Creep and ductility in an Al-Cu solid-solution alloy

High-temperature creep was investigated in an Al-3 wt pct Cu alloy at temperatures in the range of 773 to 853 K and at a normalized shear stress range extending from 10^-5 to 7 × 10^-4. The results show the presence of three distinct regions. In region I (low stresses), the stress exponent is 4.5 and the activation energy is 155 kJ/mole. In region II (intermediate stresses), the stress exponent is 3.2 and the activation energy is 151 kJ/mole. In region III (high stresses), the stress exponent is 4.5 and the activation energy is 205 kJ/mole. Creep curves obtained in the three regions exhibit a normal primary stage, but the extent of the stage is less pronounced in region II than in regions I and III. The creep characteristics in regions I and II, along with the values of the transition stresses between the two regions, are in conformity with the prediction of the deformation criterion for solid-solution alloys. While the advent of region III (high stresses) correlates well with dislocation breakaway from a solute-atom atmosphere, the creep characteristics in this region are not entirely consistent with any of the existing high-stress creep mechanisms. The plot of elongation to fracturevs initial strain rate at 853 K exhibits two peaks at strain rates of 1 × 10^-4 and 6 × 10^-4 s^-1. The first peak (1 × 10^-4 s^-1) is attributed to the variation of the stress exponent for creep in the alloy with strain rate, and the second peak (6 × 10^-4 s^-1) appears to reflect the effect of solute drag on dislocation velocity.     

Onset of recrystallization during the tensile deformation of austenitic iron at intermediate strain rates

The onset of recrystallization during the tensile deformation of austenitic iron has been fully documented for the temperature range 950 to 1350°C and strain-rate range 2.8 × 10^-5 to 2.3 × 10^-2 s^-1. Representative materials are zone-refined iron, electrolytic iron, Fe−0.05 C and Fe−5.2 Mn. In general, the strain at the onset of recrystallization decreases with increasing temperature of deformation and decreasing strain rate. The postponement of recrystallization is favored by prior annealing at temperatures above 1200°C and is greatest for the Fe−5.2 Mn alloy; however, for the range of strain rates used, it is difficult to completely eliminate recrystallization. The effects of test conditions on the onset of recrystallization are discussed in terms of a nucleation process that requires a critical amount of stored energy.     

Transition of dominant diffusion process during superplastic deformation in AZ61 magnesium alloys

The superplastic behavior of the AZ61 magnesium alloy sheet, processed by one-step hot extrusion and possessing medium grain sizes of ∼12 μm, has been investigated over the temperature range of 523 to 673 K. The highest superplastic elongation of 920 pct was obtained at 623 K and a deformation rate of 1×10⁻⁴ s⁻¹. In the lower and higher strain rate regimes, with apparent m values of ∼0.45 and ∼0.25, respectively, grain-boundary sliding (GBS) and dislocation creep appeared to dominate the deformation, consistent with the scanning electron microscopy (SEM) examination. The SEM examination also revealed that individual GBS started to operate from the very initial deformation stage in the strain rate range with m∼0.45, which was attributed to the relatively high fraction (88 pct) of high-angle boundaries. The analyses of the superplastic data over 523 to 673 K and 5×10⁻⁵ to 1×10⁻³ s⁻¹ revealed a true stress exponent of ∼2, and the activation energy was close to that for grain-boundary and lattice diffusion of magnesium at 523 to 573 K and 573 to 673 K, respectively. The transition temperature of activation energy is ∼573 K, which is attributed to the change in the dominant diffusion process from grain-boundary diffusion to lattice diffusion. It is demonstrated that the effective diffusion coefficient is a valid parameter to characterize the superplastic behavior and the dominant diffusion process.     

Hot Deformation Behavior of Beta Titanium Ti-13V-11Cr-3Al Alloy

Hot compression tests were conducted on Ti-13V-11Cr-3Al beta-Ti alloy in the temperature range of 1203 K to 1353 K (930 °C to 1080 °C) and at strain rates between 0.001 and 1 s^?1 The stress–strain curves showed pronounced yield point phenomena at high strain rates and low temperatures. The yield point elongation and flow stresses at the upper and lower yield points were related to the Zener–Hollomon parameter. It was found that dynamic recovery at low strain rates and dynamic recrystallization at high strain rates were the controlling mechanisms of microstructural evolution. The results also showed that strain rate had a stronger influence on the hot deformation behavior than temperature. The microstructural observations and constitutive analysis of flow stress data supported the change in the hot deformation behavior of the studied alloy varies with strain rate. For various applied strain rates, the activation energy for hot deformation was calculated in range of 199.5 to 361.7 kJ/mol. At low strain rates (0.001 and 0.01 s^?1), the value of activation energy was very close to the activation energy for the diffusion of V, Cr, and Al in beta titanium. The higher value of activation energy for deformation at high strain rates (0.1 and 1 s^?1) was attributed to the accumulation of dislocations and the tendency to initiate dynamic recrystallization.     

Grain coarsening in Ti-5Al-2.5Sn

Grain coarsening in a Ti-5 Al-2.5 Sn titanium alloy, deformed in tension to 13 pct uniform elongation and then heated to 1144 K (1600°F) for one h, was investigated. The influence of deformation temperature (77 to 598 K), grain size (10.7, 11.8, and 22.5 μm), and strain rate (2.67 × 10^-2, 6.67 × 10^-4, 2.67 × 10^-5 s^-1) was also studied. Critical elongation and work input values for maximum grain coarsening varied with deformation temperature. The critical elongation value increased from 9 to 12 pct as the temperature decreased from 598 to 367 K and decreased from 12 to 9 pct as temperature decreased from 367 to 77 K. The critical work energy input increased linearly with decreasing temperature.     

Low-temperature superplastic behavior of a submicrometer-grained 5083 Al alloy fabricated by severe plastic deformation

A submicrometer-grained structure was introduced in a commercial 5083 Al alloy by imposing an effective strain of ∼8 through equal channel angular pressing. In order to examine the low-temperature superplastic behavior, the as-equal channel angular pressed (as-ECAP) samples were tensile tested in the strain rate range of 10⁻⁵ to 10⁻² s⁻¹ at temperatures of 498 to 548 K corresponding to 0.58 to 0.65 T _m, where T _m is the incipient melting point. The mechanical data of the alloy at 498 and 548 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain rate sensitivity was 0.1 to 0.2 in the low- and high-strain rate regions and 0.4 in the intermediate-strain rate region, indicating the potential for superplasticity. At 523 K, instead of the sigmoidal behavior, a strain rate sensitivity of 0.4 was maintained to low strain rates. A maximum elongation of 315 pct was obtained at 548 K and 5×10⁻⁴ s⁻¹. The activation energy for deformation in the intermediate-strain rate region was estimated as 63 kJ/mol. Low-temperature superplasticity of the ultrafine grained 5083 Al alloy was attributed to grain boundary sliding that is rate-controlled by grain boundary diffusion, with a low activation energy associated with nonequilibrium grain boundaries. Cavity stringers parallel to the tensile axis were developed during deformation, and the failure occurred in a quasi-brittle manner with moderately diffusive necking.     

Mechanical and microstructural aspects of the high temperature plastic deformation of yttria-stabilized zirconia polycrystals

The high-temperature plastic deformation of 6 mol% Y₂O₃-stabilized ZrO₂ polycrystals with grain sizes of 1.8, 3.4 and 6.3 μm has been studied in compression between 1350 and 1450 °C in air at constant strain rate (between 1 × 10⁻⁵ and 2 × 10⁻⁴s⁻¹) and under constant load (between 5 and 90 MPa). Two mechanical behaviours were observed depending on strain rate or stress levels: grain boundary sliding controlled by cation bulk diffusion, with an activation energy of 560 kJ/mol, and intergranular cavitation without plastic deformation of the grains.     

Impact Response and Microstructural Evolution of 316L Stainless Steel under Ambient and Elevated Temperature Conditions

The impact response and microstructural evolution of 316L stainless steel are examined at strain rates ranging from 1?×?10³ to 5?×?10³?s^?1 and temperatures between 298?K and 1073?K (25?°C and 800?°C) using a split Hopkinson pressure bar and transmission electron microscopy (TEM). The results show that the flow behavior, mechanical strength, and work-hardening properties of 316L stainless steel are significantly dependent on the strain rate and temperature. The TEM observations reveal that the dislocation density increases with increasing strain rate but decreases with increasing temperature. Moreover, twinning occurs only in the specimens deformed at 298?K (25?°C), which suggests that the threshold stress for twinning is higher than that for slip under impact loading. Finally, it is found that the volume fraction of transformed ???? martensite increases with increasing strain rate or decreasing temperature. Overall, the results suggest that the increased flow stress observed in 316L stainless steel under higher strain rates and lower temperatures is determined by the combined effects of dislocation multiplication, twin nucleation and growth, and martensite transformation.     

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were performed at a strain amplitude of ±0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of γ′ and carbides); and microstructure B, in a double-aged condition with γ′ of 18-nm diameter and M₂₃C₆ carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered γ′ that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 × 10^-5 to 3 × 10^-3 s^-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.     

Microplasticity in Vanadium single crystals

An investigation of the activation parameters for plastic deformation in the microstrain region has been performed. Vanadium single crystals with 〈491〉 axial orientations were tested in compression by strain rate cycling at various intervals over the strain range of 1 × 10⁻⁵−2 × 10⁻³ at a strain sensitivity of 5 × 10⁻⁷. Tests were conducted between 125 and 300 K on a series of crystals containing interstitial solute concentrations in the range 486 at. ppm to 1649 at. ppm oxygen plus nitrogen content.The results indicate that in the microstrain region more than one dislocation process is occurring. The results are consistent with the theory that edge and nonscrew dislocations are the mobile species in the microstrain region at low temperatures. As deformation proceeds, the mobile densities of the edge and nonscrew segments decrease and the transition to macroflow is associated with the onset of screw dislocation motion.The data indicates that in the microstrain region edge and nonscrew dislocation mobility is strongly affected by solute interactions. In the macrostrain regions, however, flow rate is apparently controlled by screw dislocation-lattice interactions.     

Superelastic behaviour of a fine-grained,yttria-stabilized,tetragonal zirconia polycrystal (Y-TZP)

The microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated. Both hot indentation and tensile tests were carried out at temperatures between 1273 and 1923K over the strain rate range from 2.7 × 10⁻⁵ to 2 × 10⁻³ s⁻¹. It was found that the material exhibited extensive plasticity at temperatures higher than 1473K; a maximum tensile elongation of over 800% was recorded. Microstructural examination did not indicate the presence of a glassy phase at grain boundaries. Yttrium, however, was found to segregate to the grain boundaries. The microstructure of the Y-TZP was thermally unstable and appreciable grain growth was observed at emperattures higher than 1723 K; the grain growth was enhanced by external stresses, i.e. dynamic grain growth was observed. Grain growth at elevated temperatures resulted in apparent strain rate sensitivity exponents of approximately 0.33 at 1723K. This value decreased with increasing temperature. The grain size-compensated strain rate, however, was found to depend approximately on the square of the flow stress, i.e. to exhibit a true strain sensitivity value of 0.5, which suggests a grain boundary sliding mechanism. Microstructures from samples that were deformed superelastically indicated that grains remained equiaxed; this observation is consistent with a grain boundary sliding mechanism. The activation energy for superplasticity, under the conditions of constant structure, in Y-TZP was calculated to be 720 kJ/mol.     

The mechanical properties of the superplastic AI- 33 Pct Cu eutectic alloy

Experiments were conducted to determine the mechanical properties of the superplastic Al-33 Pct Cu eutectic alloy at temperatures from 673 to 723 K. Specimens were tested in a well-annealed condition and there was no evidence for grain growth even at the lowest experimental strain rate of 6.7 × 1(10^-7 s^-1. It is shown that the stress-strain curves rapidly attain a steady-state value at strain rates below ′10^-4 s^-1, and there is a sigmoidal relationship between stress and strain rate which may be obtained using several different testing procedures. The maximum elongation to failure recorded in these experiments was 1475 Pct at an initial strain rate of 1.3 × 10^-5 s^-1. The true activation energy for plastic flow is 175 ±11 kJ mol^-1 in the superplastic region II, but it increases to 299 ± 18 kJ mol^-1 at low strain rates in region I. The exponent of the inverse grain size is 2.1 ±0.3 in region II. These results show that, when the grains size is stable, there is a genuine region I in the Al-33 Pct Cu alloy at initial strain rates below ∼10^-5 s^-1.     

Hot Ductility and Compression Deformation Behavior of TRIP980 at Elevated Temperatures

The hot ductility tests of a kind of 980 MPa class Fe-0.31C (wt pct) TRIP steel (TRIP980) with the addition of Ti/V/Nb were conducted on a Gleeble-3500 thermomechanical simulator in the temperatures ranging from 873 K to 1573 K (600 °C to 1300 °C) at a constant strain rate of 0.001 s^?1. It is found that the hot ductility trough ranges from 873 K to 1123 K (600 °C to 850 °C). The recommended straightening temperatures are from 1173 K to 1523 K (900 °C to 1250 °C). The isothermal hot compression deformation behavior was also studied by means of Gleeble-3500 in the temperatures ranging from 1173 K to 1373 K (900 °C to 1100 °C) at strain rates ranging from 0.01 s^?1 to 10 s^?1. The results show that the peak stress decreases with the increasing temperature and the decreasing strain rate. The deformation activation energy of the test steel is 436.7 kJ/mol. The hot deformation equation of the steel has been established, and the processing maps have been developed on the basis of experimental data and the principle of dynamic materials model (DMM). By analyzing the processing maps of strains of 0.5, 0.7, and 0.9, it is found that dynamic recrystallization occurs in the peak power dissipation efficiency domain, which is the optimal area of hot working. Finally, the factors influencing hot ductility and thermal activation energy of the test steel were investigated by means of microscopic analysis. It indicates that the additional microalloying elements play important roles both in the loss of hot ductility and in the enormous increase of deformation activation energy for the TRIP980 steel.     

Deformation mechanisms of a rapidly solidified Al-8.8Fe-3.7Ce alloy

The mechanisms of deformation of a rapidly solidified and compacted Al-8.8Fe-3.7Ce (wt pct) alloy were investigated in the stress range 20 to 115 MPa and temperature range 523 to 623 K. The stress dependence of the steady state strain rates indicated a transition from diffusional creep to power law creep, the transition stress decreasing with increasing temperature from 70 MPa (σ/G = 3.1 × 10^-3) at 523 K to 40 MPa (σ/G = 1.9 × 10^-3) at 623 K. The activation energy in the power law creep regime was close to that of bulk self-diffusion in aluminum, while the activation energy in the diffusional creep regime was close to that of grain boundary self-diffusion in Al. The creep strain rates in the power law creep regime were found to be predicted much better by the substructure-invariant creep law (Sherby, 1981) than by the semi-empirical Dorn equation for Al, with the inclusion of a “threshold” stress. In the Coble creep regime, it was found that the cell/subgrain boundaries are inefficient vacancy sources/sinks and that their contribution to Coble creep is totally suppressed in this alloy. The Coble creep rates could be explained by using the average diameter of the powder particles as the effective grain size in the Coble creep equation.     

Removal of SO2 with oxygen in the presence of Fe(III)

A laboratory study of the aqueous oxidation of SO₂ in the presence of Fe(III) and Fe(II) has been conducted. The SO₂ concentration was 3930 ppm (3.93 × 10⁻³ atm or 398 Pa) in a gas stream with nitrogen and oxygen. The oxygen pressure was varied from 0 to 0.203 atmosphere. The initial concentration of Fe(III) ranged from 10⁻³ to 5-10⁻³ molar while that of Fe(II) was 5 × 10⁻³ molar. The temperatures were 298, 309.2, and 317.5 K. The solution pH was 1.83. The oxidation of SO₂ is intensive and yields from 90 to 97 pct recovery of incoming SO₂ when 5 × 10⁻³ molar Fe(III) and an oxygen pressure above 0.057 atmosphere are applied at 298 K. The reaction mechanism has been explained by determining the rate constants of the oxidation reactions from a kinetic model. The rate constants show that SO₂ is mostly oxidized by oxygen through formation of ferric-sulfite complex and that regeneration of ferric ion is possible under a normal oxygen pressure. The activation energy of the oxidation has been determined and has been found to be 13.5 Kcal/mole.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6×10⁻⁵ s⁻¹ to 5.6×10⁻² s⁻¹ and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

Modeling Grain Size and Strain Rate in Linear Friction Welded Waspaloy

The high-temperature deformation behavior of the Ni-base superalloy, Waspaloy, using uniaxial isothermal compression testing was investigated at temperatures above the γ′ solvus, 1333 K, 1373 K, and 1413 K (1060 °C, 1100 °C, and 1140 °C) for constant true strain rates of 0.001, 0.01, 0.1, and 1 s^?1 and up to a true strain of 0.83. Flow softening and microstructural investigation indicated that dynamic recrystallization took place during deformation. For the investigated conditions, the strain rate sensitivity factor and the activation energy of hot deformation were 0.199 and 462 kJ/mol, respectively. Constitutive equations relating the dynamic recrystallized grain size to the deformation temperature and strain rate were developed and used to predict the grain size and strain rate in linear friction-welded (LFWed) Waspaloy. The predictions were validated against experimental findings and data reported in the literature. It was found that the equations can reliably predict the grain size of LFWed Waspaloy. Furthermore, the estimated strain rate was in agreement with finite element modeling data reported in the literature.     

Evaluation of activation energy for dynamic strain aging process in 316L(N)/316(N) SS weld joints

Type 316 L(N) Stainless Steel (SS) is being currently used as a structural material for various components of Prototype Fast Breeder Reactor (PFBR). The possibility of using 316 L(N) electrodes for fabrication of 316 L(N) welding joints is being critically examined. This paper discusses about the evaluation of activation energy for Dynamic Strain Aging (DSA) process in 316L(N)/316(N) SS Weld Joints. The Gas Tungsten Arc Welding (GTAW) process was used for the root pass and Gas Metal Arc Welding (GMAW) process was used for the remaining passes. Tensile tests have been conducted in the wide temperature range from room temperature to 1023 K at a strain rate of 3 × 10⁻³ s⁻¹. Yield stress showed a continuous decrease with increasing temperature, with a plateau being observed between 823 and 923 K. A minima in elongation was also observed in this temperature range. These two properties being manifestations of dynamic strain aging, further tests at different strain rates (3 × 10⁻⁵ s⁻¹ to 3 × 10⁻² s⁻¹) were conducted in this temperature range. Detailed analyses of the results were carried out and the solute responsible for dynamic strain aging was identified to be substitutional chromium. Post test analysis of fracture surfaces and deformation substructures were correlated with the changes in tensile properties at different testing temperatures.     

Role of Plastic Deformation on Elevated Temperature Tribological Behavior of an Al-Mg Alloy (AA5083): A Friction Mapping Approach

Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10⁻³ s⁻¹ and 4 × 10⁻² s⁻¹. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at 693 K (420 °C) and 1 × 10⁻² s⁻¹ to 2.1 at 818 K (545 °C) and 4 × 10⁻² s⁻¹) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage was also examined.     

Onset of recrystallization during the tensile deformation of austenitic iron at intermediate strain rates

The onset of recrystallization during the tensile deformation of austenitic iron has been fully documented for the temperature range 950 to 1350°C and strain-rate range 2.8 x 10^-5 to 2.3 x 10^-2 s^-1. Representative materials are zone-refined iron, electrolytic iron, Fe-0.05 C and Fe-5.2 Mn. In general, the strain at the onset of recrystallization decreases with increasing temperature of deformation and decreasing strain rate. The postponement of recrystallization is favored by prior annealing at temperatures above 1200°C and is greatest for the Fe-5.2 Mn alloy; however, for the range of strain rates used, it is difficult to completely eliminate recrystallization. The effects of test conditions on the onset of recrystallization are discussed in terms of a nucleation process that requires a critical amount of stored energy.