Pre-deformed Alloy Research Articles

Effects of twin-boundary precipitates on twin stability and yield strength of a Mg-Nd-Ag-Zr alloy

The effects of precipitates at {10 1‾ 2} twin boundaries on twin stability and yield strength of a Mg-Nd-Ag-Zr alloy are investigated by transmission electron microscopy and electron back-scattered diffraction. Twin-boundary precipitates are observed in the pre-deformed alloys after subsequent ageing treatments over a temperature range of 200–450 °C. Such twin-boundary precipitates significantly enhance the thermal stability of {10 1‾ 2} twins, which remain relatively stable even at 480 °C. The stress stability of {10 1‾ 2} twins is also improved due to the pinning effect of the twin-boundary precipitates, leading to a strengthening effect on the alloy. More twin boundaries can be introduced via pre-deformation at a much higher strain rate of 1000 s−1, and such dynamic pre-compression with subsequent ageing treatments is more effective in improving the yield strength due to the formation of more stable {10 1‾ 2} twin boundaries.

Microstructural characteristics and deformation behaviors of an Al–Mg–Si alloy with improved strength and conductivity processed by continuous casting and expansion extrusion

The industrial applications of Al–Mg–Si alloys conductors still meet the big challenges of strength and conductivity trade-off. In the present work, a special continuous expansion extrusion forming (CEEF) with violent shearing deformation was performed on continuous cast 6101 Al alloy rods to process the sheets with improved mechanical and electrical properties by combining various artificial aging treatments. The results showed that refined dendrites with few intergranular phases and high supersaturation solid solubility could be obtained in the continuous casted rods with fast cooling rate. In the CEEF sheet, the gradient distribution of grain size from the core to the surface of the sheet was observed, where the grain size of processed sheet was refined remarkably by dynamic recovery (DRV) and dynamic recrystallization (DRX). When artificial aging was conducted on the as-extruded, solid-solution and pre-deformed alloy specimens respectively, different precipitation responses were found. The high-density dislocations in the pre-deformed alloy acted as pipes for rapid diffusion of solid solution atoms to accelerate their precipitation, reducing the matrix solid solution atom content and mitigating electron scattering phenomena. As a result, simultaneous improvements of strength and conductivity could be obtained by combining the above optimized strategies.

Synergy of homogeneous/heterogeneous nucleation in Al-Mg-xSi-Cu-Zn alloys with rapid age-hardening response

The low age-hardening response dilemma of aluminum alloys is an urgent challenge that needs to be overcome to save costs and improve comprehensive mechanical properties for industrial applications. In this study, we have proposed a novel low-cost strategy to improve the age-hardening response of advanced Al alloys by the synergy of homogeneous/heterogeneous nucleation. The results reveal that nano-precipitates can preferentially nucleate and grow along dislocations due to the rapid diffusion rate of solute atoms assisted by dislocations. Meanwhile, the critical nucleation size of the solute clusters in the pre-aged alloy can be decreased by pre-deformation during the isothermal aging process due to the higher energy for diffusion and higher total strain energy in the matrix. Furthermore, the interaction force between Mg and Zn atoms in the matrix of no dislocation can be enhanced. Accordingly, many fine solute clusters with a high Zn/Mg ratio can also precipitate inside the grain. As a result, remarkable bake-hardening response and excellent strain hardening properties are obtained in the pre-deformed alloys after bake-hardening treatment based on the synergy of homogeneous/heterogeneous nucleation, and the related mechanism has been proposed.

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.

An investigation on microstructure and mechanical properties of 316 stainless steel: a comparison between ultrasonic treatment and thermal annealing

ABSTRACT The effect of ultrasonic treatment (UST) and thermal annealing (THA) post-processes on the mechanical properties and the related microstructural mechanisms of the tensile pre-strained 316 stainless steel was investigated. It was shown that both processes reduce the microhardness and the yield point as well as increasing the elongation of the pre-deformed alloy. A 10% reduction of the yield point and 28% increase in the elongation was observed after the higher power UST (500 W), while an enhanced ductility of 56% and 41% reduction of the yield point was measured for the high-temperature THA (800°C) treated steel. The increased ductility was related to de-twinning and dislocation annihilation mechanisms, which increase the mean free path distance of dislocations. The de-twinning mechanism was proposed as the boundary migration mechanism and reverse gliding of the partial dislocations by cyclic shear stress for the THA and UST processes, respectively. Unlike the UST process, the high-temperature thermal annealing was associated with the formation of M23C6 precipitates, which causes depletion of alloying elements from the vicinity of grain boundaries and makes the alloy more prone to intergranular corrosion. Compared with THA, the advantages of the UST process are as follows: a rapid and straightforward process, low energy consumption, enhanced ductility without significant reduction in strength, and inhibition of grain boundary precipitation.

Evolution of grain boundary character distributions in a cold-deformed Nickel-based superalloy during electropulsing treatment

The evolution of grain boundary character distribution in a cold-deformed nickel-based superalloy during the electropulsing treatment (EPT) was studied. The results revealed that both coherent Σ3 boundaries (Σ3c) and incoherent Σ3 boundaries (Σ3ic) were formed in the recrystallized structures mainly by growth accidents during EPT, while the fraction of Σ3ic boundaries became much lower at the higher EPT temperatures. Meanwhile, the fraction of Σ3 boundaries increased with the increasing EPT temperatures till 850 °C, and decreased again at 900 °C. In addition, the high mobility of grain boundary during EPT could promote the formation of stacking errors at some locations of high angle grain boundaries (HAGBs), and thus promoting the nucleation of annealing twins. With the increasing EPT time, the fraction of Σ3 boundaries increased slightly, and the unstable Σ3ic boundaries were transformed into stable Σ3c boundaries, in which intersecting migrating SRX fronts were easier to be extended. On the other hand, a lot of low-Σ CSL grain boundaries were formed in the pre-deformed alloy with the strain of 0.1 during EPT, which was mainly attributed to the development of large grain-clusters. As the strain was large enough, the energy supplied by electropulsing could enable the cold-rolled alloy to effectively utilize the stored energy on the formation of Σ3 boundaries. Besides that, the evolution of Σ9 and Σ27 boundaries was analyzed in detail as well as the corrosion resistance of the studied alloy after different treatments.

Static coarsening behavior of a pre-deformed Ti2AlNb-based alloy during heat treatment

Microstructure evolution, static coarsening mechanism and coarsening kinetics of a pre-deformed Ti2AlNb-based alloy during aging at 945 °C and 970 °C were investigated in this study. The corresponding microstructures were characterized by scanning electron microscopy and transmission electron microscopy. The results show that the pre-deformed alloy exhibited different morphologies aged at 945 °C and 970 °C. When the alloy was aged at 970 °C, the coarsening mechanism of the O precipitates was mainly controlled by the Ostwald ripening mechanism at first aging stage; after that, it was controlled by the boundary splitting mechanism and termination migration mechanism. Due to the difference in interface energy between the terminals and the long-axis direction of the O lath, diffusion process of solute atoms leads to the coarsening and fragmenting of O lath. The dimension of the O precipitates in the studied alloy continued to grow with the prolonged time during aging at 945 °C or 970 °C while the coarsening rate decreased. The static coarsening kinetic of the studied alloy was established in term of the modified Lifshitz-Slyozov-Wagner (LSW) theory. Static coarsening behavior of the pre-deformed alloy was mainly controlled by bulk diffusion at 945 °C and interfacial diffusion at 970 °C, respectively.

Evolution of the hardening precipitates with an enclosed structure in pre-deformed Mg-Sm-Nd-Zn-Zr alloy

Precipitation evolution of pre-deformed Mg-5Sm-1.4Nd-0.4Zn-0.5Zr alloy during aging at 200 °C was investigated by using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A long lamellae βB′ phase was found in the pre-deformed alloy. The βB′ phase has a disordered hexagonal structure, and the closely packed planes in βB′ have the ABCA stacking sequence with Sm, Nd and Zn atoms in B and C planes. The βB′ precipitating on basal plane and β′/β1 precipitating on prismatic plane form an enclosed structure. This special structure has a stronger impeding effect on dislocations slip. The precipitation sequence of the pre-deformed Mg-5Sm-1.4Nd-0.4Zn-0.5Zr alloy is: supersaturated solid solution → Guinier Preston (G.P.) zones → βB′/β′ → β1 → β. The age-hardening response of the Mg-Sm-Nd-Zn-Zr alloy is enhanced significantly by pre-deformation.

Microstructure and High Temperature Tensile Properties of Mg–10Gd–5Y–0.5Zr Alloy after Thermo-Mechanical Processing

The microstructure, high-temperature tensile properties and fracture behavior of the Mg-10Gd-5Y-0.5Zr alloy after thermo-mechanical processing (pre-tension between solution and aging treatment) were investigated. The pre-deformed alloy shows the accelerated aging kinetics compared to the un-deformed alloy. Microstructure of pre-deformed samples showed not only the homogeneous nucleation of the precipitate but also heterogeneous nucleation of precipitates on the dislocation and twin boundaries. Tensile results show that the pre-deformation enhanced the strength of the alloy, while it deteriorates the ductility of the alloy. The ultimate tensile strength (UTS) of the T6 treated un-deformed and pre-deformed alloy at room temperature are 331 MPa and 366 MPa, respectively. Tensile strength of the T6 treated alloy in both un-deformed and deformed conditions was enhanced by raising the test temperature and then reduced by further raising the test temperature. The higher strength of the pre-deformed alloy could be related to the higher density of the precipitates, which grow on the twin boundaries and can hinder the dislocation movement and strengthen the alloy. The results shows that thermo-mechanical processing can significantly improve the room- and high-temperature mechanical properties and enhance the formation of precipitates in Mg-10Gd-5Y-0.5Zr alloy, which can lead to wider application of the alloy in industries such as aerospace or powertrains that need better room- and high-temperature mechanical properties.

Analysis of empirical relation between microstructure, texture evolution and fatigue properties of an Al-Cu-Li alloy during different pre-deformation processes

The microstructure and fatigue properties of Al-Cu-Li alloy sheets were investigated respectively under the conditions of without pre-deformation, 3% pre-stretching deformation and 8% pre-rolling deformation utilizing scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) in present work. Results show that the intensive Goss texture and relatively weaker Brass, Cube texture were formed in without pre-deformed alloy after aging treatment. The pole density level of Goss texture was decreased remarkably and the intensity of Brass, Cube texture was increased lightly after 3% pre-stretching deformation and 8% pre-rolling deformation. The magnitude order of fatigue crack propagation (FCP) rate of the alloy under different pre-deformation conditions was dadNpre−rolling > dadNpre−stretching > dadNnon−predeformation in stage II (Paris regime). The combined contribution of T1 phase with different size, amount and the intensity of Goss texture is the main factors affecting the FCP rate of Al-Cu-Li alloy.

预变形对汽车用Al-Mg-Si-Cu合金析出行为的影响

To reduce the weight of car body, Al- Mg- Si- Cu alloys have been widely used to produce outer body panels of automobiles due to their favorable high strength- to- weight ratio, corrosion resistance, weldability and good formability. Al- Mg- Si- Cu alloys belong to age- hardenable aluminium alloys, whose strength derives mainly from the matrix precipitation during aging treatments. However, their bake hardening response still need to be further improved to enhance their dent resistance. A novel thermo- mechanical treatment consisting of conventional pre-aging, pre-deformation and re-aging was developed to enhance the tensile properties and bake hardening increment of Al-Mg-Si-Cu alloys. In this work, the effect of pre-deformation on the precipitation behaviors of AlMg-Si-Cu alloy was studied by DSC, mechanical property measurement and TEM. The results show that, the GPzone dissolution rate decreases with increasing pre-deformation during the slow heating up process for the pre-aged alloy, the corresponding activation energies of 0, 5% and 15% pre-deformed alloy calculated by Avrami-JohnsonMehl method are 137.1, 189.5 and 141.3 k J/mol, respectively. If the pre- deformed alloys are directly bake hardened at 185 ℃ for 20 min, precipitation and bake hardening increment can be greatly improved by pre-deformation(the highest bake hardening increment is 160 MPa), but the bake hardening increment rate gradually decreases if the pre- deformation is above 10%. In addition, the GP zone dissolution rates of pre- deformed alloys after bake hardening treatment are much lower when the heat treatment temperatures are below one certain value, but if the treatment temperatures above it, the corresponding GP zone dissolution rates are higher than that of alloy without pre-deformation, finally, the activation energy changes from high value to low value even can be observed in the ln[(d Y/d T)f/f(Y)]-1/T curve. For the β″ precipitation in the alloys, with increasing pre-deformation, its activation energy gradually decreases, corresponding gradually increase of precipitation rate.

Dislocation character and operative slip systems in α-Nb 5Si 3 tested at 1673 K

Dislocation character and operative slip systems in α-Nb 5Si 3 were examined by transmission electron microscopy. Two-phase alloys comprised of (Nb) and α-Nb 5Si 3 were used in this study. Although few dislocations were present in the α-Nb 5Si 3 phase of a pre-deformed alloy, many developed after 15% of compressive deformation at 1673 K. Two types of the Burgers vectors were identified for α-Nb 5Si 3: <100] and 1/2 < 111]. The glide planes of dislocations were defined by the cross products between the Burgers vectors and the line vectors of the dislocations, by which the slip systems that operate in α-Nb 5Si 3 at 1673 K were determined as {011)<111], {100)<010], and {001)<100].

Study of the Nanostructures Formed in 2024 Alloy during Thermomechanical Treatments

A study on the microstructures developed during artificial aging of the 2024 alloy in nondeformed and 8% pre-deformed specimens is presented. The microstructural characterization was carried out using transmission electron microscopy (TEM), specifically, diffraction contrast and highresolution imaging. The S phase was found to precipitate heterogeneously on dislocations in both nondeformed and pre-deformed alloys. After the same artificial aging time, a larger density of S phase precipitates was obtained in the pre-deformed specimens. In non-deformed specimens, a high density of small nanometer-sized aggregates was found, that was absent in the pre-deformed specimens, that were identified as GP zones. To further analyze the small sized aggregates, artificially aged specimens with no previous deformation were also studied with Coincidence Doppler Broadening (CDB) of positron annihilation radiation. With this technique, information on the chemical environment around vacancy sites contained in the GP zones was obtained. Specifically, a composition close to (AlCu)2Mg was derived.

Effect of prestrain on martensitic transformation in a Ti46.4Ni47.6Nb6.0 superelastic alloy and its application to medical stents

The effect of applied strain on martensitic transformation in a superelastic Ti(46.4)Ni(47.6)Nb(6.0) alloy at room temperature was investigated by tensile tests, differential scanning calorimetry measurements, and X-ray diffraction. Reverse transformation starting (A(s)) and finishing (A(f)) temperatures increased with the application of tensile-strain over 13%, the undeformed specimen showing A(s) = -29.2 degrees C and A(f) = 17.9 degrees C, while the 13% predeformed alloy exhibited A(s) = 37.1 degrees C and A(f) = 40.2 degrees C. Furthermore, the values of the A(s) and A(f) for the predeformed alloy almost recovered to those of the undeformed alloy when heated to about 42 degrees C and then showed superelasticity again at room temperature. This characteristic is significant for application in sensors, actuators, and medical devices. Especially, medical stents with such qualities show promise as a new class of self-expandable stents with both excellent mountability and deliverability.