Abstract
The hardness, toughness, wear resistance, and fatigue behavior of materials can be improved through a deep cryogenic treatment (DCT). During this treatment, low temperatures (−100 °C to −196 °C) are maintained and then increased to room or higher. In this work, an indirect-extrusion plate of 7A99 ultra-high strength aluminum alloy was subjected to a T6 (peak aging) treatment and a T6-DCT treatment. The influence of the T6-DCT treatment on the mechanical properties, grain morphologies, precipitates, and atom–cluster distribution was investigated via tensile testing, electron backscatter diffraction, transmission electron microscopy, and three-dimensional atom probe analysis. The tensile strength (maximum: 705 deep cryogenic treatment), yield strength (maximum: 678 MPa), and elongation (maximum: 14%) of the T6-DCT-treated alloy were higher than those of the T6-treated alloy. Moreover, the T6-DCT treatment resulted in (i) grain size refinement and increased uniformity of the microstructure (homogeneous distribution of η’-MgZn2- and η-phase precipitates), and (ii) reduced segregation degree of Zn, Mg, and Cu atoms in the matrix (fraction of small atom clusters (sizes: 10–20 nm, 20–50 nm) increased, fraction of large clusters (size: >1,000 nm) decreased). Therefore, DCT can refine the precipitates and increase the uniformity of the precipitate distribution, thereby improving the strength and plasticity of the alloy.
Highlights
In recent years, with the rapid development of weaponry, 7XXX ultra-high strength aluminum alloys have been used increasingly in military fields [1,2,3,4,5]
The high-strength 7A55 alloy exhibits poor stress corrosion resistance due to the relatively high content of Cu [6,7,8]. 7A99 alloy, developed by Beijing Institute of Aeronautical Materials, is a new ultra-high strength and toughness aluminum alloy that has been used on a preliminary basis in weaponry
The tensile properties of indirectly extruded 7A99 ultra-high strength aluminum alloy subjected to T6 and T6-deep cryogenic treatment (DCT) treatments are shown in Table 4 and Figure 1
Summary
With the rapid development of weaponry, 7XXX ultra-high strength aluminum alloys have been used increasingly in military fields [1,2,3,4,5]. The tensile strength of the 7A85 alloy is relatively low compared to that of the 7A55 alloy. The high-strength 7A55 alloy exhibits poor stress corrosion resistance due to the relatively high content of Cu [6,7,8]. 7A99 alloy, developed by Beijing Institute of Aeronautical Materials, is a new ultra-high strength and toughness aluminum alloy that has been used on a preliminary basis in weaponry. Materials with extremely high strength and toughness are required due Metals 2019, 9, 631; doi:10.3390/met9060631 www.mdpi.com/journal/metals
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