合金元素对钢中NbC异质形核影响的第一性原理研究

为了探索不同合金元素对Nb C异质形核的影响,本文利用第一性原理研究了合金元素X(X=Cr,Mn,Mo,W,Zr,V,Ti,Cu和Ni)对ferrite(100)/Nb C(100)界面性质的影响,并且分析了上述合金元素掺杂前后界面的黏附功、界面能和电子结构.研究结果表明,Cr,V和Ti掺杂的界面具有负的偏聚能,说明它们容易偏聚到ferrite/Nb C界面,但Mn,W,Mo,Zr,Cu和Ni却难以偏聚到此界面.当Mn,Zr,Cu和Ni取代界面处的Fe原子后,界面的黏附强度降低,即这些合金减弱铁素体在Nb C上的形核能力.然而Cr,W,Mo,V和Ti引入界面后,其黏附功比掺杂前的界面要大,且界面能均降低,即提高了界面的稳定性.因此,W,Mo,V和Ti,尤其是Cr,能够有效地促进铁素体形核和细化晶粒.电子结构分析表明,Zr和Cu引入界面后,界面处的Zr,Cu原子和C原子的相互作用变弱;然而Cr和W引入界面后,Cr,W和C原子之间形成了很强的非极性共价键,提高了ferrite/Nb C界面的结合强度.

First-principles calculation of influence of alloying elements on NbC heterogeneous nucleation in steel

The NbC precipitated in steel is in favor of the heterogeneous nucleation of ferrite, which is affected by the alloying elements at the ferrite/NbC interface. However, it is difficult to clearly understand the effect of alloying elements on the ferrite/NbC interface behavior experimentally. Therefore, the first-principles calculation is employed to address this problem in this paper. First of all, the segregation behaviors of alloying element X (=Cr, Mn, Mo, W, Zr, V, Ti, Cu and Ni) on the ferrite(100)/NbC(100) interface are systematically explored. And then, we investigate the influences of these alloying elements on the property of the ferrite/NbC interface. The work of adhesion (W_ad), interfacial energy (γ_int) and electronic structure of ferrite/NbC interface alloyed by these elements are also analyzed. The results show that the (Cr, V, Ti)-doped interfaces have negative segregation energies, which indicates that Cr, V and Ti are easily segregated at the ferrite/NbC interface. Conversely, the Mn, W, Mo, Zr, Cu and Ni are difficult to segregate at the interface. When Mn, Zr, Cu and Ni replace the Fe atoms in the ferrite/NbC interface, the adhesive strength of the interface will decrease, thus weakening the heterogeneous nucleation of ferrite on NbC surface. However, the introduction of Cr, W, Mo, V and Ti will improve the stability of the ferrite/NbC interface due to the larger W_ad and lower γ_int. Therefore, the Cr, W, Mo, V and Ti on the ferrite side of the interface can effectively promote ferrite heterogeneous nucleation on NbC surface to form fine ferrite grain. The analysis of difference charge density indicates that after the introduction of Zr and Cu in ferrite/NbC interface, the interactions among interfacial Zr, Cu and C atoms was weaken. However, when Cr and W are introduced into the clean interface, the strong Cr-C and W-C non-polar covalent bonds are formed, which enhances the adhesion strength of the ferrite/NbC interface. In addition, the minimum Cr-C bonding length at the Cr-doped interface suggests that the interface has the highest interface strength. The Mulliken population analysis shows that for the (Cr, W, Mo, V, Ti)-doped interfaces, the transfer charges of Cr, W, Mo, V and Ti are 1.12, 0.84, 0.54, 0.33 and 0.28, respectively. Nevertheless, for the clean interface, the transfer charge of Fe is only 0.05. Therefore, the interactions among interfacial Cr, W, Mo, V, Ti and C atoms are stronger than that between interfacial Fe and C atoms, which is in good accordance with the above analysis.