High-Temperature Deformation of Naturally Aged 7010 Aluminum Alloy

This study is focused on the deformation mechanism and behavior of naturally aged 7010 aluminum alloy at elevated temperatures. The specimens were naturally aged for 60 days to reach a saturated hardness state. High-temperature tensile tests for the naturally aged sample were conducted at different temperatures of 573, 623, 673, and 723 K at various strain rates ranging from 5 × 10−5 to 10−2 s−1. The dependency of stress on the strain rate showed a stress exponent, n, of ~6.5 for the low two temperatures and ~4.5 for the high two temperatures. The apparent activation energies of 290 and 165 kJ/mol are observed at the low, and high-temperature range, respectively. These values of activation energies are greater than those of solute/solvent self-diffusion. The stress exponents, n, and activation energy observed are rather high and this indicates the presence of threshold stress. This behavior occurred as a result of the dislocation interaction with the second phase particles that are existed in the alloy at the testing temperatures. The threshold stress decreases in an exponential manner as temperature increases. The true activation energy was computed by incorporating the threshold stress in the power-law relation between the stress and the strain. The magnitude of the true activation energy, Qt dropped to 234 and 102 kJ/mol at the low and high-temperature range, respectively. These values are close to that of diffusion of Zinc in Aluminum and diffusion of Magnesium in Aluminum, respectively. The Zener–Hollomon parameter for the alloy was developed as a function of effective stress. The data in each region (low and high-temperature region) coalescence in a segment line in each region.

Kinetic Analysis of High-Temperature Plastic Flow in 2.25Cr-1Mo-0.25V Steel

High-temperature plastic flow of heat-resistant 2.25Cr-1Mo-0.25V steel was investigated by hot tension (at 500–650 °C) on a Gleeble 3800 machine. The strain rate of hot tension was set as 0.001–1 s−1. The constitutive relation of the steel was modeled by the introduction of the parameters termed “true activation energy” and “threshold stress”. Then, the kinetics of high-temperature plastic flow was analyzed based on an Arrhenius equation modified by a “threshold stress”. The stress exponent of the modified equation was equal to 5. True activation energy was estimated to be 132 kJ·mol−1. According to the slip band model, the basic mechanism behind the hot deformation of the steel was considered to be dislocation climbing, which was governed by grain boundary diffusion. This model proved to be successful in its analysis of the experimental results of hot tension tests.

Investigation on Creep Mechanisms of Alloy 709

To improve efficiency, safety, and reliability of nuclear reactors, structural materials for Gen-IV reactors are being designed and developed. Alloy 709, a 20Cr-25Ni austenitic stainless steel, has superior mechanical properties to be a preferred candidate material for Sodium Fast Reactor structural application. Creep tensile tests were performed at temperatures of 700 °C, 725 °C and 750 °C and range of stresses from 100 MPa to 250 MPa. The apparent stress exponent and activation energy were found to be 10.3±0.4 and 368.6±14.7 kJ/mol. Linear extrapolation method was used to rationalize the higher stress exponent and activation energy relative to the mechanism in power law creep yielding to a true stress exponent of 7.1 ± 0.3 and a true activation energy of 277 ± 12.8 kJ/mol which is close to the lattice diffusion of iron in Fe-20Cr-25Ni. Hence, the lattice diffusion controlled dislocation climb process is believed to be the rate controlling creep deformation mechanism in this range of stresses and temperatures. The appropriate constitutive equation was developed based on the results; however, microstructural evaluations are under investigation to confirm the rate controlling mechanism. In addition, creep tests at higher temperatures and lower stresses are being conducted to extend the stress and strain-rate ranges to observe possible transition in creep mechanism.

Transition from Superplastic Behavior - Viscous Glide - Dislocation Climb - Power-Law Break down Regimes in Friction Stir Processed AA5083

5083 Al alloy was friction stir processed (FSP) at room temperature under various experimental conditions. Two rotational speeds of 430 and 850 rpm with a single traverse feed of 90 mm/min (430-90, 850-90) were used, to investigate the effect of rotation speed. Also, another feed rate of 140 mm/min was used with a rotational speed of 430 rpm (430-140), to investigate the effect of feed rate. The processing conditions resulted in three different grain sizes of 0.95, 1.6 and 2.6 μm depending on the FSP parameters. The deformation behavior of the FSP samples was investigated at 250 C at three strain rates of 10-4, 10-3 and 10-2 s-1. The values of strain rate sensitivity, m was determined, as a function of grain size, and it decreased from 0.45 to 0.33 to 0.18 with increasing the grain size. True activation energy was calculated as 63, 95, 157 kJ/mole for the grain sizes of 0.95, 1.6 and 2.6 μm, respectively. These calculated values are comparable to grain boundary sliding of magnesium in aluminum (69 – 78 kJ mol-1), magnesium in aluminum (~115 kJ mole-1) and aluminum lattice diffusion (~143 kJ mol-1 ). The change in the deformation mechanism with grain size was discussed in some details.

Activation Energy for Hot Deformation of 15-5 PH Stainless Steel

±Abstract. By applying the new quantum mechanics and relativistic mathematical model, proposed by Muñoz-Andrade, on the experimental results reported previously by Aghaie-khafri and Adhami [5], the true activation energy for hot deformation of 15-5 PH stainless steel is obtained over the temperature range of 900-1150°C and strain rates varying between 0.001 and 0.5s-1. It is interesting to contrast the results of this theoretical work with the main results of the apparent activation energy obtained for the same data, but applying the common methodology. It is shown that the true activation energy increased as the hot deformation is increased. Moreover, the true activation energy decreased as the strain rate is increased. The mean value of the true activation energy (289 kJ/mol) at high strain rate, ξ=0.5s-1, for dynamic recrystallization over the temperature range of 900-1150°C is in a closed agreement with the value of activation energy for self-diffusion in γ iron (280 kJ/mol) in dissimilarity of the result of the apparent activation energy (49221 kJ/mol) obtained beforehand by Aghaie-khafri and Adhami [5]. The results obtained in this work by the quantum mechanics and relativistic mathematical model are widely satisfactory; because essentially they are over the crucial limitations of the common methodology to obtain the activation energy at each thermo-mechanical metalworking condition. Keywords: Activation Energy, Hot Deformation, Dynamic Recrystallization, Quantum Mechanics, Special Relativity Theory.

Static and cyclic creep behavior of in situTiB2 particulate reinforced aluminum composite

Static and cyclic creep tests of Al–15 vol% TiB2in situ composite were carried out at 573–623 K. The values of apparent stress exponent and activation energy for cyclic creep of the composite were much higher than that for static creep. Furthermore, the cyclic creep rate tended to decrease with increasing percentage of unloading amount but was independent of the loading frequencies under the frequency ranges investigated. Finally, the true stress exponent of the composite was equal to 8, and the true activation energy was close to the value for the lattice self-diffusion of aluminum by incorporating a threshold stress for the analysis.