Effect Of Copper Content Research Articles

The effect of copper content and microstructure on the hydrogen embrittlement of AI-6Zn-2Mg alloys

Two commercially-processed Al-6Zn-2Mg alloys, 7050 and a “low copper” 7050, were tested for susceptibility to embrittlement by precharged hydrogen and by simultaneous cathodic charging and straining (SET procedure). Specimens were heat treated to underaged, peak-strength aged, and overaged conditions. In 7050, the peak strength and overaged conditions were not embrittled by hydrogen, though underaged material showed marked embrittlement. All microstructures tested for the low-copper alloy were embrittled. The results agree with the microstructural rationale established through earlier work on 7075 and 2124 aluminum alloys, particularly with respect to the susceptibility of underaged material to hydrogen. As in earlier work, the extent of dislocation transport of hydrogen, and local hydrogen accumulation at grain boundaries, evidently controlled the extent and degree of brittle fracture. These three important alloys can now be ranked in the order 7050, 2124, 7075 of increasing relative susceptibility to theonset of stress corrosion cracking.

The low cycle fatigue behavior of high strength 7XXX-type aluminium alloys in the T7351 condition

The effect of copper content (from 0.01% to 2.1 %) on the low cycle fatigue behavior of four Al6Zn2MgXCu-T7351 alloys was investigated in laboratory air and dry air and compared with a previous study of the T651 condition. This research was designed so that the effect of copper content could be studied independent of variations in degree of coherency of the precipitates. Incoherent strengthening precipitates were obtained for all alloys by using the overaging T7351 treatment. Unlike the results of the T651 study, no difference in the strain-life behavior was observed for the four alloys in either environment. Cyclic softening was observed and associated with grain boundary and slip band crack initiation and subsequent microcrack coalescence. The departure from linearity observed in the strain-life plots for both the peak-aged and overaged conditions is associated with the fatigue crack initiation mechanism. Cracks are initiated at grain boundaries at high strain amplitudes and at slip bands at low strain amplitudes. Both mechanisms are possible at intermediate amplitudes.

The effect of copper content and heat treatment on the stress corrosion characteristics of Ai-6Zn-2Mg-X Cu alloys

The effect of copper and aging on the stress corrosion characteristics of Al-6 pct Zn-2 pct Mg-(0.01-2.1) pct Cu alloys in 3.5 pct NaCl was studied with other variables remaining constant. In the peak-strength condition the crack velocity in the stress-independent region decreased by two orders of magnitude when copper was increased from 0.01 to 2.1 pct. In the stress-dependent region crack velocity decreased only for copper contents higher than 1 pct. Overaging further reduced the plateau crack velocity in all the alloys studied. The results are interpreted on the basis of the effects of copper and aging on the deformation mechanism and electrochemical activity of the phases.

The effect of copper content and deformation mode on the fatigue crack propagation of Al6Zn2Mg xCu alloys at low stress intensities

The effect of copper content on the fatigue crack propagation behavior of four Al6Zn2Mg xCu (T7351) alloys was investigated in dry air and distilled water and was compared with a previous study of the T651 condition. The peak-aged treatment exhibited a lower growth rate in dry air over the range of ΔK studied and also displayed a higher threshold and resistance to fatigue cracking near the threshold in the distilled water environment. This suggests that age-hardened alloys with planar slip (dislocation shearing mechanism) will have a higher threshold than those with wavy and/or homogeneous slip. The shape of the growth rate curves can be correlated best with a transition in the fracture surface appearance which is controlled by the deformation behavior. The copper content of the four alloys in the overaged condition did not significantly affect the growth rate in either dry air or distilled water. However, in distilled water there were minor differences in the form of the growth rate curves with copper content. This is associated with electrochemical potential differences at the crack tip among the four alloys. The environmental sensitivity of the peak-aged condition was several times higher than that of the overaged treatment at the higher ΔK values and the sensitivity of the peak-aged condition was much more dependent on the copper content. This behavior is considered to be due to strain localization which occurs in the peak-aged condition and which enhances the metal-environment interactions.

The effect of copper content and degree of recrystallization on the fatigue resistance of 7XXX-type aluminum alloys II. Fatigue crack propagation

The effect of the copper content (from 0.01 to 2.1%) and of the degree of recrystallization (DR) (from 5 to 67%) of four Al6Zn2MgxCuT651 alloys on their fatigue crack propagation (FCP) behavior was investigated in dry air, in distilled water and in a 3.5% NaCl solution. The fatigue crack growth rate (FCGR) decreases with decreasing copper content in dry air; however, in corrosive environments the FCGR decreases with increasing copper content for a stress intensity range δK > 7.5 MPa m 1 2 . These results are interpreted in terms of the characteristics of the crack path and of deformation-environment interactions. The influence of the Dr on the FCGR also depends on the deformation mode and on the test environment but was only minor under our experimental conditions. Hydrogen embrittlement is believed to be associated with environmentally accelerated FCP.

The effect of copper content and degree of recrystallization on the fatigue resistance of 7XXX type aluminum alloys I. Low cycle corrosion fatigue

The effect of copper content (0.01–2.1%) and degree of recrystallization (DR) (5–45%) on the corrosion fatigue behavior of four AlZn2MgxCuT651 alloys was investigated. The cyclic strain resistance was found to increase with copper content and decrease with DR regardless of the test environment. This trend is more pronounced in aggressive environments. The improved fatigue resistance is attributed to the increased homogeneity of slip as the copper content is increased and the DR is decreased. The environmental sensitivity, which increases with decreasing copper content, is dependent on slip behavior. For alloys tested in distilled water hydrogen embrittlement is believed to be the major factor controlling the low cycle corrosion fatigue. For tests conducted in a 3.5% NaCl solution hydrogen embrittlement still plays an important role but the preferential dissolution and/or adsorption process may also have a detrimental effect.