Cr和Mo对Fe_3Al合金中微观缺陷和电子密度的影响

Effects of Cr and Mo on microdefects and valence electron densities in Fe_3Al alloys

测量了二元Fe3Al及合Cr或Mo的Fe3Al合金的正电子寿命谱参数，计算了合金基体和缺陷处的价电子密度。实验结果表明，Al和Fe结合形成Fe3Al合金时，Al原子提供价电子与Fe原子的3d电子形成局域的共价键。Fe3Al合金中金属键和共价键共存。Fe3Al合金晶界缺陷的开空间大于Fe空位或Al空位的开空间，晶界缺陷处价电子密度比基体低，晶界的键合力较弱。在Fe3Al合金中加入Mo，降低了合金中的有序度，使晶界缺陷的开空间变小．但由于Mo原子半径比Fe大，当它取代Fe后，晶格发生畸变，体积增大，导致合金基体的价电子密度降低。在Fe3Al合金中加入Cr，合金基体和晶界的价电子密度都升高，金属键合力增强。Cr元素对改善Fe3Al合金的脆性有利。

The valence electron densities of bulk and microdefects in binary Fe3Al, and Fe3Al doped with Cr or Mo have been calculated by using the positron lifetime parameters of the alloys. The result indicates that, the 3d electrons in Fe atoms have well--localized properties and tend to form covalent bonds with the valence electrons in Al atoms, the bonding nature in Fe3Al is a mixture of metallic and covalent. The large--open--volume defects occur on grain boundary in Fe3Al and the bonding cohesion of the grain boundary is weak due to the low density of valence electron there. The addition of Mo into Fe3Al results in the decrease of the ordering energy of the alloy and the open--volume defects on grain boundary. Since the radius of Mo atom is larger than that of Fe atom, when Mo atoms substitute for Fe atoms, they will distort the lattice, enlarge the volume of the lattice and therefore decrease the density of valence electron in the bulk of the alloy. The addition of Cr into Fe3Al increases the densities of valence electrons in the bulk and the grain boundary, thusincreasing the metallic bonding cohesion. Cr is found to be a beneficial element in improving the brittleness of Fe3Al alloy.