Dislocation Core Structure and the Normal Yield Behavior of L12 Ordered Alloys

1984 ◽  
Vol 39 ◽  
Author(s):  
G. Tichy ◽  
V. Vitek ◽  
D. P. Pope
Keyword(s):  
Yield Stress ◽  
High Temperatures ◽  
Flow Behavior ◽  
Dislocation Core ◽  
Screw Dislocations ◽  
Thermally Activated ◽  
Normal Behavior ◽  
Ordered Alloys ◽  
Increasing Temperature ◽  
Good Agreement

ABSTRACTA rapid increase of the yield stress with increasing temperature, often observed in L12 ordered alloys, is commonly called the “anomalous flow behavior”. This phenomenon is believed to result from the thermally activated transformation of the core of 1/2<110> screw dislocations from a glissile form to a sessile form at high temperatures. It is shown here that another class of L12 alloys exists in which these two forms of the screw dislocation core are not available. These are the alloys in which the APB on {111} planes is not stable and the atomistic studies of screw dislocations in such alloys show that their cores are always sessile. The yield stress of these alloys then increases with decreasing temperature and no increase at high temperatures occurs. Such behavior has been observed, for example, in Pt3Al. This “normal” behavior is analogous to that of b.c.c. metals and a theory of the temperature dependence of the yield stress has been developed along the same lines as in the case of b.c.c. metals. Comparison of this theory with measurements on Pt3Al single crystals shows a good agreement.

Dislocation Core Structure and the Anomalous Yield Behavior of Li2 Ordered Alloys At Elevated Temperatures

1984 ◽  
Vol 39 ◽  
Author(s):  
V. Vitek ◽  
D. P. Pope
Keyword(s):  
Flow Stress ◽  
Elevated Temperatures ◽  
Computer Modelling ◽  
Dislocation Core ◽  
Core Structure ◽  
Anomalous Increase ◽  
Screw Dislocations ◽  
Ordered Alloys ◽  
Dislocation Core Structure ◽  
Increasing Temperature

ABSTRACTIn many LI2 ordered alloys the flow stress increases with increasing temperature and is in this “anomalous” regime strongly dependent on orientation and sense of the applied stress. These dependences can be predicted from the nature of the dissociation and core structure of the 1/2<101> screw superpartials in these alloys. Computer modelling shows that two different configurations, a glissile one on {lll} planes and a sessile one on {010} planes, exist and both are described here in detail. The anomalous increase of the flow stress may then be explained by an increasing amount of core transformations from the glissile to sessile forms as the temperature increases. The theoretical model for the immobilization of screw dislocations by this mechanism is then discussed and its validity illustrated by comparison with experimental results on Ni3 (Al, Ta) single crystals.

Deformation Mechanisms and Solid-Solution Strengthening in Ordered Alloys

1990 ◽  
Vol 213 ◽  
Author(s):  
Dennis M. Dimiduk ◽  
Satish Rao
Keyword(s):  
Solid Solution ◽  
Flow Behavior ◽  
Stoichiometric Composition ◽  
Dislocation Core ◽  
Deformation Mechanisms ◽  
Solid Solution Hardening ◽  
Solid Solution Strengthening ◽  
Ordered Alloys ◽  
Solute Hardening

ABSTRACTFundamental to understanding the results of alloy design studies, is the need for understanding the intrinsic role of solutes in a particular compound. For many compounds such an understanding must be built from a systematic exploration of the role of deviations from the stoichiometric composition as well as the role of ternary solute additions on the variation of flow behavior. Within most intermetallic systems the problem is complicated since the fundamental mechanisms of flow are not well established and, in those systems where these mechanisms are known, thermal activation can lead to dislocation-core transformations and changes in the operative slip systems with temperature. In general, flow may be governed by more than one dislocation process at a given temperature and deformation twinning may be a major contributing deformation mechanism. The problem of isolating the mechanisms of solid-solution hardening may, therefore, require treatment as a problem of combined strengthening mechanisms operating in parallel. This paper reviews the key aspects of deformation mechanisms and solute strengthening in intermetallic alloys. Classical elastic theories of solute hardening serve as an origin, from which, the progress made to date in isolating the mechanisms of solute hardening in ordered alloys is discussed.

The Deformation Microstructure in NI3Si Polycrystals Strained Over the Range of Temperature of Flow Stress Anomaly

1988 ◽  
Vol 133 ◽  
Author(s):  
B. Tounsia ◽  
P. Beauchamp ◽  
Y. Mishima ◽  
T. Suzuki ◽  
P. Veysslière
Keyword(s):  
Flow Stress ◽  
Slip System ◽  
Room Temperature ◽  
Peak Temperature ◽  
Effect Of Temperature ◽  
Screw Dislocations ◽  
Thermally Activated ◽  
Cube Plane ◽  
Octahedral Slip ◽  
Increasing Temperature

ABSTRACTIn order to correlate the flow stress anomaly of Ni3Si with dislocation properties, a weakbeam study ofpolycrystalline samples deformed between ambient and the peak temperature was carried out. Samples with two extreme Ni/Si ratios were tested.The most frequently activated slip system changes progressively from octahedral to cubic with increasing temperature. The transformation of superdislocations into Kear-Wilsdorf configurations gives rise to screw dislocations that are rectilinear only after deformation at room temperature. The effect of temperature is to gradually promote bending of Kear-Wisdorf configurations in the cube plane, from a few nanometers at 230°C to several tenths of micrometers at intermediate temperature. Cube slip begins to be massively activated a little below the peak temperature. It is suggested that the flow stress anomaly is controlled by progressive exhaustion of octahedral slip by thermally-activated expansion of superdislocations on the cube cross-slip plane.

Deformation Behavior of NiAl-Based Alloys Containing Iron, Cobalt, and Hafnium

1988 ◽  
Vol 133 ◽  
Author(s):  
D. R. Pank ◽  
M. V. Nathal ◽  
D. A. Koss
Keyword(s):  
Yield Stress ◽  
Deformation Behavior ◽  
Single Phase ◽  
Flow Behavior ◽  
Iron Cobalt ◽  
Alloying Additions ◽  
Compressive Flow ◽  
Melt Spun ◽  
Increasing Temperature

ABSTRACTThe effects of alloying additions on the mechanical properties of the B2 intermetallic NiAl have been investigated in both the melt-spun ribbon and consolidated, bulk form. The study is based on a matrix of NiAl-based alloys with up to 20 a/o Co and Fe additions and with reduced Al levels in the range of 30 – 40 a/o. Characterization of the melt-spun ribbon by optical and scanning electron microscopy indicates a range of microstructures: single phase β γ, necklace phase surrounding either martensitic or β grains, and a mixture of equiaxed martensitic and γ grains. Bend ductility is present in melt-spun and annealed ribbons exhibiting the γ necklace structure and in a single phase β material containing 20 a/o Fe.The analysis of compressive flow behavior on consolidated, bulk specimens indicates that the single phase γ alloys exhibit a continuous decrease in yield stress with increasing temperature and profuse microcracking at grain boundaries. In contrast, multiphase (γ + either martensite or β) alloys tend to display a peak in flow stress between 600 and 800K with little or no signs of microcracking. In general, heat treatments which convert the martensitic grains to β + γ result in improved strength at temperatures above 600K and better resistance to crack initiation. These results are discussed in terms of the effects of β, martensite and γ on the yield stress and flow behavior of NiAl-based alloys.

PLASTIC ANISOTROPY OF TANTALUM, NIOBIUM, AND MOLYBDENUM

1967 ◽  
Vol 45 (2) ◽  
pp. 1075-1089 ◽  
Author(s):  
P. J. Sherwood ◽  
F. Guiu ◽  
H. C. Kim ◽  
P. L. Pratt
Keyword(s):  
Single Crystals ◽  
Yield Stress ◽  
Applied Stress ◽  
High Temperatures ◽  
Low Temperatures ◽  
Plastic Anisotropy ◽  
Anisotropic Behavior ◽  
Screw Dislocations ◽  
Strain Behavior ◽  
Tension And Compression

The stress–strain behavior of single crystals of tantalum, niobium, and molybdenum has been studied in both tension and compression in the temperature range 4.2–400 °K. The crystals were stressed in both the [Formula: see text] and the [Formula: see text] directions.At high temperatures, the yield stress of all three metals is independent of the direction and sense of the applied stress. At low temperatures, the yield stress depends markedly on the orientation of the crystals and the sense of the applied stress. This anisotropic behavior cannot be satisfactorily explained in terms of any of the mechanisms proposed so far, such as the mobility of jogs in screw dislocations, or the dissociation of a/2 [Formula: see text] screw dislocations on {112} planes.

Dislocation Structures and Anomalous Yielding in Ordered Alloys

1990 ◽  
Vol 213 ◽  
Author(s):  
P. M. Hazzledine ◽  
Y. Q. Sun
Keyword(s):  
Mechanical Properties ◽  
Electron Microscope ◽  
Strain Rate ◽  
Yield Stress ◽  
Work Hardening ◽  
Screw Dislocations ◽  
Ordered Alloys ◽  
Work Hardening Rate ◽  
Initial Work ◽  
Edge Dislocations

ABSTRACTThe strain, strain rate and temperature dependencies of the yield stresses of the model L12 materials Ni3Al, Ni3Ga and Co3Ti are described, as well as two associated properties, the initial work-hardening rate and the inverted creep. These mechanical properties, the electron microscope observations of <110> {111} APB coupled slip and the violations of Schmid's laws point to glissile-sessile-glissile transitions by cross slip of screw dislocations as the explanation for the yield stress anomaly. Similar explanations are probable for h.c.p. Be, B2 CoTi and tetragonal TiAl and MoSi2. A different kind of model, based on the behavior of edge dislocations is required for h.c.p. Mg, B2 CuZn and DO19 Ti3Al.

scholarly journals A CRITICAL REVIEW OF THE PEIERLS MECHANISM

1967 ◽  
Vol 45 (2) ◽  
pp. 983-1016 ◽  
Author(s):  
Pierre Guyot ◽  
John E. Dorn
Keyword(s):  
Activation Volume ◽  
Prismatic Slip ◽  
Thermally Activated ◽  
Covalently Bonded ◽  
Higher Temperature ◽  
Fe Alloys ◽  
Increasing Temperature ◽  
Peierls Model ◽  
Volume Stress ◽  
Good Agreement

A thorough review is made of the application of the Peierls model to the macroscopic plastic deformation of ionic crystals, metals, alloys, and covalently bonded crystals. The effects of the shape of the Peierls hill, kink–kink energies, and the frequency terms on the stress–temperature and activation volume–stress relationships are extended and discussed. Theory is compared with experimental results, giving special emphasis to recent advances. Single-crystal data for [Formula: see text] {110} thermally activated slip in Ta and Mo at low temperatures agree well with the dictates of the Peierls mechanism. Deformation characteristics of polycrystalline Fe alloys containing either 2 wt.% Mn or 11 at.% Mo agree with expectations based on the Peierls mechanism only at temperatures below about 200 °K. At higher temperatures, the effective stress decreases more slowly and the activation volume increases more rapidly with increasing temperature than can be accounted for by the Peierls mechanism. Over this higher temperature range, however, the experimental data are in good agreement with Escaig's mechanism based on the recombination of dissociated screw dislocations. It is also shown that low-temperature [Formula: see text] {123} slip in AgMg, prismatic slip in Ag plus 33 at.% Al, and in Mg plus 6–12 at.% Li occurs by the Peierls mechanism.

Tensile properties of finger-jointed lumber under high-temperature and oxygen-free conditions

Holzforschung ◽  
2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Kong Yue ◽  
Feng Wang ◽  
Weidong Lu ◽  
Zhongqiu Tang ◽  
Zhangjing Chen ◽  
...  
Keyword(s):  
Tensile Strength ◽  
Tensile Properties ◽  
High Temperatures ◽  
Variable Temperature ◽  
Physical Property ◽  
Solid Wood ◽  
Nitrogen Atmosphere ◽  
Thermal Physical Property ◽  
Electron Microscopy Analysis ◽  
Increasing Temperature

Abstract A model for engineered wood was developed that considers the parallel-to-grain tensile strength of finger-jointed lumber at high temperatures relevant to fire conditions. The finger-jointed lumber was composed of Douglas fir, larch, and poplar wood with phenol-resorcinol-formaldehyde (PRF) as an adhesive. The tensile properties of the finger-jointed lumber were evaluated at high temperatures under oxygen-free conditions, i.e. in a nitrogen atmosphere. A combination of chemical and thermal-physical property analysis of the PRF adhesive and microscopic observations on the glueline was used to discuss the reduction of tensile strength of the parallel-to-grain finger-jointed lumber at variable temperature. The results show that the tensile strength of the finger-jointed lumber decreased linearly with increasing temperature. The parallel-to-grain tensile strength of the PRF finger-jointed samples at 20 and 280 °C were 84 and 5% of the tensile strength of the solid wood at 20 °C, respectively. The thermal-physical properties and scanning electron microscopy analysis revealed that the pyrolysis intensity of the PRF adhesive was lower than that of the wood at 220 °C or higher.

Upward Vertical Two-Phase Flow Through an Annulus—Part II: Modeling Bubble, Slug, and Annular Flow

1992 ◽  
Vol 114 (1) ◽  
pp. 14-30 ◽  
Author(s):  
E. F. Caetano ◽  
O. Shoham ◽  
J. P. Brill
Keyword(s):  
Flow Pattern ◽  
Annular Flow ◽  
Two Phase Flow ◽  
Flow Behavior ◽  
Bubble Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Through ◽  
Physical Phenomena ◽  
Good Agreement

Mechanistic models have been developed for each of the existing two-phase flow patterns in an annulus, namely bubble flow, dispersed bubble flow, slug flow, and annular flow. These models are based on two-phase flow physical phenomena and incorporate annulus characteristics such as casing and tubing diameters and degree of eccentricity. The models also apply the new predictive means for friction factor and Taylor bubble rise velocity presented in Part I. Given a set of flow conditions, the existing flow pattern in the system can be predicted. The developed models are applied next for predicting the flow behavior, including the average volumetric liquid holdup and the average total pressure gradient for the existing flow pattern. In general, good agreement was observed between the experimental data and model predictions.

Temperature Dependence of Hole Impact Ionization Coefficient in 4H-SiC Photodiodes

2009 ◽  
Vol 615-617 ◽  
pp. 311-314 ◽  
Author(s):  
W.S. Loh ◽  
J.P.R. David ◽  
B.K. Ng ◽  
Stanislav I. Soloviev ◽  
Peter M. Sandvik ◽  
...  
Keyword(s):  
Phonon Scattering ◽  
Impact Ionization ◽  
Positive Temperature Coefficient ◽  
Positive Temperature ◽  
Different Temperatures ◽  
Ionization Coefficients ◽  
Increasing Temperature ◽  
The Impact ◽  
Good Agreement ◽  
Ionization Rates

Hole initiated multiplication characteristics of 4H-SiC Separate Absorption and Multiplication Avalanche Photodiodes (SAM-APDs) with a n- multiplication layer of 2.7 µm were obtained using 325nm excitation at temperatures ranging from 300 to 450K. The breakdown voltages increased by 200mV/K over the investigated temperature range, which indicates a positive temperature coefficient. Local ionization coefficients, including the extracted temperature dependencies, were derived in the form of the Chynoweth expression and were used to predict the hole multiplication characteristics at different temperatures. Good agreement was obtained between the measured and the modeled multiplication using these ionization coefficients. The impact ionization coefficients decreased with increasing temperature, corresponding to an increase in breakdown voltage. This result agrees well with the multiplication characteristics and can be attributed to phonon scattering enhanced carrier cooling which has suppressed the ionization process at high temperatures. Hence, a much higher electric field is required to achieve the same ionization rates.

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