Mg-Cu alloys

The effect of environmental variables upon the logarithm of velocity V5. K relationship has been examined for a few alloys in some conditions of heat treatment. While it cannot be certain that similar results would be obtained with all alloys, the results reported do show interesting features that may have points in common with all alloys. For an Al-Zn-Mg-Cu alloy (7075-T651) the stress-corrosion plateau velocity was a maximum in 5 m KI solution under potentiostatic conditions at -520 mV (v5. S.C.E.), reaching about 2 X 10 to 5 X 10 cm/s, whereas in 3% NaCl under open-circuit... 

Fig. 20.34 Light micrograph showing the highly elongated grain structure of a commercial wrought high-strength precipitation-hardening Al-Zn-Mg-Cu alloy (x 40, section perpendicular to the long transverse direction)...

Li et al. (2000) have employed nanometer scale analysis in a FEG-TEM operating at 200 kV to distinguish between true GP zones in an Al-Zn-Mg-Cu alloy and GP zone-like defects caused by electron beam irradiation in the TEM. They studied an Al-6.58Zn-2.33Mg-2.40Cu (wt%) alloy, in which it is well known that the decomposition of supersaturated solid solutions takes place via the formation of GP zones, using conventional techniques to produce thin foil specimens of aged material. 

Figure 2.40. Phase diagram of the Mg-Cu alloy system. For the alloys marked (1) (at 5 at.% Cu) and (2) (at 20 at.% Cu), the DTA curves are shown on the right. Notice that, on cooling, a sharp thermal effect due to the invariant eutectic transformation is observed. At higher temperature the crossing of the liquidus curves is detected. (The coordinates of the eutectic point are 485°C and 14.5 at.% Cu.)...

Figure 2.41. Microscopic examination of Mg-Cu alloys. The micrographs of the same two alloys considered in Fig. 2.40 are shown. Small pieces of the alloys were polished by using finer and finer abrasive powders (A1203, diamond) in order to obtain a shiny surface. This appears as a continuous white surface under a metallographic reflection microscope. When the sample, however, is gently etched by using a convenient reactant (in this specific case a dilute water-alcohohc solution of HN03) phases of different chemical composition (here Mg and the compound CuMg2) are differentiated. The more reactive phase (in this case Mg) is more deeply etched, losing its brilliance, and it appears dark under the microscope.

Scully, J.R. and Young, G.A., Jr., The effects of temper, test temperature, and alloyed copper on the hydrogen-controlled crack growth rate of an Al-Zn-Mg-(Cu) alloy, in Corrosion 2000, National Association of Corrosion Engineers, Houston, TX, 2000, paper 368. 

Table 3. Composition and mechanical properties of high-strength A1 - Zn - Mg - Cu alloys after extrusion and thermo treatment T6 [16] ...

Senatorova, O.G., Uksusnikov, A.N., Legoshina, S.F., Fridlyander, I.N. and Zhegina, I.P. (2000) Influence of different minor additions on structure and properties of high-strength Al-Zn-Mg-Cu alloy sheets, Mater. Science Forum 331-337,1249-1254. 

Milman, Yu.V., Sirko, A.I., Lotsko, D.V., Senkov, O.N. and Miracle, D.B. (2002) Micro structure and mechanical properties of cast and wrought Al-Zn-Mg-Cu alloys modified with Zr and Sc, Mater. Science Forum 396-402, 723-728. 

Key words Al-Zn-Mg-Cu alloys, effect of scandium alloying, tensile properties, microstructure, cryogenic temperatures. 

EFFECT OF FE AND SI ON THE STRUCTURE AND MECHANICAL PROPERTIES OF COMPLEX AL-ZN-MG-CU ALLOYS PRODUCED BY P/M AND CASTING TECHNIQUES... 

Al-Zn-Mg-Cu alloys with additions of Fe, as well as with combinations of Fe, Mn, Zr and Sc, were produced by induction melting using ceramic crucibles and supplementary methods of melt purification, which included ceramic filters and blowing the melt with argon. Rods of 6 mm in diameter were manufactured by extrusion at 400 C with extrusion ratio A.=84. The T6 treatment consisted in water quenching from 465 °C and aging at 120 °C for 24 h. The alloy compositions and the properties of rods in the T6 condition are given in Table 1. 

In Al-Zn-Mg-Cu alloys alloyed only with Fe, fracture had a mixed character it was ductile and semibrittle, transcrystalline with the formation of pores around the spherical second phase particles. The plasticity of these alloys was EL = 13-20 % depending on the content of Fe. As was shown by X-ray microanalysis, the phase that is situated in dimples and has spherical form contains Zn, Mg, Cu, and Fe. 

Rapidly solidified powders of Al-Zn-Mg-Cu alloys were produced by high-pressure water atomization of the melt (method WA-N) [4], This process provides cooling rates up to 106 K/s. Compaction was performed by the preliminary compaction at room temperature of powder into briquettes, followed by insertion into a capsule. The capsules were hermetically sealed, then the briquettes were subjected to hot degassing and compaction. The composition, powder size fraction used from the WA-N process, and properties of rods of 6 mm in diameter (A, = 17.7) in T6 condition are given in Table 2. 

As distinct from cast alloys, in powders of Al-Zn-Mg-Cu alloys with Fe and Si additions a uniform distribution of all elements in the powder body was observed (Fig. 3). However, precipitates of Mg2Si type were registered in T6 treated rods of alloys with Si additions. TEM investigation of T6 treated rods of both P/M and cast alloys revealed a large amount of... 

Al-Zn-Mg class of heat-treatable alloys has been attributed to Mg and Zn segregation, as well as formation of Mg-or Zn-containing grain boundary phases such as the MgZn2 phase [90]. In Al-Zn-Mg-Cu alloys, Cu depletion is also cited as a metallurgical condition leading to IGC [74, 91]. Slow quenching and certain isothermal heat treatments can lead to... 

Table I lists the mechanical properties of thermally hardened alloys of the duraluminum type (Al-Cu-Mg), as well as the Al-Zn-Mg-Cu alloys. A temperature decrease to 20 K increases the ultimate strength by 25 to 30%. The yield stress continuously increases as the temperature decreases. The relative elongation behaves in different ways in some alloys, it is practically constant, while in alloy V96 it is reduced by almost a factor of 5. Thus, hardening of alloys, such as V95 and V96, by thermal treatment produces materials with a sufficiently high level of strength, but losses in plasticity are inevitable. This is a significant limitation of their application in cryogenic equipment.

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