Characterization of phase development in commercial Al-Zn-Mg(-Mn,Fe) alloy with and without Sc,Zr-addition

Abstract

Precipitation reactions of the Al-Zn-Mg(-Mn,Fe)-based alloys with/without Sc,Zr-addition were studied by microhardness and resistivity measurements, and differential scanning calorimetry. Microstructure observation proved the Zn,Mg-containing eutectic phase at grain boundaries. Positron spectroscopy confirmed the presence of Guinier-Preston (GP) zones in the initial state. The changes in resistivity and microhardness curves as well as in heat flow are mainly caused by the formation and/or dissolution of the Guinier-Preston zones and precipitation of the particles from the Al-Zn-Mg system. Formation of the Mn,Fe-containing particles as well as of the η-and T-phase does not influence hardening significantly. The hardening effect above ∼ 300 • C reflects the Sc,Zr-addition. Heat treatment at 300 • C for 60 min and 460 • C for 45 min is insufficient for homogenization of the alloys. The apparent activation energy values were calculated: dissolution of the GP zones (∼ 106 kJ mol −1), formation of the metastable η-phase (∼ 111 kJ mol −1), formation of the stable η-phase (∼ 126 kJ mol −1

Early Stages of Precipitation in Mould-Cast, Cold-Rolled and Heat-Treated Aluminium Alloy AA7075 with Sc,Zr-Addition

“…[2][3][4][5][6][7][8]) are consistent with the following precipitation sequences: (a) supersaturated solid solution (SSS) → clusters/Guinier-Preston (GP) zones → η -phase → η-phase, (b) SSS → clusters/GP zones → T -phase → T -phase. The η -phase precipitates play a major role in the strengthening of the alloys [8]. But the formation of metastable precipitates depends on the alloy composition, artificial ageing temperature, ageing time, heat treatment etc.…”

“…Refs. [8,16,17]) particles of the metastable η -phase were observed at comparable tem- peratures both in AlZnMgScZr and AlZnMgCuScZr alloys. The hardening peak in the temperature range of 150-240 • C, as well as the exothermic B effect is associated with a formation of the η -phase.…”

“…Based on our previous study [8], it can be assumed that clusters/GP zones are being formed during cooling after casting and subsequent NA of the 7075(-ScZr) alloys. PAS measurements showed that positrons annihilated at traps in the MC state of the alloys are associated with small clusters characterized by a positron lifetime of 0.218-0.226 ns.…”

“…Figure 4a shows TEM image of the 7075-ScZr alloy in the MC state isochronally annealed up to 210 • C. Microstructure observations combined with ED and EDS proved presence of the metastable η -phase precipitates and in addition presence of the T -phase (Al 2 Zn 3 Mg 3 ) and S-phase (Al 2 CuMg) particles. In Al-Zn-Mg(-Cu)-based alloys the metastable η -phase is a typical hardening phase [2][3][4][5]8]. On the other hand, precipitation of both T -and S-phases does not lead to any significant hardening [2][3][4][5][6]8].…”

Role of Small Addition of Sc and Zr in Clustering and Precipitation Phenomena Induced in AA7075

“…The precipitation of the semi-coherent η′-and T′-phases usually cause the age-hardening of the materials in contrast to the incoherent η-and T-phases [19,20,[22][23][24][25][26][27]. Many authors use different designations for the early precipitation phases formed during the precipitation; (co-)clusters/Guinier-Preston (GP) zones of different types usually appear during ageing at temperatures lower than ~150 °C [19,20,25,[28][29][30][31][32][33].…”