| Body-centred cubic(BCC)metals and alloys are widely used in structural material field due to the special crystal structure.As polycrystalline materials,grain boundaries(GBs)play an important or even decisive role in the properties of BCC metals and alloys,and the specific contribution of each given GB to material properties stems primarily from its excess energy,i.e.,grain boundary energy(GBE).GBE is an important parameter in the design of polycrystalline materials,but how to obtain the GBE of an arbitrary GB in a rapid and accurate way has been an important problem to be solved in material field.The prediction of GBE based on the GB geometrical structure,describing by 5 macroscopic degrees of freedom,is an important way to obtain GBEs.However,it is still a challenge to establish the functional relationship between GBE anisotropy and GB geometrical parameters,due to the complexity of GB structures and the anisotropy of GBE as a function of 5 macroscopic degrees of freedom.Moreover,high temperature and the interaction between GBs and point defects also affect the structure and GBE of GBs,and may induce GB complexion transitions,resulting in the change of material properties.Based on the important effects of GB complexion transitions on material properties,Amanda et al.proposed GB complexion engineering,which is a method to improve material properties by controlling GB complexion transitions.In order to obtain GB complexion diagrams,which is an important tool for GB complexion engineering,the systematic identification of GB complexions and GB complexion transitions is a necessary prerequisite.To investigate the relationship between GBEs and GB geometrical parameters,a complete set of GBs containing symmetric tilt grain boundaries(STGBs),asymmetric tilt grain boundaries(ATGBs),twist grain boundaries(TWGBs)and mixed grain boundaries(MGBs)were created for BCC metals in a high throughput way,and the structures and energies of these GBs were calculated by Molecular Statics and Dynamics methods for W at 0 and 2400 K,Mo at 0 K and β-Ti at 1300 K.To further investigate the effects of high temperature and the interaction between GBs and point defects on the structure and energy of GBs,the initial microstructures with different GB atomic densities were created for a set of<110>STGBs of W,and the structure and energy of these GB microstructures were calculated by using Molecular Dynamics simulation of quenching.Furthermore,the GB complexion structures at different temperatures and GB atomic densities,as well as the GB complexion transitions triggered by high temperature and the change of GB atomic density,are investigated in depth for the<110>STGBs in W.The main findings and conclusions are as follows:(1)The two degrees of freedom representing GB plane orientation are coupled into a single parameter θ(the angle between lattice misorientation axis and GB plane),and the functional description of the relationship between GBE anisotropy and GB geometrical parameters is established:the GBE is a simple function of sin(θ)for the GBs with the same lattice misorientation,and the GBE functions are of the same type for the GBs with the same lattice misorientation axis but different lattice misorientation angle.These simple functions can well explain and predict the preferred GB planes when the lattice misorientation is specified.The way in which the above functional description is established also provides a new perspective on the relationship between GBEs and GB geometrical parameters.(2)It was found that local hexagonal close-packed(HCP)or face-centered cubic(FCC)structures form at some special GBs in β-Ti at high temperature,which is accompanied with a significant decrease in GBE.A similar phenomenon was found in the study of high temperature effects on a set of<110>STGBs in W.Further structure analyses show that the formation of these special GB local structures is not only strongly dependent on GB lattice misorientation,but also related to the symmetry of GBs.For<110>STGBs with the same lattice misorientation,local HCP structures can be formed at GB,only when the<2110>direction of hexagonal or quasi-hexagonal structure units of the GB is perpendicular to GB plane.(3)The increase of GB atomic density may also lead to the change of GB structure and energy.When the GBE difference caused by the change of GB atomic density is large,the GB is considered to be a weak trap for interstitial atoms,otherwise the GB is considered to be a strong trap for interstitial atoms.Furthermore,it was found that the trapped interstitials are generally located at some special locations of a GB—effective interstices.Based on the definition of these effective interstices,a new GB structure analysis method was proposed to assess the absorption capacity of GBs to interstitials.Then,it was found that the absorption capacity of GBs to interstitials is negatively related to effective interstice density,while positively correlated with effective interstice size.The proposed GB structure analysis method based on the effective interstices provides a new perspective on the assessment of the absorption capacity of GBs to point defects,which is valuable for the design of irradiation resistant materials.(4)A method for the identification of GB complexion transitions was developed,which is based on the structure analysis methods applicable to amorphous systems,and applied to analyze possible GB complexion transitions triggered by high temperature and the change of GB atomic density for 19<110>STGBs in W.In particular,the GB complexion transition analysis based on cluster-type index method(CTIM)allows the identification and characterization of different GB complexions by identifying the typical CTIM cluster components of GBs.This method not only offers the possibility to systematically study GB complexions and GB complexion transitions,as well as high-throughput map GB complexion diagrams,but also provides a new perspective to study the relationship between GB structures and properties from the GB complexions characterized by typical CTIM clusters. |
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