Stresses in a viscoelastic fluid in converging and diverging flow.
Stress in viscoelastic polystyrene in Aroclor flowing in straight, converging and diverging channels
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Hot Deformation Mechanisms of Ti22Al25Nb Orthorhombic Alloy
In this paper, the hot deformation behavior of Ti22Al25Nb was investigated by hot compression over a range of temperatures (950-1050 °C) and strain rates (0.001-1 s{sup −1}). Considering the adiabatic heating and friction, the original curves were corrected. The friction-corrected data were lower than the measured flow stress data. The flow stress data increased after the temperature correction owing to the elimination of the softening effect. In addition, the work hardening effects on the hot deformation of the normal and multi-peak flow stress curves were compared. A critical dislocation density was proposed to analyze the multi-peak phenomenon. When the dislocation density decreased to a critical value, the work hardening and dynamic recrystallization mechanisms reached a balance. Then, a new work hardening mechanism became dominant, and the flow stress increased. The effect of second-phase particles on the hot deformation mechanisms was investigated. The result showed that second-phase particles led to a high nucleation rate and a low growth rate of new recrystallization grains during the rapid hot deformation.
36 MATERIALS SCIENCE↗
A critical assessment of viscous models of trench topography and corner flow
Stresses for Newtonian viscous flow in a simple geometry (e.g., corner flow, bending flow) are obtained in order to study the effect of imposed velocity boundary conditions. Stress for a delta function velocity boundary condition decays as 1/R(2); for a step function velocity, stress goes as 1/R; for a discontinuity in curvature, the stress singularity is logarithmic. For corner flow, which has a discontinuity of velocity at a certain point, the corresponding stress has a 1/R singularity. However, for a more realistic circular-slab model, the stress singularity becomes logarithmic. Thus the stress distribution is very sensitive to the boundary conditions, and in evaluating the applicability of viscous models of trench topography it is essential to use realistic geometries. Topography and seismicity data from northern Hoshu, Japan, were used to construct a finite element model, with flow assumed tangent to the top of the grid, for both Newtonian and non-Newtonian flow (power law 3 rheology). Normal stresses at the top of the grid are compared to the observed trench topography and gravity anomalies. There is poor agreement. Purely viscous models of subducting slables with specified velocity boundary conditions do not predict normal stress patterns compatible with observed topography and gravity. Elasticity and plasticity appear to be important for the subduction process.
Zhang, J.↗
A critical assessment of viscous models of trench topography and corner flow
Stresses for Newtonian viscous flow in a simple geometry (e.g., corner flow, bending flow) are obtained in order to study the effect of imposed velocity boundary conditions. Stress for a delta function velocity boundary condition decays as 1/R(2); for a step function velocity, stress goes as 1/R; for a discontinuity in curvature, the stress singularity is logarithmic. For corner flow, which has a discontinuity of velocity at a certain point, the corresponding stress has a 1/R singularity. However, for a more realistic circular-slab model, the stress singularity becomes logarithmic. Thus the stress distribution is very sensitive to the boundary conditions, and in evaluating the applicability of viscous models of trench topography it is essential to use realistic geometries. Topography and seismicity data from northern Hoshu, Japan, were used to construct a finite element model, with flow assumed tangent to the top of the grid, for both Newtonian and non-Newtonian flow (power law 3 rheology). Normal stresses at the top of the grid are compared to the observed trench topography and gravity anomalies. There is poor agreement. Purely viscous models of subducting slables with specified velocity boundary conditions do not predict normal stress patterns compatible with observed topography and gravity. Elasticity and plasticity appear to be important for the subduction process.
Zhang, J.↗
Constitutive Behavior and Hot Workability of a Hot Isostatic Pressed Ti-22Al-25Nb Alloy during Hot Compression
A Ti-22Al-25Nb alloy was fabricated from prealloyed powders by hot isostatic pressing for 2 h at a temperature of 1050 °C and pressure of 100 MPa. The hot deformation behavior of the Ti-22Al-25Nb alloy was characterized by isothermal compression testing at deformation temperatures between 900 and 1060 °C and strain rates between 0.001 and 1 s{sup −1}. Based on the true stress–strain curves, a constitutive equation was constructed to describe the flow stress as a function of the strain rate and deformation temperature. Three-dimensional (3D) processing maps were developed based on dynamic material model theory using the stress flow data to identify the instability and optimization regions of the hot processing parameters. The results show that the flow stress decreases with increasing deformation temperature and decreasing strain rate, and the softening mechanisms are different under different deformation conditions. The apparent activation energies in the (α{sub 2} + β/B2 + O) and (α{sub 2} + B2) phase regions are calculated to be 865.177 and 590.661 kJ/mol, respectively, which suggests that the hot isostatic pressed (HIPed) Ti-22Al-25Nb alloy requires a high hot deformation activation energy. Combined with microstructural observations, the optimal processing domains are determined to be a temperature range of 1030-1060 °C and strain rate range of 0.01-0.1 s{sup −1} in the α{sub 2} + B2 phase region. Moreover, the results indicate that adiabatic shear bands and severe inhomogeneous deformation cause flow instability at lower temperatures (900-1005 °C) and higher strain rates (> 0.1 s{sup −1}).
36 MATERIALS SCIENCE↗
Mechanical Behavior of Lead at High Strain Rates
This paper presents a study on the material behaviors and constitutive models of lead at high strain rates. Quasi-static compressive tests and split Hopkinson pressure bar (SHPB) tests were conducted at room temperature. The results of the SHPB tests were verified by high-speed photography, and the error of strain is less than 3% at high strain rate (5000/s). Our results show that the yield stress and flow stress increase at high strain rates. This result indicates that lead is sensitive to the strain rate at high strain rates, but the dependence is not linear during 3000-5000/s. The strain-rate dependence of lead was fitted by a quadratic polynomial curve. To describe the nonlinear strain-rate relationship of lead, modified Johnson–Cook and Cowper–Symonds material models were used to fit the experimental stress–strain curves. The modified Cowper–Symonds model agrees better with the experimental results and can better describe the dynamic mechanical behavior of lead under high strain rates.
36 MATERIALS SCIENCE↗
Constitutive Model for Hot Deformation of the Cu-Zr-Ce Alloy
Hot compressive deformation behavior of the Cu-Zr-Ce alloy has been investigated according to the hot deformation tests in the 550-900 °C temperature range and 0.001-10 s{sup −1} strain rate range. Based on the true stress–true strain curves, the flow stress behavior of the Cu-Zr-Ce alloy was investigated. Microstructure evolution was observed by optical microscopy. Based on the experimental results, a constitutive equation, which reflects the relationships between the stress, strain, strain rate and temperature, has been established. Material constants n, α, Q and ln A were calculated as functions of strain. The equation predicting the flow stress combined with these materials constants has been proposed. The predicted stress is consistent with experimental stress, indicating that developed constitutive equation can adequately predict the flow stress of the Cu-Zr-Ce alloy. Dynamic recrystallization critical strain was determined using the work hardening rate method. According to the dynamic material model, the processing maps for the Cu-Zr and Cu-Zr-Ce alloy were obtained at 0.4 and 0.5 strain. Based on the processing maps and microstructure observations, the optimal processing parameters for the two alloys were determined, and it was found that the addition of Ce can promote the hot workability of the Cu-Zr alloy.
36 MATERIALS SCIENCE↗
Role of Chemical Disorder in High Temperature Dislocation Glide in Refractory Multi-principal Element Alloys
Refractory multi-principal element alloys (RMPEAs) combine a chemically disordered lattice with a structurally ordered, single‐phase body‐centered cubic (bcc) crystal structure. Chemical fluctuations in these alloys give rise to significant energy barriers that impede dislocation motion. In this study, we use phase field dislocation dynamics to examine how spatial temperature fluctuations compete with randomness in energy barriers to affect the motion of long screw dislocations in three equi-atomic MoNbTa‐based RMPEAs: MoNbTa, MoNbTaW, and MoNbTaVW. Over a wide range of homologous temperatures (T h ≈ 0–0.6), we determined a screw dislocation ‘flow stress’ as the minimum applied stress to sustain continuous motion over a long excursion distance within a fixed timeframe. All three RMPEAs exhibit temperature dependent flow stress with three characteristic glide regimes: at low homologous temperatures, flow stress drops sharply, and screw glide remains planar and rectilinear; at intermediate homologous temperatures, the flow stress levels off as glide becomes planar but wavy; and at high homologous temperatures, flow stress plateaus as screws exhibit nonplanar, three‐dimensional motion. Glide kinetics and transition temperatures are controlled by the chemically induced fluctuations in the energy landscape. The rectilinear to wavy transition temperature is controlled by statistically weakest local barriers in the glide plane, whereas the wavy to 3D transition temperature is governed by statistically strongest local barriers. At low and intermediate homologous temperatures, the relative spread in energy barriers governs glide behavior by controlling the local kinetics of kink pair formation and kink pinning. At high homologous temperatures, the average barrier height governs the glide behavior by controlling the number of out-of-plane excursions during 3D glide. These findings reveal how random chemical fluctuations determine screw‐driven plasticity in RMPEAs, providing critical insight for the design of high temperature structural alloys.
Defects↗
Effect of Homogenization Treatment and Microalloying with Mn on the Microstructure and Hot Workability of AA6060 Aluminum Alloys
The effects of a homogenization treatment and microalloying with Mn on the evolution of the microstructure and hot workability of AA6060 aluminum alloys were investigated. Various homogenization treatments with temperatures ranging from 520 to 610 °C and soaking times from 2 to 16 h were conducted. The results revealed that β-AlFeSi intermetallic was the dominant phase in the as-cast microstructure of the experimental alloys. During the homogenization, the fragmentation of intermetallics occurred and plate-like β-AlFeSi transformed into rod-like α-AlFeSi. In addition, a number of dispersoids precipitated in the 0.1Mn alloy in the temperature range of 520-580 °C. The flow stress behavior of the homogenized AA6060 alloys was mainly determined by the solid solution level. Increasing homogenization temperatures resulted in higher flow stresses owing to the increase in solute atoms in the aluminum matrix. The incremental Mn addition from 0 to 0.1% moderately increased the flow stress by up to 3%. Grain growth occurred in the alloys with low Mn contents (< 0.03Mn) during the high-temperature homogenization (580-610 °C), which resulted in a sudden decrease in the flow stresses and an irregular sample shape after the deformation. Microalloying with Mn (> 0.06%) can effectively prevent grain growth at such temperatures. For an alloy with Mn (0.1%) microalloying, homogenization at 550-580 °C for 6 h could be the optimal condition to balance the flow stress and desirable microstructure.
36 MATERIALS SCIENCE↗
Room-Temperature Microstructural Evolution of Extruded AM80 Magnesium Alloys under Dynamic Loading
Dynamic compression tests were conducted using a split Hopkinson pressure bar along the transverse direction at strain rates (SRs) of 3200 and 6000 s{sup −1}. The initial microstructure significantly affected the flow stress behavior and microstructural evolution of the two studied AM80 magnesium alloys. Under the same SR loading, the as-extruded alloy exhibited an apparently higher flow stress. At a SR of 6000 s{sup −1}, the flow stress of the as-extruded alloy exhibited a visible decrease (~ 34 MPa) as the strain increased to ~ 0.2, whereas a constant flow stress was exhibited by the solution-treated alloy. High-density dislocations and mechanical twins were detected in the two alloys at a SR of 3200 s{sup −1}. Interestingly, dynamic recrystallization (DRX) with an average grain size of ~ 2 µm was only identified in the as-extruded alloy as the applied SR increased to 6000 s{sup −1}. The increased localized deformation in the as-extruded alloy due to the retained strain energy and high critical twinning stress improved the DRX driving force, thereby resulting in visibly different micrographs of the two alloys when impacted at the SR of 6000 s{sup −1}.
36 MATERIALS SCIENCE↗
Study on Tensile Deformation Behavior of TC21 Titanium Alloy
To investigate the tensile deformation behavior of TC21 titanium alloy wires, the isothermal tensile deformation tests were conducted by a Geeble-1500D simulator in the temperature range of 720-880 °C and strain rates of 0.01-10 s{sup −1}. The in situ tensile deformation behavior has been studied by SEM SURPA40. The stress–strain curves indicate that the flow stress has shown an inverse relationship with temperature and a direct relationship with strain rate. The flow stress immediately reached the peak value and, then, gradually decreased due to the dynamic softening induced by dynamic recovery and dynamic recrystallization. According to the hot processing maps, the temperatures ranging from 720 to 760 °C (corresponding to the strain rates of 0.01-0.032 s{sup −1}) and 800-840 °C (corresponding to the strain rates of 0.01-0.056 s{sup −1}) are suitable for processing. The in situ tensile results reveal that the cracks initiated at the pre-fabricated defects with the concentrated stress and mainly propagated in the transgranular mode.
36 MATERIALS SCIENCE↗
Simulation of dislocation evolution in microparticle impacts over a wide range of impact velocities
We report the dynamic loading environment created by high velocity microparticle impacts is very difficult to model. The extremely high strain rates, large deformations, and high temperatures affect the flow stress of the material in ways that standard flow stress models cannot capture. Plastic deformation and strain hardening in metals is controlled by the motion of dislocations. Dislocations can be nucleated, stored as forest dislocations, or be annihilated as loading progresses. A comprehensive accounting of dislocation density change is needed to accurately describe rate and temperature dependent dislocation glide and evolution across the wide rage of loading conditions present in microparticle impact problems. Therefore, we implement and apply a newly proposed flow stress model (Hunter and Preston, 2015, 2022) that is then coupled with mobile and immobile dislocation density evolution equations. This model is implemented in Los Alamos National Laboratory’s hydrodynamics code, FLAG, to model copper-on-copper microparticle impacts. This new strength model allows for accurate simulation of particle rebound and flattening across a wide range of impact velocities.
36 MATERIALS SCIENCE↗
Dynamic Recrystallization Behavior of AISI 422 Stainless Steel During Hot Deformation Processes
In this work, hot compression tests were performed to investigate the dynamic recrystallization (DRX) process of a martensitic stainless steel (AISI 422) at temperatures of 950, 1000, 1050, 1100 and 1150 °C and strain rates of 0.01, 0.1 and 1 s{sup −1}. The dependency of strain-hardening rate on flow stress was used to estimate the critical stress for the onset of DRX. Accordingly, the critical stress to peak stress ratio was calculated as 0.84. Moreover, the effect of true strain was examined by fitting stress values to an Arrhenius type constitutive equation, and then considering material constants as a function of strain by using a third-order polynomial equation. Finally, two constitutive models were used to investigate the competency of the strain-dependent constitutive equations to predict the flow stress curves of the studied steel. It was concluded that one model offers better precision on the flow stress values after the peak stress, while the other model gives more accurate results before the peak stress.
36 MATERIALS SCIENCE↗
Microstructural Characterization and High Strain Rate Plastic Flow Behavior of SMAW Armox500T Steel Joints from Spherical Indentation Experiments
Static indentation and uniaxial compression tests have been conducted on Armox500T steel and its weldments to predict the constraint factor (CF), i.e., ratio of Meyer’s hardness to uniaxial flow stress. Series of dynamic indentation experiments were carried out at impact velocities ranging from 5 to 300 m/s to estimate dynamic hardness as a function of average strain. Subsequently, dynamic indentation (DI) test data and CF determined under static indentation conditions have been used to study the high strain rate plastic flow behavior of Armox500T weldments in comparison with the base metal. It was observed that flow stress for Armox500T and its weldments under dynamic loading conditions (10{sup 4} s{sup −1}) are significantly higher than flow stress measured under static loading conditions (10{sup −3} s{sup −1}). The plastic flow behavior computed from DI is in good agreement with the data evaluated through conventional split Hopkinson pressure bar technique. Further, the study of the microstructure of base metal and weldments by optical microscopy and SEM revealed a considerable variation in the microstructure.
36 MATERIALS SCIENCE↗
Effects of Particle Diameter and Coke Layer Thickness on Solid Flow and Stress Distribution in BF by 3D Discrete Element Method
In order to reduce ironmaking-related CO{sub 2} emissions, hydrogen-enriched blast furnace (BF) operation is currently under development. In hydrogen-enriched BF operation, coke layer thickness can be decreased to reduce CO{sub 2} emissions. However, BFs operating with thin coke layers may experience instability or discontinuous phenomena such as particle slip and gas channeling problems, so it is important to optimize the particle diameter and coke layer thickness for optimal BF operation. In this study, the effects of particle diameter and coke layer thickness on the solid flow and stress distribution in a BF were analyzed using a three-dimensional discrete element method. Furthermore, the effects of particle diameter and coke layer thickness on the burden layer stabilities, particle velocities, and particle stress distributions have been investigated. The results show that decreasing the coke layer thickness caused instability owing to the mixing of the coke and ore layers in the BF-cohesive zone and slight increases in both the average particle velocities and the average normal particle stress magnitudes. In addition, the average particle velocities and average normal particle stresses were higher for the smaller particles than for the larger ones during the simulations.
36 MATERIALS SCIENCE↗
Deformation Behavior and Microstructure Evolution of Powder Metallurgy Ti6Al4V Alloy During Hot Compression
In order to determine appropriate workability parameters for cross-wedge rolling products of powder metallurgy (PM) Ti6Al4V alloys, thermal compression experiments were carried out within the temperature range of 850-1050 °C, the strain rate range of 0.1-10 s{sup −1}, and with a height decrease of 60%. The compression deformation behavior was investigated. A strain-compensated constitutive model was established based on the flow stress curves corrected by the friction, which can predict the flow behaviors under the entirety of the experimental deformation conditions. Moreover, the processing map was developed, and stable and unstable deformation regimes were established. The microstructure analysis illustrated that globularization and dynamic recrystallization were the main deformation mechanisms. The porosity of the compressed samples was greatly reduced and influenced by the temperature and strain rate. Considering the material flow stress, hot processing map, microstructure and porosity, the appropriate hot deformation parameters for PM Ti6Al4V alloy preforms are suggested.
36 MATERIALS SCIENCE↗
Low-Temperature Plasticity in Olivine: Grain Size, Strain Hardening, and the Strength of the Lithosphere
Plastic deformation of olivine at relatively low temperatures (i.e., low-temperature plasticity) likely controls the strength of the lithospheric mantle in a variety of geodynamic contexts. Unfortunately, laboratory estimates of the strength of olivine deforming by low-temperature plasticity vary considerably from study to study, limiting confidence in extrapolation to geological conditions. Here we present the results of deformation experiments on olivine single crystals and aggregates conducted in a deformation-DIA at confining pressures of 5 to 9 GPa and temperatures of 298 to 1473 K. These results demonstrate that, under conditions in which low-temperature plasticity is the dominant deformation mechanism, fine-grained samples are stronger at yield than coarse-grained samples, and the yield stress decreases with increasing temperature. All samples exhibited significant strain hardening until an approximately constant flow stress was reached. The magnitude of the increase in stress from the yield stress to the flow stress was independent of grain size and temperature. Cyclical loading experiments revealed a Bauschinger effect, wherein the initial yield strength is higher than the yield strength during subsequent cycles. Both strain hardening and the Bauschinger effect are interpreted to result from the development of back stresses associated with long-range dislocation interactions. We calibrated a constitutive model based on these observations, and extrapolation of the model to geological conditions predicts that the strength of the lithosphere at yield is low compared to previous experimental predictions but increases significantly with increasing strain. Our results resolve apparent discrepancies in recent observational estimates of the strength of the oceanic lithosphere.
58 GEOSCIENCES↗