A Comparison of the Damage Tolerance of 7010 T7451 and 7050 T7451

2000

MSF

To improve the understanding of the relation between composition, precipitation and the balance of strength and electrical conductivity (as a measure of the stress-corrosion resistance), a number of Al-Zn-Mg-Cu-Zr aluminium alloy plates with different Zn, Mg and Cu contents were produced and studied by Differential Scanning Calorimetry (DSC). It is shown that detailed analysis of the DSC data produces valuable information on the optimal Mg, Zn and Cu contents. 1.Introduction With the ever increasing demands put on high strength aluminium alloys for aerospace applications, improvement in alloy chemistry and thermo-mechanical treatments for optimisation of the critical properties strength, toughness and stress-corrosion cracking (SCC) resistance of 7xxx alloys is an area of continued interest. Differential Scanning Calorimetry (DSC) can provide a contribution to this [1] because it is sensitive to reactions of the main phases determining these properties, GP zones, η′, η, T, S and Mg 2 Si phase. In the present paper we will investigate to what extend DSC data can provide information concerning optimising alloy chemistry and heat treatment. 2.Experimental Four Al-Zn-Mg-Cu-Zr plate alloys with different Zn, Mg and Cu contents (broadly within in the range of 7075, 7010 and 7050 compositions) were studied. The alloys were produced at DERA (Farnborough, UK) under contract through British Aluminium Plate. The chemical compositions of the alloys are shown in Table 1. All alloys were solution treated and water-quenched at DERA according to normal industrial practice.For DSC testing, we cut samples from the plates and aged them for various times at a single temperature typical for a T7 temper. For all the experiments, a pure aluminium reference (99.9%) with a mass and shape close to that of the sample was used. Baseline correction was performed employing experiments consisting of a single DSC run using pure Al samples. The heat flow was calibrated by measuring the heat of fusion of In and Zn as well as the heat capacity of pure Al. The temperature is calibrated by taking the deviation ΔT from the reference temperature, compared with the measured melting points of In and Zn. The DSC samples were machined from the 7xxx plates to discs 5mm in diameter and 1mm in height with average mass of about 63 mg. DSC experiments were performed at heating rate 10°C/min. As standard procedure, each experiment contains 3 heating runs, with the latter two runs after cooling at 20 and 2ºC/min being used mainly for heat capacity corrections (for further details, see Ref.[2]).For optical microscopy and image analysis, two alloys (B and D) were selected. For grain structure examination, polished samples were etched in 10% H 3 PO 4 in distilled water heated to 50°C for about one minute.

mentioning

confidence: 84%

Analysis of Precipitation and Dissolution in Overaged 7xxx Aluminium Alloys using DSC

2000

MSF

“…[18,22] There is no evidence of any effect of aging on this peak, as it does not seem to vary systematically with the heat treatment. This indicates that this peak is sample dependent, involving S phase formed during ingot casting.…”

Section: Temmentioning

confidence: 99%

“…The form of the crack The S-phase intermetallics are expected to promote failure path, illustrated in Figure 10, suggests that a degree of void through coarse voiding. [22,27] S phase can be-suppressed by growth occurs at the grain boundaries rather than a separation a combination of high solution-treatment temperature and a of the boundaries. It may be suggested that the presence of limited Cu and Mg content such that all S phase can be a low density of grain boundary precipitates will be the dissolved at the solution treatment temperature.…”

Section: F Fractographymentioning

confidence: 99%

Toughness-strength relations in the overaged 7449 al-based alloy

Kamp

Sinclair

2002

Metall Mater Trans A

102

This article examines the relationship between plane strain fracture toughness, K Ic , the tensile properties, and the microstructure of the overaged 7449 aluminum-plate alloy, and compares them to the 7150 alloy. The 7449 alloy has a higher content of Ј/ precipitates; and, the 7150 alloy contains a greater amount of coarse intermetallic particles, as it contains an appreciable amount of coarse S phase (Al 2 CuMg), which is largely absent in the 7449 alloy. The toughness of the alloys shows an increase on overaging, and the 7449 alloy shows a reasonably linear toughness-yield strength relation on extended overaging. Several mechanisms of failure occur: coarse voiding at intermetallics and a combined intergranular/transgranular shear fracture mode, with the former becoming more important as overaging progresses. Drawbacks of existing models for toughness are discussed, and a new model for plane strain fracture toughness, based on the microstructurally dependent work-hardening factor, K A , introduced in Ashby's theory of work hardening, is developed. This model predicts a linear relation between K Ic and K 0.85ys , where ys is the yield strength, which is consistent with the experimental data.

“…Alloying elements can remain undissolved because they are incorporated in phases that are stable at solution treatment temperatures. In 7xxx alloys this concerns mainly the S (Al 2 CuMg), T (Mg 3 Zn 3 Al 2 with some Cu dissolved in it), Al 7 Cu 2 Fe and Mg 2 Si phases [27,31,32], where the presence of the latter two is caused by impurities Fe and Si. The amounts of each phase present can be calculated/predicted using thermodynamic models [33,34] or phase diagrams [35].…”

Section: Undissolved Particlesmentioning

confidence: 99%

A model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu alloys

2003

Metall Mater Trans A

110

A physically-based model for the electrical conductivity of peak aged and overaged Al-Zn-Mg-Cu (7xxx series) alloys is presented. The model includes calculations of the η and the S phase solvus (using a regular solution model), taking account of the capillary effect and η coarsening. It takes account of the conductivity of grains (incorporating dissolved alloying elements, undissolved particles and precipitates) and solute depleted areas at the grain boundaries. Data from optical microscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) along with energy dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM) are consistent with the model and its predictions. The model has been successfully used to fit and predict the conductivity data of a set of 7xxx alloys including both Zr containing alloys and Cr containing alloys under various ageing conditions, achieving an accuracy of about 1% in predicting unseen conductivity data from this set of alloys.