A series of new achievements in the field of aluminum alloy precipitation strengthening

Recently, the team of Professor Du Yong of the Institute of Powder Metallurgy of Central South University published an original paper entitled "Atomic scale investigation of the crystal structure and interfaces of the B 'precipitate in Al-Mg-Si alloys" in the metal material journal "Acta Materialia" . Associate Professor Li Kai and Professor Du Yong of the Research Institute of Powder Metallurgy of Central South University are the co-corresponding authors of this article. For the first author unit.

Aluminum alloy has become an important lightweight material for the automobile and other industries to achieve energy saving and emission reduction due to the characteristics of light weight, high specific strength, corrosion resistance, and easy recycling. Studying the structure of the strengthening phase precipitated during the aging heat treatment of the aluminum alloy is the basis for designing the alloy composition and controlling the type of the precipitated phase to improve the mechanical properties of the aluminum alloy. Due to the low structural stability of the metastable nano-precipitated phase in aluminum alloys under the high-energy electron beam of transmission electron microscope, the atomic scale characterization of its structure is restricted.

Chen Haonan, Li Kai and others found in the previous research that under the conventional high-energy electron beam of 200-300 keV, the main strengthening precipitated phase of Al-Mg-Si alloy β is easily damaged by the electron beam. , Which greatly promotes diffusion. Therefore, by reducing the electron beam energy of the spherical aberration correction transmission electron microscope of the objective lens to 80 keV to reduce the probability of knocking out atoms, a stable long-term high-resolution transmission electron microscopy observation of β "(Micron116 ( 2019) 116–123). Based on this, under low irradiation damage conditions such as 80 keV low energy electron beam and 200 keV low dose electron beam, spherical aberration correction high resolution scanning transmission electron microscopy (HRSTEM) and atomic scale energy spectroscopy (EDX) surface scan were performed. Combined with the dual criterion of enthalpy-mismatch degree calculated by first-principles calculation, the atomic-scale structure model of the precipitated phase B ′ in the Al-Mg-Si alloy has been unclear for 20 years.

In this study, the interface structure model between B ′ and aluminum matrix was obtained, and it was found that the layered defect structure can be self-coordinated inside the precipitated phase to reduce the overall mismatch between the precipitated phase and the matrix to reduce the growth resistance. "Interestingly" (reviewer's comment), in this work, the phase transition path of U1 → U2 → B ′ was found near the coherent interface, which meets the lowest enthalpy formation criterion, and in the non-coherent interface A B ′ → U2 transition path was found nearby (see figure below), revealing the interleaving effect of multiple energy criteria on the phase transition path, and also illustrating the limitations of traditional off-site observations and the broad prospects of in-situ characterization of transmission electron microscopy.

In the same period, two other papers (and) with Li Kai and Du Yong as co-corresponding authors were published in the international journals "Materials Science and Engineering: A" and "Journal of Materials Science & Technology". The former proves for the first time that the difference in strengthening effect between the main strengthening phase β ″ and the secondary strengthening phase β ′ of Al-Mg-Si alloy depends on the degree of mismatch; the latter uses a multi-phase multiphase aluminum alloy thermodynamic database developed by Du Yong ’s group, The alloy composition of Al-Mg-Si automobile body plate alloy with comprehensive strength and toughness (yield strength 277MPa, elongation rate 20.0%) is obviously superior to the existing literature data through calculation and adjustment of alloy composition through phase diagram.