Temperature Dependence Of Yield Stress Research Articles

Strength vs temperature for refractory complex concentrated alloys (RCCAs): A critical comparison with refractory BCC elements and dilute alloys

To support the development of refractory complex, concentrated alloys (RCCAs), a clear understanding of the effect of temperature on strength is needed. Body-centered cubic (BCC) refractory metals and dilute refractory alloys show a strong temperature dependence of yield stress (σy) at low and high temperatures, with a relatively temperature-independent regime in between. RCCAs may introduce important changes in deformation and strengthening mechanisms, so it is not clear if RCCAs will show the same thermal dependencies. The objective of this work is to answer the question, “Is the temperature dependence of strength similar or different for RCCAs compared to BCC refractory elements and dilute refractory alloys ?” We evaluate σyvs. temperature for 61 RCCAs by analyzing data curated from the literature. We find that σy increases progressively from refractory BCC elements, to dilute alloys, to single- and multi-phase RCCAs. Single-phase RCCAs show the same three thermal regimes, but the stresses are higher and the intermediate plateau is shorter and is sometimes steeper. The thermal dependence of σy in multi-phase RCCAs shows more substantial differences. Six factors that contribute to the higher σy of RCCAs are discussed: (i) higher solute concentration; (ii) dispersion in atomic sizes; (iii) the shear modulus magnitude; (iv) the type of principal elements; (v) the solvus temperature of predicted secondary phases; and (vi) microstructure. Based on the present observations and analyses, suggestions for future research are made, especially regarding improved models for the effect of temperature on strength.

A new route to achieve high strength and high ductility compositions in Cr-Co-Ni-based medium-entropy alloys: A predictive model connecting theoretical calculations and experimental measurements

A new route to achieve high strength and high ductility compositions in the Cr-Co-Ni medium entropy alloys (MEAs) is proposed, by controlling the solid solution hardening parameter (Mean Square Atomic Displacement, MSAD) and twinning propensity parameter (Stacking Fault Energy, SFE), respectively. The MSAD is calculated to increase with the increase in the Cr content and with the increase in the Ni/Co ratio at high Cr concentrations, while the SFE is calculated to decrease with the increase in the Cr content and with the increase in the Co/Ni ratio at high Cr concentrations. In experiment, the strength at 0 K (derived from the temperature dependence of yield stress) increases as the Cr content increases and/or as the Ni content increases for a given high Cr content, so that a linear correlation is found between the yield strength at 0 K and MSAD. The SFE also decreases as the Cr content increases and as the Co content increases for a given high Cr content. However, while the tensile elongation increases with the decrease in SFE down to SFE values of 10–12 mJ/m2, it abruptly decreases once the SFE decreases below this value due to a change in major deformation mode from deformation twinning to deformation-induced ε-martensite transformation. Based on the established connection between the theoretical calculation and experimental measurement, outstanding combinations of strength and ductility are predicted and experimentally confirmed at high Cr compositions and at a bit Ni-rich side of the Co/Ni equi-composition line. The proposed composition (around 45Cr-20Co-35Ni) exhibits a greater 0 K strength and a superior 77 K tensile ductility by 32 % and 13 %, respectively, compared to those of the equiatomic Cr-Co-Ni alloy.

Composition dependence of positive temperature dependence of yield stress for Co3(Al, W)–Co3Ti pseudo-binary alloy with the L12 structure

Variations in the positive temperature dependence of yield stress for L12 intermetallic compounds in pseudo-binary alloys with the composition of Co3(Al, W)–Co3Ti have been investigated. Polycrystalline L12 compounds in the pseudo-binary system with a Co3(Al, W) to Co3Ti composition ratio of approximately 1:3 and 1:9 were prepared. The temperature dependence of the yield stress ranging from 293 to 1273 K was determined. The onset temperature for the positive temperature dependence of yield stress for each compound is located between that of Co3(Al, W) and Co3Ti. The activation energies for the positive temperature dependence of these compounds are evaluated to be 47.0 and 43.6 kJmol-1, respectively. The activation energies are also intermediate values between those of Co3(Al, W) and Co3Ti.The composition dependence of the positive temperature dependence of yield stress is estimated from the physical properties of Co3(Al, W) and Co3Ti. Although the activation energies estimated for the present compounds are slightly different from the experimentally determined values, the temperature dependences of the experimentally determined yield stress are almost represented from the estimated parameters. The composition dependence of the positive temperature dependence of the yield stress can be predicted from the physical properties of the base compounds.

Non-isothermal Steady Flow of Non-Newtonian Fluid in an Axisymmetric Channel

Non-isothermal steady-state flow of a viscoplastic fluid in an axisymmetric channel is studied with account for viscous dissipation at a specified constant flow rate. The rheology of the medium is described by the Herschel-Bulkley law with a temperature dependence of yield stress and consistency defined by exponential law. On the solid wall, the no-slip boundary condition and the assigned temperature are used. The mathematical statement of the problem includes the dimensionless motion and energy equations and boundary conditions. The problem is solved numerically using a finite-difference approach. The difference equations are solved by sweep method. When applying a shock-capturing method for calculating the flow, the rheological model is regularized in order to eliminate stress singularity in the regions of zero shear rates. The steady-state distributions of velocity, temperature, viscosity, and dissipative function are obtained. A limiting value of pressure drop defining the existence domain of a steady solution is proved to exist. Two problem solutions are obtained at a specified pressure drop, which are referred to as high- and low-temperature flow regimes. As a result of computations, different flow structures with unyielded regions occurring near symmetry line and in the dead zone next to a solid wall are revealed.

Failure of the ASTM E 1921 master curve to characterize the fracture toughness temperature dependence of ferritic steel and successful application of the stress distribution T-scaling method

The fracture toughness KJc of a material in the ductile-to-brittle transition temperature (DBTT) region shows a large temperature dependence, large scatter, and specimen thickness dependence. Thus, a large amount of data is necessary to satisfy the design requirements of equipment that experiences temperature changes. The ASTM E 1921 master curve (MC) method provides an engineering approach to solve these three issues and is intended to be applicable to an arbitrary ferritic material with a yield stress of 275–825 MPa. This study tested the CrMo steel JIS SCM440, which satisfies these conditions, but the ASTM E 1921 MC failed to characterize its fracture toughness temperature dependence. The material was tested at four temperatures: −55, 20, 60 and 100 °C. The obtained reference temperature T0 (i.e., the temperature at which a 25 mm thickness shows a fracture toughness of 100 MPa·m1/2) from these tests differed in the range of −9 to 89 °C. T0 from all of the test data was 16.6 °C. Thus, the MC using any of these T0 values failed to characterize the material’s fracture toughness temperature dependence. This material showed a T0 value higher than room temperature, which is a rare case for materials to which the MC has been successfully applied. The ASTM E 1921 MC is based on the Zerilli equation, which showed a poor fitting performance of the yield stress above room temperature for this material. When this yield stress temperature dependence was corrected (i.e., a temperature shift), the ASTM E 1921 MC better fitted the material’s fracture toughness test data. However, some discrepancy still remained. Because the Zerilli equation showed a small yield stress change and may differ from the experimental results above room temperature, the ASTM E 1921 MC may fail to characterize the fracture toughness temperature dependence close to or above room temperature. In contrast, the stress distribution T-scaling method was demonstrated to successfully predict the KJc temperature dependence of SCM440.

Temperature Dependence of the Yield Stress in <i>α</i>-Titanium Investigated with Crystal Plasticity Analysis

The effect of the activated slip systems on the temperature dependence of yield stress was investigated in α-Ti by using crystal plasticity finite element method. A model for finite element analysis (FEA) was constructed based on experimental results. The displacement in FEA was applied up to the nominal strain of 4% which is the same strain as the experimental one. Stress-strain curves were obtained, which corresponds to experimental data taken every 50 K between 73 K and 673 K. The used material constants which are temperature dependent were elastic constants, and lattice friction stresses. The lattice friction stresses of basal slip systems were set to be higher than that of pyramidal slip systems at 73 K. Then, the lattice friction stresses were set to be closer as the temperature increases. It was found that the activation of slip systems is strong temperature dependent, and that the yield stress depends on the number of active slip systems.

Thermal Activation Analysis for Higher-Temperature Deformation of Gamma-Iron

Recent extension of Zerilli–Armstrong (Z–A) constitutive relations to higher temperature is assessed using yield strength measurements on 0.09 C steel covering the initial deformation behaviors of metastable and stable γ-iron phases. Descriptions of temperature dependence of yield stress; stress-dependent activation energy; and, activation area agree with the Z–A formulation. The relatively weaker thermal dependence of fcc strain hardening compared to a stronger thermal dependence of yield stress for bcc metals is pointed out.

Study on Dislocation-Dopant Ions Interaction in Ionic Crystals by the Strain-Rate Cycling Test during the Blaha Effect

The interaction between a dislocation and impurities has been investigated by measurements of yield stress and proof stress, micro-hardness tests, direct observations of dislocation, internal friction measurements, or stress relaxation tests so far. A large number of investigations has been carried out by the separation of the flow stress into effective and internal stresses on the basis of the temperature dependence of yield stress, the strain rate dependence of flow stress, and stress relaxation. Nevertheless, it is difficult to investigate the interaction between a dislocation and obstacles during plastic deformation by the mentioned methods. As for the original method which combines strain-rate cycling tests with the Blaha effect measurement, the original method is different from above-mentioned ones and would be possible to clear it up. The strain-rate cycling test during the Blaha effect measurement has successively provided the information on the dislocation motion breaking away from the strain fields around dopant ions with the help of thermal activation, and seems to separate the contributions arising from the interaction between dislocation and the point defects and those from dislocations themselves during plastic deformation of ionic crystals. Such information on dislocation motion in bulk material cannot be obtained by the widely known methods so far. Furthermore, the various deformation characteristics derived from the original method are sensitive to a change in the state of dopant ions in a specimen by heat treatment, e.g., the Gibbs free energy (G0) for overcoming of the strain field around the dopant by a dislocation at absolute zero becomes small for the annealed KCl:Sr2+ single crystal (G0 = 0.26 eV) in comparison with that for the quenched one (G0 = 0.39 eV).

High-temperature tensile deformation behavior of hot rolled CrMnFeCoNi high-entropy alloy

In the present study, high temperature (700–1100 °C) tensile deformation behavior of hot-rolled CrMnFeCoNi high-entropy alloy was investigated. A single face-centered cubic phase was retained in the hot deformed specimens under different deformation conditions. The flow behavior was significantly influenced by temperature and microstructure evolution. A strong temperature dependence of yield stress was observed with a drastic drop in yield strength from 413 MPa at 700 °C to 94 MPa at 900 °C. A profound increase in ductility was observed above 700 °C and a perceptive decrease in ductility was observed above 900 °C. Electron backscatter diffraction analysis indicates incomplete dynamic recrystallization at 700 °C leading to a significant difference in the flow behavior.

Thermally activated deformation and the rate controlling mechanism in CoCrFeMnNi high entropy alloy

The nature of obstacles to dislocation motion in CoCrFeMnNi alloy was analyzed using the thermally activated deformation analyses at low temperatures. The strong temperature dependence of yield stress and small activation volume in CoCrFeMnNi favor the dislocation glide over the obstacles with high friction stress. The activation volume of CoCrFeMnNi alloy (10–100 b3) in this study is much smaller than those of conventional FCC metals (102~103 b3), but close to those observed in BCC metals (8–100 b3) and HCP metals (5–100 b3). The increase of the activation volume with strain supports overcoming the nanoscale inhomogeneity such as co-clusters and/or short range orders as the rate controlling mechanism. The transition of dislocation structure from planar array to cell structure at 20% strain in CoCrFeMnNi reported in the literature can be attributed to the prevalent shearing of nanoscale inhomogeneity with strain.

What Controls Temperature Dependence of Yield Stress in L12-Ordered Intermetallic Compounds?

ABSTRACTThe temperature dependence of yield stress and the associated dislocation dissociation in L12 intermetallic compounds are investigated in order to check the feasibility of the classification of L12 intermetallic compounds so far made in terms of the planarity of core structures of partial dislocations with b = 1/2<110> and 1/3<112> on {111} and {001} glide planes. In contrast to what is believed from the classification, the motion of APB-coupled dislocations is evidenced to give rise to the rapid decrease in yield stress at low temperatures for Pt3Al. In view of the fact that rapid decrease in yield stress at low temperatures is also observed in Co3(Al,W) and Co3Ti in which APB-coupled dislocations are responsible for deformation, the SISF-type dissociation is not a prerequisite for the rapidly decreasing CRSS for slip on (111) and the relative magnitudes of the APB energy on (111) and the SISF energy on (111) cannot be a primary factor that determines the type of the temperature dependence of CRSS for L12 compounds. The importance of the CSF energy as a factor determining the type of the temperature dependence of yield stress for L12 compounds through the changes in the planarity of the core structure of the APB-coupled partial dislocation with bp = ½[1$\overline 1$0] is discussed in the light of experimental evidence obtained from Pt3Al.

Effects of Alloying Elements on the Temperature Dependence of Yield Stress in L12-Co3(Al,W)

ABSTRACTThe effects of alloying elements (Ni/Ta) on the temperature dependence of yield stress in Co3(Al,W) with the L12 structure have been investigated through compression tests of nearly single-phase polycrystalline alloys in the temperature range between room temperature to 1,473K. Compared with a ternary Co3(Al,W), a Ni/Ta-added Co3(Al,W) alloy exhibits a higher γ΄ solvus temperature and lower onset temperature of the yield stress anomaly (positive temperature dependence of yield stress), suggesting that the CSF energy is increased by Ni/Ta addition. As a consequence, the high-temperature strength in Co3(Al,W) is considerably enhanced.

Deformation mechanism in NiAl single crystals at low temperatures

Available data on the temperature dependence of yield stress and of associated activation volume of high-purity NiAl single crystals between 76 and 205 K have been analyzed within the framework of a solid-solution hardening model, which is based on the concept of depinning of an edge-dislocation segment from several randomly dispersed, isolated, point defects simultaneously. The vacancies in NiAl single crystals act as point-defect obstacles to the stress-assisted thermally-activated glide of edge dislocations, and their concentration is estimated as about 10at.%. The product of yield stress and associated activation volume (≈0.146 eV) is found to be independent of temperature and yield stress, as envisaged in the model.

Strain-rate and temperature dependence of yield stress of amorphous solids via a self-learning metabasin escape algorithm

A general self-learning metabasin escape (SLME) algorithm (Cao et al., 2012) is coupled in this work with continuous shear deformations to probe the yield stress as a function of strain rate and temperature for a binary Lennard-Jones (LJ) amorphous solid. The approach is shown to match the results of classical molecular dynamics (MD) at high strain rates where the MD results are valid, but, importantly, is able to access experimental strain rates that are about ten orders of magnitude slower than MD. In doing so, we find in agreement with previous experimental studies that a substantial decrease in yield stress is observed with a decreasing strain rate. At room temperature and laboratory strain rates, the activation volume associated with yield is found to contain about 10 LJ particles, while the yield stress is as sensitive to a 1.5%Tg increase in temperature as it is to a one order of magnitude decrease in the strain rate. Moreover, our SLME results suggest that the SLME and extrapolated results from MD simulations follow distinctly different energetic pathways during the applied shear deformation at low temperatures and experimental strain rates, which implies that extrapolation of the governing deformation mechanisms from MD strain rates to experimental may not be valid.

Dislocation motion in tungsten: Atomistic input to discrete dislocation simulations

A computational framework for the discrete dislocation dynamics simulation of body-centered cubic (bcc) metals which incorporates atomistic simulation results is developed here on the example of tungsten. Mobility rules for the a/2〈111〉 screw dislocations are based on the kink-pair mechanism. The fundamental physical quantity controlling the kink-pair nucleation, the stress-dependent activation enthalpy, is obtained by fitting the line-tension model to atomistic data extending the approach by Gröger et al. (2008a,b) and Gröger and Vitek (2008c). In agreement with atomistic simulation, kink-pair nucleation is assumed to occur only on {110} planes. It is demonstrated that slip of the crystal along high-index planes like {112} which is often observed in experiments is obtained by the glide of the dislocation on two or more {110} planes. It is shown that such an atomistic based description of the dislocation mobility provides a physical basis to naturally explain many experimentally observed phenomena in bcc metals like the tension–compression asymmetry, the orientation dependence of loading, temperature dependence of yield stress and the crystallography of slip.

Thermal activation plasticity of nanocrystalline Ni–18.75 at. % Fe alloy in temperature range 4.2–350 K

The mechanical properties of nanocrystalline Ni–18.75 at. % Fe alloy (average grain size ∼ 22 nm) were studied under uniaxial compression in the temperature range of 4.2–350 K. During deformation with a constant rate the temperature dependence of yield stress was registered and deformation curves were analyzed. Temperature dependences of the deforming stress, strain rate sensitivity of the deforming stress, and activation volume of plastic deformation were measured for plastic strain of 2%. Thermal activation analysis of the experimental data was carried out. It was shown that the plastic deformation in the temperature range from 35 to 350 K was controlled by a single deformation mechanism. Empirical estimates of the parameters of interaction between dislocation and local barriers were obtained and internal stress values were evaluated.

Temperature Dependence of Yield Stress and Dislocation Dissociation in L12-Ordered Intermetallic Compounds

ABSTRACTThe temperature dependence of yield stress and the associated dislocation dissociation in L12 intermetallic compounds are investigated in order to check the feasibility of the classification of L12 intermetallic compounds so far reported in terms of the planarity of core structures of partial dislocations with b = 1/2<110> and 1/3<112> on {111} and {001} glide planes. In contrast to what is believed from the reported classification, the motion of APB-coupled dislocations is proved to give rise to the rapid decrease in yield stress at low temperatures for Co3Ti and Co3 (Al,W). The temperature dependence of yield stress at low temperatures is newly interpreted in terms of a thermal component of solid-solution hardening, at least, for these two L12 compounds. We have proposed a new way to describe the yield stress–temperature curves of L12 compounds with three parameters (the athermal and thermal components of solid-solution hardening and the anomalous strengthening component) when the dislocation dissociation scheme is of the APB-type.

Physical nature of the temperature dependence of yield stress

Below is one of the most significant works by V. I. Trefilov and Yu. V. Mil’man. The idea that the temperature dependence of the yield stress is determined by the thermally activated escape of potential barriers by dislocations is used to derive an equation that includes, as a particular case, the Seeger equation that describes the linear dependence of the yield stress on low temperatures and the Haasen equation that represents the exponential temperature dependence of the yield stress. The equation derived was later used to describe brittle-ductile transition and introduce the so-called deformation temperature at which the yield stress begins to strongly depend on temperature and strain rate and the ductile and brittle ranges are separated. These ideas were widely used to examine the deformation and fracture mechanisms for a range of promising materials with contained plasticity (refractory metals, ceramic materials, covalent crystals, quasicrystals, sintered materials).

Reconstruction of Dislocation Potential Relief by Means of Self-Blocking Effect

The effect of dislocation self-blocking in intermetallic compounds with an anomalous temperature dependence of yield stress has been theoretically and experimentally analyzed. In essence, this effect is a dislocation self-immersion in a deep potential relief valley (without external stress). The possibility of reconstructing the relief shape (from one-valley to two-valley) is shown and the relief parameters are reported. The allowed and forbidden regions for self-blocking are revealed. The method for determining the ratio of the valley depths by measuring the self-blocking limiting angles between the dislocation segments is proposed. Transmission electron microscopy images of the dislocation structure can be used to this end. As an example, this ratio is estimated for a superpartial dislocation sliding in the cube plane in Ni3Ge.

Molecular Dynamics Simulations of Uniaxial Compression of Ceria and Gadolinia Doped Ceria

In this paper, we investigate the mechanical properties of ceria and gadolinia doped ceria by molecular dynamics simulations. The doped concentrations and temperature dependence of yield stress and elastic modulus have been evaluated via uniaxial compression. Simulation results reveal that such properties decrease dramatically with higher temperature and doped content. In addition, the attenuated effect of doped content is more significant than that of temperature.