First-principles investigation on stability and electronic structure of Sc-doped θ′/Al interface in Al−Cu alloys

The properties of Sc-doped θ′ (Al₂Cu)/Al interface in Al−Cu alloys were investigated by first-principles calculations. Sc-doped semi-coherent and coherent θ′ (Al₂Cu)/Al interfaces (Sc doped in Al slab (S1 site), Sc doped in θ′ slab (S2 site)) were modeled based on calculated results and reported experiments. Through the analysis of interfacial bonding strength, it is revealed that the doping of Sc at S1 site can significantly decrease the interface energy and increase the work of adhesion. In particular, the doped coherent interface with Sc at S1 site which is occupied by interstitial Cu atoms has very good bonding strength. The electronic structure shows the strong Al—Cu bonds at the interfaces with Sc at S1 site, and the Al—Al bonds at the interfaces with Sc at S2 site are formed. The formation of strong Al—Cu and Al—Al bonds plays an important role in the enhancement of doped interface strength.     

Superplastic deformation of a heat-resistant Al–Cu–Mg–Ag–Mn alloy

The superplastic behavior and deformation mechanism of a heat-resistant Al–Cu–Mg–Ag–Mn alloy prepared by ingot metallurgy was investigated by using optical microscopy, scanning electron microscopy and transmission electron microscopy. It is shown that the Al–Cu–Mg–Ag–Mn alloy shows good superplastic properties at temperatures higher than 450 °C and strain rates lower than 10^?2 s^?1. A maximum elongation-to-failure of 320% was observed at 500 °C and 5 × 10^?4 s^?1, where the corresponding strain rate sensitivity index m is 0.58 and the flow stress σ is 5.7 MPa. Microstructure studies revealed that the observed superplastic behavior resulted from severe grain elongation rather than grain boundary sliding. It is suggested that this phenomenon may provide a new concept for developing superplastic materials.     

Effect of trace Ag on the microstructure and precipitation of an Al–Cu–Mg alloy

Abstract

This paper investigated the effect of different amounts of Ag addition on the microstructure, properties and precipitation processes of Al–4·6Cu–6·9Mg(wt-%) alloy using various analytical methods. It was found that Ag addition stimulated new X′ 9 and Ω phases precipitated finely and dispersively in the matrix, as a result of Mg–Ag co-clusters; the volume fraction of precipitates increased with the content of Ag addition. Such precipitation improved the mechanical performance of the Al–Cu–Mg alloy significantly. The mechanism for the formation of new precipitates is also described in this paper.     

Influence of megaplastic deformation on the structure and hardness of Al–Cu–Mg alloy after aging

Methods of electron microscopy and X-ray diffraction have been used to investigate structural and phase transformations in the aluminum alloy of grade A2024 (Al–4.5 Cu–1.37 Mg–0.61 Mn–0.07 Si–0.27 Fe–0.02 Zn–0.02 Ti (wt %)) after aging and deformation by shear under high quasi-static pressure. It has been shown that the combination of two-stage aging with megaplastic deformation leads to the refinement of the structure to a nanolevel and to strengthening of the alloy (to an increase in the microhardness to 3000 MPa). The values of true deformation at which the deformation-induced dissolution of the particles of the strengthening S phase occurs have been determined.     

Effect of pre-straining on structure and formation mechanism of precipitates in Al–Mg–Si–Cu alloy

The effect of pre-straining on the structure and formation mechanism of precipitates in an Al–Mg–Si–Cu alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Elongated and string-like precipitates are formed along the dislocations in the pre-strained Al–Mg–Si–Cu alloy. The precipitates formed along the dislocations exhibit three features: non-periodic atomic arrangement within the precipitate; Cu segregation occurring at the precipitate/α(Al) interface; different orientations presented in one individual precipitate. Four different formation mechanisms of these heterogeneous precipitates were proposed as follows: elongated precipitates are formed independently in the dislocation; string-like precipitates are formed directly along the dislocations; different precipitates encounter to form string-like precipitates; precipitates are connected by other phases or solute enrichment regions. These different formation mechanisms are responsible for forming different atomic structures and morphologies of precipitates.     

First-principles investigation of the Cu–Ni,Cu–Pd,and Ni–Pd binary alloy systems

Phase diagrams of copper–nickel–palladium binary alloys were determined by density functional theory cluster expansion method. The system has both magnetic and non-magnetic binaries and subtle phase coexistence areas between similar and different kind of lattice types. Furthermore, the CuPd binary has several ordered structures. Cluster expansion models were constructed by heuristic cluster selection for all of the fcc structures and for the CuPd_bcc structure. Both configurational and magnetic phase diagrams were determined. Small amount of nickel magnetize fcc palladium to 0.26 μ_B from which the magnetic moment rises almost linearly to that of pure Ni. In CuNi, 0.46 x-Ni is needed for the magnetic transition. In CuPd alloy in 0 K, configurational free energy difference between bcc and fcc lattice resulting to phase separation is only about 1.1 kJ/mol-atoms. Low temperature energetics and magnetic phase diagrams have good quantitative agreement with available experimental and theoretical results. Finite temperature properties of the alloys are in good qualitative agreement with experimental results.     

In situ observation of Cu segregation and phase nucleation at a solid–liquid interface in an Al alloy

This paper presents a systematic study comparing experimental in situ transmission electron microscopy observation of microstructural and compositional evolution with complementary thermodynamic calculations, to better understand the redistribution of solute elements and the nucleation behavior of different phases in a commercial Al-alloy powder (AA390). The results show that Cu segregation to the solid Si–liquid Al interface, as well as the significant undercooling achieved in the liquid under non-equilibrium conditions because the Al phase cannot nucleate homogeneously, play a important roles in nucleating Al₂Cu at the interface prior to the Mg₂Si phase in the alloy. Although Cu segregation can occur at various locations along the interface, the Al₂Cu phase appears to preferentially nucleate at a high-index Si–liquid interface as opposed to a low-index one. The Cu concentration during segregation remains essentially constant with time, indicating that the observed segregation behavior is a thermodynamic and not a kinetic phenomenon. These in situ observations and complementary thermodynamic calculations substantially enhance our understanding of potential crystal nucleation and growth processes.     

The structure and mobility of the intervariant boundaries in 18R martensite in a Cu–Zn–Al alloy

Detailed crystallographic analysis was carried out on the martensitic transformation and the various variant combinations in 18R martensite in a Cu–Zn–Al alloy. The self-accommodation of martensitic shear strain is quite perfect within a variant group, but not effective or even does not exist for variant combinations which belong to different groups. Twenty-three unique variant combinations between 24 martensite variants can be divided into four groups, i.e. reflection twin, 180° rotation twin, 120° rotation twin and 90° rotation twin. TEM and HREM observations show that the A/C boundary is straight, well-defined and perfectly coherent, the A/B boundary is irrational, coherent and gradually curved, and the A/D boundary is stepped. The A/C and A/B boundaries have obvious mobility, and the mobility is not effective for A/D boundary. The interplate group boundaries are curved, blurred and immobile. The morphology, structure and mobility of interplate boundary are all related to the degree of self-accommodation and the misorientation of twin boundary.     

Kinetics and fracture behavior under cycle loading of an Al–Cu–Mg–Ag alloy

The behavior of aluminum alloy AA2139 subjected to T6 treatment, including solution treatment and artificial aging, has been studied using cyclic loading with a constant total strain amplitude. Upon low-cyclic fatigue in the range of total strain amplitudes ε_ac of 0.4–1.0%, the cyclic behavior of the AA2139-T6 alloy is determined by the processes that occur under the conditions of predominance of the elastic deformation over plastic deformation. The AA2139 alloy exhibits stability to cyclic loading without significant softening. The stress-strained state of the alloy upon cyclic loading can be described by the Hollomon equation with the cyclic strength coefficient K' and the cyclic strain-hardening exponent n' equal to 641 MPa and 0.066, respectively. The dependence of the number of cycles to fracture on the loading amplitude and its components (amplitudes of the plastic and elastic deformation) is described by a Basquin–Manson–Coffin equation with the parameters σ′/E = 0.014, b =–0.123, ε′_f= 178.65, and c =–1.677.     

Enhanced creep performance in an Al–Cu–Mg–Ag alloy through underageing

Tests at 130 °C and 150 °C have shown that the creep resistance of an Al–Cu–Mg–Ag alloy is significantly increased if it is heat-treated at an elevated temperature to an underaged condition rather than the fully hardened, T6 temper. This beneficial effect of underageing is manifest in reduced rates of secondary creep. Similar results have been obtained for the commercial alloy 2024. Delays at ambient temperature after underageing and before testing lead to secondary precipitation and a progressive decrease in creep performance that eventually reverts to close to that for the T6 condition. This detrimental effect may be overcome by slow cooling from the underageing temperature, which arrests or impedes subsequent secondary precipitation. Microstructural observations suggest that the enhanced creep resistance in the underaged condition is a consequence of the presence of “free” solute in solid solution that is not yet involved in precipitation.     

Oxidation behavior of a Zr–Cu–Al–Ni amorphous alloy in air at 300–425 °C

The oxidation behavior of Zr–30Cu–10Al–5Ni bulk metallic glass and its crystalline counterpart was studied over the temperature range of 300–425 °C in dry air. In general, the oxidation kinetics of both amorphous and crystalline alloys followed a two- or three-stage parabolic rate law at T⩾350 °C, while at 300 °C the amorphous alloy oxidized following a linear behavior. The oxidation rate constants for the amorphous alloy are slightly higher than those for the crystalline alloy at 350–400 °C. The scale formed on the amorphous alloy consists of mainly tetragonal-ZrO₂ at 300 °C, while a mixture of monoclinic-ZrO₂ (m-ZrO₂) and tetragonal-ZrO₂ (t-ZrO₂) and some CuO were detected at higher temperatures. The scale formed on the crystalline alloy, on the other hand, consists of mainly Al₂O₃, some tetragonal-ZrO₂, and a slight amount of monoclinic-ZrO₂ at 300 °C. At higher temperatures, the crystalline alloy consists of mainly monoclinic-ZrO₂, some CuO and Cu₂O, and limited tetragonal-ZrO₂. It is suggested that the formation of Al₂O₃ (at 300 °C) and CuO/Cu₂O (at 350-400 °C) on the crystalline alloy is responsible for the reduced oxidation rates as compared with those of amorphous alloy.     

Evolution of bonding interface in solid–liquid cast-rolling bonding of Cu/Al clad strip

Cu/Al clad strips are prepared using solid–liquid cast-rolling bonding (SLCRB) technique with a d160 mm × 150 mm twin-roll experimental caster. The extent of interfacial reactions, composition of the reaction products, and their micro-morphology evolution in the SLCRB process are investigated with scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD). In the casting pool, initial aluminized coating is first generated on the copper strip surface, with the diffusion layer mainly consisting of α(Al)+CuAl₂ and growing at high temperatures, with the maximum thickness of 10 μm. After sequent rolling below the kiss point, the diffusion layer is broken by severe elongation, which leads to an additional crack bond process with a fresh interface of virgin base metal. The average thickness is reduced from 10 to 5 μm. The reaction products, CuAl₂, CuAl, and Cu₉Al₄, are dispersed along the rolling direction. Peeling and bending test results indicate that the fracture occurs in the aluminum substrate, and the morphology is a dimple pattern. No crack or separation is found at the bonding interface after 90°–180° bending. The presented method provides an economical way to fabricate Cu/Al clad strip directly.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Experimental investigation of phase equilibria in the Ru–Fe–Al system at 1473 K

The low-Al part of the ternary Ru–Fe–Al phase diagram at 1473 K is established in this work. Due to the very promising properties of B2 ruthenium aluminide, the investigation of the B2 region of this system is of special interest. The experimental work includes diffusion methods, as well as quenching of annealed single-phase and two-phase alloys. The results of the different methods are in good agreement. Optical and scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction are used to investigate the samples. It is shown in this work that a three-component B2 phase exists over a wide composition range.     

Strength properties and structure of a submicrocrystalline Al–Mg–Mn alloy under shock compression

The results of studying the strength of a submicrocrystalline aluminum A5083 alloy (chemical composition was 4.4Mg–0.6Mn–0.11Si–0.23Fe–0.03Cr–0.02Cu–0.06Ti wt % and Al base) under shockwave compression are presented. The submicrocrystalline structure of the alloy was produced in the process of dynamic channel-angular pressing at a strain rate of 10⁴ s^–1. The average size of crystallites in the alloy was 180–460 nm. Hugoniot elastic limit σ_HEL, dynamic yield stress σ_y, and the spall strength σ_SP of the submicrocrystalline alloy were determined based on the free-surface velocity profiles of samples during shock compression. It has been established that upon shock compression, the σ_HEL and σ_y of the submicrocrystalline alloy are higher than those of the coarse-grained alloy and σ_sp does not depend on the grain size. The maximum value of σ_HEL reached for the submicrocrystalline alloy is 0.66 GPa, which is greater than that in the coarse-crystalline alloy by 78%. The dynamic yield stress is σ_y = 0.31 GPa, which is higher than that of the coarse-crystalline alloy by 63%. The spall strength is σ_sp = 1.49 GPa. The evolution of the submicrocrystalline structure of the alloy during shock compression was studied. It has been established that a mixed nonequilibrium grain-subgrain structure with a fragment size of about 400 nm is retained after shock compression, and the dislocation density and the hardness of the alloy are increased.     

The effect of long term creep exposure on the microstructure and properties of an underaged Al–Cu–Mg–Ag alloy

An underaged Al–Cu–Mg–Ag alloy shows zero secondary creep after 20,000 h at 130 °C and a stress of 200 MPa. Specimens exposed with and without an applied load reveal that dynamic precipitation of θ^′ and S^′(S) occurs on dislocations during primary creep, whereas σ phase forms in the matrix of the unloaded specimen.