First-principles Investigation On Lattice Reorientation And Alloying Effect In Ti And Mg

By providing an independent slip system,the {1012}<1011>twin acts as an important deformation mode of the hexagonal lattice.The manipulation of this twinning mode has practical significance to the plasticity of hexagonal metals and alloys.For instance,with the profusion and paucity of {1012} twins in Mg and Ti,respectively,either strength or plasticity can be improved by its hindrance or promotion.At variance with traditional models,recently studies have identified a new deformation mode(twinning-like lattice reorientation)which produces an orientational relationship similar to that of the conventional {1012} twinning in hexagonal Mg,Ti and Co.This mode nucleates via direct lattice reorientation and results in the basal/prismatic boundary.The vertical orientation relationship between the parent and twin lattices,and the transformations at the basal/prismatic interfaces may have implications for alloy design.However,some important details remain unclarified,e.g.,the twinning path and alloying effect.Hence,in this dissertation,some issues closely related to the new deformation mode was studied by first-principles calculation.Firstly,high-throughput ab initio calculations were employed to systematically investigate the energy paths of {1012} twin-related lattice reorientation in a series of hexagonal metals Be,Mg,Sc,Ti,Co,Y,Zr,Tc,Ru,Gd,Tb,Dy,Ho,Er,Tm,Lu,Hf,Re,and Os.Among the studied systems,reorientation consumes the lowest and highest energies in Mg and Os,respectively.The lattice reorientation energy increases in the order of Mg,Gd,Tb,Dy,Zr,Tc,Ti,Ho,Y,Co,Er,Sc,Be,Tm,Lu,Hf,Re,Ru and Os.The results indicate that lattice reorientation requires different activation energies in various hexagonal metals,and which are consistent with the fact that reorientation has been experimentally observed in compressed Mg nanopillars,but only in atomic scale simulations under high strain rate in Ti.We then separated the shear and shuffle components of the reorientation in pure Ti and Mg.The result indicates that the shuffle component always contributes a significant part of the reorientation energy in Mg,whereas in Ti with sufficient shear strain,subsequent reorientation process becomes energy-downhill.Concerning the significance of shuffle,the studied hexagonal metals are divided into two groups.In the first group,which includes Ti,Tc,Be,Y,Gd,Tb,Dy,Ho,Zr,Er,Sc,Hf,Lu and Tm,shuffle becomes an energy-downhill process if shear component reaches an adequate level(at least 60%),while in the second group,which includes Mg,Co,Ru,Re and Os,regardless of the shear amount,subsequent shuffle is an energy-uphill process.These results qualitatively explain the present observation of lattice reorientation in hexagonal metals,and sheds light on a general understanding on the{1012} twinning behavior in the aim of improving materials properties.Subsequently,the alloying effect was studied in great detail.In alloyed Mg and Ti,appropriate alloying significant reduce the reorientation energy,and especially in alloyed Ti,there is significant negative correlation between the reorientation energy and certain properties of the alloying elements.In alloyed Ti,with relative shear>0.65 and regardless of the alloying,the barrier energy of shuffle vanishes,whereas in alloyed Mg,only with relative shear>0.95 and alloying La,Zr and Mn,does the barrier energy of shuffle vanish.It was shown that,in Mg and its alloys,the shuffle energy is always a significant part of the overall reorientation energy.Hence at relatively low temperature,shuffle is hindered despite the relative activity of twin system,resulting in unperceivable temperature-dependency of {1012}twins in Mg and its alloys compared with Ti and its alloys.In hexagonal close-packed metals,c/a ratio is an important factor which has strongly correlation with many properties,such as dislocation and twinning,which are the extremely important behavior in the structural materials.Therefore,in this dissertation,high-throughput ab initio calculations were employed to calculate the c/a ratio in titanium and magnesium alloys.The results indicate that the effects of these alloying elements on the c/a ratio are periodical in alloyed Ti and Mg.Among these alloying elements,Mn,Fe,Ru,Ir and W exhibit relatively strong influence on the c/a ratio in alloyed Ti,with relatively weak influence in alloyed Mg.These results contribute to the facilitation of alloy design and property prediction.