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PROPERTIES AND NANOSTRUCTURES OF NANO-MATERIALS PROCESSED BY SEVERE PLASTIC DEFORMATION (SPD)

Description: Metallic materials usually exhibit higher strength but lower ductility after being plastically deformed by conventional techniques such as rolling, drawing and extrusion. In contrast, nanostructured metals and alloys processed by severe plastic deformation (SPD) have demonstrated both high strength and high ductility. This extraordinary mechanical behavior is attributed to the unique nanostructures generated by SPD processing. The combination of ultrafine grain size and high-density dislocations appears to enable deformation by new mechanisms not active in coarse-grained metals and alloys. These results demonstrate the possibility of tailoring the microstructures of metals and alloys by SPD to obtain superior mechanical properties. Nanostructured metals and alloys processed by SPD techniques have unique nanostructures not observed in nano-materials synthesized by other techniques such as the consolidation of nanopowders. The SPD-generated nanostructures have many features related to deformation, including high dislocation densities, and high- and low-angle grain boundaries in equilibrium or non-equilibrium states. Future studies are needed to investigate the deformation mechanisms that relate the unique nanostructures with the superior mechanical properties exhibited by SPD-processed metals and alloys.
Date: March 1, 2001
Creator: ZHU, Y. T.
Partner: UNT Libraries Government Documents Department

PROPERTIES AND NANOSTRUCTURES OF NANO-MATERIALS PROCESSED BY SEVERE PLASTIC DEFORMATION (SPD).

Description: Metallic materials usually exhibit higher strength but lower ductility after being plastically deformed by conventional techniques such as rolling, drawing and extrusion. In contrast, nanostructured metals and alloys processed by severe plastic deformation (SPD) have demonstrated both high strength and high ductility. This extraordinary mechanical behavior is attributed to the unique nanostructures generated by SPD processing. The combination of ultrafine grain size and high-density dislocations appears to enable deformation by new mechanisms not active in coarse-grained metals and alloys. These results demonstrate the possibility of tailoring the microstructures of metals and alloys by SPD to obtain superior mechanical properties. Nanostructured metals and alloys processed by SPD techniques have unique nanostructures not observed in nanomaterials synthesized by other techniques such as the consolidation of nanopowders. The SPD-generated nanostructures have many features related to deformation, including high dislocation densities, and high- and low-angle grain boundaries in equilibrium or nonequilibrium states. Future studies are needed to investigate the deformation mechanisms that relate the unique nanostructures with the superior mechanical properties exhibited by SPD-processed metals and alloys.
Date: January 1, 2001
Creator: Zhu, Y. T. (Yuntian Theodore)
Partner: UNT Libraries Government Documents Department

PROCESSING OF NANOSTRUCTURED COPPER BY REPETITIVE CORRUGATION AND STRAIGHTENING (RCS)

Description: A new process, Repetitive Corrugation and Straightening (RCS), has been developed to create bulk, nanostructured copper. In this investigation, a high purity (99.99%). copper bar measuring 6 x 6 x 50 mm with an average grain size of 765 {micro}m was used as the starting material. It was repetitively corrugated and straightened for 14 times with 90{degree} rotations along its longitudinal axis between consecutive corrugation-straightening cycles. The copper was cooled to below room temperature before each RCS cycle. The grain size obtained after the RCS process was in the range of twenty to a few hundred nanometers, and microhardness was increased by 100%. Both equilibrium and non-equilibrium grain boundaries are observed. This work demonstrates the capability of the RCS process in refining grain size of metal materials. The RCS process can be easily adapted to large-scale industrial production and has the potential to pave the way to large-scale structural applications of nanostructured materials.
Date: October 1, 2000
Creator: Zhu, Y.T. & Jiang, H.
Partner: UNT Libraries Government Documents Department

Direct fiber strengthening in three dimensional random-oriented short-fiber composites

Description: A theory for direct fiber strengthening in random-oriented short-fiber composites is developed. It adopts a maximum load composite failure criterion and takes into account the fiber orientation effect on the probability of a fiber being intercepted by a specimen cross-section. The strain and load of short fibers with different inclination angles with respect to the loading direction were first calculated, and their contribution in carrying load toward the composite load direction was integrated to give the total load. The fibers with smaller inclination angles bear greater stress and break first. This load is then transferred to fibers with larger inclination angles. Direct fiber strengthening component of the composite strength was calculated from the maximum total load these short fibers can carry. The present theory predicts a much greater direct short-fiber strengthening than does previous theories, and provides useful information for composite design and strength assessment.
Date: May 1, 1995
Creator: Zhu, Y.T. & Blumenthal, W.R.
Partner: UNT Libraries Government Documents Department

Properties and Nanostructures of Materials Processed by SPD Techniques

Description: Metallic materials usually exhibit higher strength but lower ductility after being plastically deformed by conventional techniques such as rolling, drawing and extrusion. In contrast, nanostructured metals and alloys processed by severe plastic deformation (SPD) have demonstrated both high strength and high ductility. This extraordinary mechanical behavior is attributed to the unique nanostructures generated by SPD processing. It demonstrates the possibility of tailoring the microstructures of metals and alloys by SPD to obtain superior mechanical properties. The SPD-generated nanostructures have many features related to deformation, including high dislocation densities, and high- and low-angle grain boundaries in equilibrium or non-equilibrium states. This paper reviews the mechanical properties and the defect structures of SPD-processed nanostructured materials. Keywords: strength, ductility, nanostructures, SPD, non-equilibrium grain boundary
Date: January 1, 2001
Creator: Liao, Xiaoshan; Huang, J. (Jianyu) & Zhu, Y. T. (Yuntian Theodore)
Partner: UNT Libraries Government Documents Department

Modeling of statistical tensile strength tensile of short-fiber composites

Description: This Paper develops a statistical strength theory for three-dimensionally (3-D) oriented short-fiber reinforced composites. Short-fiber composites are usually reinforced with glass and ceramic short fibers and whiskers. These reinforcements are brittle and display a range of strength values, which can be statistically characterized by a Weibull distribution. This statistical nature of fiber strength needs to be taken into account in the prediction of composite strength. In this paper, the statistical nature of fiber strength is incorporated into the calculation of direct fiber strengthening, and a maximum-load composite failure criterion is adopted to calculate the composite strength. Other strengthening mechanisms such as residual thermal stress, matrix work hardening, and short-fiber dispersion hardening are also briefly discussed.
Date: October 5, 1995
Creator: Zhu, Y.T.; Blumenthal, W.R.; Stout, M.G. & Lowe, T.C.
Partner: UNT Libraries Government Documents Department

Deformation mechanisms at Different grain sizes in a cryogenically ball-milled Al-Mg alloy.

Description: An Al-7.5 wt. % Mg alloy was ball-milled in liquid N2 for eight hours and its microstructures were investigated using transmission electron microscopy. Electron diffraction confirmed that the resulting powder is a supersaturated Al-Mg solid solution with a face-centered cubic structure. Three nanostructures with different grain size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. The reasons for the different deformation mechanisms were discussed. Keywords: Aluminum alloy; Cryogenic ball milling; Transmission electron microscopy; Microstructure.
Date: January 1, 2001
Creator: Liao, Xiaozhou; Huang, J. (Jianyu); Zhu, Y. T. (Yuntian Theodore); Zhou, F. (Fei) & Lavernia, Enrique J.
Partner: UNT Libraries Government Documents Department

Quantitative analysis of the effects of strain-state on the microstructure and J{sub c} of BSCCO tapes

Description: After considerable optimization efforts, conventional thermomechanical processing of long, high temperature superconductors has not produced critical current densities (J{sub c}) adequate for most liquid nitrogen temperature applications. New approaches are needed to improve the J{sub c} of superconducting tape produced by co-deforming a ductile silver sheath containing the superconducting oxide using the powder-in-tube process. This study investigates improvements in J{sub c} generated by modifying the strain-state during rolling of silver-sheathed Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10+x} (BSCCO-2223) tape using quantitative image analysis of the different phases. Pure compression and combined compression-shear loading was achieved by embedding BSCCO-2223 tapes at different locations within thick steel blocks. High hydrostatic compressive stress was imposed by confining the tape width. Tapes deformed with combined shear-compression exhibited measurably higher J{sub c} values than tapes subjected to pure compression, but their microstructures showed little difference in the amount of nonconducting (including porosity) phase content. However, constraining the tape width resulted in the most significant increase in J{sub c} which corresponded to a much lower porosity and nonconducting phase volume in the oxide near the tape edge.
Date: April 1, 1996
Creator: Blumenthal, W.R.; Zhu, Y.T.; Sebring, R.J.; Lowe, T.C. & Asaro, R.J.
Partner: UNT Libraries Government Documents Department

On the statistical strength of nicalon fibers and its characterization

Description: Nicalon fibers are one of the most attractive ceramic fibers for reinforcing high temperature structural composites. Experiment show that the diameter variation (from 8 to 22 {mu}m) in a tow of commercial Nicalon fibers has an effect on statistical strength distribution of Nicalon fibers. Therefore, an appropriate characterization of the statistical distribution of fiber strength, capable of accounting for the effect of diameter variation, is required. The two-parameter single-modal Weibull distribution is found inadequate for characterizing the strength of Nicalon fibers. This work demonstrates that a 3-parameter modified Weibull distribution can successfully characterize the strength of Nicalon fibers; this distribution yields a higher {beta} value, which indicates less scatter in fiber strength than with the 2-parameter single modal Weibull distribution. It more accurately treats the strength variation caused by diameter variation. It is also much easier to use than the 4-parameter bimodal. Moreover, it is seen to more accurately treat the strength variation caused by diameter variation than the single modal 2-parameter analysis.
Date: February 1, 1997
Creator: Zhu, Y.T.; Taylor, S.T.; Stout, M.G.; Butt, D.P.; Blumenthal, W.R. & Lowe, T.C.
Partner: UNT Libraries Government Documents Department

Innovative Composites Through Reinforcement Morphology Design - a Bone-Shaped-Short-Fiber Composite

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The objective of this project is to improve the strength and toughness of conventional short-fiber composites by using innovative bone-shaped-short (BSS) fibers as reinforcement. We fabricated a model polyethylene BSS fiber-reinforced polyester-matrix composite to prove that fiber morphology, instead of interfacial strength, solves the problem. Experimental tensile and fracture toughness test results show that BSS fibers can bridge matrix cracks more effectively, and consume many times more energy when pulled out, than conventional-straight-short (CSS) fibers. This leads to both higher strength and fracture toughness for the BSS-fiber composites. A computational model was developed to simulate crack propagation in both BSS- and CSS-fiber composites, accounting for stress concentrations, interface debonding, and fiber pullout. Model predictions were validated by experimental results and will be useful in optimizing BSS-fiber morphology and other material system parameters.
Date: June 29, 1999
Creator: Zhu, Y.T.; Valdez, J.A.; Beyerlain, I.J.; Stout, M.G.; Zhou, S.; Shi, N. et al.
Partner: UNT Libraries Government Documents Department

Some new perspective on the strength and fracture of Nicalon fibers

Description: Nicalon{trademark} SiC fibers, processed by melt-spinning, are attractive reinforcing materials for high-temperature structural composites. This paper studies the effects of fiber diameter on fracture and statistical strength distribution of the fibers, by means of fractography on 8-22 {mu}m fibers fractured under tensile load. Flaw population and location effects are also studied.
Date: July 1, 1996
Creator: Taylor, S.T.; Zhu, Y.T.; Butt, D.P.; Stout, M.G.; Blumenthal, W.R. & Lowe, T.C.
Partner: UNT Libraries Government Documents Department

The influence of structural defects on intragranular critical currents of bulk MgB[sub 2].

Description: Bulk MgB2 samples were prepared under different synthesis conditions and analyzed by scanning and transmission electron microscopy. The critical current densities were determined from the magnetization versus magnetic field curves of bulk and powder-dispersed-in-epoxy samples. Results show that through a slow cooling process, the oxygen dissolved in bulk MgB2 at high synthesis temperatures can segregate and form nanometer-sized coherent precipitates of Mg(B,O)2 in the MgB2 matrix. Magnetization measurements indicate that these precipitates act as effective flux pinning centers and therefore significantly improve the intra-grain critical current density and its field dependence.
Date: January 1, 2002
Creator: Serquis, A. (Adriana); Liao, Xiaozhou; Civale, L. (Leonardo); Zhu, Y. T. (Yuntian Theodore); Coulter, J. Y. (James Y.); Peterson, D. E. (Dean E.) et al.
Partner: UNT Libraries Government Documents Department

Critical currents, vortex dynamics and microstructure in MgB[sub 2].

Description: One key issue in optimizing critical current density (J,) in MgB2 is to determine which structural features are the relevant pinning centers. Likely sources of vortex pinning include grain boundaries and intra-grain defects. Detailed studies of the field (H) and temperature (T) dependence of pinning in microstructurally well-characterized samples are required to clarify this point. In this work we explore the influence of microstructures on the vortex dynamics of MgB2 bulk samples prepared either at ambient or at high pressure (HIP). Scanning and transmission electron microscopy indicate the presence of several types of defects. Both un-HIPed and HIPed samples contain a large number of intra-grain Mg(B,0)2 precipitates coherent with the matrix, with sizes ranging from 5 nm to 100 nm, which are very well suited to act as pinning centers. The HIP process further improves flux pinning by eliminating the porosity, dispersing the MgO present at the grain boundaries of the un-HIPed samples, and generating dislocations. We also present a detailed study of the T, H and current density (J) dependence of the normalized time relaxation rate, S=dlnJ/dlnt. The intermediate transition temperature T, - 39K makes MgB2 attractive for exploring vortex dynamics in a regime of intermediate influence of thermal fluctuations. At low T, we observe a linear S(T), from which we extract a pinning energy U, that is weakly T dependent and decreases monotonically with H. The extrapolations to T=O indicate that the quantum creep rate is small. At higher T, the activation energy U(J) shows the divergent behavior at J -> 0 that characterizes the glassy phases. The results are contrasted with the expectations of various collective creep scenarios to extract information on the characteristics of the pinning centers.
Date: January 1, 2002
Creator: Serquis, A. (Adriana); Civale, L. (Leonardo); Liao, Xiaozhou; Maley, M. P. (Martin P.); Zhu, Y. T. (Yuntian Theodore); Nesterenko, V. F. et al.
Partner: UNT Libraries Government Documents Department

Influence of reinforcement morphology on the mechanical properties of short-fiber composites

Description: A major problem of short-fiber composites is that the interfaces between the fiber and matrix become a limiting factor in improving mechanical properties such as strength. For a short fiber, a strong interface is desired to effectively transfer load from matrix to fiber, thus reducing the ineffective fiber length. However, a strong interface will make it difficult to relieve fiber stress concentration in front of an approaching crack. Stress concentrations result in fiber breakage. The authors report in this paper an innovative approach to overcome this problem: reinforcement morphology design. Short-fibers with enlarged ends are processed and used to reinforce a polyester matrix. The initial results show that the bone-shaped short-fibers produce a composite with significantly higher strength than can be attained with conventional short, straight fibers.
Date: December 1, 1997
Creator: Zhu, Y.T.; Valdez, J.A.; Shi, N.; Lovato, M.L.; Stout, M.G.; Zhou, S. et al.
Partner: UNT Libraries Government Documents Department