Precipitation behaviour in an Al-Zn-Mg-Cu alloy subjected to high strain rate compression tests

Highlights: • The precipitates average size is increased after compression at 1.0×10^-3 s^-1. • The growth of precipitates and overlapping are dominant after high strain rate compression. • Coarse precipitates are formed either in the vicinity of dislocations or across the grain boundaries. • The SRS increases with increasing strain rates from 1×10^-3 s^-1 to 4.0×10³ s^-1 and becomes negative after 5.0×10³ s^-1. A peak aged (T6 heat treatment: 743 K /1.5 h + 393 K/24 h) Al-Zn-Mg-Cu alloy was subjected to compression tests at different strain rates from 1.0 × 10⁻³ s⁻¹ to 5.0 × 10³ s⁻¹. The yield strength of the Al alloy is increasing with strain rates from 1.0 × 10⁻³ s⁻¹ to 4.0 × 10³ s⁻¹and is decreasing with increasing strain rate from 4.0 × 10³ s⁻¹ to 5.0 × 10³ s⁻¹ showing strain rate sensitivity (SRS). A large number of Al₃Zr, GP zones, η', T (Al₂Mg₃Zn₃) and S (Al₂CuMg) precipitates are present in the Al alloy after compression at 1.0 × 10⁻³ s⁻¹ and Al₃Zr, η', η, T and S phases are dominant after high strain rate compression from 3.5 × 10³ s⁻¹ to 5.0 × 10³ s⁻¹. The precipitates average size is increased after compression at 1.0 × 10⁻³ s⁻¹. High strain rate compression leads to the growth of precipitates and overlapping. Coarse precipitates are mainly found along the grain boundaries and dislocations. The increasing SRS with increasing strain rates up to 4.0 × 10³ s⁻¹ owes to the growth of precipitates which are hindering dislocations motion and generating more dislocations. Increasing strain rates to 5.0 × 10³ s⁻¹, the coarse precipitates are no longer effective in pinning dislocations, leading to a decrease in yield strength and a negative SRS value.