Molecular Simulations Of Grain Boundary Structures And Shear Behaviors Of NiAl Twin Films

Serving as a kind of elevated-temperature structural material, NiAl exhibits plentyof excellent physical and mechanical performance that has attracted interest forapplications in many large engineering fields. However, its severe room temperaturebrittleness limits its practical use. For revealing the intrinsic brittleness, improving theductility and providing theoretical basis and research thought for investigatingpolycrystal, it is necessary to develop an understanding of microstructures, energetics,and mechanical behaviors of NiAl bicrystals.In this dissertation, molecular dynamics simulations are employed to investigate thestructures of grain boundaries and the deformation mechanisms of shear of NiAlbicrystalline films with different grain orientations. The influences of grain boundarystructures and temperature on the mechanical behaviors are discussed. The mainachievements are shown as follows:⑴An analysis of the structures of grain boundaries indicates that the grainboundary length and free volume are independent in describing the grain boundary. It hasa direct correlation between the length and the plane normal index of the grain boundary.The grain boundary energies at0K are calculated and mapped as a function of grainmisorientation. It shows indications of cusps at36.87(310),53.13(210) and67.38(320). The results reveal that the free volume and the presence of particularstructural units have significant impacts on the grain boundary energy.⑵For shear of the bicrystalline films, various kinds of structure evolution behaviorare found depending on the structures of grain boundaries:①grain boundary coupling motion;and②atomic shuffling accompanied by lattice dislocation emission from thegrain boundary. It turns out that yield strength of singular grain boundary is much greaterthan that of others. An detailed analysis suggests that the transformation between E and Bstructural units is the main mechanism of grain boundary migration,and the structureevolution behavior can be attributed to several elementary transformation modes relatedto the transformation between the two units.