Publications

2009
J. - K. Chang, Taleff, E. M., and Krajewski, P. E., “The Effect of Microstructure on Cavitation during Hot Deformation of a Fine-Grained Aluminum-Magnesium Alloy as Revealed through Three-Dimensional Characterization,” Metallurgical and Materials Transactions A, vol. 40, pp. 3128–3137, 2009. LinkAbstract
The effect of microstructure on cavitation developed during hot deformation of a fine-grained AA5083 aluminum-magnesium alloy is investigated. Two-point correlation functions and three-dimensional (3-D) microstructure characterization reveal that cavitation depends strongly on the mechanism that controls plastic deformation. Grain-boundary-sliding (GBS) creep produces large, interconnected cavities rapidly during plastic straining. Solute-drag (SD) creep produces isolated cavities with less total volume fraction at a given strain. The 3-D microstructure data reveal adjacency between various microstructural features. Cavities are observed to be preferentially adjacent to large Al6(Mn,Fe) particles and to Mg-Si particles of all observed sizes. These data suggest that cavities preferentially nucleate at Mg-Si particles and at large Al6(Mn,Fe) particles. This result may be applied to reduce cavitation in commercial hot-forming operations utilizing aluminum-magnesium alloys.
E. M. Taleff, Hector, Louis G., J., Bradley, J. R., Verma, R., and Krajewski, P. E., “The Effect of Stress State on High-Temperature Deformation of Fine-Grained Aluminum-Magnesium Alloy AA5083 Sheet,” Acta Materialia, vol. 57, pp. 2812–2822, 2009. LinkAbstract
The effect of stress state on high-temperature deformation of fine-grained aluminum–magnesium alloy AA5083 sheet is investigated over a range of temperatures and strain rates for which the grain-boundary-sliding and solute-drag creep mechanisms govern plastic flow. Experimental data from uniaxial tension and biaxial tension are used in conjunction with finite-element-method simulations to examine the role of stress state. Three different material constitutive models derived from uniaxial tensile data are used to simulate bulge-forming experiments. Comparison of simulation results with bulge-forming data indicates that stress state affects grain-boundary-sliding creep by increasing creep rate as hydrostatic stress increases. Thus, creep deformation is faster under biaxial tension than under uniaxial tension for a constant effective stress. No effect of stress state is observed for solute-drag creep. A new material model that accounts for the effect of stress state on grain-boundary-sliding creep is proposed. Keywords Finite element modeling; Constitutive equations; Superplasticity; Creep; High-temperature deformation
J. - K. Chang, Taleff, E. M., Krajewski, P. E., and Ciulik, J. R., “Effects of Atmosphere in Filament Formation on a Superplastically Deformed Aluminum-Magnesium Alloy,” Scripta Materialia, vol. 60, pp. 459–462, 2009. LinkAbstract
Filaments with diameters of the order of 1 μm were observed on fracture surfaces of a fine-grained aluminum–magnesium alloy following superplastic deformation in air. No filaments were observed on fracture surfaces of the same material after superplastic deformation at identical temperature and strain rate in vacuum. Filaments contain oxygen and more magnesium than does the as-received specimen surface. These data support the theory that filaments are formed on fracture surfaces by growth of magnesium-rich oxide. Keywords Aluminum; Superplastic; Fracture; Filament; Oxide
W. Grigsby, Bowes, B.T. andDalton, D. A., Bernstein, A. B., Bless, S., Downer, M. C., Taleff, E. M., Colvin, J., and Ditmire, T., “Picosecond Time Scale Dynamics of Short Pulse Laser-Driven Shocks in Tin,” Journal of Applied Physics, vol. 105, pp. 1–10, 2009. LinkAbstract
The dynamics of high strain rate shock waves driven by a subnanosecond laser pulse in thin tin slabs have been investigated. These shocks, with pressure up to 1 Mbar, have been diagnosed with an 800 nm wavelength ultrafast laser pulse in a pump-probe configuration, which measured reflectivity and two-dimensional interferometry of the expanding rear surface.Time-resolved rear surface expansion data suggest that we reached pressures necessary to shock melt tin upon compression. Reflectivity measurements, however, show an anomalously high drop in the tinreflectivity for free standing foils, which can be attributed to microparticle formation at the back surface when the laser-driven shock releases.
2008
T. R. McNelley, Oh-oishi, K., Zhilyaev, A. P., Swaminathan, S., Krajewski, P. E., and Taleff, E. M., “Characteristics of the Transition from Grain Boundary Sliding to Solute Drag Creep in Superplastic AA5083,” Metallurgical and Materials Engineering A, vol. 39, pp. 50–64, 2008. LinkAbstract
Superplastic tensile ductility has been attained when specially-processed AA5083 materials are strained in tension at relatively high strain rates, in the range of the transition from grain-boundary sliding (GBS) to solute drag creep (SDC) control of deformation. Quick plastic forming (QPF) technology involves deformation at such strain rates, and the relative contributions of GBS and SDC to the strain during deformation in this strain rate regime have been examined in this investigation. The additive, independent contributions of GBS and SDC to the elevated temperature deformation of fine-grained materials are reviewed. The transition from GBS to SDC in grain-refined AA5083 materials was evaluated by several methods, including the assessment of initial transients during straining and of transients during strain-rate change tests; the strain-rate dependence of the flow stress; the dependence of ductility on strain rate; flow localization behavior and fracture mode; cavitation growth; the evolution of microstructure and microtexture during deformation; and comparison with phenomenological models for the GBS-to-SDC transition.
N. Du, Bower, A. F., Krajewski, P. E., and Taleff, E. M., “The Influence of a Threshold Stress for Grain Boundary Sliding on Constitutive Response of Polycrystalline Al during High Temperature Deformation,” Materials Science and Engineering A, vol. 494, pp. 86–91, 2008. LinkAbstract
A continuum polycrystal plasticity model was used to estimate the influence of a threshold stress for grain boundary sliding on the relationship between macroscopic flow stress and strain rate for the aluminum alloy AA5083 when subjected to plane strain uniaxial tension at 450 °C. Under these conditions, AA5083 deforms by dislocation glide at strain rates exceeding 0.001 s−1, and by grain boundary sliding at lower strain rates. The stress–strain rate response can be approximated by View the MathML sourceε˙=Aσn, where A and n depend on grain size and strain rate. We find that a threshold stress less or equal to 4 MPa has only a small influence on flow stress and stress exponent n in the dislocation creep regime (a threshold stress of 2 MPa increases n from 4.2 to 4.5), but substantially increases both flow stress and stress exponent in the grain boundary sliding regime (a threshold stress of 2 MPa increases n from 1.5 to 2.7). In addition, when the threshold stress is included, our model predicts stress versus strain rate behavior that is in good agreement with experimental measurements reported by Kulas et al. [M.A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, T.R. McNelley, Metall. Mater. Trans. A 36 (2005) 1249]. Keywords Superplasticity; Finite element method; Aluminum alloy; Grain boundary sliding; Threshold stress; Strain rate sensitivity
Y. Takigawa, Aguirre, J. V., Taleff, E. M., and Higashi, K., “Cavitation during Grain-Boundary-Sliding Deformation in an AZ61 Magnesium Alloy,” Materials Science and Engineering A, vol. 497, pp. 139–146, 2008. linkAbstract
Cavitation behavior has been investigated in a relatively coarse-grained AZ61 alloy deformed under two conditions for which grain-boundary sliding (GBS) creep controls plastic flow and which produce the same flow stress of 10MPa. At a strain rate of 10-5 s-1 and a temperature of 573 K, GBS creep is rate controlled by grain-boundary diffusion, DGB. At a strain rate of 210-4 s-1 and a temperature of 648 K, GBS creep is rate controlled by lattice diffusion, DL. Test conditions and selected results are reported in Table 1. Tensile elongation is slightly greater when DGB accommodates GBS deformation. Fig. 1 presents data for cavity volume fraction as a function of true strain for data from the present investigation, DGB controlling rate, and a previous investigation, DL controlling rate. These data indicate a very similar rate of increase in cavity volume fraction with strain between the two test conditions. Cavity areal number densities are given in Fig. 2 as a function of cavity radius. Data are plotted on semi logarithmic scales for the two test conditions under consideration. Cavity areal number density distributions are similar between the different deformation conditions when strain is a constant. Fig. 3 provides data for the average radius of the 10 largest cavities observed at each strain level studied. The logarithm of the average cavity radius is plotted against true strain for both deformation conditions of interest. Both the DGB and the DL specimen data are in very close agreement, suggesting similar cavity growth rates. To further investigate the mechanism of cavity growth, the fit to data shown in Fig. 3 was used to calculate cavity growth rate (dr/d) as a function of cavity radius at each strain for which data are shown in Fig. 3. These results are shown in Fig. 4 as the logarithm of cavity growth rate against the logarithm of cavity radius. Also shown in Fig. 4 are predictions for diffusive growth and for plasticity-controlled growth. Cavitation of the AZ61 magnesium sheet material is clearly plasticity controlled when deformation is by GBS, regardless of whether accommodation of sliding is by DL or by DGB.
N. Du, Bower, A. F., Krajewski, P. E., and Taleff, E. M., “The Influence of a Threshold Stress for Grain Boundary Sliding on Constitutive Response of Polycrystalline Al during High Temperature Deformation,” Materials Science and Engineering A, vol. 494, pp. 86–91, 2008. LinkAbstract
A continuum polycrystal plasticity model was used to estimate the influence of a threshold stress for grain boundary sliding on the relationship between macroscopic flow stress and strain rate for the aluminum alloy AA5083 when subjected to plane strain uniaxial tension at 450 °C. Under these conditions, AA5083 deforms by dislocation glide at strain rates exceeding 0.001 s−1, and by grain boundary sliding at lower strain rates. The stress–strain rate response can be approximated by View the MathML sourceε˙=Aσn, where A and n depend on grain size and strain rate. We find that a threshold stress less or equal to 4 MPa has only a small influence on flow stress and stress exponent n in the dislocation creep regime (a threshold stress of 2 MPa increases n from 4.2 to 4.5), but substantially increases both flow stress and stress exponent in the grain boundary sliding regime (a threshold stress of 2 MPa increases n from 1.5 to 2.7). In addition, when the threshold stress is included, our model predicts stress versus strain rate behavior that is in good agreement with experimental measurements reported by Kulas et al. [M.A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, T.R. McNelley, Metall. Mater. Trans. A 36 (2005) 1249]. Keywords Superplasticity; Finite element method; Aluminum alloy; Grain boundary sliding; Threshold stress; Strain rate sensitivity
D. A. Dalton, Brewer, J. L., Bernstein, A. C., Grigsby, W., Milathianaki, D., Jackson, E. D., Adams, R. G., Rambo, P., Schwarz, J., Edens, A., Geissel, M., Smith, I., Taleff, E. M., and Ditmire, T., “Laser-induced Spallation of Aluminum and Al Alloys at Strain Rates above 2 × 10^6 s^-1,” Journal of Applied Physics, vol. 104, pp. eid 013526, 2008. LinkAbstract
Material microstructure is a significant determinant of the tensile stress at which materials fail. Using a high-energy laser to drive shocks in thin slabs, we have explored the role material microstructure plays on the spall strength of high-purity and alloyed aluminum at strain rates of (2–7.5)×106s−1. Slabs of pure recrystallized Al and recrystallized or cold worked Al+3wt%Mg were shock driven using the Z-Beamlet Laser at Sandia National Laboratories. Velocity interferometermeasurements determined the spall strength of the materials, and postshot target analysis explored the microscopic fracture morphology. We observed the greatest spall strength for large-grained, recrystallized high-purity aluminum, with the dominant failure mode being ductile and transgranular. We observe for the first time at these strain rates fracture features for a fine-grained Al+3wt%Mg that were a combination of brittle intergranular and ductile transgranular fracture types. Postshot analysis of target cross sections and hydrocode simulations indicate that this mixed-mode failure results from spall dynamics occurring on spatial scales on the order of the grain size. Differences in spall strength between these Al samples were experimentally significant and correlate with the damage morphologies observed.
2007
S. Agarwal, Briant, C. L., Krajewski, P. E., Bower, A. F., and Taleff, E. M., “Experimental Validation of Two-Dimensional Finite Element Method for Simulating Constitutive Response of Polycrystals during High Temperature Plastic Deformation,” Journal of Materials Engineering and Performance, vol. 13, pp. 170–178, 2007. LinkAbstract
A finite element method was recently designed to model the mechanisms that cause superplastic deformation (A.F. Bower and E. Wininger, A Two-Dimensional Finite Element Method for Simulating the Constitutive Response and Microstructure of Polycrystals during High-Temperature Plastic Deformation, J. Mech. Phys. Solids, 2004, 52, p 1289–1317). The computations idealize the solid as a collection of two-dimensional grains, separated by sharp grain boundaries. The grains may deform plastically by thermally activated dislocation motion, which is modeled using a conventional crystal plasticity law. The solid may also deform by sliding on the grain boundaries, or by stress-driven diffusion of atoms along grain boundaries. The governing equations are solved using a finite element method, which includes a front-tracking procedure to monitor the evolution of the grain boundaries and surfaces in the solid. The goal of this article is to validate these computations by systematically comparing numerical predictions to experimental measurements of the elevated-temperature response of aluminum alloy AA5083 (M.-A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, and T.R. McNelley, Deformation Mechanisms in Superplastic AA5083 materials. Metall. Mater. Trans. A, 2005, 36(5), p 1249–1261). The experimental work revealed that a transition occurs from grain-boundary sliding to dislocation (solute-drag) creep at approximately 0.001/s for temperatures between 425 and 500°C. In addition, increasing the grain size from 7 to 10μm decreased the transition to significantly lower strain rates. Predictions from the finite element method accurately predict the effect of grain size on the transition in deformation mechanisms.
M. - A. Kulas, Krajewski, P. E., Bradley, J. R., and Taleff, E. M., “Forming Limit Diagrams for AA5083 under SPF and QPF Conditions,” Journal of Materials Engineering and Performance, vol. 551–552, pp. 129–134, 2007. LinkAbstract
No abstract available
M. - A. Kulas, Krajewski, P. E., Bradley, J. R., and Taleff, E. M., “Forming-Limit Diagrams for Hot-Forming of AA5083 Aluminum Sheet: Continuously-Cast Material,” Journal of Materials Engineering and Performance, vol. 13, pp. 308–313, 2007. LinkAbstract
Fine-grained AA5083 aluminum sheet is used for hot-forming automotive body panels with gas pressure in the superplastic forming (SPF) and quick plastic forming (QPF) processes. Deformation under QPF conditions is controlled by two fundamental creep mechanisms, grain-boundary-sliding (GBS) and solute-drag (SD) creep. The failure mechanisms of AA5083 materials under QPF conditions depend strongly on these deformation mechanisms and on the applied stress state. Failure can be controlled by flow localization, cavitation development or a combination of both. There is interest in using continuously cast (CC) AA5083 materials instead of direct-chill cast (DC) materials in QPF operations as a means of reducing material cost. However, CC and DC AA5083 materials can produce significantly different ductilities under hot forming. Rupture-based forming-limit diagrams (FLDs) have been constructed for a CC AA5083 sheet material under hot-forming conditions. Forming limits are shown to be related to the controlling deformation mechanisms. Differences between FLDs from DC and CC AA5083 materials are investigated. The differences in FLDs between these materials are related to differences in cavitation development.
J. L. Brewer, Dalton, A. D., Jackson, E. D., Bernstein, A. C., Grigsby, W., Taleff, E. M., and Ditmire, T., “Influence of Microstructure on the Spall Failure of Aluminum Materials,” Metallurgical and Materials Transactions A, vol. 38, pp. 2666–2673, 2007. LinkAbstract
Laser-shock-induced spall failure is studied in thin aluminum targets at strain rates from 2to 5×106s−1. Targets were prepared from high-purity aluminum in the recrystallized condition and a low-impurity aluminum alloy containing 3wt pctmagnesium in both recrystallized and cold-rolled conditions. The effects of material and microstructure on spall fracture morphology are investigated. Recrystallized pure aluminum produced spall fracture surfaces characterized by transgranular ductile dimpling. Recrystallized aluminum-magnesium alloy with a 50-μm grain size produced less ductile spall surfaces, which were dominated by transgranular fracture, with some isolated transgranular ductile dimpling at fast strain rates. Transgranular ductile dimpling regions disappeared in recrystallized alloy specimens with a 23-μm grain size tested at faster rates. Cold-rolled alloy material produced spall failure surfaces consisting of brittle intergranular and transgranular fractures. Measured spall strength increases with increasing ductile fracture character. Spall failure preferentially follows grain boundaries, making grain size an important factor in spall fracture surface character.
J. R. Ciulik and Taleff, E. M., “Power-Law Creep of Powder-Metallurgy Grade Molybdenum Sheet,” Materials Science and Engineering A, vol. 463, pp. 197–202, 2007. LinkAbstract
Creep behavior of commercial-purity, powder-metallurgy grade molybdenum (Mo) sheet has been investigated at temperatures between 1300 and 1600°C (0.56–0.63Tm) using tensile testing at controlled strain rates. Strain-rate-change tests were performed at constant-temperatures over true-strain rates from 1.0×10−6 to 5.0×10−4s−1. Results agree with previously published data indicating that Mo follows power-law creep with a stress exponent of about 5; however, the present results address a temperature range not previously documented. The activation energy for creep was determined to be 240kJ/mol within this temperature range, which is lower than previously published values and approximately half the value reported for self-diffusion, indicating that diffusion mechanisms faster than lattice diffusion are active. It is shown that Mo creep data from a variety of investigations converge closely to a single line on a master plot of strain rate normalized using an activation energy of 240kJ/mol when plotted against stress normalized by the temperature-dependent elastic modulus. This activation energy for creep is attributed to an effective diffusivity that fits the creep data obtained during this study as well as from previously published creep data from commercial-purity molybdenum.
2006
P. W. Green, Kulas, M. - A., Niazi, A., Oh-ishi, K., Taleff, E. M., Krajewski, P. E., and McNelley, T. R., “Deformation and Failure of a Superplastic AA5083 Aluminum Material with a Cu Addition,” Metallurgical and Materials Transactions A, vol. 37, pp. 2727–2738, 2006. LinkAbstract
A modified AA5083 aluminum sheet material containing a Cu addition of 0.61 wt pct has been investigated under conditions relevant to commercial hot-forming technologies. This material was produced by continuous casting followed by industrial hot and cold rolling into sheet. Deformation and failure mechanisms at elevated temperatures were investigated through mechanical testing, thermal analysis, and microscopy. The effects of Cu addition are evaluated by comparisons with data from AA5083 sheet materials without Cu addition, produced both by continuous and direct-chill (DC) casting techniques. At low temperatures and fast strain rates, for which solute-drag (SD) creep governs deformation, the Cu addition slightly increases tensile ductility at 450 °C but does not otherwise alter deformation behaviors. At high temperatures and slow strain rates, for which grainboundary-sliding (GBS) creep governs deformation, the Cu addition decreases flow stress and, at 450 °C, improves tensile ductility. A strong temperature dependence for tensile ductility results from the Cu addition; tensile ductility at 500 °C is notably reduced from that at 450 °C. The Cu addition creates platelike particles at grain boundaries, which produce incipient melting and the observed mechanical behavior.
J. R. Ciulik and Taleff, E. M., “Dynamic Abnormal Grain Growth in Commercial-Purity Molybdenum,” in Proceedings of the 2006 International Conference on Tungsten, Refractory & Hardmetals {VI}, Orlando, Florida, 2006, pp. CD ROM. Similar link, UT RespositoryAbstract
In this experimental investigation, the tensile creep behavior of commercial-purity molybdenum sheet at temperatures between 1300°C and 1700°C is critically evaluated, based upon experimental creep testing and microstructural characterizations. The high-temperature properties of molybdenum are of interest because there are many applications in which molybdenum and molybdenum alloys are used at elevated temperatures. Understanding of the creep mechanisms and the constitutive relations between stress and strain at elevated temperatures is needed in order to determine if molybdenum is an appropriate choice for a given high-temperature design application and to accurately predict its creep life. The creep behavior of two commercially-available grades of molybdenum was determined using short-term creep tests (1/2 to 14 hours) at slow to moderate true-strain rates of 10⁻⁶ to 10⁻⁴ s⁻¹ and temperatures between 1300°C and 1700°C. High-temperature, uniaxial tensile testing was used to produce data defining the relationship between tensile creep strain-rate and steady-state flow stress at four temperatures: 1340°C, 1440°C, 1540°C, 1640°C. Microstructural changes that occurred during creep testing were evaluated and compared to changes resulting from elevated temperature exposure alone. Mechanisms for dynamic abnormal grain growth that occurred during creep testing and the causes of the microstructural changes that occurred as a function of temperature are discussed.
E. M. Taleff, “Overview of 5000-Series Aluminum Materials for Hot Forming in the Automotive Industry,” in Aluminum Wrought Products for Automotive, Packaging, and Other Applications - The James Morris Honorary Symposium, 2006, pp. 87–96.Abstract
Not available.
A. Niazi, “The Effect of Cu, Zn, and Bi Additions on the High-Temperature Deformation Behavior of Al-3.0Mg Alloys,” The University of Texas at Austin, 2006.Abstract
Not available.
J. R. Ciulik and Taleff, E. M., “Method for Growing Single Crystals of Metals”, 2006. LinkAbstract
A method for growing large single crystals of metals is disclosed. A polycrystalline form of a metal specimen is initially heated in a non-oxidizing environment. A minimum plastic strain is then applied to the heated metal specimen to initiate the growth of a selected grain within the heated metal specimen. Additional plastic strain is subsequently applied to the heated metal specimen to propagate the growth of the selected grain to become a large single crystal.
J. L. Brewer, “Effect of Microstructure and Alloying on the Spall of Aluminum,” The University of Texas at Austin, 2006.Abstract
Not available

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