Effect of Temper Condition on Stress Relaxation Behavior of an Aluminum Copper Lithium Alloy

Deformation behavior of an Al-Cu-Li alloy in different temper conditions (solutionized and T8) is investigated using stress relaxation tests. Fundamental parameters such as the apparent and physical activation volume, strain rate sensitivity, effective stress, and exhaustion rate of mobile dislocation density are determined from single and multiple relaxation tests. It was found that dislocation–dislocation interaction controls the kinetics of plastic deformation in the solutionized sample, whereas dislocation–precipitate interaction is the overriding factor in the presence of T₁ precipitates. The apparent activation volume was found to be significantly lower in the presence of T₁ precipitates compared with solutionized samples. Strain rate sensitivity and effective stress were found to be higher in the presence of T₁ precipitates. In addition, multiple relaxation tests showed that irrespective of microstructural features (solutes, semi-coherent precipitates), the mobile dislocation density reduces during the relaxation period. Further evidence regarding reduction in mobile dislocation density is obtained from uniaxial tensile tests carried out after stress relaxation tests, where both solutionized and T8 samples show an increase in strength. Additional discussion on relaxation strain is included to provide a complete overview regarding the time-dependent deformation behavior of the Al-Cu-Li alloy in different temper conditions.     

Strain hardening during low temperature creep of 304 stainless steel

A study has been done of the strain hardening of 304 stainless steel during low temperature creep. Hardening is measured by additional loading (above the creep stress) after various creep times. Both the level and the shape of the strain versus stress curve are altered by creep; higher creep strain is linked to a higher offset yield stress and to a longer inelastic transient during the early stages of deformation during reloading. A theory is described in which the mobile dislocation density is determined by a competition between stress rate dependent injection and velocity dependent trapping. This theory predicts both the creep curve and the hardening effects of creep with good quantitative accuracy.     

Strain aging and load relaxation behavior of type 316 stainless steel at room temperature

The strain aging and load relaxation behavior of type 316 stainless steel (SS) at room temperature were studied. It is shown that rapid aging occurs in 316 SS at room temperature to an extent that affects the load relaxation behavior of the material. Qualitatively, the aging behavior was found to agree with those reported earlier for Fe-Ni-C-alloys, and the observed aging characteristics could be explained by using an earlier proposed vacancy-interstitial mechanism. The load relaxation behavior is analyzed in terms of Hart’s state variable model. Effects of strain aging and strain hardening on the load relaxation behavior and the scaling of the relaxation curves are determined. It is shown that aging can be accounted for by a time-dependent change in a model parameter, which is dependent on the mobile dislocation density and the dislocation mobility. In addition, a dependency on plastic state of the same parameter previously held constant was found. It is concluded that this phenomenon, which in 316 SS could be rationalized in terms of increasing forest dislocation density, is likely to be more general, and a provision for it should be made in the state variable theory. S. P. Hannula formerly Research Associate in the Department of Materials Science and Engineering, Cornell University, Ithaca, NY M. A. Korhonen, formerly Visiting Assistant Professor in the Department of Materials Science and Engineering, Cornell University, Ithaca, NY     

The Contribution of Dislocation Density and Velocity to the Strain Rate and Size Effect Using Transient Indentation Methods and Activation Volume Analysis

Constant load indentation creep and load relaxation tests were performed on several FCC Al, Ag, and Ni metals that exhibit indentation size effect (ISE) to examine the coupled relationship between the activation volume V* at specific loads, the dislocation density ρ, and the dislocation velocity (v) from kinetics-based perspective. The influence of the ISE on the dislocation velocity and the activation volume is thoroughly examined using the two independent indentation creep and load relaxation experiments. This study is carried out based on the general experimental and theoretical hypothesis that the ISE is driven by a dislocation mechanism, specifically the increase in the geometrically necessary dislocation density at shallow depth of indentation due to the presence of a large strain gradient. Geometrically necessary dislocations dominate the material’s propensity to work harden when their density exceeds the density of statistically stored dislocations and are primarily considered responsible for the size effects observed in indentation. Based on the preestablished bilinear behavior and the decrease in the activation volume with hardening due to dislocation–dislocation interaction in indentation creep experiments by Elmustafa and Stone, 2003, we demonstrate that the dislocation velocity exhibits a bilinear behavior when plotted vs hardness using the Orowan’s relation. Ag and Ni distinctively depict a bilinear behavior in the dislocation velocity with hardness, whereas Al exhibited a rather linear behavior. This can be explained by the fact that aluminum’s work-hardening rate is higher due to the increase in the rate and intensity of cross-slip and dislocation climbing.

     

Operation of near-surface dislocation sources

Numerous experimental observations have suggested that there is a higher dislocation density,i.e., a hard layer, in the near-surface region of a plastically deformed sample. In this study, we considered a dislocation model which could possibly account for this hard layer. A computer simulation investigation of a dynamic near-surface dislocation source operation resulted in the formation of a hard layer, 10 to 500 μm in thickness. This hard layer disappeared with time, at room temperature, when the stress was removed,i.e., relaxation. The relaxation time for Cu is approximately two weeks; for Fe-Si and Mo the relaxation time is 2.5 and 10 years, respectively.     

Analysis of Strain Rate Sensitivity of Ultrafine-Grained AA1050 by Stress Relaxation Test

Commercially pure aluminum sheets, AA 1050, are processed by accumulative roll bonding (ARB) up to eight cycles to achieve ultrafine-grained (UFG) aluminum as primary material for mechanical testing. Optical microscopy and electron backscattering diffraction analysis are used for microstructural analysis of the processed sheets. Strain rate sensitivity (m-value) of the specimens is measured over a wide range of strain rates by stress relaxation test under plane strain compression. It is shown that the flow stress activation volume is reduced by decrease of the grain size. This reduction which follows a linear relation for UFG specimens, is thought to enhance the required effective (or thermal) component of flow stress. This results in increase of the m-value with the number of ARB cycles. Strain rate sensitivity is also obtained as a monotonic function of strain rate. The results show that this parameter increases monotonically by decrease of the strain rate, in particular for specimens processed by more ARB cycles. This increase is mainly linked to enhanced grain boundary sliding as a competing mechanism of deformation acting besides the common dislocation glide at low strain rate deformation of UFGed aluminum. Recovery of the internal (athermal) component of flow stress during the relaxation of these specimens seems also to cause further increase of the m-value by decrease of the strain rate.     

On the deformation of the 〈100〉 orientated γ′ strengthening phase of the CMSX2 superalloy

Both conventional macroscopic deformation tests and in situ straining experiments were devoted to the plastic behaviour study of the 〈100〉 orientated γ′ strengthening phase of the CMSX2 superalloy. Tensile test experiments, performed at various temperatures, reproduce the anomalous variation of the flow stress. Specimen unloading/reloading experiments show that the flow stress anomaly does not ensue from the exhaustion of the mobile superdislocation population. Stress relaxation experiments suggest that at both low and high temperatures, the superdislocation motion is controlled by a thermally activated mechanism, while in the temperature range around 300°C, the laws of thermal activation fail and instabilities occur on both the stress/strain and stress relaxation curves. In situ straining experiments, performed at 25°C and 320°C, lead to a microscopic scale approach of the deformation. At room temperature, a locking/unlocking mechanism controls the superdislocation motion. At 320°C, in addition to the locking/unlocking mechanism, local pinning points (either of extrinsic or intrinsic nature) affect the superdislocation motion. The in situ experiments permit us to propose various superdislocation core configurations so as to account for the experimental facts described above.     

The relationship between discontinuous yielding and cyclic behavior in polycrystalline molybdenum

The fatigue properties of polycrystalline molybdenum at room temperature have been determined by axial tension-compression tests in both stress and strain control modes to give lives of up to 10⁶ cycles. Under the appropriate conditions, the endurance limit can be considerably reduced,e.g., the stress to cause failure in 10⁶ cycles decreases from ～42,000 psi at 5 cps to ～28,000 psi at 0.5 cps. The fatigue resistance as assessed by either strain control or by stress control is discussed in relation to the tensile and creep behavior, and it is concluded that the reduction in the long life fatigue resistance with decrease in the cyclic frequency is due to the time dependent generation of a mobile dislocation population. The previously accepted value of 0.6 for the ratioFL/UTS for bcc metals can be attributed to this frequency effect. A lower value for this ratio, 0.4, can be derived from the high frequency data by the use of an extrapolation technique. This lower value is confirmed by long life, low frequency tests, in agreement with the value for fcc metals. The importance of obtaining fatigue data under comparable states of mobile dislocation density is emphasized, particularly for materials exhibiting pronounced tensile yield points, in order that meaningful comparisons can be made between different materials.     

Theory of mobile dislocation density: Application to the deformation of 304 stainless steel

Tensile and creep deformation of 304 stainless steel have been studied at room temperature in a soft tensile machine. The strain-stress curve shows a long inelastic transient, over nearly two hundred megapascals, in which the slope and the strain rate increase by about a factor of 3. Creep is characterized by large creep strain, transient strain rate, and a weak stress dependence. The stress rate applied during loading, on the other hand, has a strong effect on the subsequent creep. These results, in combination with earlier studies, have suggested a new model for the evolution of mobile dislocation density. Mobile dislocations are injected into the sample as a consequence of the increase of stress; the dislocations move under the influence of an effective stress over a statistical mean free path and then are trapped in a network. The effects of trapping are reduced mobile density, strain hardening, and a decrease in the effective stress. Application of the model provides a quantitative prediction of the principal experimental results.     

Strain distribution in copper tensile specimens prestrained in rolling

Sequences of orthogonal rolling-tension experiments were performed on polycrystalline copper sheets. The effect of strain path change on subsequent yield and flow behavior has been investigated. Optical microscopy and transmission electron microscopy (TEM) were used to clarify the physical mechanisms occurring during the second deformation. The observed increase in yield stress in reloading was related to the change of slip systems corresponding to the glide of dislocations with a Burgers vector, which had not been active during prestrain. The transient observed in the workhardening behavior after the path change corresponds to the appearance of disorganization in the dislocation microstructure. It was shown that no special feature of slip behavior inside the grains can be related to the nonhomogeneous surface deformation observed at the beginning of reloading. Also, the plastic instability of prestrained samples corresponding to the maximum load in tension does not seem to be directly controlled by the developed local substructure. The nonuniform deformation observed in reloading was studied using a simplified macroscopic two-zone model. It takes into account the presence of geometrical defects in the samples and considers the importance of the mechanical behavior. The macroscopic results, concerning the delay of starting deformation in some regions, are explained by the model, which allows formulation of an analytical condition necessary for deformation to spread through the length of the sample before necking takes place.     

Dislocations in continuous filament reinforced W/NiAl and Al2O3/NiAl composites

Continuous filament reinforced W/NiAl and Al₂O₃/NiAl composites (as-processed, annealed, and thermally cycled) have much higher dislocation densities than that of monolithic NiAl. These higher dislocation densities resulted from the relaxation of thermal residual stress, which developed during the cooling of the sample from elevated temperatures and was caused by the difference in the coefficients of thermal expansion between the matrix and the reinforcement. The dislocation density in the region adjacent to the matrix-filament interface was high and decreased only slightly with distance from the interface in the 30 vol pct composites. The as-processed and annealed composites exhibited a rather homogeneous dislocation density in the matrix. After thermal cycling, these composites showed no large difference in the dislocation density and morphology. However, there were local regions of lower dislocation densities. This difference was examined in relationship to filament fracture, surface matrix cracking, and degree of bonding.     

The microstructural evolution during the equal channel angular pressing process and its relationship with the tensile behavior of oxygen-free copper

In the present study, the tensile behavior of nanograin-sized, pure copper produced by the equal channel angular pressing (ECAP) process, which is at present one of the most popular methods for producing nanograined bulk material, was examined as related to the microstructural evolution. It was found that the yield and tensile strength values of 99.99 pct pure, oxygen-free copper increased with the increasing number of ECAP cycles due to the strain hardening in the initial stage. Further ECAP process promoted the formation of equiaxed grains accompanied with the gradual decrease in dislocation density. Once the equiaxed grain formed, the decrease in dislocation density would be further accelerated due to the extremely high rate of dynamic recovery with the grain boundary area acting as a dislocation sink. The strain hardening mechanism would then stop to operate and the fine grain boundary hardening mechanism would begin to dominate after the forth cycle of the ECAP process, resulting in an increase in the tensile ductility without sacrificing the strength. This research was supported by Grant No. 03K1501-00310 from the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Programs of the Ministry of Science and Technology, Korea.     

热循环温度场作用下1420A1-Li合金焊缝组织和拉伸性能的演变规律

利用外约束型模拟空间热循环温度场试验设备对1420Al-Li合金焊缝进行了热循环(77～393 K)试验,测量了热循环前后焊缝的拉伸性能,并观察了热循环前后焊缝的显微组织,讨论了热循环对焊缝组织和拉伸性能的影响.试验结果表明,经1000次或3000次热循环后,焊缝的强度和延伸率显著降低.热应力使焊缝显微组织产生损伤,主要表现为从晶界处向晶内发射位错,在晶界处形成位错塞积群,晶粒内位错密度逐渐升高.随着热循环次数的增加,组织损伤的不断累积导致在晶界处产生的应力集中程度增大,这是导致合金焊缝强度和延伸率下降的主要原因.     

热循环温度场作用下1420Al-Li合金焊缝组织和拉伸性能能的演变规律

利用外约束型模拟空间热循环温度场试验设备对1420Al-Li合金焊缝进行了热循环（77～393K）试验，测量了热循环前后焊缝的拉伸性能，并观察了热循环前后焊缝的显微组织，讨论了热循环对焊缝组织和拉伸性能的影响。试验结果表明，经1000次或3000次热循环后，焊缝的强度和延伸率显著降低。热应力使焊缝显微组织产生损伤，主要表现为从晶界处向晶内发射位错，在晶界处形成位错塞积群，晶粒内位错密度逐渐升高。随着热循环次数的增加，组织损伤的不断累积导致在晶界处产生的应力集中程度增大，这是导致合金焊缝强度和延伸率下降的主要原因。     

The effects of H on the mechanical behavior of austenitic stainless steels at room temperature

A stable face-centered cubic (fcc) stainless steel was H charged by exposing it to 9.8 MPa H gas at 300 °C for 14 days. The concentration of H was determined to be 65 ppm. The mechanical testing at room temperature has shown that after H charging, the yield strength of the steel increases, and the behavior of the transient creep and the short-term stress relaxation becomes more intensive. All the results can be interpreted qualitatively, and part of the results can be quantitatively calculated by the theory of mobile dislocation density proposed by Alden. The analysis indicates that the essential effect of H is to obstruct the movement of dislocations, so that the strength and inelastic viscousness of the steel increase.     

Cyclic evolution of the defect structure in a deformation macrolocalization zone in an HCP Zr-Nb alloy

The microstructure of a Zr-1% Nb is studied in a deformation macrolocalization zone during its transformation into a neck. The related dislocation transformations are found to be cyclic, and this cyclicity is accompanied by oscillatory changes in the volumes occupied by different dislocation substructures, the scalar dislocation density, the subboundary density, and periodic relaxation of internal stresses as a result of the decomposition of low-angle subboundaries and dislocation redistribution.     

Effect of Solute Clusters on Stress Relaxation Behavior in Cu-Ni-P Alloys

In this study, the ultrafine structures in Cu-P and Cu-Ni-P alloys have been characterized using a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM), and the stress relaxation behavior of these alloys has been explored. The results show that low-temperature annealing greatly improved the stress relaxation performance, especially in the Cu-Ni-P alloys. The presence of Ni-P clusters in the Cu-Ni-P alloys has been revealed. The overall improvement in properties has been analyzed in terms of variations in the dislocation density and solute atom cluster density within these materials. It is shown that clusters with small average spacing give rise to significant improvements in the stress relaxation performance, without requiring significant change in the dislocation density.     

Cyclic stress strain relations and strain-controlled fatigue of 4140 steel

The strain controlled low cycle fatigue properties and cyclic stress-strain response of a 4140 steel were investigated as functions of tempering temperature. While as-quenched 4140 steel specimens showed cyclic hardening, tempering at 200°C or higher resulted in cyclic softening which was a maximum for 400°C tempering. The hardening in the quenched steel was attributed to dynamic strain aging. Cyclic softening for the 400 and 550°C tempers resulted in part from rearrangement of the dislocation substructure and reduction of dislocation density as shown by transmission electron microscope. Mechanical removal of the yield point also contributed to cyclic softening and was the dominant mode for specimens tempered at 650°C. Removal of the yield point was attributed to an increase in the number of mobile dislocations.On tempering at 650°C, a failure of the Coffin-Manson relation was observed. Agreement was obtained when the plastic strain amplitude was plotted vs cycles to crack initiation rather than cycles to failure. For tempering at 350°C and higher, it is suggested that the endurance limit is essentially equal to the cyclic yield stress.