Abstract
This study reports on the effect of interrupted quenching on the microstructure and mechanical properties of plates made of the AlZnMg(Cu) alloy AA7050. Rapid cooling from the solution heat treatment temperature is interrupted at temperatures between 100 and 200 °C and continued with a very slow further cooling to room temperature. The final material’s condition is achieved without or with subsequent artificial ageing. The results show that an improvement in the strength–toughness trade-off can be obtained by using this method. Interrupted quenching at 125 °C with peak artificial ageing leads to a yield strength increase of 27 MPa (538 MPa to 565 MPa) compared to the reference material at the same fracture toughness level. A further special case is the complete omission of an artificial ageing treatment with interrupted quenching at 200 °C. This heat treatment exhibits an 20% increase in fracture toughness (35 to 42 MPa m−1/2) while retaining a sufficient yield strength of 512 MPa for industrial applications. A detailed characterization of the relevant microstructural parameters like present phases, phase distribution and precipitate-free zones is performed using transmission electron microscopy and atom probe tomography.
Highlights
AlZnMg(Cu) alloys belong to the group of age-hardenable alloys and are characterized by high specific strength, sufficient ductility, and satisfactory corrosion resistance
As the yield strength of the material is influenced by the distribution of the hardening phases while the fracture toughness is governed by the void nucleation, it is theoretically possible to improve the fracture toughness without significant loss in strength and, vice versa, increase the strength level without loss in toughness
Interrupted Quenching leads to similar microstructures and properties as obtained by Interrupted Ageing at low temperatures and over-ageing or Retrogression and Re-ageing at higher temperatures
Summary
AlZnMg(Cu) alloys belong to the group of age-hardenable alloys and are characterized by high specific strength, sufficient ductility, and satisfactory corrosion resistance. Due to this combination of properties, they are widely used as rolled, pressed and forged products in structural applications, especially in aircraft constructions [1]. Recent developments focus on improving various performance criteria, higher yield strength in combination with increased fracture toughness. These properties are highly dependent on the actual microstructure, which can be deliberately influenced by heat treatments. The precipitation sequence upon ageing is generally given by [2]: SSSS → cluster → GP-zones (GPI, GPII) → η’ → stable η (MgZn2 ) or T ((Al,Zn) Mg32 )
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