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The age hardening responses of the Al-5.2Mg-0.45Cu alloy with Zn addition have been investigated in this study. It is found that the age hardening process of the Zn-containing Al-5.2Mg-0.45Cu alloys can be divided into four stages, namely the initial jump(Ⅰ), the linear increase (Ⅱ), the accelerated increase to the peak (Ⅲ) and the overage (Ⅳ). The addition of Zn to the Al-5.2Mg-0.45Cu alloy leads to an enhanced and accelerated age-hardening response in two aspects: firstly introducing the T-[Mg 32 (Al, Zn) 49 ] phase resulting from the high Mg/Zn ratio and relatively high aging temperature; secondly stimulating the precipitation of the Guinier-Preston-Bagaryatsky (GPB) zones by increasing the supersaturation upon quenching, and accelerating the precipitation of S-Al 2 MgCu phase by increasing Cu/Mg ratio in the matrix due to the formation of the T phase. The peak hardness is from the synergetic effects of the S and T phases. The clustering and partitioning behaviors of solutes after 1 h aging treatment have been investigated in detail using atom probe tomography (APT) and high resolution transmission electron microscopy (HRTEM). It is shown that the formation of Mg-Cu clusters (precursor of the S phase) precedes that of Mg-Zn clusters (precursor of T phase), and the Mg-Cu clusters are responsible for the age hardening during the first two age hardening stages in the Cu-bearing alloys.

Designing high-performance Al-based alloys based on the entropy concept offers a promising strategy for next-generation advanced structural materials. The main idea of entropy increasing is to increase the content of alloying elements. For traditional high-strength Al–Zn–Mg–Cu alloy, however, significantly increasing the content of alloying elements, especially when the Zn content is increased to above 10 at.% (atomic percent), leads to poor deformability and poor tensile property. Here, we overcome this trend by designing a quinary alloy based on the idea of high-entropy alloys. We fabricate a high-Zn-content Al80Zn14Li2Mg2Cu2 alloy with the Zn content above 10 at.%. It shows Zn, and Al–Zn precipitates in the as-cast state, and it is observed that Al3(Li, Mg), MgZn2, and Zn nano-precipitates form after rolling. The precipitation transformation contributes to high strength and high deformability. Our present findings provide not only a fundamental understanding of precipitation transformation in the high-Zn-content Al alloys but valuable guidance for the development of lightweight alloys as well.

The early precipitation behavior of long-range-ordered (LRO) Ni 2 (Cr,Mo) strengthening phase in a Ni–21Cr–17Mo (wt%) superalloy that was subjected to a patent two-step heat treatment (705 °C/16 h/furnace cooling – 605 °C/32 h/air cooling) was revealed by means of transmission electron microscopy (TEM) and electron diffraction pattern (EDP) analysis. The coherent interfacial structure between the long-range-ordered Ni 2 (Cr,Mo) precipitate and the Ni-based matrix leads to the excellent strengthening effect of the alloy. High-spatial-resolution energy-disperse X-ray spectrum (EDX) analysis reveals that the Ni 2 (Cr,Mo) precipitates were slightly rich in Mo. The minor microstructure change after the key first-step heat treatment was especially focused on. It was found that weak streak fringes were present along 〈 311 〉 Ni - base ⁎ directions in the EDP along the [112] zone axis and the ordered nano-regions parallel to {311} Ni-base planes were distributed homogeneously in the matrix. Combined with the detailed microstructural characteristic, it is concluded that Mo atoms might enrich at every two {311} Ni-base planes during the first step of aging at high temperature (705 °C), which could increase the transition temperature of Ni 2 Cr resulting in its formation when aging at the second aging step of 605 °C. Meanwhile, Ni 2 (Cr,Mo) phase precipitates by the substitution of Mo atoms for Cr atoms every three {311} Ni-base planes in Ni 2 Cr phase with the continuous long-range diffusion of Mo atoms.