Abstract
Aluminium alloy 7010 is subjected to retrogression and re-ageing (RRA) heat treatment to study the influence of microstructural changes on hardness. Retrogression is performed at 190 °C for different time intervals ranging from 10 to 60 minutes. Optimum time for retrogression treatment is estimated based on the retrogression time that result with equivalent mechanical properties as that of peak aged (T6) condition. Retrogression performed for 30 minutes resulted with micro hardness of 203 HV, which is equivalent to that obtained by following T6 treatment. Microstructural characterization done with the help of transmission electron microscope (TEM) indicates RRA treatment results with the coarsened and discontinuous precipitates along the grain boundary which is similar to over aged (T7) condition, where as fine and densely populated precipitates in the matrix similar to T6 condition. Coarse and discontinuous grain boundary precipitates (GBP’s) improves resistance to stress corrosion cracking. Fine and dense precipitates in the matrix ensures hardness equivalent to that of T6.
Highlights
Aluminium 7xxx series alloys is one of the high strength aluminium alloys used in aerospace structural applications mainly due to their high specific strength, damage tolerance and corrosion resistance properties [1]
The ή precipitates formed during the precipitation sequence is the metastable precipitate of MgZn2, which is semicoherent with the matrix and is the major strengthening phase of the alloy
The matrix of the T6 treated sample is highly occupied with GP zones and fine scale metastable precipitates ή of size 3-5 nm
Summary
Aluminium 7xxx series alloys is one of the high strength aluminium alloys used in aerospace structural applications mainly due to their high specific strength, damage tolerance and corrosion resistance properties [1]. The ή precipitates formed during the precipitation sequence is the metastable precipitate of MgZn2, which is semicoherent with the matrix and is the major strengthening phase of the alloy These alloys were earlier used in peak ageing (T6) condition, but during service, they are prone to stress corrosion cracking (SCC) due to the microstructural features like GBP’s and coarse intermetallic particles [3]. The aim of the present research work is to optimize the time of retrogression at 190 °C so as to obtain equivalent hardness as that of T6 treated sample and to study the microstructural changes during RRA
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