Effects of Al₃Ni and Al₇Cr Intermetallics and T₆ Heat Treatment on the Microstructure and Tensile Properties of Al-Zn-Mg-Cu Alloy

In this study, the effects of Ni, Cr additions and T6 heat treatment on the microstructure and tensile properties of Ti-refined Al-12%Zn-2.5%Mg-2.5%Cu alloy were investigated. Microstructural and tensile properties evaluation was conducted by optical microscopy and scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry, x-ray diffractometry and standard tensile testing. The microstructural observations showed that the base alloy consists of lamellar eutectic structure in dendritic regions (α(Al) + η(MgZn₂)) and some interdendritic phases (T(Al₂Mg₃Zn₃)/S(Al₂CuMg)). Addition of Ni and Cr elements (from 0 to 5 wt.%) introduced two new intermetallic compounds in the microstructure as Al₃Ni and Al₇Cr, respectively. With the enhancement of Ni and Cr contents from 3 to 5 wt.%, the size and volume fraction of Al₃Ni and Al₇Cr particles increased from 2.1 μm and 8% (for Ni) and 32 μm and 3.2% (for Cr) to 5 μm and 18% and 45 μm and 12%, respectively. Tensile testing revealed that the highest ultimate tensile strength (UTS) and elongation (El.%) values are achieved by adding 3 wt. % Ni and 0.5 wt.% Cr to the alloy, which altered the quantities from 215 MPa and 5.5% for the base alloy to 295 MPa and 7.1% (for 3 wt.% Ni) and 230 MPa and 5.8% (for 0.5 wt.% Cr). These interesting results were concluded from size and morphology of new intermetallic particles, in which Ni additions had more effective influences on tensile properties of the alloy. In addition, T6 heat treatment had the adequate influence on the increment of the UTS and elongation to fail values by breaking eutectic structure and dissolution of η-phase and homogeneous distribution of new intermetallics. SEM images of several fractured surfaces showed an overall brittle failure mechanism with cracking mainly through the interdendritic region or within the intermetallics. Al₃Ni phases caused an improvement in the fracture mode to more ductile, whereas Al₇Cr intermetallics turned the mode to more brittle form.