Solute Redistribution In Ultrafine/Nano-grained Al-Mg Alloys With Different Grain Boundaries

With the rapid development of science and technology,global warming and energy crisis are becoming increasingly severe.Lightweight aluminum alloys are favored by researchers.Grain refinement not only makes the material excellent in strength and plasticity,but also leads to solute redistribution,which has a great impact on the mechanical properties and thermal stability.In this paper,five Al-Mg alloys with different composition are used to study the solute redistribution law and mechanism in the alloy after high pressure torsion(HPT).Lattice parameters and average micro-strains of different alloys are calculated based on X-ray diffraction(XRD)results;Microstructure and grain size of these alloys are investigated by transmission electron microscope(TEM);Grain boundaries of interest are extracted by focused ion beam(FIB).High-angle annular dark-field and bright field image of scanning transmission electron microscopy(HAADF-STEM),Electron Energy Loss Spectroscopy(EELS),High Resolution Transmission Electron Microscopy(HRTEM)and Atom Probe Tomography(APT)were used to analyze the solute redistribution of Al-8.0Mg alloy.Combining with geometric phase analysis(GPA)to explore the solute redistribution at grain boundaries(GBs),mechanism and cluster growth mechanism.The main conclusions obtained are:(1)The microstructure and grain size of Al-Mg alloys with different compositions after HPT were observed by TEM and analyzed by XRD.The results show that there are dislocation tangles in multiple regions and high angle sub-grain boundaries composed of dislocations.With the increase of Mg content from 1.0 wt.%to 10.0 wt.%,the grain size of these alloys has been reduced from 224.9 nm to 55.3nm,and has been refined into ultrafine crystals or nanocrystals.(2)Five Al-Mg alloys were observed by HAADF-STEM.The results show that with the increase of Mg content,the enrichment degree of solute atoms at the grain boundary(GB)increases gradually.When the Mg content was 8.0 wt.%,the solute content at the GB is twice that in the matrix.However,when the Mg content is 10.0wt.%,the solute segregation degree at the GB decreases and is lower than Al-8.0Mg alloy.(3)The solute redistribution at GB of nanocrystalline Al-8.0Mg alloy was observed by HAADF-STEM,EDX and EDS.The results show that there are three different solute element distribution phenomena at the grain boundaries: solute atom depletion,solute atom enrichment and solute atoms are not uniformly distributed.These three GBs were analyzed by GPA,and the tooth tips of the serrated grain boundary depleted in solute elements shows alternating tension and compression strains;the low angle grain boundary enriched by solutes has only a little strain field;the grain boundary where solute atoms is unevenly distributed shows irregular strain fields,The solute atoms are enriched in the tensile strain field and depleted in the compressive strain field.(4)The GBs with different structures observed by HAADF were removed by FIB,and the HRTEM,EELS and GPA were used for observation and systematical analysis.The results obtained that the LAGB with disordered atomic arrangement presented a compressive strain field,and the solute atoms were depleted.The solute atoms are enriched at the HAGB,and there is almost no strain field at the center of the enrichment region.Combined with the EELS line scanning,it can be seen that there is a short-range ordered magnesium-rich structure at the HAGB.There is a Mg-rich region in the center of the trigeminal GB.However,slight solute depletion and tensile strain field generated between the two grains adjacent to the trigeminal junction,which is due to the formation of the Mg-rich region in the trigeminal center.(5)Grain boundary migration is an important way for solute redistribution.The greater the difference in orientation of the grain boundary,the easier it is to migrate.The large-angle grain boundaries of the nanocrystalline Al-8.0Mg alloy account for more than 95% of all grain boundaries,so a large number of solute atoms are enriched.Theoretically,small-angle grain boundaries can also achieve solute redistribution through migration,but the applied stress is too large,so it relies more on the strong interaction between solute atoms and defects to promote the enrichment of solute atoms in the grain boundaries.(6)The redistribution of solute atoms at the grain boundaries of nanocrystalline Al-Mg aluminum alloys is very complicated.The redistribution rules of solute elements may be very different for different grain sizes or different grain boundary structures.Based on the analysis of APT and other research results in the existing literature,several mechanisms for the redistribution of solute atoms at the grain boundaries of nanocrystalline Al-Mg aluminum alloys have been proposed:(a)dislocation guidance;(b)vacancy induction;(c)non-equilibrium grain boundary migration and "attraction".For the first time,APT analysis was used to observe the presence of high magnesium dislocation lines near the grain boundaries of nanocrystalline Al-8Mg,which provides direct evidence for magnesium atoms to diffuse to the grain boundary enrichment through "dislocation-guided".At the same time,using HAADF-STEM and other methods,preliminary observations were made that the “non-equilibrium grain boundary migration and attraction” affects the redistribution of solute elements near the nanocrystalline Al-8Mg grain boundary.(7)Based on the APT analysis of magnesium-rich clusters in nanocrystalline Al-8.0Mg alloy,the growth mechanism of magnesium-rich clusters in nanocrystalline Al-Mg aluminum alloy is proposed: two adjacent clusters attract free solutes The volume of the element and the matrix gradually increase,and they gradually approach and form interconnected channels during the movement process,and finally merge into a new cluster with relatively stable structure and composition.