Influence of precipitation on serrated flow in Al-5Zn-1Mg alloy

Abstract

An Al-5Zn-1Mg alloy was aged at three different temperatures, 70 ° C, 150 ° C, and 200 ° C to obtain different states of precipitation. Serrated flow behaviour was studied with progressive aging at these three different aging temperatures. A comparison of serrated flow behaviour for the three different aging condition was made in the peak aged condition. Finally, serrated flow behaviour in Al-5Zn-1Mg alloy was compared with that in Al-Li alloys studied earlier. It is concluded that all the features of serrated flow can be explained only by considering the combined effect of solute - dislocation and precipitate - dislocation interaction.     

Effect of hydrogen charging on microstructural evolution and corrosion behavior of Ti-4Al-2V-1Mo-1Fe alloy

The effect of hydro gen charging on microstructural evolution and corrosion behavior of a Ti-4Al-2V-1Mo-1Fe alloy in a 3.5 wt.% NaCl solution was investigated.The results showed that the hydrogen charging induced the formation and growth of γ-TiH and δ-TiH₂ phases,leading to the initiation and propagation of hydrogen-induced cracks.It was also found that hydrogen charging can change the passivity of this alloy and increase its pitting corrosion susceptibility.The main reason for these was attributed to the fo rmation of hydrides in α phase in the Ti-4Al-2V-1Mo-1Fe alloy,leading to the preferential dissolution of the α phase and thus the deterioration in the protective ability of passive film.     

The hydrogen content of austenite after cathodic charging

We have made direct measurements of deuterium concentration profiles and have shown that the concentration of hydrogen (deuterium) in the near-surface layers of 310 austenitic steel following room-temperature cathodic charging in poisoned electrolytes is 0.5 to 0.8 hydrogen atoms per metal atom. There is also considerable microstructural damage, including the formation of an unstable martensite. With no arsenic poison in the electrolyte the hydrogen ratio is about 0.15, and there is low associated damage structure. These hydrogen (deuterium) concentrations are many orders of magnitude greater than those required to embrittle ferritic steels, implying a different embrittlement mechanism in the austenite. The observations of a hydrogen-induced martensite phase in this supposedly phase-stable alloy indicates that hydrogen embrittlement mechanisms involving martensite are no longer excluded.     

The grain size dependence of fracture toughness in an Al-6.0% Zn-2.5% Mg alloy

The grain size dependence of the fracture toughness (K _IC) of an aged Al-6.0% Zn-2.5% Mg alloy was studied experimentally. K _IC depended strongly upon grain size (L _G) in two ways. In the small grain size region K _IC decreased with increasing average grain size. In contrast, K _IC increased with increasing average grain size for large grain sizes. The increase in K _IC with increasing grain size arose as a result of the presence of abnormally large grains compared to the average grain size in the large-grained specimens.     

The relationship between ductility and grain-boundary precipitates in an Al-6.0% Zn-2.6% Mg alloy

The relationship between ductility and material parameters such as grain-boundary (GB) precipitate size and dimple size in an Al-6.0% Zn-2.6% Mg alloy has been studied. Under the condition of a given GB precipitate size, the ductility increases with decreasing number of GB precipitates and dimples per unit area. The critical GB precipitate sizes acting as the nuclei for void formation are deduced.     

Tensile and fatigue fracture behaviour of a spot welded Al- Mg alloy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Hydrogen trapping and repelling in an Al-6 wt % Zn-2 wt % Mg alloy

Hydrogen trapping in an Al-6 wt % Zn-2 wt % Mg alloy aged up to typical stages in the agehardening curve has been studied by measuring the tritium release rate after charging. The distribution of hydrogen in the aged alloy has been studied by tritium electron microautoradiography. It has been found that the Guinier-Preston zones in the alloy do not act as trapping sites but as a repeller for hydrogen, and that precipitate does not trap hydrogen, but the interface between the matrix and precipitate acts as a trapping site for hydrogen. Dislocation has been found to be capable of trapping hydrogen, while trapped hydrogen by the grain boundary has not been observed.     

On the decomposition behaviour of Al-4.5 at% Zn-2 to 3 at% Mg alloys during continuous heating

The decomposition processes taking place in the Al-4.5 at % Zn-2 to 3 at % Mg alloys were studied during continuous heating by means of electrical resistivity, XSAS and DSC measurements and by TEM investigations. It was found that the room temperature pre-ageing has no significant influence on the processes taking place above 230° C. Several temperature ranges were determined in which the decomposition of the solid solution and/or the transformation of the different particles of the second phases take place by different mechanisms.     

The effect of exposure on the mechanical properties of the Ti-6Al-4v alloy

The mechanical properties of TJ-6A1-4V specimens were investigated after their exposure to air at 900°C. It was found that even short exposure causes a marked drop in ductility and decrease in the ultimate tensile strength, while the yield strength remains uneffected. The loss of ductility is attributed to small surface cracks which appear in the early stages of the tensile test along the entire gauge length of the specimens.     

The high-temperature creep behaviour of an Al-1 wt% Cu solid-solution alloy

The creep behaviour of an Al-1 wt% Cu solid-solution alloy is investigated at a temperature of 813 K under stress range of 0.5–5 MPa. The creep characteristics of the alloy including the stress dependence of the steady-state creep rate (n=4.4), the shape of creep curve (normal primary stage), the transient creep after stress increase, and the value of the true activation energy for creep, suggest that some form of dislocation climb is the rate-controlling process at higher stresses above 1 MPa. However, at low stresses (< 1 MPa), the creep curves show no distinguished steady state, and the stress dependence of the minimum creep rate is as high as ～ 8. The creep behaviour of the alloy is discussed based on recent theories available for describing creep in solid-solution alloys.     

Correlation between the mechanical properties and microstructural changes of aged Al-5.8% Mg alloy

Mechanical properties of aged Al-5.8% Mg alloy at 423 and 473 K, over the ageing time range 1–35 h, have been investigated to assess the effect of ageing temperature on deformation in the presence of precipitation. The results indicate that the 0.2% yield strength, ultimate tensile strength, flow stress, work hardening exponent and ductility increase with ageing time reaching a maximum value, and then decrease to minimum value, followed by an increase at longer ageing times. The variation in yield, ultimate tensile strength, work hardening exponent of the flow, as well as the hardness at different degrees of deformation, were recorded as functions of experimental variables. Electron microscopic investigations revealed that the strengthening and loss in ductility of the alloy may be attributed to the precipitation of different shapes of MnAl₆, Mg₂Al₃ and ɛ-Mg₂₃Al₃₀, whose size, quantity and morphology depend on the experimental conditions. An attempt has been made to correlate strength, ductility and structural changes at different ageing times. The Brinell hardness increases and the recrystallization temperature decreases with deformation. From parabolic stress-strain relation, the σ-ɛ^1/2 curves could be divided into two linear parts.     

The effect of lattice defects induced by cathodic hydrogen charging on the apparent diffusivity of hydrogen in pure iron

The variation of apparent hydrogen diffuoivity with the applied current density and the types and amounts of hydrogen -induced damage caused by hydrogen during the cathodic charging of hydrogen in pure iron are investigated by thermal analysis techniques, and analysed in the light of theoretical models. Internal microcracks and microvoids are generated predominantly below 1 mA cm^–2 and are the major trapping sites of hydrogen in pure iron when charging hydrogen cathodically. Blisters on the surface of iron specimens are found tD be interconnected to the surface of the specimen through microcracks remaining in the vicinity of blisters. The peak temperature of hydrogen released from an internal microcrack or microvoid decreased as the applied current density is increased. The apparent diffusivity of hydrogen at 458 K decreases linearly with the reciprocal value of the square root of applied current density during cathodic charging. This implies that the amounts of internal microcracks or microvoids are linearly proportional to the lattice hydrogen solubility or the square root of the applied current density.     

On the intergranular fracture mechanism due to ledge formation in an Al-6.0% Zn-2.5% Mg alloy

The ledges observed on the over-load fracture surfaces of an Al-6.0% Zn-2.5% Mg alloy variously heat treated are discussed. As pointed out by Ryum and Baardseth the ledges are formed by the reaction between slip bands and grain boundaries. The morphology of the ledges changes from a sharp saw-toothed to a sine-curved shape, and the facets of the ledges vary from a flat face to a dimpled one with the degree of ageing. Grain-boundary precipitates were frequently observed on the flat facets. The fracture stress, calculated from a modified equation of Stroh's theory, supports the possibility of fracture occurring at a stress a little beyond yielding.     

Influence of hydrogen charging on mechanical properties of gas tungsten arc weldments of aluminium-lithium alloy 8090

The effect of hydrogen charging on the mechanical properties of gas tungsten arc welds (GTAW) of aluminium-lithium alloy 8090 (2 mm thick rolled sheets) was studied using cathodic hydrogen charging. To stimulate an increased amount of hydrogen into welds, the charging current density was increased through a galvanostatic circuit. The deleterious effect of hydrogen on ductility is documented in terms of degradation in tensile ductility (reduction in area and elongation-to-failure). Microscopic analysis was performed to characterize the microstructure and grain morphology of the weldments. Hardness measurements revealed an increase in hardness of the charged welds over the uncharged counterpart. Scanning electron microscopy observations of uncharged welds revealed a mixed mode failure with predominantly ductile rupture. Although, the charged welds exhibited a near similar mode of failure to that of the uncharged welds, extensive planar slip deformation was observed near the outer surface of the uncharged welds. The change in fracture mode from the outer surface to the central portion of the charged welds is attributed to intrinsic differences in hydrogen densities. An attempt has been made to rationalize the role of hydrogen on tensile properties and quasi-static fracture behaviour of the GTAW welds.