Hardness-Lattice Parameter Correlation for Aged Al-Zn-Mg Alloys

Two ternary Al-2.2Zn-0.95Mg and Al-5.5Zn-2.1Mg(in wt pct) alloys,with Zn:Mg ratios close to 2.5 were produced by conventional ingot casting metallurgy.The ingots were solution heat treated at 500℃ for 0.5 h and aged at 180℃ for times between 0.5 and 80 h.The structural characterization was carried out by X-ray diffraction(XRD),transmission electron microscopy(TEM),selected area electron diffraction(SAED) and Vickers microhardness measurements(HV).The study was focused on the investigation of the precipitates formation and the relationship between hardness and lattice parameter for α-Al.The results showed that there was an inverse correlation for all the experimental conditions,and the aged peaks coincided with lattice parameter minima.Significant precipitates formation only occurred for the alloy containing 5.5 wt pct Zn and 2.1 wt pct Mg,provoking an important strengthening and variations in the lattice parameter,however,this was not observed for the alloy containing 2.2 wt pct Zn and 0.95 wt pct Mg.A plausible explanation of the increment of hardness values could be the presence of a well distributed η phase(MgZn2).At initial stages of the precipitation process,η was the most abundant precipitate while the phase τ was observed at overaged conditions.These results showed that the aging response of the conventionally cast Al-Zn-Mg alloys could be obtained using the lattice parameter of the α-Al matrix,even for alloy systems with low precipitates formation.