Phase-Field Crystal Study On Grain Boundary Segregation And Precipitation Mechanism Of Al-Li Alloy At The Early Stage Of Aging

Al-Li alloys are widely used in the manufacture of aerospace vehicles due to their comprehensive properties such as high specific modulus,high specific strength and specific stiffness,and good corrosion resistance.Grain boundary（GB）segregation and precipitation are easily to occur during the aging process,and the size and distribution of the precipitates can greatly affect the properties of Al-Li alloys.However,it is difficult to observe the structural transformation process of the precipitated phase in situ by experimental characterization.Therefore,in this paper,the Phase-field crystal method is used to study the GB segregation and precipitation behavior of Al-Li binary alloys during the aging process on the atomic scale.We revealed the atomic scale mechanisms for solute segregation and precipitation at different GBs.The main conclusions are as follows:（1）GB segregation is closely related to GB structure.The degree of GB segregation is proportional to the GB misorientation angle of the low-angle GB,and gradually converges at the high-angle GBs,but minima would appear in the lowΣGBs such asΣ3（111）GB andΣ9（114）GB.GB energy is the driving force for GB segregation,and our study confirms that the degree of GB segregation is positively correlated with GB energy.The GB atomic number density reflects the excess volume of the GB and represents the ability of the GB to accommodate solute atoms.We found that the GB atomic number density and GB segregation are negatively correlated with the misorientation angle.The higher the GB density,the lower the capacity of the GB to accommodate solute atoms and the lower degree of GB segregation.（2）The atomic-scale microscopic mechanism of Li atom segregation at GBs is revealed.The segregated of Li atoms in Al-Li alloys is generally concentrated at the dislocations in the GBs and the compressive stress of the structural units.It is distributed periodically with the arrangement of dislocations and structures,which is closely related to the GB structure.（3）The precipitation on the low-angle GB occurs near the dislocation.Around the dislocation region,a disordered precursor phase is evolved from the Al-based solid solution first,and then the precipitation phase nucleates and grows on the basis of the disordered phase.On the high-angle GB,the solute segregates uniformly on the GB at the early stage of aging,and then a two-dimensional spinodal decomposition occurs at the GB,resulting in periodically fluctuated concentration at the GB,and disordered precursors in the solute-enriched region which then evolve to uniformly distributed precipitate particles.The orientation relationship for the precipitation on the<100>tilt GB is Pitsch{112}_fcc//{110}_bcc.For the precipitation on the<110>tilt GB,the orientation relationship of theδphase precipitation is almost K-S{111}_fcc//{110}_bcc e.g.,theΣ9（114）GB and the low-angle GBs,but in some special GBs likeΣ3（111）GB exhibits N-W{111}_fcc//{110}_bcc.