Ultrafine-grained Aluminum Alloy Research Articles

巨大ひずみ加工により結晶粒を微細化したアルミニウム合金の耐孔食性

巨大ひずみ加工により結晶粒を微細化したアルミニウム合金の耐孔食性

Temperature-Dependent Mechanical Response of an UFG Aluminum Alloy at High Rates

High temperature (298 K–573 K) and high strain rate (4000 s−1) compression experiments were performed on a cryomilled ultra-fine grained (UFG) Al-5083 using a modified Kolsky bar with a heating system designed to reduce “cold contact” time. The resulting stress strain curves show a reduction in strength of approximately 300 MPa at the highest temperature tested. This softening has been related to a thermally activated deformation mechanism. In addition, an experimental procedure was developed to investigate the microstructure evolution during the preheating, prior to mechanical loading, so as to identify the intrinsic mechanical response of the material at high temperatures. The results of this procedure are in good agreement with a TEM study on material that has been heated but not loaded.

Hot Workability of Ultrafine-Grained Aluminum Alloy Produced by Severe Plastic Deformation of Al6063 Powder and Consolidation

Al6063 powder was subjected to severe plastic deformation via high-energy mechanical milling to prepare ultrafine-grained (UFG) aluminium alloy. Uniaxial compression test at various temperatures between 300 and 450 °C and strain rates between 0.01 and 1 s-1 was carried out to evaluate hot workability of the material. Microstructural studies were performed by EBSD and TEM. The average activation energy and strain rate sensitivity of the hot deformation process were determined to be 280 kJ mol-1 and 0.05, respectively. The deformation temperature and applied strain rate significantly affected the grain structure of UFG Al alloy. A finer grain structure was obtained at lower temperatures and higher strain rates. The formation of highly misoriented and equiaxed grains also revealed that dynamic recrystallization occurred upon hot deformation. Furthermore, elongated grains with high dislocation density were observed that disclosed partial dynamic recrystallization of the aluminum matrix.

Enhanced Thermal Stability and Mechanical Properties of Ultrafine-Grained Aluminum Alloy

The paper reports on microstructure, strength and fatigue of ultrafine-grained (UFG) samples of the Al-Cu-Mg-Si aluminum alloy processed by high pressure torsion (HPT) at various temperatures. Application of the HPT treatment led to strong grain refinement, as well as to a raise of the mean-root square strains and dynamic precipitation. In case of optimal HPT treatment the UFG samples have demonstrated the enhanced thermal stability, an increase in ultimate tensile strength in 2.5 times and enhancement in fatigue endurance limit by 20 % in comparison with coarse-grained alloy subjected to standard treatment. It is shown that the regime of the HPT treatment governs the volume fraction of precipitates and segregations, thereby affecting a grain size and thermal stability of ultrafine-grained structure.

A stress–strain model for a two-phase ultrafine-grained aluminum alloy

Two-phase ultrafine-grained (UFG) aluminum alloys with different precipitate and low-angle boundary distributions were examined for their monotonic stress–strain behavior. Precipitates in combination with low-angle grain boundaries were found to determine the deformation micromechanism. It is proposed that thermally activated vacancy movement plays a central role in this regard. The experimental stress–strain curves matched well with the trend predicted by a modified Blum–Zheng model based on vacancy-assisted dislocation annihilation mechanism.

Tailoring materials properties of UFG aluminium alloys by accumulative roll bonded sandwich-like sheets

Accumulative roll bonding (ARB) as a method of severe plastic deformation is a well-established process to produce ultrafine-grained (UFG) sheet materials with extraordinary mechanical properties. In this work ARB is applied to combine different sheet materials in order to tailor the materials properties by producing sandwich-like structures. The high strength aluminium alloy AA5754, after 4 ARB cycles (N4), is used as a core material. To achieve high corrosion resistance and good visual properties, it is cladded with commercially pure aluminium AA1050A (N4) at room temperature and alternatively with AA6014 (N4) at 230 °C. All materials are UFG and satisfactory bonding between the different layers of aluminium alloys is achieved. Nanoindentation measurements reveal that there is a sharp transition in hardness at the interface. The yield and tensile strength of the core material are fully retained in the case of the AA6014/AA5754 sandwich. The strength of the AA1050A/AA5754 sandwich is slightly lower compared to the core material but still twice as high as the clad material. The serrated yielding effect which is strongly visible in tensile tests on the pure AA5754 alloy completely disappears in the sandwich sheets, which means the surface quality is strongly enhanced.

Superplastic deformation mechanism of an ultrafine-grained aluminum alloy produced by friction stir processing

An ultrafine-grained (UFG) Al–4Mg–1Zr alloy with a grain size of ∼0.7 μm with predominantly high-angle boundaries of 97% was produced by friction stir processing (FSP). The UFG Al–4Mg–1Zr retained submicrometer grains even after static annealing at 425 °C, and exhibited excellent superplasticity at 175–425 °C. High strain rate and low-temperature superplasticity of >1200% were observed at 1 × 10 −2–1 × 10 −1 s −1 and 300–350 °C. Even at 425 °C, a superplasticity of 1400% was achieved at 1 s −1. A linear relationship between log ε ˙ opti and T was observed (where ε ˙ opti is the optimum strain rate, and T is the temperature). The analyses on the superplastic data revealed the presence of threshold stress, a stress exponent of 2, an inverse grain size dependence of 2, and an activation energy of 142 kJ mol –1. This indicated that the dominant deformation mechanism was grain boundary sliding, which was controlled by lattice diffusion. Based on this notion, a constitutive equation has been developed. A new superplastic deformation mechanism map for FSP aluminum alloys is proposed.

Superstrength of ultrafine-grained aluminum alloys produced by severe plastic deformation

Superstrength of ultrafine-grained aluminum alloys produced by severe plastic deformation

Unusual super-ductility at room temperature in an ultrafine-grained aluminum alloy

Processing by severe plastic deformation (SPD) typically increases the strength of metals and alloys drastically by decreasing their grain size into the submicrometer or nanometer range but the ductility of such materials remains typically low. This report describes the first demonstration that it is possible to increase the room temperature ductility of aluminum-based alloys processed by SPD and to attain elongations to failure of >150% while retaining the enhanced strength. This unique combination of properties is due to the occurrence of grain boundary sliding at room temperature. The sliding was obviously achieved by introducing a grain boundary wetting of the aluminum/aluminum grain boundaries.

Fabrication of MEMS components using ultrafine-grained aluminium alloys

A novel process for the fabrication of a microelectromechanical systems (MEMS) metallic component with features smaller than 10 µm and high thermal conductivity was investigated. This may be applied to new or improved microscale components, such as (micro-) heat exchangers. In the first stage of processing, equal channel angular pressing (ECAP) was employed to refine the grain size of commercial purity aluminium (Al-1050) to the ultrafine-grained (UFG) material. Embossing was conducted using a micro silicon mould fabricated by deep reactive ion etching (DRIE). Both cold embossing and hot embossing were performed on the coarse-grained and UFG Al-1050. Cold embossing on UFG Al-1050 led to a partially transferred pattern from the micro silicon mould and high failure rate of the mould. Hot embossing on UFG Al-1050 provided a smooth embossed surface with a fully transferred pattern and a low failure rate of the mould, while hot embossing on the coarse-grained Al-1050 resulted in a rougher surface with shear bands.

Plastic deformation mechanism of ultra-fine-grained AA6063 processed by equal-channel angular pressing

Equal-channel angular pressing (ECAP) is an established method to produce ultra-fine-grained (UFG) materials with extraordinary mechanical properties. However, different methods to characterize the complex microstructure give contradictory results. Therefore an ECAP-processed UFG aluminum alloy AA6063 was characterized by various electron-microscopy-based methods such as backscattered electron contrast, electron backscatter diffraction, transmission electron microscopy and low-voltage scanning transmission electron microscopy. Only a skilful combination of all methods reveals the complete information about the microstructure which is needed to understand the results of the mechanical testing by nanoindentation, tensile tests and strain-rate jump test. The main difference is the amount of low-angle grain boundaries and high-angle grain boundaries which determine hardness, tensile and yield strength and strain-rate sensitivity of ECAP materials produced by different numbers of passes and routes. This is explained by the interaction of dislocations with the different grain boundaries.

An evaluation of creep behavior in ultrafine-grained aluminum alloys processed by ECAP

An evaluation of creep behavior in ultrafine-grained aluminum alloys processed by ECAP

Aluminum tailored heat treated blanks

Tailored Heat Treated Blanks (THTB) are blanks, which exhibit locally different material properties optimized for the succeeding sheet metal forming process. The distribution of the material properties is obtained by a local, short-term heat treatment. As a result of the optimized property gradients in the sheet metal’s plain, THTB allow enhancing the forming limits significantly. The research work covered in this paper focuses on THTB made out of aluminum alloys. The article presents the microstructural mechanisms due to a short-term heat treatment as well as design principles for the heat treatment layout for THTB made out of AlMgSi alloys and ultra-fine grained (UFG) aluminum alloys produced by the Accumulative Roll Bonding process.

Contribution of grain boundary sliding in low-temperature superplasticity of ultrafine-grained aluminum alloys

Ultrafine-grained (0.6 mu m) Al-Mg-Sc alloy with predominant high-angle boundaries was produced by friction stir processing. A superplastic elongation of 210% was obtained at 175 degrees C, and the optimum strain rate and maximum elongation increased with increasing temperature. Marker lines were scratched on the polished specimen surfaces using a nano-indenter to measure grain boundary sliding (GBS) offsets during deformation, which indicated that the GBS contribution to the strain exceeded 50% at 175 degrees C and increased with increases in strain and temperature. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Change in the microstructure of ultrafine-grained aluminum alloys 1420 and 1421 under the action of pulse laser irradiation

The microstructure of ultrafine-grained aluminum alloys 1420 and 1421 subjected to pulse laser irradiation is studied. Comparative analysis of changes in the structure of the alloys in ultrafine-grained and coarse-crystalline states after laser treatment is performed.

On the Occurrence of Portevin–Le Châtelier Instabilities in Ultrafine-Grained 5083 Aluminum Alloys

The Portevin–Le Châtelier (PLC) instability is commonly observed in Al–Mg alloys and is manifested in serrated flow within the stress–strain response. We investigate the persistence of this instability with reduction in grain size by studying an ultrafine-grained (ufg) aluminum alloy (Al5083) and a conventional grain size Al5083. Micro-scale tensile tests combined with digital image correlation (DIC) reveal strength anisotropy and heterogeneity of the deformation in the three material directions (extrusion, rolled, and transverse). For the same applied displacement rate, the PLC effect in ufg-Al5083 is observed only over a small strain range immediately following the yield, while the coarse-grained Al5083 exhibits serrated flow over nearly the entire plastic strain range. These observations are explained using the stability analysis of Hahner (Acta Mater 45:3695–3707, 1997), and implications for nanocrystalline (nc) alloys are discussed.

Structure and mechanical properties of strips and shapes from ultrafine-grained aluminum alloy 1421

The structure and mechanical properties of high-strength shapes from ultrafine-grained aluminum alloys for the aircraft industry are studied. The initial material is commercial alloy 1421 in coarse-crystal state. An ultrafine-grained structure is formed in preforms 40 mm in diameter and 160 mm long by equichannel angular pressing at 370°C with degree of accumulated strain e ≥ 10. Strips with ultrafine-grained structure with a thickness of 2 mm are fabricated by rolling with e = 2.7 at 350°C. The strips are used for making shapes with Π-section under conditions of superplastic deformation.

Strengthening mechanisms in cryomilled ultrafine-grained aluminum alloy at quasi-static and dynamic rates of loading

To achieve lightweight and high-strength materials, Al alloy has been cryomilled and processed to obtain refined microstructures. The microstructure and strengthening mechanisms of the resulting ultrafine-grained material are investigated in this work. Contributions from the several mechanisms (boundary strengthening, solid solution strengthening, precipitate strengthening and dislocation strengthening) are discussed and estimated using simplified models. Comparison with experimental data suggests some directions for materials design.

Solute-enhanced tensile ductility of ultrafine-grained Al–Zn alloy fabricated by friction stir processing

The pronounced beneficial effect of the addition of Zn on the tensile strength and ductility of ultrafine-grained aluminum alloy prepared by friction stir processing is investigated. Experimental evidence suggests that increasing the Zn content from 0 to 15 wt.% can effectively enhance tensile elongation from 10 to >30% as a result of increased work hardening rate in the initial tensile stage and the effect of numerous fine slip bands.

Effect of temperature and strain rate on tensile behavior of ultrafine-grained aluminum alloys

Ultrafine-grained Al–4Y–4Ni and Al–4Y–4Ni–0.9Fe (at.%) alloys were synthesized by the consolidation of atomized powders and subsequent hot extrusion. The mechanical behavior of these two alloys has been studied by performing uniaxial tension tests ranging from room temperature to 350°C. These alloys, with high volume fraction of second-phase particles, exhibited ambient temperature tensile strength ranging from 473 to 608MPa and plastic elongation ranging from 6.7 to 9.6% at an initial strain rate of 1×10−3s−1. However, lower ductility was observed with decreasing strain rate at the intermediate temperature ranging from 150 to 250°C for Al–Y–Ni–Fe alloys due to limited work hardening.