Pre-deformation Level Research Articles

Effect of pre-deformation on microstructure and mechanical properties of 2050 Al−Li alloy

The microstructural evolution and mechanical properties of 2050 Al−Li alloy subjected to different rolling pre-strains (0−20%) and aging (T8) were studied. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the microstructure. The results show that the grains were significantly refined along the normal direction (ND) and the intragranular distortion increased with the increase of pre-strain level. The entanglement of defects, e.g., dislocations, provides a large number of nucleation sites for the precipitation of T1 phases. This leads to larger number of precipitates and narrower distance between precipitates, thus improving the hindering effects on dislocation motions and the strength of the alloy. When the pre-deformation level reaches 16%, the alloy shows the highest density of T1 phases and strength (the yield strength and tensile strength reach 568 and 584 MPa, respectively, and elongation is 8.6%). When the amount of pre-deformation is further increased to 20%, the atomic diffusion channels are reduced due to dislocation entanglement, and the reduction in the number of T1 phases eventually leads to a decrease in strength of the alloy.

Investigation of anisotropy evolution of an aluminium‑lithium alloy with increasing pre-deformation in creep age forming

The 3rd generation aluminium‑lithium (Al-Li) alloy demonstrates promising potential in various applications due to its advanced synergy of strength, toughness, corrosion resistance and weight reduction. However, like its predecessors, this alloy also suffers from the anisotropy issue which can exert significant impact on the manufacturing processes. This study seeks to investigate the anisotropy evolution of a T8 temper 3rd generation Al-Li alloy when subjecting to increased tensile pre-deformation at 0°, 45°, and 90° to the rolling direction. Uniaxial creep-ageing and tensile tests were conducted to reveal the anisotropic behaviours of the material during creep and tensile deformation. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) methods were used to investigate the microstructural evolutions and their influence on the material anisotropy evolvement with treatments of tensile pre-deformation and creep-ageing. It was found that with the increase of tensile pre-deformation level, the creep deformation increased at 0° while decreased at 45° and 90°, due to different configurations of the dislocation substructures at 0°, 45° and 90°. Furthermore, the in-plane anisotropy (IPA) was reduced in both the creep deformation and material strength with increasing tensile pre-deformation and subsequent creep-ageing. The enhanced creep deformation at 0° and decreased deformation at 45° and 90°, as well as the reduction of anisotropy, can be attributed to the reconfiguration of dislocation structures and different extents of precipitation promotion at different directions when subjected to tensile pre-deformation and creep-ageing.

An Investigation of Compressive Creep Aging Behavior of Al-Cu-Li Alloy Pre-Treated by Compressive Plastic Deformation and Artificial Aging

In this paper, the effects of compressive pre-deformation and successive pre-artificial aging on the compressive creep aging behavior and microstructure evolution of the Al-Cu-Li alloy have been studied. Severe hot deformation mainly occurs near the grain boundaries during the compressive creep initially, which steadily extends to the grain interior. After that, the T1 phases will obtain a low radius-thickness ratio. The secondary T1 phases in pre-deformed samples usually only nucleate on dislocation loops or Shockley incomplete dislocations induced by movable dislocations during creep, which are especially prevalent in low plastic pre-deformation. For all pre-deformed and pre-aged samples, two precipitation situations exist. When pre-deformation is low (3% and 6%), solute atoms (Cu and Li) can be consumed prematurely during pre-aging at 200 °C, with dispersed coherent Li-rich clusters in the matrix. Then, the pre-aged samples with low pre-deformation no longer have the ability to form secondary T1 phases in large quantities during subsequent creep. When dislocation entangles seriously to some extent, a large quantity of stacking faults, together with a "Suzuki atmosphere" containing Cu and Li, can provide the nucleation sites for the secondary T1 phase, even when pre-aged at 200 °C. The sample, pre-deformed by 9% and pre-aged at 200 °C, displays excellent dimensional stability during compressive creep because of the mutual reinforcement of entangled dislocations and pre-formed secondary T1 phases. In order to decrease the total creep strain, increasing the pre-deformation level is more effective than pre-aging.

Quantitative effects of pre-deformation prior to non-isothermal aging on the mechanical properties-microstructure relationships in an Al-3.51Cu-1.01Li alloy

The effects of increasing levels of pre-deformation on the mechanical properties and microstructure evolution (dominantly T1 phase) of an extensively used aerospace Al-Cu-Li alloy have been systematically investigated during the non-isothermal aging. Quantitative descriptions extracted from the microstructure parameters further revealed the separated strengthening contributions and mechanism. Pre-deformation prior to the NIA significantly accelerated the aging kinetics and improved the strength, and yet showed a limited benefit due to the generally decreased precipitation strengthening. Elevated pre-deformation in conjunction with the NIA process reduced the T1 diameter and interprecipitate spacing, the interprecipitate diffusion was gradually activated, responsible for the undulatory mean thickness. The experimental yield strength was successfully described by invoking the shearing mechanism in the pre-deformed samples (4–20 %). However, the strengthening transition from shearing to by-passing occurred in the 0 % sample, the by-passing model should be considered simultaneously owing to the larger diameter T1 precipitates. The critical diameter of the T1 precipitate for this transition ranged from 84 to 117 nm and was approximately 100 nm.

Superposed hardening from precipitates and dislocations enhances strength-ductility balance in Al-Cu alloy

The age hardening response and tensile properties in an Al-Cu alloy pre-deformed by 20-80% and post-aged at 120-165°C have been examined to study the effect of mixed dislocations/precipitates on the strength-ductility balance. As compared to the T8 temper, both yield strength and uniform elongation are significantly improved by a pre-deformation of 20-40%, followed by ageing at 120°C. As compared to the T6 condition, the yield strength is higher (by about 50%), while the uniform strain is similar. The dislocation self-organization evolves from forests to tangles and cell boundaries with the pre-deformation level increasing to 60-80% and keeps preserved after post-ageing. The heterogeneous formation of θ' phases at dislocations and the homogeneous matrix precipitation of θ"/GP zones constitute the hardening particles. The quantitative analysis suggests dislocations and θ' precipitates are the major strength contributors. Off-alignment between the deformed precipitate and the un-deformed one occurs for both θ' and θ" during tensile straining. The dislocations tend to accumulate around large θ', eventually leading to the θ' rotation by 2-5° and severe matrix distortion near the precipitate. The fine and relatively sparse θ' phases in the 20-40% pre-deformed alloy post-aged at 120°C firstly block and then transmit dislocations, as is evidenced by the shearing at multiple locations, resembling the θ" phases. The correlation between the precipitate-dislocation interaction and the obtained properties supports that localized dislocation pile-up at hard barriers, e.g. large θ' and cell boundaries, could initiate void and deteriorate the plasticity. Suppressing the dense formation of non-shearable precipitates delivers exceptional strength-ductility synergy in the pre-deformed and post-aged alloy.

Effect of cryogenic pre-deformation on the stress relaxation response and mechanical/corrosion properties in Al–Zn–Mg–Cu alloy

The stress relaxation ageing behavior and mechanical/corrosion properties of the alloys pre-treated by conventional T4 temper without pre-deformation and cryogenic pre-deformation with different levels (20%–80%) have been investigated and compared by hardness test, stress relaxation ageing test, tensile test, intergranular corrosion test and microstructure characterization. Pre-deformation by cryo-rolling could increase the steady-rate relaxation rate at 120 °C by 35%–360% and effectively prolong the duration of the variable-rate relaxation stage compared with the T4 treatment. The dislocation movement facilitated by the recovery is responsible for the significant increase in the stress relaxation rate. Increasing the pre-deformation level markedly improves the strength and localized corrosion resistance but lowers the elongation in the stress relaxation aged alloy. Cryogenic pre-deformation promotes the heterogeneous nucleation of relatively large η2-type phases at dislocations apart from the uniform formation of fine η2-type phases in the matrix. High density dislocations retain after stress relaxation ageing at the investigated temperature. The dislocation hardening could offset the softening due to the precipitate coarsening and leads to a higher strength in the alloy pre-deformed by 50–80%. The modified grain boundary precipitation consisting of narrower precipitation-free zones and discontinuous grain boundary precipitates contribute to the better corrosion resistance in the cryo-rolled alloy.

A model to unravel the beneficial contributions of trace Cu in wrought Al–Mg alloys

The softening and strengthening contributions in pre-deformed and aged Al–Mg–Cu alloys containing 3 wt.%Mg and 0.5 wt.%Cu are evaluated by a combination of microscopy, mechanical testing and modelling. A refined phenomenological model for the work hardening response, accounting for the separate effects of recovery and precipitation, is shown to be suitable for an unambiguous determination of the precipitation hardening contribution in these alloys. Significantly, it is found that the mechanical response of these alloys is not strongly impacted by Cu content (in the low Cu content regime), pre-deformation level or aging temperature meaning that the alloys are robust with respect to variations in composition. This is interesting from the perspective of alloy design concepts based on ‘recycling friendly’ compositions in applications that include paint-baking.

Dislocation-Induced Precipitation and Its Strengthening of Al–Cu–Li–Mg Alloys with High Mg

Generally, the good combination of pre-deformation and aging can improve the mechanical strength of the Al–Cu–Li–Mg alloys. However, the effects of pre-deformation on competitive precipitation relationship and precipitation strengthening have not been clarified in detail in Al–Cu–Li–Mg alloys with high Mg. In the present study, the effects of pre-deformation level on the microstructure and mechanical properties of an Al–2.95Cu–1.55Li–0.57Mg–0.18Zr alloy have been investigated. It is found that the introduction of dislocation by 5% pre-deformation can facilitate the precipitation of new successive composite precipitates and T1 precipitates along the sub-grain boundaries or dislocations and inhibit the precipitation of dispersive GPB zones which is the main precipitates of the alloys without pre-deformation. The introduction of 5% pre-deformation can enhance the mechanical properties considerably. When the pre-deformation level increases from 5 to 15%, the number density of the successive composite precipitates and T1 precipitates increases, and the aspect ratio of T1 precipitates decreases. The decrease in T1 precipitate aspect ratio and the increment of the successive composite precipitates result in the reduction in precipitation strengthening. Therefore, the increase in pre-deformation level from 5 to 15% does not further improve the mechanical properties of the alloys, although the dislocation strengthening increases continuously.

Optimization of the grain boundary character distribution of pure copper by low-strain thermomechanical processing

Abstract Although thermomechanical processing has been widely used in grain boundary engineering, the relationship between thermomechanical parameters and grain boundary character distribution is still not well understood. In the present study, electron backscatter diffraction was used to study the grain boundary character distribution in pure copper after lowstrain thermomechanical treatments. It was found that the low-R coincidence site lattice frequency and grain size decreases in the following sequence, 10% compression > 15% compression > 5% compression, except for 700 0C. During thermomechanical treatments of copper, strain-induced grain boundary migration and grain growth may occur during annealing of 5% compressed copper, and recrystallization dominates during annealing of 15% compressed copper, while the annealing mechanism of 10% compressed copper changes from strain-induced grain boundary migration and grain growth to recrystallization when the annealing temperature exceeds 600 0C. The results indicated that during single-step low-strain thermomechanical treatments, straininduced grain boundary migration and grain growth would gradually change to recrystallization with the increase of pre-deformation level and annealing temperature. Among the three mechanisms, strain-induced grain boundary migration seems to be more effective than recrystallization and grain growth in the optimization of grain boundary character distribution, and it is suggested that this is due to the high boundary migration rate of strain-induced grain boundary migration.

Optimization of the grain boundary character distribution of pure copper by low-strain thermomechanical processing

Abstract Although thermomechanical processing has been widely used in grain boundary engineering, the relationship between thermomechanical parameters and grain boundary character distribution is still not well understood. In the present study, electron backscatter diffraction was used to study the grain boundary character distribution in pure copper after lowstrain thermomechanical treatments. It was found that the low-R coincidence site lattice frequency and grain size decreases in the following sequence, 10% compression > 15% compression > 5% compression, except for 700 0C. During thermomechanical treatments of copper, strain-induced grain boundary migration and grain growth may occur during annealing of 5% compressed copper, and recrystallization dominates during annealing of 15% compressed copper, while the annealing mechanism of 10% compressed copper changes from strain-induced grain boundary migration and grain growth to recrystallization when the annealing temperature exceeds 600 0C. The results indicated that during single-step low-strain thermomechanical treatments, straininduced grain boundary migration and grain growth would gradually change to recrystallization with the increase of pre-deformation level and annealing temperature. Among the three mechanisms, strain-induced grain boundary migration seems to be more effective than recrystallization and grain growth in the optimization of grain boundary character distribution, and it is suggested that this is due to the high boundary migration rate of strain-induced grain boundary migration.

A phenomenological constitutive model for the viscoelastic deformation of elastomers

This study proposes a one-dimensional constitutive model for elastomeric materials based on recent observations regarding the separation of elastic and viscous contributions in uniaxial cyclic tensile experiments on EPDM rubber. The focus is on capturing the changes in constitutive behavior and energy dissipation associated with the Mullins effect. In the model, this is achieved through the evolution of both permanent set and hyperelastic parameters of an Edwards-Vilgis function to account for the Mullins effect, and with a viscosity associated with the effective stretch rate of the network to describe the non-linear flow stress. The simulations are able to reproduce the observed constitutive response and its change with increasing levels of pre-deformation. The model is less able to accurately reproduce the virgin loading response, which is achieved via extrapolation to zero pre-strain. However, for practical purposes, where scragging of elastomeric products is the norm, the model is able to predict the cyclic response and the dissipated energy, and their change with different scragging levels in good agreement with experimental data.

Effect of pre-compression on microstructural evolution, mechanical property and strengthening mechanism of AZ31 alloy

The effect of pre-compression deformation on the microstructural evolution, mechanical property and deformation behavior of AZ31 magnesium alloy was studied. The alloy was microstructurally characterized with a combination of optical microscopy, electron backscatter diffraction, X-ray diffraction and energy-dispersive spectrometer. The results indicate that average number of twins per grain and dislocation density increases with an increase in compressive pre-deformation level. Stress–strain curves of low pre-strained samples (0%, 1% and 3%) present concave-up features, whereas concave-down shapes are detected for high pre-strained samples (6% and 8%) in uniaxial compression tests. The corresponding underlying deformation mechanism transists from a twin-dominated mechanism to a slip-dominated mechanism. This deformation mechanism transition is related to {10–12} twinning, which leads to grain refinement and crystal orientation change, as well as increased dislocation density. The yield strength linearly increases with increasing equivalent grain size regardless of the deformation mechanisms. Quantitative analysis reveals that the contributions from texture strengthening, twinning/grain boundary strengthening and dislocation strengthening increase as pre-compression level increases. Texture strengthening is the dominant strengthening mechanism for high pre-strained samples.

Effect of Pre-deformation on the Multiaxial Low-Cycle Fatigue Resistance of 2024-T4 Aluminum Alloy

Abstract The effect of pre-deformation on the fatigue properties of 2024-T4 aluminum alloy is evaluated, including pre-tension of 1, 2, 3, and 4 % as well as pre-torsion of 10°, 20°, 30°, and 40°. Multiaxial low-cycle fatigue (LCF) tests are carried out under a constant amplitude sinusoidal wave loading with a tension-torsion ratio √3 in air at room temperature. The results indicate that the multiaxial LCF life of the specimen reduces with the pre-deformation level. The observed mechanical behavior and associated phenomena are directly linked to microstructure characteristics, such as dislocation density, surface defects, and microcracks. The analysis of mechanism reveals a competition between beneficial effects that are due to hardening and detrimental influences that are due to defects that are caused by the pre-deformation. However, the multiaxial loading makes the detrimental influence more prominent. The typical fracture morphology of multiaxial fatigue can be observed. With the consideration of the effect of pre-deformation on fatigue limit, a modified criterion is developed to predict the multiaxial fatigue life of 2024-T4 aluminum alloy.

Giant reversible stress-induced change of resistivity in Ni-Mn-In-Co alloys

Ni43Mn37.8In12.2Co7 and Ni43Mn37.65In12.35Co7 polycrystalline alloys were tested mechanically in uniaxial compression in order to determine the stress–strain response and accompanying changes of electrical resistivity. Compression of the specimen by 9% at room temperature resulted in a 250% increase of resistivity followed by almost full recovery to its predeformation level upon heating to 400 K. Microstructural observations revealed that giant reversible changes of electrical resistivity occurred due to stress-induced martensitic transformation and shape recovery of plastically deformed grains induced by heating.

Evolution of Grain Boundary Character Distribution in Pure Copper during Low-Strain Thermomechanical Processing

In order to get optimal grain boundary character distribution (GBCD) and grain boundary properties, thermomechanical processing (TMP) is usually adopted in grain boundary engineering. However, the mechanism behind the TMP treatments and GBCD optimization is still unclear. The present study has conducted a series experiments involving low-strain TMPs to study the relationship between TMP parameters and the behind microstructural evolution. The experimental results indicate that in the scope of low-strain TMP, strain induced boundary migration (SIBM) is the most effective process for GBCD optimization. Besides, SIBM and grain growth would gradually transfer to recrystallization with the increase of pre-deformation level and annealing temperature. Further quasi in-situ EBSD results infer that SBIM is activated locally in some region with high stored energy, and further gradual initiation of SIBM from one region to another contributes to the gradual increase of special boundaries with annealing time.

Effect of pre-deformation on the pre-corrosion multiaxial fatigue behaviors of 2024-T4 aluminum alloy

Pre-damages which contain pre-deformation and pre-corrosion have influences on multiaxial fatigue properties. In this paper, the effect of pre-tension of 1%, 2%, 3% and 4% as well as pre-torsion of 10°, 20°, 30° and 40° on fatigue life of 2024-T4 aluminum alloy under corrosion conditions is evaluated. Multiaxial fatigue tests are performed under constant amplitude sinusoidal wave loading with tension-torsion ratio of 3 in air at room temperature. The results indicate a reduction tendency of pre-corrosion multiaxial fatigue life with the pre-deformation level in the same pre-corrosion time. With the same pre-deformation level, the pre-corrosion multiaxial fatigue life decreases as pre-corrosion time increases. Pitting caused by corrosion is observed on the surface of specimens. The observed mechanical behavior and associated phenomena are directly linked to microstructure characteristics such as surface defects, corrosion pits and micro-cracks. The analysis of mechanism reveals that the synergistic combination of pre-deformation and pre-corrosion is more detrimental than that of either one acting separately. The alternate cyclic hardening and softening appear in the axial direction. The cyclic softening in the tangential direction becomes more serious with the increase of pre-deformation. The typical morphology of corrosion fatigue can be observed on the fracture. Involving the damage factors of single pre-damage conditions, which include pre-deformation and pre-corrosion effects, the damage factor for pre-corroded multiaxial fatigue after pre-deformation is deduced. Fine results are achieved according to the above methods with most predicted life in 2× scatter band.

Assessment of strain hardening in copper single crystals using in situ SEM microshear experiments

The effect of a pre-strain on the plasticity of copper single crystals subjected to in situ microshear deformation in a scanning electron microscope (SEM) is investigated. Pre-strains of 6.5 and 20% are imposed using [1 0 0] tensile testing. During tensile pre-deformation, several slip systems are activated and irregularly spaced slip bands form. A trace analysis revealed the presence of several slip bands on the tensile specimen near the grips while one family of slip bands parallel to the (1 1 1) crystallographic plane were detected in the middle of the tensile specimen. From the middle of the pre-deformed tensile specimens double microshear samples were prepared using focused ion beam (FIB) machining such that the [0 -1 -1] (1 -1 1) slip system could be directly activated. The results show how microshear behavior reacts to different levels of tensile pre-deformation. Sudden deformation events (SDEs) are observed during microshear testing. The critical stress associated with the first SDE is shown to increase with increasing pre-deformation as a result of an increasing number of slip bands introduced during pre-deformation per shear zone. The results allow also to obtain information on the interaction between dislocations activated during microshearing ([0 -1 -1] (1 -1 1)) and those which were introduced during tensile pre-deformation ([1 0 -1] (1 1 1) and [1 -1 0] (1 1 1)). When these slip systems interact glissile junctions and Lomer-Cottrell locks are likely to form. In the light of this analysis, we rationalize the occurrence of sudden deformation events based on piled up dislocation assemblies which overcome Lomer-Cottrell lock barriers.

The effect of thermo-mechanical processing on grain boundary character distribution in Incoloy 800H/HT

In this study, we applied a thermo-mechanical process to alter the grain boundary characteristic distribution (GBCD) with a view to the feasibility of grain boundary engineering in Incoloy 800H/HT. In order to optimize the GBCD through increasing the low Σ coincidence-site lattice (CSL) boundaries, we applied various thickness reductions with two different rolling modes followed by annealing. We used Electron Backscattered Diffraction (EBSD) to analyze the GBCD and CSL boundaries. We found that the coincidence-site lattice boundaries, particularly Σ3 and its variants, increased with the pre-deformation level in cross-rolled (CR) samples. In contrast, the fraction of these CSL boundaries had an optimum in 50% reduction for unidirectionally rolled (UDR) samples. In fact, different Σ3n interactions led to different GBCD in UDR and CR processed samples. Low to medium deformation with UDR and medium to high deformation with CR on the Incoloy 800H/HT samples showed potential for grain boundary engineering.

The effects of pre-deformation on the subsequent fatigue behaviors of SUS 430 Stainless Steel in load-control

The purpose of this study is to investigate the fatigue behavior of the SUS 430 Stainless Steel after the tensile pre-deformation is subjected to a symmetric loading. The investigations have been performed in three levels of pre-deformation. Cyclic deformation following pre-deformation was carried out in tension-compression fatigue test experiment at room temperature. The pre-deformation was induced on the specimen by a tensile test under strain control at 5%, 8%, and 12% strain level at a strain rate of 10−4s−1, respectively. In this paper, the effects of the cyclic deformation behaviors with different pre-strain levels were compared with the fatigue responses without pre-strain. Based on the comparison, it is found that the pre-strain caused evident changes in fatigue behaviors such as cyclic softening response, cyclic creep response, and cycles to failure. Generally, as compared to the as-received material, the softening response increased with increasing pre-strain level. A pronounced cyclic compressive creep occurred in the specimen with and without pre-strain level effect in all applied loading cases. This could be attributed to an-isotropy between tension and compression, resulting in the occurrence of compressive cyclic creep on the tested material during cyclic loading. Moreover, the stable total compressive creep strain scaled with increasing tensile pre-strain level for same cyclic loading. For the effect of pre-strain on fatigue resistance, a decrease in the number of cycles to failure was observed and the extent of the decrease also increased with increasing tensile pre-strain level under the same loading amplitude condition. In this paper, three damage parameters, ΔWp, Wf, and σaεa, are respectively used to predict the number of cycles to failure of the tested material with tensile pre-strain effect. In the plot of observed versus predicted cycles, it is observed that most of the data points are located within the bound of factor two for the three damage parameters. Consequently, it is confirmed that the three damage parameters could provide satisfactory predictions for the tested material with a tensile pre-strain effect.

Effect of Overaging on Solute Distributions and Bake Hardening Phenomenon in Bake Hardening Steels

Specimens of two different kinds of bake hardening steels (BH-Mn and BH-P) were prepared and treated with different annealing processes (water quenching and overaging). A novel technique of three dimensional atom probe was used to investigate solute distributions in these steels. The results indicate that C concentration decreases, whereas V increases during overaging in both bake hardening steels. The conclusion that no vanadium carbides precipitate during the overaging is therefore originally obtained by microanalysis in bake hardening steels. Moreover, bake hardening values of all the specimens were tested by tensile experiments with 2% pre-deformation. However, those of overaged specimens were further measured with higher levels of pre-deformation because no bake hardening phenomenon was present at 2% pre-deformation. As the pre-deformation increases from 2% to 6% and 8%, both overaged steels show bake hardening values, and the value data are almost the same.