Mechanisms governing cyclic fracture in an Al–Cu–Li alloy

Abstract

The fracture behaviour of a peak-aged, partially recrystallized Al–4·5Cu–1·21Li–0·51Mn–0·20Cd alloy has been investigated as a function of strain amplitude, stress intensity, and environment. It was found that the failure was predominantly intergranular separation, regardless of the environment, stress intensity, or strain amplitude, and that the fracture behaviour was influenced mostly by intrinsic microstructural features, rather than the nature of the environment. The shearable nature of matrix strengthening precipitates, large recrystallized grains, and precipitate-free zones along the high-angle grain boundaries aid in localizing the deformation, resulting in low-energy intergranular fracture. The iron- and silicon-rich intermetallic precipitates in the alloy promote void nucleation following fracture of the particle. A model is proposed which suggests the need for high stresses and strains for the initiation and spontaneous growth and coalescence of microvoids. The mechanisms of fracture behaviour of the alloy are discussed in terms of several concurrent processes involving strength of the material, intrinsic microstructural effects, deformation behaviour, state of stress, and strain.

MST/497     

Influence of high-pressure torsion on microstructural evolution in an Al–Zn–Mg–Cu alloy

A commercial age-hardenable Al-7136 alloy was successfully processed by high-pressure torsion (HPT) at room temperature through 1/8 to 4 turns. Microhardness measurements showed significant hardening even after 1/8 turn with the average hardness value reaching a maximum after 1 turn and then slowly decreasing. Higher hardness values were attained by processing the alloy through one pass of equal-channel angular pressing in a supersaturated condition at room temperature and then applying HPT for 1 or 2 turns. Microstructural observations revealed the possibility of achieving true nanometer grain sizes of <100 nm after processing at room temperature. There were variations in hardness with imposed strain due to the fragmentation and subsequent growth of precipitates during processing.     

Critical conditions for the occurrence of quench cracking in an Al–Zn–Mg–Cu alloy

The occurrence of quench cracking in small cuboidal samples of aluminium alloy AA7150 was determined to be related to the maximum temperature difference (?T _max) between various locations within samples during quenching. When ?T _max between different locations is between 96 and 124 °C, there is some risk of quench cracking under various quenching conditions. When the ?T _max value is higher than 124 °C, quench cracks cannot be avoided. Quench cracks preferentially occur at sample corners and edges and are preferentially propagating in the short transverse direction–long transverse direction plane. Finite element modelling results indirectly indicate that the quench cracking should occur at the very early stages of the quenching process. Microscopy reveals that the quench cracking mode is intergranular, and cracks preferentially occur at high-angle grain boundaries with an average misorientation angle of ~42°. Moreover, quench cracks can penetrate through the whole thickness of a sample quenched from 495 into 20 °C water. Fractography reveals that no constituent particles exist in the quench fracture region, indicating that, unlike impact fracture, the occurrence of quench cracks is not dependent on the presence of coarse particles.     

Eutectic spacing and faults of directionally solidified Al–Al3Ni eutectic

The Al–Al₃Ni eutectic was directionally solidified at a thermal gradient of 4.5 K/mm in a vacuum Bridgman–type furnace in order to study eutectic spacing selection criterion.The microstructure was examined in transverse and longitudinal sections and the interrod spacings were measured at different growth velocity. It has been shown that the interrod spacing is not unique and displays a limited range for rodlike Al–Al₃Ni eutectic alloy. The initial growth velocities are not responsible for the eutectic spacing range, while such faults as branching, endingand diameter change have a significant influence on the eutectic spacing adjustment.     

Dynamics of a Charged Fluctuator in an Al–AlOx–Al Single-Electron Transistor

We report detailed observations of random-telegraph charge fluctuations in a two-junction Al–AlO_x–Al single-electron transistor (SET). We measured the fluctuations from 85 mK to 3 K and observed that the SET switched between two states, causing charge shifts of Q_o=0.1±0.025 e on the SET's island. The transition rate out of each state was periodic in the gate voltage, varied non-monotonically with the device bias voltage, and was independent of the temperature below about 0.3 K. We discuss two effects which could contribute to the behavior of the transition rates, including heating of the defect by the island conduction electrons and inelastic scattering between the defect and electrons flowing through the SET.     

Eutectic spacing and faults of directionally solidified Al–Al3Ni eutectic

The Al–Al₃Ni eutectic was directionally solidified at a thermal gradient of 4.5 K/mm in a vacuum Bridgman-type furnace in order to study eutectic spacing selection criterion. The microstructure was examined in transverse and longitudinal sections and the interrod spacings were measured at different growth velocity. It has been shown that the interrod spacing is not unique and displays a limited range for rodlike Al–Al₃Ni eutectic alloy. The initial growth velocities are not responsible for the eutectic spacing range, while such faults as branching, ending and diameter change have a significant influence on the eutectic spacing adjustment.     

Dissolution kinetics of primary silicon in hypereutectic Al–Si melt

The dissolution process of primary silicon particles in Al–18%wt silicon alloy was studied both by a melt overheating experiment and by theoretical analysis. A dissolution model of primary silicon in the melt was established based on atomic diffusion and taking account to interface reaction and curvature of particles. The results show that the theoretical curve agrees with the experimental curve at an overheating temperature of 1100°C. However, there was some deviation at 700°C due to retained silicon clusters in the melt at lower temperature. Therefore, the model is in accord with experiment when not considering the influence of retained silicon clusters.     

Dissolution kinetics of primary silicon in hypereutectic Al–Si melt

The dissolution process of primary silicon particles in Al-18%wt silicon alloy was studied both bya melt overheating experiment and by theoretical analysis. A dissolution model of primary silicon in the melt was established based on atomic diffusion and taking account to interface reaction and curvature of particles. The results show that the theoretical curve agrees with the experimental curve at an overheating temperature of 1100ºC. However, there was some deviation at 700ºCdue to retained silicon clusters in the melt at lower temperature. Therefore, the model is in accord with experiment whennot considering the influence of retained silicon clusters.     

Effect of creep-aging processing on corrosion resistance of an Al–Zn–Mg–Cu alloy

Creep-aging forming, combining both the aging treatment and forming process, has recently drawn much attention of researchers. In this study, the effects of creep-aging processing on the corrosion resistance of an Al–Zn–Mg–Cu alloy are studied. Results show that the corrosion resistance of the studied Al–Zn–Mg–Cu alloy is sensitive to creep-aging processing parameters (creep-aging temperature and applied stress). With the increase of creep-aging temperature, the corrosion resistance first increases and then decreases. Increasing the applied stress can deteriorate the electrochemical corrosion resistance and improve the exfoliation corrosion resistance. The creep-aging processing can change the size and distribution of precipitates in the aluminum matrix, which significantly affects the corrosion resistance. The discontinuous grain boundary precipitates and narrow precipitate-free zones can enhance the corrosion resistance.     

DSC and TEM characterizations of thermal stability of an Al–Cu–Mg–Ag alloy

Assessment of long-term stability of an aluminium alloy exposed to elevated temperatures is important in the design of lightweight aerospace structures. The manner in which differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) are used together in monitoring microstructural evolution, and thereby assess phase stabilities in an Al–5.1Cu–0.8 Mg–0.5 Ag–0.7 Mn–0.13 Zr (wt%) alloy, are described. DSC thermograms of the alloy, spanning room temperature to 400°C, revealed the presence of two endotherms and an exotherm. TEM investigation has identified these thermal events to be associated with , S, and precipitates. Quantitative TEM was used to measure diameter, thickness, number density, and volume fraction of the precipitates in the alloy exposed at 135°C for times as long as 3000 h. The quantitative TEM results are correlated with the DSC signatures relating to precipitation, dissolution, and coarsening reactions affecting the , S, and precipitates in the exposed alloy.     

Grain refinement of Cu–Zn–Al and Cu–Al–Ni by Ti addition

Abstract

To obtain fine grained Cu based shape memory alloys after thermomechanical processing, Ti is added to β-Cu–Zn–Al or β-Cu–Al–Ni as a particle forming element. This work consists of a study of the mechanism that controls the grain growth limiting effect during the final annealing treatment. A critical evaluation of the grain growth models in particle containing materials and comparison with the experimental results lead to the conclusion that the grain growth inhibition is mainly attributable to the effect of the second phase particles but also to the influence of Ti atoms in solid solution.

MST/678     

Influence of cryogenic thermal treatment on mechanical properties of an Al–Cu–Mg alloy

In this study, a cryogenic thermal treatment is developed and its effects on mechanical properties and precipitates are investigated. Water-quenched samples were immersed in liquid nitrogen and reheated in hot oil at 180°C or boiling water for 5?min. Finally, the samples were artificially aged at 190°C for 12?h. The results indicated a notable increase of about 75?MPa in the ultimate tensile strength in comparison to T6 heat-treated alloy. TEM observations revealed that the S(S′) precipitates were fine and uniformly distributed in the microstructure due to reheating in hot oil and subsequent aging treatment.     

Hot tensile behavior of an extruded Al–Zn–Mg–Cu alloy in the solid and in the semi-solid state

This is the first reported research into the tensile behavior of as-deformed Al–Zn–Mg–Cu alloy in the semi-solid state. Tensile tests of extruded 7075 aluminium alloy were carried out in the high temperature solid and semi-solid states. Based on the tensile results and microstructural examination, the tensile behavior can be divided into three stages according to the effect of liquid: one behaves in predominantly ductile character between 400 and about 520 °C (f_l ∼ 0.31%), one is governed by both of solid and liquid between 520 and 550 °C (f_l ∼ 2%), and almost completely dominated by liquid above ∼550 °C. A brittle temperature range (519–550 °C) is proposed, in which the as-deformed Al–Zn–Mg–Cu alloy exhibits large crack probability. An equation based on ultimate tensile stress and temperature is proposed.     

Relativity of medium range order in molten Cu–Sn alloys and phase diagram

Abstract

Evolution of the medium range order (MRO) in Cu-Sn alloy melts with different compositions at the same superheat temperature has been studied. A prepeak, which characterises the MRO, appeared in front of the main peak. Based on the positions Q_pp and heights H_pp of the prepeak on the structure factors of liquid Cu-Sn alloys, it was found that the Q_pp was higher around Cu₅Sn, and the highest at Cu₅Sn while the H_pp was smaller, and a minimum at Cu₅Sn. As proved by the first peak of the pair correlation functions and the coordination numbers, the molten structure of Cu-Sn changes from order to disorder largely when the tin concentration is in the range of 35.3-60 wt-%. In addition, a shoulder appears on the right side of the first peak of the pair correlation function and shifts to small r (real space).     

Microstructure evolution in abrasion-induced surface layer on an Al–Zn–Mg–Cu alloy

An altered surface layer forms on an Al–Zn–Mg–Cu alloy during surface preparation by abrasion with grinding paper. Strain-induced dissolution of η′/η precipitates and formation of nano-sized subgrains were observed in the surface layer with thickness of several hundred nanometers. The segregation of solute elements along dislocations and subgrain boundaries and the precipitation of Al₂Cu phase at the sub-boundaries and the free surface were related to enhanced diffusion accelerated by deformation-induced vacancies, dislocations and subgrain boundaries. The microstructure evolution in this layer is mainly attributed to the shear strain and is modified by temperature rise during surface abrasion. The unique surface microstructural changes produced by abrasion might alter the surface properties.     

Microstructural evolution and mechanical properties of dissimilar Al–Cu joints produced by friction stir welding

5A02 aluminum alloy and pure copper were joined by friction stir welding (FSW). A defect-free joint was obtained when one of process parameters, i.e. the traverse speed was lowered from 40 mm/min to 20 mm/min. A good mixing of Al and Cu was observed in the weld nugget zone (WNZ). A large amount of fine Cu particles were dispersed in the upper part of the WNZ producing a composite-like structure. In the lower part, nano-scaled intercalations were observed and identified by transmission electron microscopy (TEM). These layered structures were subsequently confirmed as Al₄Cu₉ (γ), Al₂Cu₃ (ε), Al₂Cu (θ), respectively. Formation of these microstructures caused an inhomogeneous hardness profile. Particularly, a distinct rise in hardness was noticed at the Al/Cu interface. Excellent metallurgical bonding between Al and Cu gave rise to good behaviors in the tensile and bending strength.     

Structure of metastable precipitate in some Al–Cu–Mg–Ag alloys

Abstract

The high strength of some Al–Cu–Mg–Ag alloys has been attributed to very thin (~2·5 nm), but broad, hexagonal-shaped precipitates. Previous work has shown that the precipitates have a hexagonal unit cell, but different lattice parameters have been reported. In the present paper, the intensities of X-ray diffraction reflections from the precipitates have been measured on Buerger precession photographs, and it is shown that the crystal structure is monoclinic (space group P2/m) with the parameters a = b = 0·496 nm, c = 0·848 nm, γ = 120°. The special values of these parameters confer a hexagonal symmetry on the lattice. This unusual structure is a slightly distorted form of θ-CuAl₂, to which it appears to change after long aging times at 200°C.     

Nucleation and growth of precipitates in Al–Cu–Mg–Ag alloys

Abstract

Adding small amounts (<1wt-%) of both magnesium and silver to an aluminium alloy containing about 4 wt-% Cu causes precipitates with a hexagonal structure (Ω-phase) to form on {111} planes of the aluminium lattice. Precipitation of θ′ on {100} planes may also occur, the relative proportions of the two types of precipitate being dependent on the levels of magnesium and silver, e.g. ~0·7 wt-% of each element almost entirely suppresses θ′ formation. Even when θ′ does form in parallel with Ω-phase, on prolonged aging it tends to dissolve in favour of Ω growth. Using an X-ray technique to establish foil thickness, the relative amounts of Ω and θ′ precipitate have been measured as a function of aging time, analysis of the data showing that growth is diffusion controlled with an activation energy of 136± 15 kJ mol^?1.

MST/648     

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression was characterized in present work by high-temperature testing and transmission electron microscope (TEM) studies. The true stress–true strain curves exhibited a peak stress at a critical stain. The peak stress decreased with increasing deformation temperature and decreasing strain rate, which can be described by Zener–Hollomon (Z) parameter in hyperbolic sine function with the deformation activation energy 277.8 kJ/mol. The processing map revealed the existence of an optimum hot-working regime between 390 and 420 °C, under strain rates ranging from 0.1 to 1 s⁻¹. The main softening mechanism of the alloy was dynamic recovery at high lnZ value; continuous dynamic recrystallization (DRX) occurred as deformed at low lnZ value. The dynamic precipitation of Al₃Zr and Al₂₀Cu₂Mn₃ dispersoids during hot deformation restrained DRX and increased the hot deformation activation energy of the alloy.