Mahmoud S. Soliman

Creep transitions in an Al-Zn alloy

The creep behavior of an Al-10 at. pct Zn alloy was investigated at temperatures in the range 573 to 800 K and in a normalized shear stress range, τ/G, extending from 10•6 to 10•3, where τ is the applied shear stress andG is the shear modulus. The results reveal the presence of four deformation regions: region I (very low stresses), region II (low stresses), region III (intermediate stresses), and region IV (high stresses). In regions II and III, the creep characteristics including the stress exponent, the shape of the creep curve, and the nature of creep transients after stress reductions are consistent with dislocation climb and viscous glide, respectively. The experimental transition stresses between region II (climb) and region III (glide) were compared with those predicted from the deformation criterion for solid-solution alloys, and it is demonstrated that the correlation between experiment and prediction becomes satisfactory when corrections concerning the appropriate diffusion coefficients for describing glide-controlled creep and climb-controlled creep, the stacking fault energy of the alloy, and the contributions of Suzuki interaction and Fisher interaction to solute atom drag forces are incorporated in the analysis. Experimental data in region IV are not sufficient to unambiguously identify the rate process, but calculations show that the values of the transition stress between region III and region IV agree reasonably well with those of the critical stress for dislocation breakaway from a solute-atom atmosphere. The characteristics of deformation in region I are still under investigation.

An investigation of the stress exponent and subgrain size in Al after stress reduction

Abstract Stress-change and uninterrupted tests were performed on Al over the high-temperature range 0·65–0·99T m, where T m is the melting point, and at stresses varying from 7 × 10−6 G to −2 × 10−4 G, where G is the shear modulus. In addition, dislocation substructure that developed during creep was examined by means of the etch-pit technique and transmission electron microscopy. By comparing the mechanical and substructural data of stress-reduction tests with those obtained in uninterrupted tests, two observations are made. First, the stress exponent inferred from stress-reduction tests is essentially equal to that obtained from uninter rupted tests. Second, as a result of stress reduction, the subgrain size coarsens and reaches the new steady-state value whicli would be attained in an uninterrupted test at the reduced stress. Evidence of subgrain coarsening is demonstrated not only by changes in size, but also by relevant substructural phenomena, such as subgrain-boundary dissolution, which are noted d...

Correlation between creep behavior and substructure in an Al-3at.%Mg solid solution alloy

Abstract The dislocation substructure formed during the creep of an Al-3at.%Mg alloy under steady state conditions was investigated. The stress dependence of the substructure was examined by an etch pit technique and transmission electron microscopy over a normalized stress range extending from τ /G = 10 −5 to τ /G = 10 −3 (where τ is the applied shear stress and G is the shear modulus). On the basis of this examination, the substructural behavior of the alloy is divided into three distinct regions, depending on the value of the normalized stress. In region I ( τ /G > 4 × 10 −4 ), the substructure consists of an essentially random distribution of long curved dislocations with no significant tendency to form subboundaries in the interiors of grains. In region III ( τ /G −5 ), well-developed subgrains are formed and dislocations sometimes form irregularly spaced networks inside subgrains. In region II (2 × 10 −5 τ /G −4 ), which represents a transition region between region I and region III, the substructure consists of a large number of long curved dislocations and subboundaries in the process of development. It is demonstrated that the present substructural observations together with the mechanical evidence are in conformity with the prediction that the creep behavior of a solid solution alloy would change from control by viscous glide to control by dislocation climb with decreasing stress.

Effect of Equal-Channel Angular Pressing Process on Properties of 1050 Al Alloy

Annealed 1050 Al samples with coarse-grained microstructure of 600 µm were equal-channel angular pressing (ECAP) processed using two routes, A and BC. The samples were processed up to four passes through a die with an internal angle of 90o using both routes. Electron back-scattered diffraction (EBSD) technique was used to study the developed microstructure after ECAP processing. The cell size distribution, misorientation, and the fraction of high angle boundaries were determined. The microstructure study was conducted on both the extrusion direction and the shear plane. The produced microstructure depends on the used route and number of passes. A study of mechanical behavior was conducted by using tensile and compression specimens from the specimens produced by ECAP in the extrusion direction. Enhanced strength was observed but with anisotropic behavior between tension and compression. The dimple size and distribution on fractured surface of tensile specimens was affected by the ECAP route and number of passes.

The high-temperature creep behaviour of an Al-1 wt % Cu solid-solution alloy

The creep behaviour of an Al-1 wt% Cu solid-solution alloy is investigated at a temperature of 813 K under stress range of 0.5–5 MPa. The creep characteristics of the alloy including the stress dependence of the steady-state creep rate (n=4.4), the shape of creep curve (normal primary stage), the transient creep after stress increase, and the value of the true activation energy for creep, suggest that some form of dislocation climb is the rate-controlling process at higher stresses above 1 MPa. However, at low stresses (< 1 MPa), the creep curves show no distinguished steady state, and the stress dependence of the minimum creep rate is as high as ∼ 8. The creep behaviour of the alloy is discussed based on recent theories available for describing creep in solid-solution alloys.

On the creep behavior of uranium dioxide

The creep data for stoichiometric polycrystalline uranium dioxide (UO2) obtained in several compression investigations are analyzed using the Dorn equation together with deformation mapping, and it is shown that the data are divisible into three regions of deformation, depending on the value of the normalized stress. At high normalized stresses (σ/G > 10−3 where σ is the applied stress and G is the shear modulus), the stress exponent is greater than 5 and the creep behavior seems to be consistent with the breakdown of the creep power law. At intermediate normalized stresses (5 × 10−4 < σ/G < 10−3), the stress exponent is about 4.5 and the creep behavior is in accord with that attributed to dislocation creep. At low normalized stresses (σ/G < 5 × 10−4), the stress exponent is about 1, but the creep characteristics are not entirely compatible with the Nabarro-Herring process as originally documented in the literature on creep.

Breakdown of the power-law creep in a Class I Al-10 at % Zn alloy

The creep behaviour of Al-10 at% Zn at 573 K is divisible into three deformation regions; low stress region, intermediate stress region and high stress region. The creep characteristics of the low stress region and intermediate stress region are consistent with dislocation climb and viscous glide, respectively. In the high stress region, the stress exponent,n increases with stress, the activation energy is higher than those observed in the other two regions, the activation area is slightly decreasing with stress and the internal stress is almost negligible. Present analysis shows that these characteristics are consistent with the thermally-activated glide motion of dislocations as a rate controlling mechanism at high stresses.[/p]

Effect of Cu concentration on the high-temperature creep behavior of AlCu solid solution alloys

Abstract The creep behavior of AlCu alloys containing 2 and 4 wt.% Cu was investigated at a temperature of 813 K and over a stress range of 0.5–10 MPa. The present results, along with previous data on AlCu alloy, suggest the presence of three regions of deformations: region I at low stresses, region II at intermediate stresses and region III at high stresses. In region I, the creep curves showed the characteristics of dynamic recrystallization and the stress dependence of minimum creep rate n is approx. 6.5. The concentration dependence of creep rate in this region is a reflection of a decrease in stacking fault energy with solute concentration. In region II, the creep characteristics including the stress exponent, n (∼ 3) and the shape of the creep curve (sigmodial primary creep) are consistent with viscous glide control. The transition stress between regions I and II is inversely proportional the square root of Cu concentration, in agreement with the prediction of the climb-viscous glide criterion for solid solutions. In region III the creep characteristics are not entirely consistent with any of the existing deformation mechanisms. The transition from region II to region III is due to the dislocations breakaway from their solute atmospheres.

Effect of Solution Heat Treatment on the Hot Workability of Al–Mg–Si Alloy

The current work presents a detailed study on the high temperature processing of solution treated Al–Mg–Si alloy in the temperature range of 623 K to 773 K and at different strain rates in the range of 5 × 10−5 to 6 × 10−2 s−1. A constitutive relation that can be used in modeling the forming process of this alloy under similar hot working conditions is established. Also, the prevailing deformation mechanism was investigated through relations of the steady state stress dependence on strain rate which revealed a stress exponent of 8.5 (strain rate sensitivity; m ∼ 0.12). This stress exponent is higher than what is usually observed in Al and Al–Mg alloys under similar experimental conditions. This high stress exponent may arise from the presence of threshold stress that results from dislocation interaction with second phase particles (Mg2Si), precipitating during the deformation at high temperatures. The values of threshold stress showed an exponential increase with decreasing temperature and a dependence with an energy term Qo = 38 kJmol−1. The apparent activation energy for solution treated condition was calculated to be about 320 kJmol−1, which is higher than the activation energy for self-diffusion in Al (Qd = 143 kJmol−1) and for the diffusion of Mg in Al (115–130 kJmol−1). By incorporating the threshold stress in the analysis, the true activation energy was calculated to have a value of 111 kJmol−1, and the normalized strain rates can be represented by a power function of the effective stress with stress exponent of ∼3. Ductility was documented to reveal the best working condition for this alloy in solution treated condition. The ductility exhibited a maximum value of about 120% at 773 K at a strain rate of 0.064 s−1. The results of the current work is, also, compared to the results of another heat treatment condition (T4-naturally aged) to reveal which ever condition holds better hot forming characteristics.

Effect of stress reduction ratio on the creep behaviour of Al

Abstract Stress-reduction and uninterrupted experiments have been conducted on Al in the temperature range 573–923 K; in performing stress-reduction experiments, stress-reduction ratios (τ1-τ2)/τ1, (where τ1 is the initial shear stress and τ2 is reduced shear stress) ranging from 0.05 to 0.75 were used. The stress exponent inferred from stress-reduction experiments, n∗, was compared with that obtained from uninterrupted tests, n. On the basis of this comparison three regions of behaviour, depending on temperature, were identified. At high temperatures (T≥673K), n∗ = n regardless of the value of the stress-reduction ratio. At intermediate temperatures (600K n for small reduction ratios (≤0.15); for small reductions, the lower the reduction ratio, the larger the discrepancy between n∗ and n. At low temperatures (T n for all stress-reduction ratios used (whether small or large). It is suggested that the discrepancy between n∗ and n at ...