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C J Boehlert - One of the best experts on this subject based on the ideXlab platform.

  • The tensile and Creep behavior of Mg–Zn Alloys with and without Y and Zr as ternary elements
    Journal of Materials Science, 2007
    Co-Authors: C J Boehlert
    Abstract:

    Tensile–Creep experiments were conducted in the temperature range 100–200 °C and stress range 20–83 MPa for a series of magnesium–zinc–yttrium (Mg-Zn-Y) and mangnesium-zinc–zirconium (Mg-Zn-Zr) alloys ranging from 0 to 5.4 wt% Zn, 0 to 3 wt% Y, and 0 to 0.6 wt.% Zr. The greatest tensile–Creep resistance was exhibited by an Mg–4.1Zn–0.2Y alloy. The room-temperature yield strength increased with increasing Y content for Mg–1.6–2.0Zn alloys. The greatest tensile strength and elongation was exhibited by Mg–5.4Zn–0.6Zr. This alloy also exhibited the finest grain size and the poorest Creep resistance. The measured Creep Exponents and activation energies suggested that the Creep mechanisms were dependent on stress. For applied stresses greater than 40 MPa, the Creep Exponents were between 4 and 8. For applied stresses less than 40 MPa, the Creep Exponent was 2.2. The calculated activation energies (Qapp) were dependent on temperature where the Q _app values between 100 and 150 °C (65 kJ/mol) were half those between 150 and 200 °C for the same applied stress value (30 MPa). Deformation observations indicated that the grain boundaries were susceptible to cracking in both tension and tension-Creep, where at low applied stresses grain boundary sliding was suggested where strain accommodation occurred through grain boundary cracking. Thus grain size and grain boundaries appeared to be important microstructural parameters affecting the mechanical behavior. Microstructural effects on the tensile properties and Creep behavior are discussed in comparison to other Mg-based alloy systems.

  • the microstructure tensile properties and Creep behavior of mg zn alloys containing 0 4 4 wt zn
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2006
    Co-Authors: C J Boehlert, K Knittel
    Abstract:

    Abstract This paper describes the microstructure, tensile, and tensile–Creep behavior of a series of Mg–Zn alloys ranging from 0 to 4.4 wt.% Zn. The microstructures consisted of equiaxed hexagonal-close-packed grains with fine precipitates preferentially located at grain boundaries. Some of the microstructures contained fine laths within the equiaxed grains. The finest grain sizes were observed for a Zn composition of 4 wt.%. Tensile experiments were performed at room temperature and 150 °C while Creep experiments were conducted at 150 °C for applied stresses between 30 and 50 MPa. The greatest tensile and Creep resistance was exhibited by Mg–4.1Zn which contained 0.2 wt.% Y. The measured Creep Exponent for the Mg–4.1Zn alloy was 4.2, suggesting dislocation climb as the dominant Creep mechanism. Overall, Zn proved to be a potent grain refiner and strengthener for Mg where 4 wt.% appeared to be the optimal Zn content for tensile and Creep strengthening over the range of alloying additions examined.

  • microstructure tensile and Creep behavior of o bcc ti2alnb alloys processed using induction float zone melting
    Journal of Materials Processing Technology, 2001
    Co-Authors: C J Boehlert, J F Bingert
    Abstract:

    Abstract The microstructure, tensile, and tensile–Creep behavior were studied for orthorhombic (O) plus body-centered cubic (BCC) Ti–22Al–24Nb and Ti–26Al–27Nb (at.%) alloys processed using induction-float-zone melting (IFZM). Microstructure studies were performed using scanning and transmission electron microscopy (SEM and TEM), automated electron back-scattered diffraction (EBSD), and X-ray diffraction (XRD). Upon solidification, the BCC phase evolved with [1 0 0] oriented nearly parallel to the longitudinal rod direction. During the slow cool through the O+BCC-phase field, O-variants formed in a fine lath network within the parent BCC. The as-processed Ti–26Al–27Nb rod was strongly textured with an approximately equal distribution of six resolvable O-variants. The retained BCC phase, which was sandwiched between O laths, maintained a higher volume fraction (Vf∼0.20) in Ti–22Al–24Nb compared to Ti–26Al–27Nb (Vf∼0.05). The tensile and Creep behavior of the as-processed microstructures were evaluated with the tensile axis oriented parallel to the longitudinal rod direction. The fully lath O+BCC Ti–22Al–24Nb microstructure exhibited a room-temperature yield strength of 836 MPa and an elongation-to-failure of 4.5%. Surface slip traces revealed that slip was compatible between the O and BCC phases. An untransformed-BCC Ti–26Al–27Nb microstructure, oriented with [1 0 0] nearly parallel to the tensile axis, exhibited localized deformation bands resulting in an elongation of more than 5.9% and an average yield strength of 828 MPa. In terms of the Creep behavior, the secondary Creep rates revealed that the fully lath O+BCC Ti–26Al–27Nb microstructure significantly outperformed that for all other O-based alloys. For applied stresses greater than 300 MPa, an activation energy of 346 kJ/mol and a Creep Exponent of 5.1 were measured, while for lower applied stresses the Creep Exponent transitioned to a value of 2.3. Overall, this work shows that induction-float-zone processing produces textured fully lath O+BCC microstructures containing an attractive balance of room- and elevated-temperature properties for Al concentrations as high as 26 at.%.

Linzhong Zhuang - One of the best experts on this subject based on the ideXlab platform.

  • physically based constitutive analysis and microstructural evolution of aa7050 aluminum alloy during hot compression
    Materials & Design, 2016
    Co-Authors: Shushen Wang, J S Zhang, Linzhong Zhuang
    Abstract:

    Abstract The hot compression tests of AA7050 aluminum alloy were conducted under conditions of 603–693 K and 0.001–10 s − 1 , and the related microstructures were observed. Physically based constitutive analysis was conducted to describe the flow behaviors, which can relate the microstructural evolution with flow behaviors for high stacking fault energy (SFE) and/or precipitation-strengthened alloys. A revised model considering the coupling effects of lattice diffusion and grain boundary diffusion was proposed to characterize the transition of diffusion mechanisms under different deformation conditions. The main diffusion mechanism is determined as lattice diffusion at 633–693 K and grain boundary diffusion at 603 K. The microstructural evolution can be reflected by the deviation of Creep Exponent n ' from the theoretical value ( n ' =5). The reasons for the Creep Exponent n '  > 5 could be related to the change of internal stress and Creep rate by dynamic precipitates at lower temperatures. At higher strain rates, it could be related to the impediment of dislocations motion by defects and the change of rate controlling mechanism. The operation of grain boundary sliding (GBS) may lead to n '

X Hong - One of the best experts on this subject based on the ideXlab platform.

  • domain wall roughness and Creep in nanoscale crystalline ferroelectric polymers
    arXiv: Materials Science, 2013
    Co-Authors: Zhiyong Xiao, Shashi Poddar, Stephen Ducharme, X Hong
    Abstract:

    We report piezo-response force microscopy studies of the static and dynamic properties of domain walls (DWs) in 11 to 36 nm thick films of crystalline ferroelectric poly(vinylidene-fluoride-trifluorethylene). The DW roughness Exponent {\zeta} ranges from 0.39 to 0.48 and the DW Creep Exponent {\mu} varies from 0.20 to 0.28, revealing an unexpected effective dimensionality of ~1.5 that is independent of film thickness. Our results suggest predominantly 2D ferroelectricity in the layered polymer and we attribute the fractal dimensionality to DW deroughening due to the correlations between the in-plane and out-of-plane polarization, an effect that can be exploited to achieve high lateral domain density for developing nanoscale ferroelectrics-based applications.

  • Domain wall roughness and Creep in nanoscale crystalline ferroelectric polymers
    Applied Physics Letters, 2013
    Co-Authors: Zhihua Xiao, Shashi Poddar, Stephen Ducharme, X Hong
    Abstract:

    We report piezo-response force microscopy studies of the static and dynamic properties of domain walls (DWs) in 11 to 36nm thick films of crystalline ferroelectric poly(vinylidene-fluoride- trifluorethylene). The DW roughness Exponent f ranges from 0.39 to 0.48 and the DW Creep Exponent l varies from 0.20 to 0.28, revealing an unexpected effective dimensionality of ?1.5 that is independent of film thickness. Our results suggest predominantly 2D ferroelectricity in the layered polymer and we attribute the fractal dimensionality to DW deroughening due to the correlations between the in-plane and out-of-plane polarization, an effect that can be exploited to achieve high lateral domain density for developing nanoscale ferroelectrics-based applications.

E H Brandt - One of the best experts on this subject based on the ideXlab platform.

  • a novel method for the determination of the flux Creep Exponent from higher harmonic ac susceptibility measurements
    Physica C-superconductivity and Its Applications, 2005
    Co-Authors: K M Schindler, Holger Hochmuth, Michael Ziese, Pablo Esquinazi, M. Lorenz, K. Zimmer, E H Brandt
    Abstract:

    Abstract The critical current density and flux-Creep activation energy of YBa 2 C 3 O 7 rings were determined from ac-susceptibility measurements. A novel approach was developed theoretically and experimentally to derive the flux-Creep Exponent from higher susceptibility harmonics. An alternative method using a scaling approach yielded similar sample-to-sample variations, but qualitatively different values for the flux-Creep Exponent. This is discussed considering the limits of validity of the logarithmic approximation to the flux-line activation energy.

  • susceptibility of superconductor disks and rings with and without flux Creep
    Physical Review B, 1997
    Co-Authors: E H Brandt
    Abstract:

    for rings with general hole radius a1 and general Creep Exponent n. Next the exact formulation for rings in a perpendicular applied field Ha(t) is presented in the form of an equation of motion for the current density in thick rings and disks or the sheet current in thin rings and disks. This method is used to compute general magnetization curves m(Ha) and ac susceptibilities x of rings with and without Creep, accounting also for nonconstant Jc(B). Typical current and field (B) profiles are depicted. The initial slope of m(Ha) ~the ideal diamagnetic moment! and the field of full penetration are expressed as functions of the inner and outer ring radii a1 and a. A scaling law is derived which states that for arbitrary Creep Exponent n the complex nonlinear ac susceptibility x(H0 ,v) depends only on the combination H021 /v of the ac amplitude H0 and the ac frequency v/2p. This scaling law thus connects the known dependencies x5x(v) in the ohmic limit (n51) and x5x(H0) in the Bean limit ( n!‘). @S0163-1829~97!01921-8#

Rolf Sandstrom - One of the best experts on this subject based on the ideXlab platform.

  • formation of a dislocation back stress during Creep of copper at low temperatures
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017
    Co-Authors: Rolf Sandstrom
    Abstract:

    Abstract Copper gives Creep strain versus time curves at 75 °C that look very similar to those recorded at much higher temperatures. Thus, for example, an extended secondary stage where the strain rate is constant is observed. Considering the high Creep Exponent that can be up to 75, one would expect a Creep curve with rapidly increasing strain rate but that is not found. The difference to Creep of pure metals at high temperatures is so large that we can talk about an entirely new material class with respect to Creep. To explain the observations a recently developed dislocation model (Sandstrom, 2017) for cell structures is used. A new Creep model is presented where a back stress based on the dislocations in the cell walls is introduced. Unbalanced sets of dislocations without matching dislocations of opposite signs are formed in the cell walls. Since the unbalanced content is not exposed to static recovery, it forms a stable back stress. It is shown that the computed back stress can fully explain the observations and reproduce both Creep curves and results for slow strain rate tensile tests.

  • precipitation evolution and Creep strength modelling of 25cr20ninbn austenitic steel
    Materials at High Temperatures, 2015
    Co-Authors: Stojan Vujic, Rolf Sandstrom, Christof Sommitsch
    Abstract:

    25Cr-20Ni-Nb-N is a high strength and oxidation-resistant austenitic stainless steel intended for Ultra-Supercritical (USC) power plants. In this work, the precipitation evolution, and Creep strength at 650 and 750°C for up to 100 000 h are predicted. Six precipitates are considered in the thermokinetic calculation by MatCalc: M23C6, η (Cr3Ni2SiN), σ, G, Z, Nb(C,N). For the Creep strength prediction, three hardening mechanisms are taken into account: dislocation, precipitation, and solid solution hardening. Both matrix composition and precipitation evolution, calculated with MatCalc, are used for modelling the precipitation and solid solution hardening. It is found that the dislocation hardening, followed by precipitation hardening gives the largest contribution to the Creep strength. The most important precipitates strengthening phases are found to be Z-Phase and Nb(C,N), which are nucleated at the dislocations. The model for the Creep rate can represent how the Creep Exponent is raised with increasing a...

  • Creep in phosphorus alloyed copper during power law breakdown
    Journal of Nuclear Materials, 2008
    Co-Authors: Rolf Sandstrom, Henrik Andersson
    Abstract:

    Abstract During the first phase of storage, Creep will take place in the copper canisters in the KBS-3 package for nuclear waste. The temperatures are below 100 °C, and the Creep is well inside the power-law breakdown regime. Creep models for this situation have been developed. The analysed material is pure copper with about 50 ppm phosphorus. Constitutive equations for Creep and other plastic deformation have been set up based on a generalised Norton expression and Kocks–Mecking’s model for the back stress. A model for the minimum Creep rate based on fundamental principles for climb and glide has been derived. This model gives the correct order of magnitude for the Creep rate in the temperature range from 400 to 20 °C without the use of fitted parameters. The Creep Exponent varies from 5 to 105 in this interval. The constitutive equations have also been formulated for multiaxial stress states.