Influence of retrogression and reaging on microstructure and properties of 8090 Al–Li alloy

Abstract

An investigation was made into the influence of a retrogression and reaging treatment on the microstructure, tensile properties, and stress corrosion cracking resistance of 8090 Al–Li alloy. The results show that retrogression of the material at 230°C for 40 min or 325°C for 1·5 min, and then reaging to the peak aged condition, can result in an improved combination of tensile strength and stress corrosion cracking resistance. Through retrogression and reaging treatment, the alloy almost achieves the strength of the peak aged state and the stress corrosion cracking resistance of the overaged state. Transmission electron microscopy indicates that the δ′ phase dissolves during retrogression and reprecipitates during reaging, thus increasing the strength. The T₂ phase precipitates and grows during both retrogression and reaging, which results in the increase of stress corrosion cracking resistance.

MST/1670     

Effect of microstructure on fatigue behaviour of cast Al–7Si–Mg alloy

Abstract

The fatigue behaviour of a cast Al–7Si–Mg alloy, conforming to A356, has been studied. Specimens of this material were tested in both the as cast condition and a solution treated and aged condition. It was observed that the size, number, and position of casting defects influenced the fatigue life very strongly. This marked effect nearly hides that of the heat treatment. Nevertheless, if the analysis is carried out considering only results obtained from sound specimens it is revealed that the heat treatment causes an improvement in the fatigue resistance of the alloy.     

Effect of predeformation on microstructure and mechanical properties of Al–Cu–Li–Zr alloy containing Sc

Abstract

The effects of prior cold deformation on the microstructures and the room temperature mechanical properties of an Al–3·5Cu–1·5Li–0·22(Sc + Zr) alloy have been observed by using TEM and tensile test at room temperature. The results show that the alloy has the character of aging hardening, and the major phase of precipitation and strengthening is T₁ phase. The result also show that prior cold deformation leads to more dispersive and uniform distribution of T₁ precipitations. It accelerates aging response, causes earlier aging peak occurrence, and enhances strength greatly. However, the plasticity of the alloy is declined with prior cold deformation. In contrast, excessive prior cold deformation causes coarsening and heterogeneous distribution of T₁ phase. It also reduces the strength of the alloy, therefore, influences the composite properties of the alloy. The favourable prior cold deformation is about 3·5% under the experimental condition.     

Microstructural development and superplasticity in Al–Li 8090 alloy

Abstract

The mechanical behaviour of an Al–Li–Mg–Cu–Zr 8090 alloy at a temperature of 515°C and strain rates in the range 10^?4?10^?2s^?1 was measured by tensile testing. The greatest strain rate sensitivity was measured in the middle of that strain rate range, and did not change significantly with strain. Large abrupt changes in strain rate during testing showed that the strain rate history had a significant effect, especially slow prestraining which gave a relative increase in flow stress and a reduction in rate sensitivity compared with testing at a constant rate to the same strain. The evolution of grain size was measured, and there was evidence that this aspect of the material microstructure could be used to explain the observed behaviour. This view was reinforced by the ability of a transition model of superplasticity, together with a simple model of the evolution of grain size distribution, to reproduce the essential features observed in testing with large changes in strain rate.

MST/3351     

Microstructural evolution and superplastic behavior in friction stir processed Mg–Li–Al–Zn alloy

A Mg–Li–Al–Zn alloy was friction stir processed (FSP) under water, and the microstructures and superplastic behavior in the FSP alloy were investigated. The FSP Mg–Li–Al–Zn alloy consisted of a mixed microstructure with fine, equiaxed, and recrystallized α (hcp) and β (bcc) grains surrounded by high-angle grain boundaries, and the average grain size of the α and β grains was ~1.6 and ~6.8 μm, respectively. The fine α grains played a critical role in providing thermal stability for the β grains. The FSP Mg–Li–Al–Zn alloy exhibited low-temperature superplasticity with a ductility of 330 % at 100 °C and high strain rate superplasticity with ductility of ≥400 % at 225–300 °C. Microstructural examination and superplastic data analysis revealed that the dominant deformation mechanism for the FSPed Mg–Li–Al–Zn alloy is grain boundary sliding, which is controlled by the grain boundary diffusion in the β phase.     

Influence of Sc and Zr additions on microstructure and properties evolution of Al–Zn–Mg alloy

Journal of Materials Science - In this study, the effects of different Sc?+?Zr compound addition on the tensile properties, impact toughness, stress corrosion cracking (SCC) properties,...     

Effect of stamping deformation on microstructure and properties evolution of an Al–Mg–Si–Cu alloy for automotive panels

The effect of 5 % tensile deformation, which simulates the stamping process of Al–Mg–Si–Cu automotive outer panels, on the microstructural evolution during age strengthening, has been investigated. In addition, its benefit on key mechanical properties including hardness, yield strength, ductility, and corrosion resistance has been linked to the microstructural features. It was found that the aging precipitation sequence, SSSS → clusters and G.P. zones → β″ → β′ + Q′ → Q, was not influenced by the dislocations introduced through the stamping deformation prior to aging. On the other hand, stamping deformation could promote the formation of precipitates and refine the precipitates because of the enhanced heterogeneous nucleation and the accelerated precipitation kinetics, leading to superior strength of the alloy at the early stage. Meanwhile, the larger amount of Cu incorporated into nanoprecipitates leads to better intergranular corrosion resistance of the stamped alloy compared with the unstamped one. Due to the reduction in free Si amount at grain boundaries, the formation of fine subgrain structures and the increase of dislocation accumulation, the ductility of the stamped alloy was increased.     

Effect of joint stiffness on peel strength of diffusion bonded joints between Al–Li 8090 alloy sheet

Abstract

The peel behaviour of diffusion bonded joints between Al–Li 8090 alloy sheet depends upon joint geometry, sheet thickness, and the local stiffness of the bonded joint. The local stiffness was increased by bonding 8090 metal matrix composite onto the faces of the joint. At the superplastic forming temperature of 530°C the peel strengths of solid state or liquid phase diffusion bonded joints at peak load were increased from 5–7 N mm^?1 to >8 N mm^?1. This led to superplastic deformation of the sheet without peel fracture at the bonded joint. After air cooling and aging, the corresponding room temperature peel strengths were 174–252 N mm^?1, compared with 30–54 N mm^?1 for an unstiffened joint, an increase by a factor of 3·2–8·4. It was concluded that stiffened bonded joints would enable multiple thin sheet structures to be manufactured in Al–Li 8090 alloy via a diffusion bonding/superplastic forming (DB/SPF) technique. A DB/SPF technique for a three sheet structure is described.

MST/1687     

Compressive creep behaviour of Mg–Li–Al alloy

Abstract

The compressive creep behaviour of as cast Mg–14Li–1·3Al (wt-%) alloy was investigated in the temperature range of 20?85°C and under different compressive stress in the range of 37·3–74·6 MPa with special apparatus. Primary creep deformation and steady creep rate increase with temperature and applied stress. The compressive creep behaviour obeys an empirical equation ln t=C?nln σ + Q/RT, where t is the time to a selected creep strain, σ is the applied stress, T is the absolute temperature, R is the gas constant, and C, n, and Q are constants for the experimental alloy. The average values of the exponent n and the creep activation energy Q are 4·33 and 101·13 kJ mol^?1 respectively. The creep rate controlling mechanism is the dislocation climb and the lattice diffusion of Li in the experimental alloy under the testing conditions.     

Influence of chromium on microstructure and etching behaviour of new Ni–Fe–Al based alloy

In the present study, the influence of chromium on the microstructure and etching behaviour of the polycrystalline alloy Ni–13Fe–8Al–4Ti (at-%) has been examined. The alloy was recently designed for nanomembrane fabrication, but although it showed the necessary γ/γ′ microstructure with cubic, well aligned γ′ precipitates, it proved to be unsuitable for nanomembrane fabrication as the γ matrix was dissolved during chemical etching. To obtain the passivation of the γ matrix, chromium has been added in further modifications containing 1, 2, 3 and 4 at-% chromium. Moreover, the influence of heat treatment and the different cooling rates of heat treatment in air/vacuum have been investigated. For chemical phase extraction, the application of the chemical etchants MoO₃ acid and ‘G’ etchant has been examined, the formability was characterised by Vickers hardness testing. The main purpose of the present study, namely the passivation of the γ matrix, could be achieved by the addition of 4 at-% chromium and etching with ‘G’ etchant.     

Effect of Al on the microstructure and properties of Sn–0.7Cu solder alloy

Effect of Al on the microstructure and mechanical properties were investigated. The results showed that Al could depress the formation of eutectic phase in Sn–Cu–Al solder alloy. The intermetallic compounds of Sn–0.7Cu–0.03Al were refined compared with that of Sn–0.7Cu–0.015Al. Segregated CuAl intermetallic compound was observed in Sn–0.7Cu–0.15Al and Sn–0.7Cu–0.5Al solder alloy. Sn-whisker was observed on the polished surface of Sn–0.7Cu–0.15Al and Sn–0.7Cu–0.5Al. The ultimate tensile strength of Sn–0.7Cu–0.03Al and Sn–0.7Cu–0.5Al was found to be higher than that of Sn–0.7Cu–xAl (x = 0, 0.015 and 0.15). The elongation of Sn–0.7Cu–0.015Al was the highest. The creep performance of Sn–0.7Cu–0.03Al and Sn–0.7Cu–0.5Al was similar and higher than that of Sn–0.7Cu and Sn–0.7Cu–0.15Al.     

Effect of rolling speed on microstructure and age-hardening behaviour of Al–Mg–Si alloy produced by twin roll casting process

In the present investigation the microstructure and age-hardening behaviour of Al–Mg–Si alloy prepared by twin roll casting (TRC), varying rolling speed (i.e., 3, 4, and 5 rpm), were studied. The as-cast samples were subjected to optical microscopy (OM) to understand the effect of rolling speed on the alloy microstructure. Significant difference in grain size and shape was observed for all the alloys in as-cast condition. The as-cast samples were solutionized at 540 °C for 2 h followed by isothermal heating at 180 °C for different time intervals. Thereafter, the as-cast and solutionized samples were subjected to scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). Segregation of solute atoms at grain boundaries were observed for both as-cast as well as solutionized samples. Age-hardening results show that time to attain peak hardness decreases for the alloy produced by higher rolling speed.     

Effect of thermomechanical processing on evolution of superplastic microstructures in Al–Cu–Zr alloys

Abstract

The role of alloying elements and the effect of prior cold work on microstructural evolution in Supral type alloys during high temperature straining has been investigated by examining a range of binary and ternary alloys containing varying amounts of Cu and/or Zr. After hot compression, specimens were rapidly cooled to retain the evolved structure and then examined using a range of techniques including optical microscopy, electron microscopy (scanning and transmission), and boundary misorientation measurements from electron backscattering patterns. The high temperature evolution of a fine grain, high angle boundary structure, without nucleation, and growth, was seen to be strain induced and required prior work and the presence of both Cu and Zr. Of the various mechanisms which have been proposed for the continuous recrystallisation of Supral type alloys, the present work is consistent with the importance of strain induced geometrical dynamic recrystallisation in microstructural evolution, and has identified the role of the alloying additions and the processing variables.     

Effects of microadditions of vanadium and boron on age hardening and mechanical properties of Al–Li alloy 8090

Abstract

The effects of micro additions of vanadium and boron on the age hardening, microstructure, mechanical properties, deformation, and fracture behaviour of Al–Li alloy 8090 have been investigated. The addition of 0·6% V to 8090 alloy increases the peak hardness by 13%, the yield strength by 23%, and the ductility by 85% when optimally aged (190°C for 22·5 h). The enhancement of these properties is attributed to the refinement of S′ and δ′ precipitates and the promotion of extensive cross-slip during deformation of the vanadium containing alloys. The same amount of vanadium reduces the impact toughness of 8090 by 50%, which is attributed to the presence of coarse particles of primary Al₁₁V precipitate. Increasing the cooling rate during solidification, or adding 0·1%B, restores and slightly increases the impact toughness of the vanadium containing alloy. The effect of cooling rate is related to the amount and size of Al₁₁V precipitates, which decrease with increasing cooling rate. The addition of boron appears to modify the morphology of the Al₁₁V precipitate.

MST/1584     

Effect of two step strain rate on superplastic behaviour of Al–Zn–Mg–Cu–Zr alloy 7010 containing Sc

Abstract

A suitable thermomechanical process, different from the Rockwell route, before the superplastic deformation together with the utilisation of a ‘two step strain rate’ during the superplastic deformation has been employed to evaluate the total percentage elongation in a high strength Al–Zn–Mg–Cu–Zr alloy containing Sc. It is shown that the utilisation of a two step strain rate under appropriate combinations of temperature and strain rate results in superior values of elongations.     

Effects of enhanced solution treatment on microstructure and mechanical properties of Al–Cu–Li–Sc alloy

Two enhanced solution treatments (ESTs) were applied to an Al–Cu–Li–Sc alloy. Results showed that the ESTs reduced the amount and size of the soluble phases, and promoted the recrystallisation of the α–Al matrix and the precipitation of the Al₂CuLi precipitate (T₁), which improved the yield strength, tensile strength and elongation of the alloy. Although the precipitation strengthening of the T₁ phase and the strengthening resulting from grain refinement of the α–Al matrix caused by the recrystallisation contributed equally to the strength increment, the EST process led to a greater proportional increase in the strengthening resulting from grain refinement than it did in the precipitation strengthening of the alloy.     

Effect of in situ phases on microstructure and properties in Al–Bi immiscible alloy

Al–Bi immiscible alloy is of particular interest as potential self-lubricating wear materials with a homogeneous distribution of minority phase. However, it is difficult to obtain a homogeneous microstructure by conventional casting methods due to liquid phase separation of Al–Bi immiscible alloy. We have developed a new strategy to restrain liquid phase separation and improve the properties of Al–Bi immiscible alloy by in situ phases. The in situ AlB₂ phase acts as heterogeneous nucleation site to accelerate the nucleation and slow down the velocity of the Bi-rich droplet, resulting in a significant size reduction and a homogeneous microstructure of Al–Bi immiscible alloy. The self-lubricating wear resistance of Al–Bi immiscible alloy can be further enhanced by in situ Al₂Cuphase.     

Effect of nickel on the microstructure and mechanical property of die-cast Al–Mg–Si–Mn alloy

The effect of nickel on the microstructure and mechanical properties of a die-cast Al–Mg–Si–Mn alloy has been investigated. The results show that the presence of Ni in the alloy promotes the formation of Ni-rich intermetallics. These occur consistently during solidification in the die-cast Al–Mg–Si–Mn alloy across different levels of Ni content. The Ni-rich intermetallics exhibit dendritic morphology during the primary solidification and lamellar morphology during the eutectic solidification stage. Ni was found to be always associated with iron forming AlFeMnSiNi intermetallics, and no Al₃Ni intermetallic was observed when Ni concentrations were up to 2.06 wt% in the alloy. Although with different morphologies, the Ni-rich intermetallics were identified as the same AlFeMnSiNi phase bearing a typical composition of Al_[100–140](Fe,Mn)_[2–7]SiNi_[4–9]. With increasing Ni content, the spacing of the α-Al–Mg₂Si eutectic phase was enlarged in the Al–Mg–Si–Mn alloy. The addition of Ni to the alloy resulted in a slight increase in the yield strength, but a significant decrease in the elongation. The ultimate tensile strength (UTS) increased slightly from 300 to 320 MPa when a small amount (e.g. 0.16 wt%) of Ni was added to the alloy, but further increase of the Ni content resulted in a decrease of the UTS.     

Effect of cooling aging on microstructure and mechanical properties of an Al–Zn–Mg–Cu alloy

In the present work, Al–Zn–Mg–Cu alloy was aged by non-isothermal cooling aging treatment (CAT). At high initial aging temperature (IAT), the hardness was decreased with the decreased cooling rate. However, when IAT was lower than 180 °C, the hardness was increased with the decreased cooling rate. Conductivity was increased with the decreased cooling rate regardless of IAT. The tensile strength, yield strength and conductivity of Al alloy after (200–100 °C, 80 °C/h) CAT were increased 2.9%, 8.1% and 8.3% than that after T6 treatment, respectively. With an increase of IAT and decrease of cooling rate, the fine GP zone and η′ phase were transformed to be larger η′ and η precipitates. Moreover, continuous η phase at grain boundary was also grown to be individual large precipitates. Cooling aging time was decreased about 90% than that for T6 treatment, indicating cooling aging could improve the mechanical properties, corrosion resistance and production efficiency with less energy consumption.