Studies on the microstructures and properties in phase transformation of homogenized 7050 alloy

The evolutions of mechanical properties and the morphologies of secondary phases in homogenized 7050 alloy were studied in detail by conductivity measurement, tensile test, scanning electron microscope (SEM), energy dispersive spectrometry (EDS), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Both σb and δ of the 7050 alloy increase with increasing homogenized temperature, and reach the saturation values at 440 °C/2 h. The secondary phases in as-cast 7050 alloy consist of T (Al4Mg2CuZn or Al3Mg3CuZn2), S (Al2CuMg) and Al7Cu2Fe. During homogenization, the region mainly situated at the margin of T phase having (wt%) 2.3-5.8 Mg, 72.2-82 Al, 5.4-14.8 Cu and 6.3-7.2 Zn sluggishly dissolves into the α(Al) matrix; at the same time, an elemental diffusion cell network appears within T phase, composed of the white cell wall having (wt%) 12.2-14 Mg, 44.9-51.6 Al, 20.4-27.4 Cu and 11.4-15.8 Zn and the light gray cell interior (wt%) 13.8-15.2 Mg, 43.9-48.1 Al, 26-33.3 Cu and 5.6-10 Zn. The network fades gradually when the light gray cell interior transforms to the gray phase and then to the dark gray phase, and disappears finally when the composition of the equilibrium S phase approaches. The diffusion of Zn happens even at 380 °C, and while the obvious diffusions of Mg and Cu begin at 440 °C. S phase contains 77% of the residual phases in 7050 alloy homogenized at 450 °C/2 h.