Viscous Glide Research Articles

Unraveling hot deformation behavior and microstructure evolution, flow stress prediction of powder metallurgy BCC/B2 Al1.8CrCuFeNi2 HEA

In this work, hot compress tests were conducted at the strain rate range of 1–0.001 s−1 and the temperature range of 900–1050 °C to unravel the hot deformation behavior of a powder metallurgy (PM) Al1.8CrCuFeNi2 high entropy alloy with BCC/B2 structure. Arrhenius model and radial basis function artificial neural network (RBF-ANN) machine learning (ML) method were used to model flow stress. RBF-ANN ML method possesses superior predictability. Elongated grains almost change to equiaxed grains under the low strain rate of 0.001 s−1 due to the activity of continuous dynamic recrystallization (CDRX). There are no significant changes in the distribution, size and content of fine B2 and BCC eutectic phases during deformation. Stress index ranges form 2.2–2.6 at various strains. Dislocation viscous gliding and grain-boundary sliding (GBS) are predominant deformation modes. The contribution of GBS to deformation is more significant at low strain rate. For dislocation slip, multiple slip systems are activated,{123̅}<111> slip system has higher Schmid factor. Hot processing ability is improved at high temperature and low strain rate. Absorbing of dislocation at grain boundaries and the dominance of GBS deformation mode under high temperature and low strain rate condition can avoid instability.

Through-thickness heterogeneity in creep properties of friction stir welding 7B50-T7451 aluminum alloy thick plate joint: Experiments and modeling

Through-thickness heterogeneity in creep properties of 7B50-T7451 aluminum alloy Friction Stir Welding (FSW) joints was investigated. Creep tests for three slices of the FSW joint were conducted at the temperature of 150–200 °C and applied stress of 60–225 MPa. The theta projection method was used to predict creep curves and minimum creep rate. The results show that the minimum creep rate increases and creep rupture life decreases with the increase of creep temperature and applied stress. Creep properties of the FSW joint deteriorate along the thickness direction from the top to the bottom. The threshold stress of all three slices of the FSW joint decreases with the increase of creep temperature and even disappears at 200 °C for the bottom slice. Creep activation energy approaches the activation energy of the lattice self-diffusion of aluminum. The value of true stress exponent for different slices is approximately equal to three. The predominant creep mechanism of the FSW joint is dislocation viscous glide by lattice self-diffusion. What is more, a constitutive model is established based on the theta method to accurately describe creep behavior of different slices of the FSW joint.

Creep Behavior of Squeeze-Cast Mg–15Gd Alloy

The creep behavior of a binary Mg-15 wt.% Gd alloy was investigated over the temperature range from 523 K to 743 K, i.e., in both the single-phase region (the hexagonal close-packed solid solution of Gd in Mg) and the two-phase region (the solid solution plus Mg5Gd precipitates). The alloy was prepared by the squeeze casting technique. In the higher temperature range, at 723 and 743 K, the specimens were solution treated by in situ annealing prior to testing. At the temperature of 673 K and below, the alloy was tested in the cast state. In the higher temperature range, the behavior was interpreted in terms of the viscous glide, where the dislocation motion was constrained by the presence of solute atmospheres. The dislocation motion was controlled by the rate of the cross slip from the basal to the prismatic planes. At the temperatures of 623 K and 673 K, the creep behavior was rationalized by introducing the threshold stress concept. At the temperatures of 523 K and 573 K, the stresses required to achieve experimentally measurable creep rates were such that dislocations broke away from the atmospheres of foreign atoms. Comparison with a series of magnesium alloys prepared by squeeze casting and creep-tested by the same technique showed that gadolinium can be a favorable creep-resistance enhancing element.

Creep behavior of 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy under constant load

In the last three decades, aerospace industry has required shape memory alloys with higher phase transformation temperatures. In response to this technical challenge, High Temperature Shape Memory Alloys that can be actuated at higher temperatures were developed. However, high temperature actuation applications also lead to observe much higher dislocation motion and thus plastic deformation due to creep and phase transformation. In this study, 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy was used for the investigation of creep deformation under tensile loads corresponding to stress magnitudes between 200 MPa and 500 MPa. Deformation behavior of the alloy in austenitic phase was investigated by means of creep deformation experiments at 450 °C, 550 °C, and 600 °C. 50at%Ni 25at%Ti 25at%Hf alloy has similar creep behavior as NiTi despite higher loading conditions. Almost no noticeable creep strain values were observed under 200 MPa and 300 MPa at 550 °C and 600 °C. Creep parameters such as the stress exponent and the activation energy values were calculated using power law creep model and viscous glide creep was found to be the main mechanism in 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy.

High temperature creep deformation behavior of heat-treated (α + β)-Mg-9Li-4Al-1Zn alloy

In this study, the high temperature uniaxial tensile creep deformation behavior of (α + β)-Mg-9 mass%Li-4 mass%Al-1 mass%Zn (LAZ941) alloy was investigated. The creep strength of the LAZ941 alloy was significantly improved by heat treatment. The creep deformation behavior of the heat-treated LAZ941 alloy was unique. As the results of creep tests, it is found that the stress exponent of LAZ941 was about 3. The activation energies of creep at the early and middle stages of deformation were 114.7 and 84.6 kJ･mol −1 , respectively, indicating that the diffusing elements controlling the viscous glide of the dislocation changed during creep deformation. • The creep strength of the LAZ941 alloy is improved by heat treatment. • The time to reach 0.1 creep strain is about 9 times longer due to heat treatment. • The creep deformation behavior of heat-treated LAZ941 alloy is unique. • The activation energy of creep for LAZ941 alloy changes during deformation.

ZnO nanoparticles and compositional dependence of structural, thermal, mechanical, and electrical properties for eutectic SAC355 lead-free solder alloys

The purpose of this paper is to investigate the reliability of mechanical, thermal, and electrical properties of the eutectic Sn96Ag3.5Cu0.5 (SAC355) alloys doping with ZnO nanoparticle as a function of doping concentration. SAC355 and (SAC355)100-x(ZnO)x (x = 0.1, 0.3, 0.5, 0.7 and 1 wt%) composite solders were prepared by melt-spinning technique, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and dynamic resonance techniques (DRT) were used to examine their structural, thermal and mechanical properties after the addition of ZnO NPs, respectively. The results show that by adding ZnO NPs to the solder alloy, the grains can be refined to a certain degree, and generate some significant improvements in internal friction (Q−1), microhardness (Hv), yield stress (σ), melting temperature (Tm) and electrical resistivity (ρ) enhancement. Elastic modulus (E) fluctuation may be due to variation in particle size for β-Sn. The Debye temperature (θD) was determined by Lindemann's melting theory from the values of elastic constants. The stress exponent (n) values were in the range 1.83–3.45 which indicates grain boundary sliding and viscous glide mechanisms. According to their favorable physical properties, the addition of ZnO NPs may be useful in many future structural applications.

TEM study of the dislocation structure at the transition from discontinuous to viscous glide in Cu–Al and Cu–Mn alloys at elevated temperatures

Abstract The dislocation structures developed during plastic deformation in the regimes of discontinuous glide (Portevin – Le-Chatelier effect, PLC) as well as viscous glide at elevated temperatures (600 – 800 K) have been studied by transmission electron microscopy in polycrystals of Cu-5Al, Cu-10Al, Cu-15Al and Cu-4.1Mn alloys (at.%). Discontinuous glide is characterized by correlated dislocation movement in few active slip planes due to breakaway of aged and rapid multiplication of fresh dislocations, while beyond the transition, when solute diffusion is fast enough to continuously reconstitute the solute cloud during dislocation motion, a homogeneous distribution of smoothly moving dislocations is observed in all alloys. Owing to the decrease of stacking-fault energy and increase of short-range order in the Cu–Al alloys a change from cell structure (Cu-5Al) to planar slip (Cu-10Al and Cu-15Al) occurs in the discontinuous glide regime; in the Cu-4.1Mn alloy with a stacking-fault energy like Cu and negligible short-range order, development of cells prevails also during discontinuous glide. The observations are discussed in terms of current views on deformation in the PLC regime.

Study on the three-dimensional tensile creep anisotropy of a hot-rolled Mg-0.9Mn-1.5Ce alloy sheet

In this work, the three-dimensional tensile creep anisotropy of a hot-rolled Mg-0.9Mn-1.5Ce (wt%) alloy sheet was investigated. Creep tests were carried out at 453 K, 473 K and 493 K under various applied stresses. The loading directions were parallel to the transverse direction (TD), rolling direction (RD), and normal direction (ND). Experimental results showed that creep behavior had a strong dependence on the loading direction under the given ranges of stress and temperature, in which the creep resistance was ranked in the order of TD ≈ RD> ND. Thermodynamic calculations and microstructure characterization illustrated that the creep mechanism displayed an intense dependence on stress and temperature in the TD and RD samples, which exhibited similar creep behaviors. Specifically, the viscous gliding of dislocations dominated the creep at 453 K. Pyramidal< c + a> slip started to activate and gradually controlled the creep because of the thermal activation at 473 K. As the temperature increased to 493 K, dislocation climb and pyramidal< c + a> slip together controlled the creep. Conversely, the creep mechanism barely changed in the ND sample, which included {101̅2} twinning and cross-slip. The dense twins developed in the ND sample directly caused early creep fracture, and< a> dislocation cross-slipping between the basal plane and prismatic plane supplied a large creep strain and accelerated creep rate. Unexpectedly, the heavy interactions between pyramidal dislocations and dynamic precipitates offered an extra strengthening factor for creep resistance in the TD and RD samples. Consequently, the poorest creep resistance was obtained in the ND sample. In addition, the breakaway of dislocations from the solute atmosphere or precipitates by climbing was the main reason for the power-law breakdown at 493 K in the TD and RD samples.

Microstructure Features and Superplasticity of Extruded, Rolled and SPD-Processed Magnesium Alloys: A Short Review

In this study, an overview of microstructure features such as grain size, grain structure, texture and its impact on strain rate sensitivity, strain hardening index, activation energy and thermal stability for achieving superplasticity of Mg alloys are presented. The deformation behavior under different strain rates and temperatures was also elaborated. For high elongation to fracture grain boundary sliding, grain boundary diffusion is the dominant deformation mechanism. In contrast, for low-temperature and high strain rate superplasticity, grain boundary sliding and solute drag creep mechanism or viscous glide dislocation followed by GBS are the dominant deformations. In addition, the results of different studies were compared, and optimal strain rate and temperature were diagnosed for achieving excellent high strain rate superplasticity.

Tensile creep behavior and mechanism of CoCrFeMnNi high entropy alloy

As a potential novel superalloy, the study on the creep of high entropy alloys is of great significance. Equiatomic ratio CoCrFeMnNi with a mass of 6.7 kg was fabricated by vacuum induction levitation melting. Dendrite structure is obtained for as-cast alloy while a homogeneous equiaxed grain structure can be formed after annealing at 1273 K for 6 h. The tensile creep behavior of as-annealed alloy at 773–973 K and 50–340 MPa is systematically studied. Apparent dynamic recovery and recrystallization occur for creep at 973 K while high dislocation density can be observed for creep at 923 K and lower temperatures. Results of EDS analyses on fractured surface, stress exponent and activation energy for steady state creep rate suggest that the creep mechanism should be the dislocation viscous glide dragged by solute Mn at low stresses and low temperatures and be the dislocation climb associated with Mn diffusion at high stresses and high temperatures.

Evolution of microstructure during tensile creep deformation of nickel-based disk superalloy

The aim of the present investigation is to study the tensile creep behavior of nickel-based disk superalloy, which is widely used in the manufacturing of turbine disks for the aero engines. Tensile creep tests were performed over a wide range of stresses (100–900 MPa) and temperatures (625–850 °C) and rupture time varies from 93.1 to 9473 h. Furthermore, stress rupture strength was predicted using the time-temperature parameter at different temperatures for longer durations. The stress dependence of minimum creep rate for the alloy is found to follow the power law. The stress exponent (n) decreased from 31.2 to 7.8 with increase in the test temperature from 625 °C to 700 °C. Subsequently, the rate-controlling mechanism of creep is identified as dislocation climb by adopting the threshold stress analysis in the temperature range of 625–700 °C. At higher temperature, the n values are drastically reduced to 2.9, 2.7 and 2.2 at 750, 800 and 850 °C, respectively, due to dislocation annihilation and dissolution of precipitates. This indicates that the rate-controlling mechanism of creep changes from viscous glide to grain boundary sliding (n = 2.2). The activation energy for creep (QC) has been determined by using the modified power law in the temperature range of 625–700 °C and is found to be 598 kJ/mol, whereas the obtained QC is also decreased significantly to 435 kJ/mol at higher temperature range (750–850 °C). The calculated QC values are found to be 52.5% and 34.7% higher than the activation energy for lattice self-diffusion of nickel (284 kJ/mol). Post creep microstructural examination using transmission electron microscopy (TEM) revealed extensive deformation in the microstructure is accommodated through the γ′ precipitates, formation of stacking faults and deformation twins within the larger γ′ precipitates. The major findings are well in strong agreement with the hardness characterization, where prominent increase in hardness within localized deformation is mainly due to extensive dislocation-interactions with the γ′ precipitates.

Creep Behavior of Near α High Temperature Ti-6.6Al-4.6Sn-4.6Zr-0.9Nb-1.0Mo-0.32Si Alloy

This study focuses on the microstructure characteristics and tensile and creep properties of a near α high temperature Ti-6.6Al-4.6Sn-4.6Zr-0.9Nb-1.0Mo-0.32Si alloy. Microstructure characteristics were quantitatively investigated using optical microscopy, scanning electron microscope, and transmission electron microscopy. Tensile properties were carried out at room and high temperature. Creep properties were detected under applied stresses ranging from 100–350 MPa at 873–973 K, respectively. Results showed that Widmanstätten microstructure was obtained after hot forged and heat treatment. The strength decreases and the elongation rises with temperature increasing. The ultimate strength and elongation were 1010 MPa, 12% at room temperature, and 620 MPa, 20% at 923 K, respectively. The steady state creep rates rise correspondingly with stress and temperature. Stress exponents are measured within the range of 3.0–3.5. Thus, the creep mechanism is diffusion-controlled viscous glide of dislocation. Ti3Al precipitates are observed. The boundaries and precipitates can obstruct dislocation movement to improve the creep properties. Fracture mechanism of creep is intergranular. The creep mechanism varied from climb of dislocation to sliding of dislocation solution.

Creep behavior and microstructural evolution of 8030 aluminum alloys compressed at intermediate temperature

The compression creep behavior of 8030 aluminum alloys at intermediate temperature was studied under the deformation temperature of 200∼250 °C and compression stress ranging from 20 to 40 MPa. The apparent stress exponent (na), the true stress exponent (nt) and the activation energy Qc for creep were na = 3.6, nt = 3 and Qc = 145.5 kJ/mol, respectively, indicating that the dislocation viscous glide controlled by lattice self-diffusion was the dominant creep mechanism of the samples at 200 °C-20 MPa. A creep constitutive equation was introduced with the threshold stress σ0 = 5.1 MPa. The microstructure characterization of selected samples was investigated by transmission electron microscopy (TEM), indicating that the sub-grain contours became ambiguous and the dislocations with long curved morphologies were homogeneously distributed within grain inners after creep for 100h.

Modeling for cyclic plasticity and rate dependence of metal materials with a new non-unified viscoplasticity model

The classic unified viscoplasticity model has achieved great success in areas such as cyclic plasticity, time-dependent behavior, and creep–fatigue interaction. However, the unified viscoplasticity model may be inadequate to describe inelastic strain rate with static recovery term. Our previous study [Chen, Kitamura, Feng (Int J Fatigue 112:70–77, 2018)] of 316L stainless steel has shown that adding a static recovery term predicts an accelerating creep strain rate in the creep–relaxation interaction tests, which is inconsistent with the experimental test results. In this study, a modified non-unified viscoplasticity (NUV) model is proposed based on the mechanism of viscous glide. The parameter determination process and the role of different parts of the model are illustrated using experimental data drawn from Al 5083; then, the proposed model is verified by simulating the test of 316L stainless steel. The modeling results show that the proposed NUV model accurately predicts the strain rate evolution during the creep–relaxation interaction tests and other behaviors, i.e., cyclic plasticity and loading rate sensitivity. As with fewer parameters and more concise equations, the proposed NUV model is a good choice for engineering applications.

Dynamic recrystallization and hot deformation mechanisms of a eutectic Al0.7CoCrFeMnNi high-entropy alloy

The deformation mechanisms and microstructural evolution of an Al0.7CoCrFeMnNi high-entropy alloy (HEA) composed of dual phases (FCC phase: 56.6 vol% and BCC phase: 43.4 vol%) have been studied during hot compressive deformation in the temperature range between 1173 and 1373 K and in the strain rate range between 10−3 and 10 s−1 up to a strain of 0.8. Among the AlxCoCrFeMnNi HEAs (with x = 0, 0.5, 0.7 and 1.0) from earlier studies and in the present study, the Al0.7CoCrFeMnNi alloy produced the finest grain size (1–46 µm) and the highest fraction of dynamically recrystallized grains (39–99%) after hot deformation, implying that the most effective grain refinement can be achieved from a eutectic microstructure. Continuous dynamic recrystallization (CDRX) was the main DRX mechanism in both the FCC and BCC phases, but the kinetics of CDRX was faster in the BCC phase, resulting in formation of a higher fraction of dynamically recrystallized grains in the BCC phase under the same deformation condition. The stress exponent associated with plastic flow at low strain rates was approximately 3. The steady-state constitutive relation linking the flow stress and the strain rate could be well interpreted in terms of either viscous glide creep with antiphase boundary interactions (when the BCC phase is assumed to be the B2 phase at high temperatures) or viscous glide creep with Cottrell-Jaswon interactions (when the BCC phase is assumed to be the disordered BCC (A2) phase at high temperatures).

Microstructural evolution, flow stress and constitutive modeling of Al−1.88Mg−0.18Sc−0.084Er alloy during hot compression

To explore the hot compression behavior and microstructural evolution, fine-grained Al−1.88Mg−0.18Sc− 0.084Er (wt.%) aluminum alloy wires were fabricated with Castex (continuous casting−extrusion) and ECAP-Conform, and their hot compression behavior was investigated at temperatures of 673−793 K and strain rates of 0.001−10 s−1; the microstructures were characterized by optical microscope, X-ray diffractometer, transmission electron microscope, and electron backscattered diffractometer, and the flow stresses were obtained by thermal compression simulator. Microstructural evolution and flow curves reveal that dynamic recovery is the dominant softening mechanism. Continuous dynamic recrystallization followed by dynamic grain growth takes place at a temperature of 773 K and a strain rate of 0.001 s−1; the yielding drop phenomenon was discovered. Hyperbolic sine constitutive equation incorporating dislocation variables was presented, and a power law constitutive equation was established. The stress exponent is 3.262, and the activation energy for deformation is 154.465 kJ/mol, indicating that dislocation viscous glide is the dominant deformation mechanism.

Hot deformation constitutive analysis and processing maps of extruded Mg–Gd binary alloys

Hot shear deformation behavior and optimum processing parameters of the binary Mg–xGd (x = 0.5, 1, and 2 wt%) alloys were studied through the development of constitutive equations and processing maps. Shear punch tests (SPT) were performed in the temperature range of 573–723 K and shear strain rate range of 1.7 × 10−2–1.3 × 10−1 s−1. It was observed that increasing the Gd content retarded the dynamic recrystallization (DRX), leading to smaller DRXed grain size, higher amounts of unDRXed elongated patches, as well as higher shear strength. Based on the stress exponent and deformation activation energy values, calculated through constitutive analysis, the dominant controlling mechanisms were suggested as climb-controlled deformation for the Mg–0.5Gd alloy, and dislocation viscous glide for the Mg–1Gd and Mg–2Gd alloys. The processing maps were plotted to obtain the optimum processing conditions for all tested materials. The results show that the temperature of the safe region with maximum efficiency increased by increasing the amount of Gd addition.

A dislocation assisted self-consistent constitutive model for the high-temperature deformation of particulate metal-matrix composite

ABSTRACT A dislocation assisted self-consistent model based on Tandon and Weng approach and Bergstrom dynamic recovery model for particulate-reinforced composites has been extended to consider the matrix evolution during high-temperature deformation on flow stress. The impact of main influential processing parameters such as temperature, strain, and strain rate in addition to reinforcement characteristics, including particle size, and volume fraction, were successfully taken into account in the constitutive model. Moreover, the effect of particle fracture, diffusion relaxation around particles, dynamic recrystallization, and dynamic recovery of the matrix during deformation were precipitated as the softening factors in the presented model. It was found that the occurrence of particle stimulating nucleation mechanism can destroy the load transfer mechanism, which results in flow curve softening for a limited range of deformation conditions. It was shown that deformation mechanisms in single-phase alloy and metal matrix composite are the same, which are viscous glide and dislocation climb.

Hot deformation behavior and processing map of a Sn0.5CoCrFeMnNi high entropy alloy with dual phases

The hot compressive deformation behavior of a cast Sn0.5CoCrFeMnNi high-entropy alloy composed of dual phases, an FCC solid solution phase and a MnNiSn-rich ordered FCC phase (with a volume fraction of 30%), was investigated in temperature and strain rate ranges of 1023–1248 K and 10-3–10 s−1, respectively. During hot deformation, the MnNiSn-rich phase with a relatively low melting temperature (1297 K) underwent a significantly higher degree of dynamic recrystallization (DRX) than the FCC solid solution phase. Continuous DRX occurred in the FCC solid solution phase, but it was pronounced only at the highest temperature of 1248 K. The Sn0.5CoCrFeMnNi alloy exhibited dislocation climb creep at low strain rates and high temperatures and power-law breakdown at high strain rates and low temperatures. The flow stresses of Sn0.5CoCrFeMnNi were notably lower than those of CoCrFeMnNi, though they shared similar deformation mechanisms, by a factor of 0.37–0.71, and this could be explained by the rule of the mixture model considering the contribution of the FCC solid solution phase and the MnNiSn-rich phase to the total deformation rate under the isostress condition. Compared to the Al element in Al0.5CoCrFeMnNi, the Sn element is anticipated to create a larger atomic size misfit in Sn0.5CoCrFeMnNi but promote less viscous glide (solute drag) creep·. The analysis for explaining this reason was conducted from the perspective of factors that affect solute drag creep and breakaway stresses of dislocations from the solute atmosphere.

Constructing a Three-Dimensional Histogram Describing Dislocation Viscous Glide and Climb Mechanisms

Creep is the process by which material occurs at high temperatures and its slow deformation process. -2 since the slow...