Ultrafine-grained Aluminum Alloy Research Articles

Low-Temperature Superplasticity of Ultrafine-Grained Aluminum Alloys: Recent Discoveries and Innovative Potential.

The last two decades have witnessed significant progress in the development of severe plastic deformation techniques to produce ultrafine-grained materials with new and superior properties. This review examines works and achievements related to the low-temperature superplasticity of ultrafine-grained aluminum alloys. The examples are provided of the possibility to observe low-temperature superplasticity in aluminum alloys at temperatures less than 0.5 Tmelt and even at room temperature, and herein, we demonstrate the cases of achieving high ductility and high strength in aluminum alloys from processing utilizing severe plastic deformation. Special emphasis is placed on recent studies of the formation of segregations of alloying elements at grain boundaries in UFG Al alloys and their influence on the development of grain boundary sliding and manifestation of low-temperature superplasticity. In addition, the current status and innovative potential of low-temperature superplasticity in aluminum alloys are observed.

Enhanced strength-ductility combination in the aluminum-gold system by heterogeneous distribution of nanoparticles via ultra-severe plastic deformation and reactive interdiffusion

Ultrafine-grained aluminum alloys are of interest due to their high strength-to-weight ratio, but they usually suffer from poor uniform ductility. In this study, an Al-Au alloy with a good combination of strength and ductility is produced by the heterogeneous distribution of Al2Au nanoparticles in an aluminum matrix. To generate such heterogeneity, the alloy is synthesized by ultra-severe plastic deformation of aluminum and gold powders via the high-pressure torsion (HPT) method. Reactive interdiffusion occurs during the process leading to the heterogeneous formation of intermetallic particles and a good strength-ductility synergy (200 MPa yield stress and 15% uniform elongation). Nanoparticles gradually distribute within the matrix and once a uniform nanoparticle distribution is achieved, the alloy shows no further increase in strength, but it completely loses its ductility. It is concluded that not only the presence of nanoparticles but more importantly the heterogeneity of their distribution can positively influence the strength-ductility combination in ultrafine-grained aluminum alloys. The findings of this study suggest that future studies on heterogeneous precipitation hardening can be a solution to achieve ductile precipitation-hardened alloys.

Effect of Sc/Zr ratio on superplastic behavior of ultrafine-grained Al–6%Mg alloys

Al–6%Mg-Sc-Zr alloys with a combined Sc and Zr ratio of 0.32 wt% make up the focus of this research. The content of Sc and Zr varied with an increment of 0.02%. Their ultrafine-grained (UFG) microstructure was formed with Equal Channel Angular Pressing (ECAP). Superplasticity tests were performed at temperatures ranging from 300 to 500 °C and at a strain rate varying between 3.3·10−3 and 3.3·10−1 s−1. The values of strain hardening factor (n), strain rate sensitivity factor (m), and superplastic deformation threshold stress (σp) were determined. The microstructure and failure patterns of the alloys post superplasticity tests were studied. The effect of the Sc/Zr ratio on the superplastic behavior, cavitation fracture and dynamic grain growth of UFG Al–6%Mg alloys was investigated. The satisfiability of Hart's criterion for calculating uniform deformation value under superplastic conditions was verified. Grain boundary sliding and intragranular deformation make comparable contributions to superplastic flow in the UFG aluminum alloys under study. Al–6%Mg-0.20%Sc-0.12%Zr (Sc/Zr = 3.3 at.%) and Al–6%Mg-0.18%Sc-0.14%Zr (Sc/Zr = 2.8 at.%) alloys exhibit the highest superplasticity. Shown that changes in the Sc/Zr ratio affect the precipitating mechanism, spatial distribution and composition of the precipitating particles Al3(ScxZr1-x). The optimal Sc/Zr ∼2.6–3.3 (at.%) ratio for the Al–6%Mg alloy corresponds to an Al3(Sc0.5Zr0.5) and Al3Sc precipitated particles. The large Al3Zr particles formed through the discontinuous precipitation mechanism was detected at Sc/Zr < 1 (at.%).

EVOLUTION OF THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AL-B COMPOSITE WITH THE ULTRAFINE-GRAINED ALUMINUM MATRIX

The paper examines the microstructural evolution of alloy 1565ch of the Al-Mg-Mn-Zn-Zr system during thermomechanical treatment, including severe plastic deformation by high-pressure torsion or equal channel angular pressing according to the conform scheme and subsequent isothermal rolling at 200°C. Formation of the nanostructured and ultrafine-grained states in alloy 1565ch with the controlled distribution of Al3Mg2, Al6Mn and Al3Zr phases both inside grains and at their boundaries allows for the superplastic effect at the temperatures 250 and 300°C and strain rates 5 × 10-2, 10-2, and 5 × 10-3 s-1. Microstructural analysis by transmission electron microscopy shows that superplastic deformation at the temperatures 250 and 300°C allows a homogeneous ultrafine-grained state to be preserved. The studied ultrafine-grained aluminum alloy 1565ch has a high strength and the ability to relieve stresses, and therefore it can be favorably used as the matrix material in composites reinforced with continuous boron fibers. In the paper, we use this alloy to study special features of production of a multilayer metal matrix composite according to the foil-fiber-foil scheme by isothermal pressing in the low-temperature superplastic mode. This method has a positive effect on the mechanical properties of the composite, such as ultimate strength at 200°C, impact strength at room temperature, and fracture toughness at room temperature.

On the formation of large grain structure after friction stir processing of ultrafine-grained aluminium alloy

ABSTRACT A tendency for formation of large grain structure after friction stir processing (FSP) of ultrafine-grained aluminium produced by severe plastic deformation was thoroughly discussed. To do so, microstructural assessments using electron backscattered diffraction (EBSD) analysis at various regions, including ahead of and beneath the FSP tool were carried out. It was found that ultrafine-grained microstructure of the base metal played an important role in dynamic recrystallisation (DRX) occurring in the stir zone. There are some enlarged grains just ahead of the FSP tool prior to being stirred by it. Partial static recrystallisation followed by grain growth as well as strain-induced grain boundary migration was responsible for the formation of such grains. The large grains could postpone grain subdivision process through continuous and geometrical DRX mechanisms. Moreover, the remained part of the stored strain provided driving force for grain growth after grain subdivision in the stir zone.

Mechanical Properties and Microstructural Evolutions of High-Pressure Torsion-Processed Al7068 Alloy

The combination of severe plastic deformation, deformational heat, and frictional heat generated during the high-pressure torsion process has a great effect on microstructural evolutions, including grain refinement, dynamic recrystallization and recovery. In particular, the low melting temperature metallic alloys (e.g., aluminum and magnesium) also induce nano-precipitations in the matrix during high-pressure torsion, which results in both unique microstructure and mechanical properties of the nanocrystalline metallic materials. In this study, the mechanical properties and microstructural evolution of high-pressure torsion-processed aluminum 7068 alloy at room temperature were investigated. In the early deformation stage, both grain refinement and nano-precipitates were formed that increase the strength of ultrafine-grained aluminum alloy, and a maximum strength of 844 MPa was obtained for the 5T sample. However, in the later deformation stage, tensile strength decreases with the increase in shear strain due to grain growth, and coarse precipitates act as crack initiation sites during plastic deformation. In summary, the mechanical properties of high-pressure torsion represent the strength-ductility trade-off as the increase of the number of revolutions. Their mechanical properties are superior to those of recent severe plastic deformation-processed aluminum alloys. This result shows that the severe plastic deformation of the HPT process positively contributes to their mechanical properties by inducing multiple strengthening paths, such as grain refinement and nano-precipitation of the aluminum alloy.

Investigation on uniaxial ratcheting fatigue behaviors and microstructure evolution of ultrafine-grained 6061 aluminum alloy

The motivation of the present study is to explore ratcheting-fatigue behavior and the corresponding micro-deformation mechanism of ultrafine-grained aluminum alloys 6061 (UFG AA6061) under two stress-controlled low-cycle fatigue (LCF) loading modes, i.e., mean tension stress (MTS) and mean compressive stress (MCS). The uniaxial ratcheting tests demonstrate that the fatigue-ratcheting damage of UFG AA6061 is prone to occur under MCS-LCF loading, rather than MTS-LCF loading. Detailed microstructure characterizations by Electron backscatter diffraction (EBSD) techniques under these two loading modes show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries increase remarkably in the case of MCS-LCF loading. These observations are mainly ascribed to the room temperature continuous dynamic recrystallization (CDRX), in which low angle boundaries evolve into high angle grain boundaries and the formation of equiaxed grains is accelerated. In contrast, there were no obvious change in the grain aspect ratios or the fractions of high angle boundaries at MTS-LCF loading condition, while the Schmid factor of non-octahedral slip system {100} <110> increases and the texture evolve gradually. The deformation mechanism is grain boundary sliding (GBS) and grain rotation would accommodate the process of GBS. An analysis of the microstructure-properties relationship showed the superposition effect of residual shear deformation and uniaxial LCF ratcheting deformation is responsible for the CDRX at room temperature. And the occurrence of GBS is mainly caused by the activation of non-octahedral slip system.

Micromechanics-Based Low Cycle Fatigue Life Prediction Model of ECAPed Aluminum Alloy

Ultrafine-grained aluminum alloys (UFG AA) show great potential in the design of fatigue-resistant lightweight alloys, and the methodology to assess low-cycle fatigue (LCF) life remains to be studied. In this work, a micromechanics-based LCF life prediction model is presented by conducting crystal plasticity finite element simulation (CPFEM). The fatigue indicator parameter (FIP) of maximum accumulated equivalent plastic strain energy, modulated by triaxiality, is developed to assess the material damage in the microstructure. Particularly, a new multiaxial strain parameter is proposed by considering the combined influence of the mean strain and non-proportional cyclic additional hardening effect, and then directly embedding into the cyclic J-integral. Finally, the reformulated Manson-Coffin relationship is theoretically constructed by correlating the crack tip opening displacement to the crack propagation equation. The results show the scatter fatigue life of UFG AA6061 is not only related to the inhomogeneous evolution of plastic deformation but also to the local stress state. Since the proposed approach considers both the deformation mechanisms at the micro-scale and the corresponding macroscopic responses, it can predict the LCF life of UFG AA with reasonable accuracy.

An Approach for Predicting the Low-Cycle-Fatigue Crack Initiation Life of Ultrafine-Grained Aluminum Alloy Considering Inhomogeneous Deformation and Microscale Multiaxial Strain.

In this paper, a low-cycle-fatigue (LCF) crack initiation life prediction approach that explicitly distinguishes nucleation and small crack propagation regimes is presented for ultrafine-grained (UFG) aluminum alloy by introducing two fatigue indicator parameters (FIPs) at the grain level. These two characterization parameters, the deformation inhomogeneity measured by the standard deviation of the dot product of normal stress and longitudinal strain and the microscale multiaxial strain considering the non-proportional cyclic additional hardening and mean strain effect, were proposed and respectively regarded as the driving forces for fatigue nucleation and small crack propagation. Then, the nucleation and small crack propagation lives were predicted by correlating these FIPs with statistical variables and cyclic J-integrals, respectively. By constructing a microstructure-based 3D polycrystalline finite element model with a free surface, a crystal plasticity finite element-based numerical simulation was carried out to quantify FIPs and clarify the role of crystallographic anisotropy in fatigue crack initiation. The numerical results reveal the following: (1) Nucleation is prone to occur on the surface of a material as a result of it having a higher inhomogeneous deformation than the interior of the material. (2) Compared with the experimental data, the LCF initiation life of UFG 6061 aluminum alloy could be predicted using the new parameters as FIPs. (3) The predicted results confirm the importance of considering the fatigue behavior of nucleation and small crack propagation with different deformation mechanisms for improving the fatigue crack initiation life prediction accuracy.

Еffect of methods and modes of severe plastic deformation on structural features of ultrafine-grained aluminum alloys, which determine their increased mechanical properties

The effect of severe plastic deformation on the structure of aluminum alloys 1420 and V96T s1, as well as the nanostructure parameters of these metal systems, which determine their increased mechanical properties, is shown: tensile strength of 775 and 800 MPa, elongation of 14 and 20 %, respectively, for 1420 and V96Ts1 alloys.

Enhanced Ductility of High-Strength Ultrafine-Grained Aluminum Alloys at Ambient Temperature (Review)

Bulk nanostructured, or ultrafine-grained metals and alloys structured by severe plastic deformation (SPD) methods usually demonstrate high strength and reduced ductility. The poor ductility is a critical issue which limits their practical applications. Significant efforts were made to improve tensile ductility of the SPD-processed metallic materials while keeping sufficiently high strength. In this paper we present a short overview of the developed approaches for simultaneous improvement of the strength and ductility of Al-based alloys with an emphasis on the recent finding and physical reasons of the plasticity enhancement. The main attention is paid to achieving increased ductility of high strength aluminum alloy at room temperature.

Influence of deformation at elevated temperatures on stability of microstructure and mechanical properties of UFG aluminum alloy

Ultrafine-grained (UFG) state with a mean grain size of 180 nm was produced in a 6060 alloy of the Al-Mg-Si system by high pressure torsion (HPT). The UFG Al 6060 alloy demonstrated high strength: microhardness of 120 Hv and ultimate tensile stress of 525 MPa at room temperature. The tensile tests were carried out at temperatures 120, 150 and 180 °C with the strain rate 10−2, 5 × 10−3, 10−3, 5 × 10−4 and 10−4 s−1 to determine the best conditions for the manifestation ductility and study of changes in strength of the study material. It established, that the UFG alloy retains an increased 80% of level of strength at room temperature after deformation at elevated temperatures in the temperature range 120–150 °C. The results obtained are compared with changes of the microstructure and strength of the UFG alloy after annealing under similar temperature–time conditions.

Friction stir welding of ultrafine grained aluminum alloys: a review

Severe plastic deformation (SPD) has been one of promising routes to fabricate ultrafine-grained (UFG) materials, especially aluminum alloys. However, the SPD products often suffer from their small size. This issue implies the necessity of welding of UFG aluminum alloys for making them usable in complex, large forms in industries. Among various welding processes, those based on solid state welding seem to be more consistent with UFG materials. This is associated with the instability of UFG materials upon intense heating cycle that is common in fusion state welding processes. Friction stir welding (FSW) as a well-known process in the category of solid state welding is widely used for welding of aluminum alloys. This review paper provides an overview of the state-of-the-art of FSW of UFG aluminum alloys. To do so, specific attention is given to microstructural and textural evolutions, effect of secondary particles, and cooling medium. Applying cryogenic cooling medium as well as secondary nanoparticles could inhibit excessive grain growth in the stir zone, which were beneficial to improve the strength of the stir zone without remarkable decrease in the ductility. These processing routes did not affect the main recrystallization mechanism of the stir zone.

Examination of inverse Hall-Petch relation in nanostructured aluminum alloys by ultra-severe plastic deformation

To have an insight into the occurrence of inverse Hall-Petch relationship in ultrafine-grained (UFG) aluminum alloys produced by severe plastic deformation (SPD), ultra-SPD (i.e. inducing several ten thousand shear strains via high-pressure torsion, HPT) followed by aging is applied to an Al-La-Ce alloy. Average nanograin sizes of 40 and 80 nm are successfully achieved together with strain-induced Lomer-Cottrell dislocation lock formation and aging-induced semi-coherent Al11(La,Ce)3 precipitation. Analysis of hardening mechanisms in this alloy compared to SPD-processed pure aluminum with micrometer grain sizes, SPD-processed Al-based alloys with submicrometer grain sizes and ultra-SPD-processed Al-Ca alloy with nanograin sizes reveals the presence of two breaks in the Hall-Petch relationship. First, a positive up-break appears when the grain sizes decrease from micrometer to submicrometer which is due to extra hardening by solute-dislocation interactions. Second, a negative down-break and softening occur by decreasing the grain sizes from submicrometer to nanometer which is caused by weakening the dislocation hardening mechanism with minor contribution of the inverse Hall-Petch mechanism. Detailed analyses confirm that nanograin formation is not necessarily a solution for extra hardening of Al-based alloys and other accompanying strategies such as grain-boundary segregation and precipitation are required to overcome such a down-break and softening.

An investigation of thermal stability of structure and mechanical properties of Al-0.5Mg–Sc ultrafine-grained aluminum alloys

This paper presents the results of studying the thermal stability of the grain structure, mechanical properties, and specific electrical resistivity of the Al-0.5 wt%Mg-xSc ultrafine-grained (UFG) alloys with varying scandium content (x = 0, 0.2, 0.3, 0.4, and 0.5 wt.%). The alloys were produced by induction casting and Equal-Channel Angular Pressing (ECAP). The alloys with Sc concentration above 0.3 wt% contained submicron Al3Sc primary particles, the amount of which increased with increasing Sc concentration. The decomposition kinetics of the Al–Sc solid solution in the UFG alloys was determined by the diffusion intensity along the dislocation cores in the lattice as well as at the grain boundaries. High thermal stability of the UFG structure in the alloys was ensured by ECAP at 225 оC whereas the recrystallization onset temperature was 375 оC. The UFG alloys exhibited high plasticity at the room and elevated temperatures and good combination of strength and electrical conductivity.

Microstructure evolution of ultrafine grained aluminum alloy prepared by large strain extrusion machining during annealing

As a new type of severe plastic deformation (SPD) method for efficiently preparing ultrafine grained (UFG) material, large strain extrusion machining (LSEM) has attracted the attention of researchers. At the same time, the UFG material prepared by the process has a small size, and the influence of external heat is involved in the subsequent utilization. Therefore, in this paper, the microstructure evolution of UFG 6061 Al alloy chips prepared by LSEM during annealing has been investigated. The UFG chips were subjected to isochronous and isothermal annealing treatments. The microstructure of UFG chips was characterized by scanning electron microscope (SEM) with electron backscattered diffraction (EBSD) system. The results showed that with the increase of annealing temperature, the UFG aluminum alloy chips underwent three processes of recovery, recrystallization and grain growth. When the annealing temperature was 160 °C, with the extension of annealing time, the UFG aluminum alloy chips remained at the recovery stage, the point defects and dislocations were reduced, the dislocation density was reduced, and the grain size was not changed significantly.

The effect of ultrafine-grained states on superplastic behavior of Al-Mg-Si alloy

Abstract In this study the ultrafine-grained aluminum alloy 6061 of the Al-Mg-Si system with a grain size of 350 nm and a regulated distribution of the secondary hardening phase Mg2Si, processed by equal-channel angular pressing in parallel channels (ECAP-PC), was investigated. Under tensile stress at 160⋯250 °C, this ultrafine-grained structure gives rise to signs of superplastic behavior at lower temperature with a total elongation of 240%, the parameter of strain rate sensitivity (m) equals to 0.31. The forming behavior of ultrafine-grained alloy 6061 at room temperature was studied by Eriksen test method, which is a simulation of cold sheet stamping.

Structure and mechanical behavior of ultrafine-grained aluminum-iron alloy stabilized by nanoscaled intermetallic particles

Ultrafine-grained aluminum alloys offer interesting multifunctional properties with a combination of high strength, low electrical resistivity, and low density. However, due to thermally induced grain coarsening, they typically suffer from an intrinsic poor thermal stability. To overcome this drawback, an Al-2%Fe alloy has been selected because of the low solubility of Fe in Al and their highly positive enthalpy of mixing leading to the formation of stable intermetallic particles. The two-phase alloy has been processed by severe plastic deformation to achieve simultaneously submicrometer Al grains and a uniform distribution of nanoscaled intermetallic particles. The influence of the level of deformation on the microstructure has been investigated thanks to transmission electron microscopy and atom probe tomography and it is shown that for the highest strain a partial dissolution of the metastable Al6Fe particle occurred leading to the formation of a Fe super saturated solid solution. The thermal stability, and especially the precipitation of particles from the ultrafine-grained solid solution and the way they pin grain boundaries has been investigated both from static annealing and in-situ transmission electron microscopy experiments. The correlation between microstructural features and microhardness has been established to identify the various strengthening contributions. Finally, it is shown that ultrafine grained high purity Al with less than 0.01 at. % Fe in solid solution could preserve a grain size only 300 nm after 1 h at 250 °C.

Annealing-Induced Grain Rotation In Ultrafine-Grained Aluminum Alloy

Abstract Results of in situ transmission electron microscopic studies of annealing-induced evolution of grain boundary misorientations in ultrafine-grained aluminum alloy Al-4%Cu-0.5%Zr (wt.%) processed by high pressure torsion are presented. An experimental procedure based on the single reflection technique was applied for the measurements of orientations of individual grains before and after annealing that allowed for determining misorientations between them. The measurements revealed that relaxation processes occurring in non-equilibrium grain boundaries during short-time annealing are accompanied by a change of grain boundary misorientations, grain rotation, grain migration as well as grain growth. Obtained results were compared with theoretical studies considering the kinetics of grain rotation in discrete-dislocation as well as continuum approaches.

Adiabatic Shear Localization and Microstructure in Ultrafine Grained Aluminum Alloy at Cryogenic Temperature

Adiabatic shear localization plays an important role in the deformation and failure of ultrafine grained 6061 aluminum alloy processed by friction stir processing. To understand the effects of temperature and strain on adiabatic shear localization in the ultrafine grained 6061 aluminum alloy, it has been investigated dynamic mechanical behavior of ultrafine grained 6061 aluminum alloy under the controlled shock loading experiments. Deformation characteristics and microstructures in the shear band were performed by optical microscopy and transmission electron microscopy. The shear band in the ultrafine grained aluminum alloy is a long and straight band distinguished from the matrix. The width of the shear band decreases with increasing nominal strain. The results show that the grains in the boundary of the shear band are highly elongated along the shear direction and form the elongated cell structures (0.2 μ in width), and the core of the shear band consists of a number of recrystallized equiaxed grains 0.2-0.3 μ in diameters and the second phases distribute in both the boundary and the inner of the equiaxed new grains. The calculated temperature in the shear band is about 692 K. Rotational dynamic recrystallization mechanism is responsible for the formation of the microstructure in the shear band.