Over-aged Tempers Research Articles

The effect of ageing on tensile behaviour, mode I and mixed mode I/III fracture toughness of 7010 aluminium alloy

Abstract The effect of ageing on the tensile behaviour, mode I fracture toughness and mixed mode I/III fracture toughness of 7010 aluminium alloy plate was investigated. It was found that the 0.2% proof stress in this alloy increased from the under-aged temper to the peak-aged (T6) temper and then subsequently decreased in the over-aged (T73) temper. On the other hand, the ductility exhibited a monotonic decrease from the under-aged temper to the over-aged temper. The Ludwigson equation was found to best describe the flow behaviour in all the three ageing tempers. Under mode I as well as mixed mode I/III loading, the highest fracture toughness was seen in the under-aged temper, whereas the fracture toughness values in the peak-aged and over-aged tempers were comparable.

Influence of Aging Treatments on Alterations of Microstructural Features and Stress Corrosion Cracking Behavior of an Al-Zn-Mg Alloy

7xxx series Al-Zn-Mg-(Cu) alloys have higher strength in their peak-aged (T6) states compared with other age-hardenable aluminum alloys; however, the maximum strength peak-aged state is more susceptible to stress corrosion cracking (SCC) which leads to catastrophic failure. The over-aged (T7) temper with 10-15% lower strength has higher resistance to SCC requiring oversized structural aerospace component applications. The medium-strength AA7017 Al-Zn-Mg weldable alloy without Cu is also prone to SCC under certain environmental conditions. In the present investigation, the SCC behaviors of an AA7017 Al-Zn-Mg alloys of different tempers have been assessed. Specific aging schedules have been adapted to an AA7017 alloy to produce various tempers, e.g., under-, peak-(T6), over-(T7), and highly over-aged tempers. Artificial aging behavior of the AA7017 alloy has been characterized by hardness, electrical conductivity measurements, x-ray diffraction, differential scanning calorimetry, and electrochemical studies. Slow strain rate test technique was used to assess the SCC behaviors of the AA7017 alloys of under-, T6, T7, and highly over-aged tempers in 3.5 wt.% NaCl solution at free corrosion potential (FCP) and at applied anodic potential, as well. Results revealed that the AA7017 alloy tempers are not susceptible to SCC in 3.5 wt.% NaCl solution at FCP, but severely damaging to SCC at applied anodic potentials. Microstructural features, showing a non-recrystallized grain structure and the presence of discrete, widely spaced, not-interconnected η precipitates at the grain boundaries, are the contributive factors by virtue of which the alloy tempers at FCP did not exhibit SCC. However, the applied anodic potential resulted in rapid metal dissolution from the grain boundary region and led to SCC. The local anodic dissolution (LAD) is believed to be the associated SCC mechanism.

Microstructural, mechanical and electrochemical behaviour of a 7017 Al–Zn–Mg alloy of different tempers

The aim of the investigation is to assess the microstructural features and associated physical, mechanical and electrochemical properties of a 7017 Al–Zn–Mg alloy of various tempers. A 7017 Al–Zn–Mg alloy was subjected to different ageing schedules to produce under-(T4), peak-(T6), over-(T7) and highly over-aged tempers. Optical microscopy, hardness measurement, electrical conductivity measurement, tensile testing and SEM fractographs, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and electrochemical polarization studies have been used to characterize the alloy tempers. Hardness measurement and tensile testing showed the characteristic age hardening phenomenon of aluminium alloys. Optical and TEM micrographs have revealed the variation in size of matrix strengthening η′ (MgZn2) and also the size and distribution of grain boundary η (MgZn2) precipitate with ageing time. DSC thermograms exhibiting exothermic and endothermic peaks indicated the characteristic solid state reaction sequence of the 7017 alloy. Potentiodynamic polarization study of the 7017 alloy of various tempers in 3.5wt.% NaCl solution at near neutral pH showed typical active metal dissolution behaviour, but at pH12 an active–passive–transpassive transition behaviour has been observed.

Quantitative Investigation on Nano-Scale Precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr Alloy under Various Aging Tempers

In the present study, a systematic study of nanoscale precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy have been carried out for various typical aging tempers, including T6, T76, T74, T73 and RRA treatments, by combining synchrotron-based small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Based on the TEM observations, quantitative and statistical structural information, including precipitate size, volume fraction, number density and inter-precipitate distance have been extracted from SAXS data through model fitting. The results show that the T6 peak-aged alloy with the smallest precipitate size has the highest number density and lowest volume fraction of precipitates. Under two-step T7X over-aged tempers, with the deepen of aging degree, the precipitate size, volume fraction and inter-precipitate distance increases, but the number density decreases. The size distribution of precipitates for the RRA-treated alloy is in between that of T6 and T76. The results also show that as the degree of over-aging deepens, the precipitate size distribution interval becomes broader.

Effect of Surface Roughness on Breakdown Behavior of Al-Zn-Mg-Cu Alloy

The surface microstructure and corrosion breakdown behavior of AA7055 Al alloy with different surface roughness were studied using scanning transmission electron microscope and polarization measurements. Atom probe tomography was used to study the compositions of the solid solution matrix of AA7055 alloy in three typical tempers. The matrix Cu content decreased from peak-aged to over-aged tempers, which resulted in a reduction in pitting potential. The AA7055-T77 sample abraded to 320 grit exhibited two breakdown potentials, whereas the 400 grit abraded sample exhibited only one. Surface layer attack occurred for both samples, but was absent for the 1200 grit abraded sample, which exhibited one breakdown potential. Abrasion-induced surface layers containing various microstructural features were observed on all surfaces. Rougher surfaces exhibited thicker surface layers with more Zn-enriched sub-boundaries and fewer η′ precipitates. These changes in surface microstructure with surface roughness resulted in difference in susceptibility of the surface layer, and in the activity gap between the surface layer and underlying bulk matrix, which altered the breakdown behavior of AA7055 alloy.

Mechanism of Localized Breakdown of 7000 Series Aluminum Alloys

The localized breakdown behavior of AA7055 alloy, and an Al-Zn-Mg alloy having no Cu, with different tempers was studied in 3.5 wt% NaCl solution by an in situ observation system during polarization. Three potentials at which the current increased rapidly were found for AA7055-T6 (peak-aged) samples. The first breakdown potential corresponded to the transient dissolution of an active surface layer formed during surface preparation by abrading with grinding paper. The second potential at which the current increased rapidly was associated with crevice corrosion and was not a true breakdown potential. The second breakdown potential was associated with pitting of the underlying bulk alloy. However, no surface layer attack and a single breakdown potential were observed for AA7055-T73 (over-aged) samples and for the Cu-free Al-Zn-Mg samples in the peak-aged condition. All three samples contained surface layers consisting of nano-grains with the solute-rich bands formed by surface preparation. Both the Zn-rich bands in the surface layer and the Cu content in matrix solid solution of the underlying bulk alloy are responsible for surface layer attack. The change in pitting potential with over-aged tempers could be rationalized based on the Cu content in the matrix.

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (Ea) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys ( ~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.

Fatigue crack growth of AA2219 under different aging conditions

The fatigue crack growth of commercial AA2219 has been examined under different aging treatments, namely, naturally aged (NA), under aged (UA), peak aged (PA) and over aged (OA) conditions. From the near threshold stress intensity range (Δ K NTH), the alloy in the NA condition is found to have the highest resistance to fatigue crack initiation. The crack growth rate increases and the near threshold stress intensity range decreases with advancing aging. This observation is found to be consistent with lower levels of crack closure and decreasing levels of tortuosity in crack path for PA and OA tempers. The inhomogeneous transcrystalline slip in the UA condition results in the slower crack growth at low stress intensity range (Δ K). The fracture morphology changes from crystallographic facets near the threshold region to clearly developed ductile striations in the Paris power-law regime to microvoid coalescence in the high Δ K regions.

Aging Behavior of Mg-9Gd-4Y-0.6Mn Alloy

Microstructures and properties of rolled Mg-9Gd-4Y-0.6Mn（nominal wt%）sheets solution treated at 793K for 12h, and aged at various combinations of temperatures and time were investigated by hardness measurement, tensile test and TEM. The evolution of the precipitates and their effect on the mechanical properties of the pre-, peak-, and over-aged tempers were determined, and the optimal yield tensile strength (YTS) and ultimate tensile strength (UTS) were 276MPa and 287MPa at room temperature, respectively. It has been found that the addition of Mn into Mg-Gd-Y system contributes to block behavior of matrix diffuse and increase potential of heat resistance of the alloy.

The effect of ageing on tensile behaviour, mode I and mixed mode I/III fracture toughness of 7010 aluminium alloy

Abstract The effect of ageing on the tensile behaviour, mode I fracture toughness and mixed mode I/III fracture toughness of 7010 aluminium alloy plate was investigated. It was found that the 0.2% proof stress in this alloy increased from the under-aged temper to the peak-aged (T6) temper and then subsequently decreased in the over-aged (T73) temper. On the other hand, the ductility exhibited a monotonic decrease from the under-aged temper to the over-aged temper. The Ludwigson equation was found to best describe the flow behaviour in all the three ageing tempers. Under mode I as well as mixed mode I/III loading, the highest fracture toughness was seen in the under-aged temper, whereas the fracture toughness values in the peak-aged and over-aged tempers were comparable.

Retrogression, reaging, and mechanical behaviour of a 1441 Al–Li–Cu–Mg–Zr alloy

Abstract A 1441 Al–Li–Cu–Mg–Zr alloy was subjected to retrogression treatment and the retrogressed states were immediately reaged to various tempers. Hardness measurement, tensile testing, and SEM fractography have been carried out to conventional peak-aged, retrogressed, retrogressed and reaged (RRA), and over-aged tempers of the alloy. Retrogression treatment applied to peak-aged tempered alloys resulted in a decrease in hardness and tensile strength, an increase of ductility, and a serration phenomenon in the plastic region. These are primarily due to the dissolution of matrix strengthening δ' precipitates into solid solution upon retrogression treatment. Retrogressed states have been reaged to various under-, peak-, and over-aged tempers. Reaging the retrogressed state to peak-aged temper has resulted in attaining the original peak-aged strength and ductility value. SEM fractography study has been carried out and fracture surfaces exhibited predominantly intergranular fracture in presence of few dimpl...

Comparison of crack growth behaviour between spray-formed and commercial plate AlZnMgCu alloy 7150 : Gingell, A.D.B. and King, J.E. Mater. Sci. Forum (1996) 217–222 (3), 1605–1610 Conference: Aluminium Alloys—Their Physical and Mechanical Properties, ICAA5, Part 3, Grenoble, France (1–5 July 1996)

The aim of the investigation is to assess the microstructural features and associated physical, mechanical and electrochemical properties of a 7017 Al–Zn–Mg alloy of various tempers. A 7017 Al–Zn–Mg alloy was subjected to different ageing schedules to produce under-(T4), peak-(T6), over-(T7) and highly over-aged tempers. Optical microscopy, hardness measurement, electrical conductivity measurement, tensile testing and SEM fractographs, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and electrochemical polarization studies have been used to characterize the alloy tempers. Hardness measurement and tensile testing showed the characteristic age hardening phenomenon of aluminium alloys. Optical and TEM micrographs have revealed the variation in size of matrix strengthening η′ (MgZn2) and also the size and distribution of grain boundary η (MgZn2) precipitate with ageing time. DSC thermograms exhibiting exothermic and endothermic peaks indicated the characteristic solid state reaction sequence of the 7017 alloy. Potentiodynamic polarization study of the 7017 alloy of various tempers in 3.5 wt.% NaCl solution at near neutral pH showed typical active metal dissolution behaviour, but at pH 12 an active–passive–transpassive transition behaviour has been observed.

EXTRUSION PROCESSING OF AN Al-Li-Cu-Mg-Zr ALLOY AA 2091

This communication reports work covering the variation in structure and properties with extrusion processing of an Al-Li alloy, AA 2091. An homogenisation temperature was established and used as standard throughout the work. Extrusions were performed over a range of temperature compensated strain rates. After solution treatment, a series of ageing curves was obtained for both stretched (2%) and unstretched specimens at various ageing temperatures, thus establishing the ageing response with respect to underage, peak age and overage tempers. A natural ageing (at room temperature) curve was also produced after solution treating and tensile tests performed at one and two week intervals. The effect of introducing a 2% stretch prior to artificial ageing was investigated in terms of microstructural changes and variations in the mechanical properties (tensile and fracture properties) for peak aged and overaged tempers at different ageing temperatures. All the extrusions had fully recrystallised surface layers. Up to 190°C, ageing at a higher temperature led to lower strength levels but the response was faster, and stretching gave an increase in strength. At 210°C the strength was not only higher than 190°C but was maintained even up to 1000 hours although a pre-age stretch caused a drop in strength after about 200 hours. This is due to the more copious precipitation of homogeneous S phase at 210°C and a reduced amount if stretched prior to ageing because the precipitation of heterogeneous S phase reduces the amounts available for homogeneous precipitation. Very high strengths were obtained especially in the peak-aged specimens. For the peak-aged condition, the introduction of a pre-age stretch did not necessarily improve toughness. TEM revealed wide-spread heterogeneous S phase precipitation right up to the grain boundaries for the stretched specimens.