness under small-scale bridging conditions were in close agreement with experimental results.

The variation of dislocation density with stacking fault energy (SFE) was measured in shock-deformed Cu and Cu–Al alloys. A differential scanning calorimeter (DSC) was used to measure the stored energy, from which the dislocation density was estimated. The energy released during recrystallization in the DSC experiments was attributed primarily to the annihilation of dislocations with the energy contribution from recovery, deformation twins and point-defects estimated to be relatively small. The dislocation density in the 10 GPa shock-deformed materials first increased and then decreased with increasing Al content (decreasing SFE) while the dislocation density in the 35 GPa shock-deformed materials initially decreased and then remained constant with increasing Al content. This variation in dislocation density in the shock-deformed materials is attributed to the nature of shock-deformation, the influence of stacking fault energy on the dislocation storage mechanisms, and the propensity for deformation twinning.

This work describes the integrated experimental and modeling effort at PNNL to enhance the room-temperature formability of aluminum alloys by taking advantage of formability improvements generally associated with high-strain-rate forming. Al alloy AA5182-O sheets were deformed in near plane-strain conditions at strain-rates exceeding 1000 /s using the electrohydraulic forming (EHF) technique, and at quasi-static strain-rates via a bulge test. A novel capability, combining highspeed imaging with digital image correlation technique, was developed to quantify the deformation history during high-rate forming. Sheet deformation under high rates was modeled in Abaqus and validated with experimentally determined deformation data. The experimental results show a ~2.5x increase in formability at high rates, relative to quasi-static rates, under a proportional loading path that was verified by the experimental data. The model shows good correlation with the experimentally determined strain path. It is anticipated that such integrated experimental and modeling work will enable room-temperature forming of Al and industrial implementation of high-rate forming processes.

This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening.

The role of stacking fault energy (SFE) in deformation twinning and work hardening was systematically studied in Cu (SFE ∼78 ergs/cm2) and a series of Cu-Al solid-solution alloys (0.2, 2, 4, and 6 wt pct Al with SFE ∼75, 25, 13, and 6 ergs/cm2, respectively). The materials were deformed under quasi-static compression and at strain rates of ∼1000/s in a Split-Hopkinson pressure bar (SHPB). The quasi-static flow curves of annealed 0.2 and 2 wt pct Al alloys were found to be representative of solid-solution strengthening and well described by the Hall-Petch relation. The quasi-static flow curves of annealed 4 and 6 wt pct Al alloys showed additional strengthening at strains greater than 0.10. This additional strengthening was attributed to deformation twins and the presence of twins was confirmed by optical microscopy. The strengthening contribution of deformation twins was incorporated in a modified Hall-Petch equation (using intertwin spacing as the “effective” grain size), and the calculated strength was in agreement with the observed quasi-static flow stresses. While the work-hardening rate of the low SFE Cu-Al alloys was found to be independent of the strain rate, the work-hardening rate of Cu and the high SFE Cu-Al alloys (low Al content) increased with increasing strain rate. The different trends in the dependence of work-hardening rate on strain rate was attributed to the difference in the ease of cross-slip (and, hence, the ease of dynamic recovery) in Cu and Cu-Al alloys.

Modern transmission electron microscopy (TEM) and spectroscopy techniques routinely probe the microstructure and local compositions in nanocrystalline materials using various analytical techniques such as electron energy-loss spectroscopy (EELS) or energy dispersive x-ray spectroscopy (EDX) maps. Electron diffraction is another technique the can help to identify phases in materials, but in applying such a technique, it is cumbersome to obtain statistically relevant information from nanomaterials. With the development of precession electron diffraction (PED) mapping and the use of image correlation for pattern identification, novel information can be obtained relatively rapidly that covers a much larger number of grains [1]. Combined with elemental mapping, this provides a powerful technique to characterize the distribution of phases and chemical distribution in nanomaterials.

This work describes recent advances in our understanding of sheet metal behavior during electro-hydraulic forming (EHF) process. Two sets of experiments were performed using AA5182-O Al sheet material. In the first set, 1 mm thick sheet samples were subjected to a single pressure-pulse or two consecutive pressure-pulses with the deformation being carried out under free-forming or inside a conical die. In the second set of experiments employing 2 mm sheet samples, a circular region at the center of the sheet was pre-thinned to 1 mm thickness and the sheet was subjected to a single pressure-pulse under free-forming conditions. The sheet deformation history for both sets of experiments was quantified using a recently developed technique that combines high-speed imaging and the digital image correlation (DIC) techniques. The results from the first set of experiments show that the manner in which the discharge is created can influence the strain-rates and hence, the deformation history experienced by the sheet materials. The results of the multi-pulse experiments demonstrate the applicability of the EHF technique for re-strike operations. The results from the second set of experiments show that the pre-thinned region is analogous to a reduced gauge section with the resulting strain-rate (in the pre-thinned region) exceeding that in the adjacent homogeneous sheet by more than 50%.Copyright © 2013 by ASME

Vacancy-mediated diffusion of an Al atom in pure Mg matrix is studied using the atomistic, on-lattice self-learning kinetic Monte Carlo (SLKMC) method. Activation barriers for vacancy-Mg and vacancy-Al atom exchange processes are calculated on-the-fly using the climbing image nudged-elastic band method and binary Mg-Al modified embedded-atom method interatomic potential. Diffusivities of an Al atom obtained from SLKMC simulations show the same behavior as observed in experimental and theoretical studies available in the literature, that is, Al atom diffuses faster within the basal plane than along the c-axis. Although, the effective activation barriers for Al-atom diffusion from SLKMC simulations are close to experimental and theoretical values, the effective prefactors are lower than those obtained from experiments. We present all the possible vacancy-Mg and vacancy-Al atom exchange processes and their activation barriers identified in SLKMC simulations. A simple mapping scheme to map an HCP lattice on to a simple cubic lattice is described, which enables the simulation of HCP lattice using on-lattice framework. We also present the pattern recognition scheme which is used in SLKMC simulations to identify the local Al atom configuration around a vacancy., Comment: 17 pages, 9 figures

The deformation behavior and texture evolution during forming of an advanced high-strength steel (DP600 grade) were characterized. The deformation history of DP600 during electro-hydraulic forming (EHF) was quantified using a unique experimental capability developed at PNNL. The texture evolution during quasi-static and high-strain-rate deformation was determined using the electron backscatter diffraction (EBSD) technique. The deformation history of EHF formed steel sheets shows an amplification of the strain-rate, relative to free-forming conditions, when the forming was carried out inside a conical-die. This strain-rate amplification was attributed to the focusing action of the conical die. The undeformed DP600 sheet was found to possess a {111} fiber texture in the sheet-normal direction. Quasi-static deformation was found to strengthen the pre-existing texture whereas high-rate forming using EHF had a lesser influence. The results of this work demonstrate the unique capability to correlate deformation history during high-strain-rate metal forming processes with the corresponding microstructural evolution. It is expected that results of this work can help fill-in the gaps in our understanding of high-rate forming processes, leading to development of accurate and validated numerical models.Copyright © 2012 by ASME

This report examines and ranks a total of seven materials processing techniques that may be potentially utilized to consolidate the undissolved solids from nuclear fuel reprocessing into a low-surface area form. Commercial vendors of processing equipment were contacted and literature researched to gather information for this report. Typical equipment and their operation, corresponding to each of the seven techniques, are described in the report based upon the discussions and information provided by the vendors. Although the report does not purport to describe all the capabilities and issues of various consolidation techniques, it is anticipated that this report will serve as a guide by highlighting the key advantages and disadvantages of these techniques. The processing techniques described in this report were broadly classified into those that employed melting and solidification, and those in which the consolidation takes place in the solid-state. Four additional techniques were examined that were deemed impractical, but were included for completeness. The techniques were ranked based on criteria such as flexibility in accepting wide-variety of feed-stock (chemistry, form, and quantity), ease of long-term maintenance, hot cell space requirements, generation of additional waste streams, cost, and any special considerations. Based on the assumption of ~2.5 L of waste to be consolidated per day, sintering based techniques, namely, microwave sintering, spark plasma sintering and hot isostatic pressing, were ranked as the top-3 choices, respectively. Melting and solidification based techniques were ranked lower on account of generation of volatile phases and difficulties associated with reactivity and containment of the molten metal.

IDDRG - Conference Proceedings - Tools and Technologies for the Processing of ultra high strength steels