Study On The Co-segregation Behavior Of Non-metallic And Metallic Elements In Ni ∑5 [001](210)grain Boundary And Its Influence

Nanograined(NG)nickel materials have excellent mechanical and peculiar physicochemical properties,and thus possess a bright application prospect.However,the low stability of NG materials has been a prominent bottleneck restricting their practical applications,which is mainly related with the large number of unstable grain boundaries(GBs).Studies have shown that GB segregation is a feasible way to improve the stability of NG metallic materials.However,the GB segregation behavior of non-metallic impurity elements commonly found in Ni alloys and their influences on the segregation behavior of main alloying elements,as well as the influence of the interactions of non-metallic elements with metallic elements and co-segregation behaviors on GB stability,are not fully understood,hindering the development and application of precise control of NG nickel stability based on GB segregation.Therefore,based on the theoretical framework of GB segregation engineering,the first-principles total energy calculation method based on density functional theory were performed in this thesis to systematically study the segregation behavior of six common non-metallic impurity elements X(X = H,C,N,O,B and S)in Ni ∑5 [001](210)symmetrical tilted GB,as well as their interactions and cosegregation behavior with metal elements(i.e.,X-Re and B-X’,X’=Zr,Hf,Ti,Ta,Nb and W).The influencing factors of GB segregation and its influence mechanism on GB stability and mechanical properties were studied in depth.Our work is not only expected to provide reliable theoretical support for improving the stability of NG nickel based on solute segregation,but also provide useful reference for the composition design and optimization of high-performance NG nickel.Main results are summarized as follows:(1)All the six kinds of non-metallic elements have the tendency of GB segregation and GB stabilizing effect.The GB segregation ability of these six elements decreases in the order of S > B > O > N > C > H.Due to the significant GB segregation ability of S element,the increase of annealing temperature and the decrease of bulk concentration have no effect on the equilibrium segregation concentration of S at the Ni GB.The calculation results on the work of separation before and after GB segregation showed that,except for B element,the other five elements are all GB embrittlers.(2)Re element originally has no segregation tendency to the pure Ni ∑5[001](210)GB,while the segregation of non-metallic elements X in the GB can induce the GB segregation of Re element.The calculated results of GB energy and fracture energy show that the segregation of Re element can further reduce the excess energy of X-containing GBs and improve their fracture strengthes.All the X-Re co-segregations show synergistic enhancement effects on GB stabilization.However,in terms of GB strengthening,N/O/B/S-Re cosegregations show antagonistic effects,while H/C-Re co-segregations dispaly synergistic enhancement effects.The results of the binding energy between X and Re atoms show that the mutual attraction between C/N/B and Re atoms has a promoting effect on Re segregation.The mechanism of the effect of Re segregation on the stabilizing and strengthening of X-containing GBs was attributed to the change in the Fermi energy level location and the increase in charge density in-between the fracture layers.(3)The presence of non-metal element B in the Ni ∑5 [001](210)GB can enhance the segregation ability of 6 transition metal elements X’,especially inducing the segregation of W.The co-segregation of B-X’ shows synergistic effect in improving GB stability,but exhibits antagonistic effect in enhancing GB fracture strength.The reason why Zr/Hf and Nb/Ta have similar segregation ability and GB energy in B-containing GBs is because they belong to the same group of elements,with similar atomic properties and electronic structures.The co-segregation of B-W shows the most significant strengthening effect on GB,as the W element loses the least electrons among all X’ elements in the Bcontaining GB.