Publications

2014
A. R. Antoniswamy, Carter, J. T., Louis G. Hector, J., and Taleff, E. M., “Static Recrystallization and Grain Growth in AZ31B-H24 Magnesium Alloy Sheet,” in Proceedings of the 2014 TMS Annual Meeting, San Diego, CA,, 2014. Publisher's VersionAbstract
e The effects of static annealing on recovery, recrystallization and grain growth in a magnesium AZ31B-H24 alloy sheet are investigated at 50°C to 450°C. Full recrystallization is observed after annealing at 250°C or higher temperatures. Recrystallized grain size increases with temperature through normal grain growth. Room-temperature hardness drops abruptly following recrystallization and then decreases with increasing grain size. Predictive relationships are proposed for recrystallized grain size as a function of temperature and time and for hardness as a function of recrystallized grain size.The effects of recrystallization and grain growth on plastic flow and anisotropy will also be discussed.
A. J. Carpenter, Antoniswamy, A. R., Carter, J. T., Louis G. Hector, J., and Taleff, E. M., “A Mechanism-dependent Material Model for the Effects of Grain Growth and Anisotropy on Plastic Deformation of Magnesium Alloy AZ31 Sheet at 450°C,” Acta Materialia, vol. 68, pp. 254–266, 2014. LinkAbstract
Utilization of wrought magnesium sheet alloys for structural components in transportation industries has been severely limited by poor room-temperature formability, a result of the slip behaviors inherent to Mg’s hexagonal close-packed crystal structure. Today, the only production technology to clearly overcome this limitation uses hot forming to activate additional (non-basal) slip systems in Mg alloy sheet. The absence of an accurate material constitutive model that captures the complex mechanical response of Mg sheet alloys at elevated temperatures has been a persistent barrier to accurate forming simulations. This study addresses that issue using experimental measurements and mechanism-based modeling. The mechanisms of plastic deformation in a Mg AZ31 wrought alloy sheet at 450 °C across strain rates from 10−4 to 10−1 s−1 are identified as grain-boundary-sliding (GBS) creep and five-power dislocation-climb (DC) creep. GBS creep is subject to hardening from grain growth, and DC creep produces texture-dependent plastic anisotropy. Based on these mechanisms, a new material constitutive model for Mg AZ31 at 450 °C is constructed to predict plastic response under general multiaxial loading. A unique aspect of this new model is that it accounts for hardening and plastic anisotropy by linking these effects to the two mechanisms controlling deformation. The model is validated against independent experimental data and provides accurate predictions for hot forming of a simple test shape. The new material model is the first for Mg AZ31 sheet that accurately predicts deformation at an elevated temperature under both uniaxial and biaxial stress states.
2013
T. J. Watt, Yasuda, S., Ichitani, K., Takata, K., Carpenter, A., Jodlowski, J., and Taleff, E. M., “The Effect of Magnesium Content on Microstructure Evolution during Hot Deformation of Aluminum Alloys,” in Proceedings of the 2013 TMS Annual Meeting, San Antonio, TX, 2013, pp. 499–503. Publisher's Version
A. J. Carpenter, Carter, J. T., Louis G. Hector, J., and Taleff, E. M., “Gas-pressure Bulge Forming of Mg AZ31Sheet 450°C,” in Proceedings of the 2013 TMS Annual Meeting, San Antonio, TX, 2013, pp. 139–144. Publisher's Version
A. R. Antoniswamy, Carpenter, A. J., Carter, J. T., Louis G. Hector, J., and Taleff, E. M., “The Influence of Deformation Mechanisms on Rupture of AZ31B Magnesium Alloy Sheet at Elevated Temperatures,” Proceedings of the 2013 TMS Annual Meeting, pp. 211–215, 2013. Publisher's Version
M. A. Morovat, Engelhardt, M. D., Helwig, T. A., and Taleff, E. M., “Influence of Creep on the Stability of Steel Columns Subjected to Fire,” in Proceedings, Annual Stability Conference, St. Louis, MO, 2013. Publisher's Version
E. M. Taleff and Pedrazas, N. A., “Perspectives: A New Route for Growing Large Grains in Metals,” Science, vol. 341, pp. 1461–1462, 2013. Publisher's VersionAbstract
(Non-Refereed) Most metallic materials consist of a network of small single crystals, or grains, connected by grain boundaries. This microstructure, which spans length scales from a few nanometers to hundreds of micrometers, controls many of the properties of the metal. Mechanical processing and thermal treatments can be used to alter this microstructure, but the evolution of grains during processing of a material is governed by phenomena that are so complex (relative to our present scientific understanding) that the outcome cannot be reliably predicted. On page 1500 of this issue, Omori et al. (1) describe a wholly unexpected microstructure that arises from synergies among multiple phenomena. They created very large grains in a copper-based shape-memory alloy—a material that will spontaneously recover large strains upon a temperature change—by thermal cycling across temperatures that produce solid-state phase transformations. The subtle mechanisms that apparently act together at elevated temperature to produce this microstructure include internal-stress plasticity (2) and abnormal grain growth (3). This discovery has potential for technological applications that depend on long service lives of shape-memory alloys.
T. McNelley, Oh-ishi, K., Swaminathan, S., Bradley, J., Krajewski, P. E., and Taleff, E. M., “Characteristics of the GBS-SDC Transition during Bi-axial Forming of AA5083,” Materials Science Forum, vol. 735, pp. 43–48, 2013. Publisher's VersionAbstract
Thermomechanical processing to enable superplasticity in AA5083 materials includes cold working followed by heating prior to hot blow forming. Upon heating for forming at 450°C, a B-type ({110}) rolling texture is replaced by a near-random texture with a weak superimposed cube orientation parallel to the sheet normal. The presence of refined grains 7 – 8μm in size reflects the predominance of particle-stimulated nucleation of recrystallization prior to forming. The subsequent evolution of microstructure, texture and cavitation behaviour during biaxial deformation in the solute drag creep (SDC) and grain boundary sliding (GBS) regimes will be presented.
D. L. Worthington, Pedrazas, N. A., and Taleff, E. M., “Dynamic Abnormal Grain Growth in Molybdenum,” Metallurgical and Materials Transactions A, vol. 44, pp. 5025–5038, 2013. Publisher's VersionAbstract
A new abnormal grain growth phenomenon that occurs only during continuous plastic straining, termed dynamic abnormal grain growth (DAGG), was observed in molybdenum (Mo) at elevated temperature. DAGG was produced in two commercial-purity molybdenum sheets and in a commercial-purity molybdenum wire. Single crystals, centimeters in length, were created in these materials through the DAGG process. DAGG was observed only at temperatures of 1713 K (1440 °C) and above and occurred across the range of strain rates investigated, ~10−5 to 10−4 s−1. DAGG initiates only after a critical plastic strain, which decreases with increasing temperature but is insensitive to strain rate. Following initiation of an abnormal grain, the rate of boundary migration during DAGG is on the order of 10 mm/min. This rapid growth provides a convenient means of producing large single crystals in the solid state. When significant normal grain growth occurs prior to DAGG, island grains result. DAGG was observed in sheet materials with two very different primary recrystallization textures. DAGG grains in Mo favor boundary growth along the tensile axis in a <110> direction, preferentially producing single crystals with orientations from an approximately <110> fiber family of orientations. A mechanism of boundary unpinning is proposed to explain the dependence of boundary migration on plastic straining during DAGG.
J. Lee, Morovat, M., Hu, G., Engelhardt, M., and Taleff, E., “Experimental Investigation of Mechanical Properties of ASTM A992 Steel at Elevated Temperatures,” Engineering Journal, American Institute of Steel Construction, vol. 50, pp. 249–272, 2013. Publisher's VersionAbstract
This paper presents the results of a detailed experimental study into the mechanical properties of ASTM A992 structural steel at elevated temperatures. Critical testing issues, including temperature measurement, temperature control, and extensometer use, along with the testing equipment and procedures are briefly explained. Tensile steady-state temperature tests are conducted on samples of ASTM A992 steel at temperatures up to 1000 °C. Full stress-strain curves, representing steel coupons tested to fracture at elevated temperatures, are generated. Important mechanical properties such as yield stress, tensile strength, proportional limit, elastic modulus and elongation are obtained from the stress-strain curves. Results are compared with elevated-temperature properties specified by Eurocode 3 and by the AISC Specification. When defined as the stress at 2% total strain, the measured yield stress values agree reasonably well with the corresponding values from Eurocode 3 and the AISC Specification. However, for more conventional definitions of yield stress, such as the 0.2% offset yield stress, the agreement is poor. It is observed that the yield stress of steel at elevated temperatures up to about 600 °C is highly dependent on the manner in which yield stress is defined. The effects of displacement loading rates on steel strength and static yielding behavior are also investigated. It is shown that the displacement rate has a large impact on the steel strength at elevated temperatures, especially at temperatures higher than 600 °C. Further work is needed to fully characterize the time-dependent effects on the elevated-temperature stress-strain response of structural steel. Additionally, this paper presents results of Charpy V-Notch (CVN) tests on ASTM A992 steel at elevated temperatures.
A. R. Antoniswamy, Carpenter, A. J., Carter, J. T., Louis G. Hector, J., and Taleff, E. M., “Forming-Limit Diagrams for Magnesiusm AZ31B and ZEK100 Alloys Sheets at Elevated Temperatures,” Journal of Materials Engineering and Performance, vol. 22, pp. 3389–3397, 2013. Publisher's VersionAbstract
Modern design and manufacturing methodologies for magnesium (Mg) sheet panels require formability data for use in computer-aided design and computer-aided engineering tools. To meet this need, forming-limit diagrams (FLDs) for AZ31B and ZEK100 wrought Mg alloy sheets were developed at elevated temperatures for strain rates of 10−3 and 10−2 s−1. The elevated temperatures investigated range from 250 to 450 °C for AZ31B and 300 to 450 °C for ZEK100. The FLDs were generated using data from uniaxial tension, biaxial bulge, and plane-strain bulge tests, all carried out until specimen rupture. The unique aspect of this study is that data from materials with consistent processing histories were produced using consistent testing techniques across all test conditions. The ZEK100 alloy reaches greater major true strains at rupture, by up to 60%, than the AZ31B alloy for all strain paths at all temperatures and strain rates examined. Formability limits decrease only slightly with a decrease in temperature, less than 30% decrease for AZ31B and less than 35% decrease for ZEK100 as the temperature decreases from 450 to 300 °C. This suggests that forming processes at 250-300 °C are potentially viable for manufacturing complex Mg components.
2012
P. A. Sherek, Carpenter, A. J., Louis G. Hector, J., Krajewski, P. E., Carter, J. T., Lasceski, J., and Taleff, E. M., “The Effects of Strain and Stress Sate in Hot Forming of Mg AZ31 Sheet,” in Magnesium Technology 2012, TMS Annual Meeting, Orlando, FL, 2012. Publisher's VersionAbstract
Wrought magnesium alloys, such as AZ31 sheet, are of considerable interest for light-weighting of vehicle structural components. The poor room-temperature ductility of AZ31 sheet has been a hindrance to forming the complex part shapes necessary for practical applications. However, the outstanding formability of AZ31 sheet at elevated temperature provides an opportunity to overcome that problem. Complex demonstration components have already been produced at 450°C using gas-pressure forming. Accurate simulations of such hot, gas-pressure forming will be required for the design and optimization exercises necessary if this technology is to be implemented commercially. We report on experiments and simulations used to construct the accurate material constitutive models necessary for finite-element-method simulations. In particular, the effects of strain and stress state on plastic deformation of AZ31 sheet at 450°C are considered in material constitutive model development. Material models are validated against data from simple forming experiments.
A. Morovat, Lee, J., Engelhardt, M., Taleff, E., Helwig, T., and Segrest, V., “Creep Properties of ASTM A992 Steel at Elevated Temperatures,” Proceedings, Second International Conference on Structures and Building Materials, vol. 446-449, pp. 786–792, 2012. Publisher's VersionAbstract
In moving towards an engineered performance-based approach to structural fire safety, a sound knowledge of the elevated-temperature properties of structural steel is crucial. Of all mechanical properties of structural steel at elevated temperatures, material creep is particularly important. Under fire conditions, behavior of steel members and structures can be highly time-dependent. As a result, understanding the time-dependent mechanical properties of structural steel at high temperatures becomes essential. This paper presents preliminary results of a comprehensive on-going research project aimed at characterizing the material creep behavior of ASTM A992 steel at elevated temperatures. Such creep properties are presented in the form of strain-time curves for materials from the web and the flanges of a W4×13 wide flange section and from the web of a W30×99 section. The test results are then compared against material creep models for structural steel developed by Harmathy, and by Fields and Fields to evaluate the predictions of these models. The preliminary results clearly indicate that material creep is significant within the time, temperature, and stress regimes expected in a builing fire. The results also demonstrate the need for a more reliable creep model for steel for strcutural-fire engineering analysis.
A. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “Investigation of Time-Dependent Buckling of Steel Columns Exposed to Fire Temperatures,” Proceedings, 2012 Structures Congress, pp. 2095–2106, 2012. Publisher's VersionAbstract
One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of applied load. The phenomenon of time-dependent buckling can have a significant impact on the safety of steel columns subjected to fire. This phenomenon has received relatively little research attention, and is not currently explicitly considered in code-based design formulas for columns at elevated temperatures, such as those in the Eurocode 3 or those in the AISC Specification. This paper presents some results of on-going research, which aims at developing analytical, computational and experimental predictions of the phenomenon of creep buckling in steel columns subjected to fire. Analytical solutions using the concept of time-dependent tangent modulus are developed to model time-dependent buckling behavior of steel columns at elevated temperatures. Results from computational creep buckling studies using Abaqus are also presented, and compared with analytical predictions. Material creep data on ASTM A992 steel is also presented in the paper and compared to existing creep models for structural steel at high temperatures. Both analytical and computational methods utilize material creep models for structural steel developed by Harmathy, by Fields and Fields, and by the authors. Predictions from this study are also compared against those from Eurocode 3 and the AISC Specification. Results of this work show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.
J. Lee, Morovat, A., Engelhardt, M., and Taleff, E., “Creep Behavior of ASTM A992 Steel at Elevated Temperatures,” in Proceedings, 7th International Conference on Structures in Fire (SiF), Zurich, Switzerland, 2012. Publisher's Version
A. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “High-Temperature Creep Buckling Phenomenon of Steel Columns Subjected to Fire,” in Proceedings, 7th International Conference on Structures in Fire (SiF), Zurich, Switzerland, 2012. Publisher's Version
J. T. Lee, Carpenter, A. J., Jodlowski, J. P., and Taleff, E. M., “Predicting Hot Deformation of AA5182 Sheet,” in In Proceedings of the 13th International Conference on Aluminum Alloys (ICAA-13), Pittsburgh, PA, 2012. Publisher's Version
M. A. Morovat, Lee, J., Engelhardt, M. D., Taleff, E. M., Helwig, T., and Segrest, V., “Creep Properties of ASTM A992 Steel at Elevated Temperatures,” vol. 446–449, pp. 786–792, 2012. Publisher's VersionAbstract
In moving towards an engineered performance-based approach to structural fire safety, a sound knowledge of the elevated-temperature properties of structural steel is crucial. Of all mechanical properties of structural steel at elevated temperatures, material creep is particularly important. Under fire conditions, behavior of steel members and structures can be highly time-dependent. As a result, understanding the time-dependent mechanical properties of structural steel at high temperatures becomes essential. This paper presents preliminary results of a comprehensive on-going research project aimed at characterizing the material creep behavior of ASTM A992 steel at elevated temperatures. Such creep properties are presented in the form of strain-time curves for materials from the web and the flanges of a W4×13 wide flange section and from the web of a W30×99 section. The test results are then compared against material creep models for structural steel developed by Harmathy, and by Fields and Fields to evaluate the predictions of these models. The preliminary results clearly indicate that material creep is significant within the time, temperature, and stress regimes expected in a builing fire. The results also demonstrate the need for a more reliable creep model for steel for strcutural-fire engineering analysis.
M. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “Investigation of Time-Dependent Buckling of Steel Columns Exposed to Fire Temperatures,” 2012, pp. 2095–2106. Publisher's VersionAbstract
One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of applied load. The phenomenon of time-dependent buckling can have a significant impact on the safety of steel columns subjected to fire. This phenomenon has received relatively little research attention, and is not currently explicitly considered in code-based design formulas for columns at elevated temperatures, such as those in the Eurocode 3 or those in the AISC Specification. This paper presents some results of on-going research, which aims at developing analytical, computational and experimental predictions of the phenomenon of creep buckling in steel columns subjected to fire. Analytical solutions using the concept of time-dependent tangent modulus are developed to model time-dependent buckling behavior of steel columns at elevated temperatures. Results from computational creep buckling studies using Abaqus are also presented, and compared with analytical predictions. Material creep data on ASTM A992 steel is also presented in the paper and compared to existing creep models for structural steel at high temperatures. Both analytical and computational methods utilize material creep models for structural steel developed by Harmathy, by Fields and Fields, and by the authors. Predictions from this study are also compared against those from Eurocode 3 and the AISC Specification. Results of this work show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.
T. McNelley, Oh-ishi, K., Swaminathan, S., Bradley, J., Krajewski, P. E., and Taleff, E. M., “Characteristics of the GBS-SDC Transition during Bi-axial Forming of AA5083,” Materials Science Forum, vol. 735, pp. 43–48, 2012. Publisher's VersionAbstract
Thermomechanical processing to enable superplasticity in AA5083 materials includes cold working followed by heating prior to hot blow forming. Upon heating for forming at 450°C, a B-type ({110}) rolling texture is replaced by a near-random texture with a weak superimposed cube orientation parallel to the sheet normal. The presence of refined grains 7 – 8μm in size reflects the predominance of particle-stimulated nucleation of recrystallization prior to forming. The subsequent evolution of microstructure, texture and cavitation behaviour during biaxial deformation in the solute drag creep (SDC) and grain boundary sliding (GBS) regimes will be presented.

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