Microstructure and properties of cast ingots of Al-Zn-Mg-Cu alloys modified with Sc and Zr

Abstract

The effect of combined additions of Sc and Zr on the microstructure and tensile properties of the direct chill (DC) cast ingots of developmental Al-Zn-Mg-Cu alloys has been evaluated in this work. The properties in both the longitudinal and transverse directions of the cast ingots were determined in as-cast and cast plus heat-treated conditions, at room and cryogenic temperatures.Extensive microstructural evaluation was carried out using optical and electron microscopy, including orientation image microscopy by the electron backscatter diffraction technique. The Sc-containing developmental cast alloys showed the tensile properties, which are much better than the properties of any commercial cast Al alloys and are similar or even superior to the properties of 7075-T6 alloy forgings. The microstructural evolution, the strengthening mechanisms, optimum content of the dispersoid-forming elements, and the processing-structureproperty correlations are discussed.

Correlation between Electrical Resistivity, Particle Dissolution, Precipitation of Dispersoids, and Recrystallization Behavior of AA7020 Aluminum Alloy

“…It is clear that all the dispersoid interspacing is greater than 100 nm, regardless of the homogenization treatment, which is in agreement with the results of other research on AA7050, [10,11] AA7475, [31] and other alloys. [14] The results of EDX analysis on dispersoids formed at different homogenization conditions are shown in Table II. It is possible to identify at least four different types of dispersoids, Zr-(type 1), Cr-(type 2), and Mn-containing (type 3) dispersoids, and those that have a combination of various elements (type 4).…”

“…[5][6][7][8][9] In the case of 7XXX-series aluminum alloys, the formation of Zr-and Sc-containing dispersoids has been investigated. [10][11][12][13][14][15][16] For example, Robson et al [10,11] investigated the effect of Zr addition on the dispersoid formation and recrystallization fraction after hot deformation. It was concluded that, when using an optimum two-step homogenization treatment, a smaller fraction of recrystallization could be obtained.…”

“…Robson [12] also considered the effect of Sc on the formation of dispersoids, because Sc was expected to eliminate the dispersoid-free zones, observed in scandium-free 7050, thus greatly increasing the recrystallization resistance. Senkova et al [13] and Senkov et al [14] investigated the effect of minor additions of Sc and Zr on the tensile properties of two developmental Al-Zn-Mg-Cu alloys in the temperature range of À196°C to 300°C. They concluded that, due to the presence of Sc and Zr in a fine-dispersoid form, both low-temperature and elevated-temperature strengths of these alloys were much higher than those of similar 7XXX-series alloys that did not contain these elements.…”

“…Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirm the formation about small particles during homogenization, which affects the electrical resistivity. [10][11][12][13][14][15][16] Therefore, the electrical resistivity analysis can be used as an indirect means of evaluating the volume fraction of dispersoids and the dissolution of constitutive particles during the homogenization of aluminum alloys.…”

Evolution of Grain Boundary Phases during the Homogenization of AA7020 Aluminum Alloy

“…For example, Lim et al [10] investigated the effects of constitutional changes and preheat conditions on the evolution of constitutive particles, the M, T, and S phases, and dispersoids in AA7175 and AA7050 alloys. Senkov et al [21] studied the effect of homogenization treatment on the microstructural evolution of four newly developed 7XXX series aluminum alloys to obtain optimized conditions. Jackson and Sheppard [22] studied the effect of homogenization treatment on the microstructural changes of AA7075, 7150, and 7049.…”