Study On The Formation And Deformation Mechanisms Of Fivefold Twins In Nickel Base Alloy

The fivefold deformation twins(FTs)have excellent properties based on their special crystal symmetry structure.They can not only enhance the hardness,strength and other mechanical properties of the materials,but also improve the physical and chemical properties,such as electricity,optics and catalysis,resulting in wide attention from countries all over the world.However,the related reports of the fivefold twins mainly focus on the study of the face-centered cubic(FCC)single elemental metals,such as Cu,Ag,etc.,or the mechanical behavior of the fivefold-twin structures.There are few study on the formation of fivefold twins in the alloy,and much less research on the fivefold deformation twinning surface of the alloy.Therefore,in order to apply fivefold-twin structures into engineering alloy better,the molecular dynamics(MD)simulations are used to study nickel-based alloy C-2000 in this paper.The formation mechanism,develop rules and deformation mechanism of fivefold deformation twins in the alloy are analyzed.Nanoindentation experiments are implemented to explore the changes of structure and properties of nickel-based alloy.Firstly,the nanoindentation simulations are carried out on the nickel-base alloy C-2000 model to investigate the formation mechanism and regularity of the fivefold twins.The researches show that fivefold deformation twins are formed at the intersection of four different {111} slip planes.During the indentation process,energy accumulation and stress concentration occur at the intersection center,and the incoherent twin boundaries(ITBs)are formed near the intersection center.The white atoms with high energy in the the intersecting center emit a partial dislocation under the action of shear stress,the stacking faults(SFs)are formed along {111} plane.The white atoms continue to emit partial dislocations,the twins get nucleated and grown,and the five deformation twins are finally formed.Particularly,the formation of fivefold deformation twinning surface for the alloy is closely related to the energy increment in the surface of the alloy during the indentation process.Besides,the formation of fivefold deformation twins are the result of the synthesis of reasonable simulation conditions.Adjusting the simulation parameters such as the indenting velocity,the radius of the indenter,and the indenting crystal orientation in the corresponding range will affect the shape and size,position and number of the fivefold twins formed in the alloy.These studies provide guidance for the preparation of fivefold-twin structures in the experiment.Then,the tensile simulations are put into effect on the nickel-based alloy C-2000single-crystalline(SC)and fivefold-twinned(FT)nanowires models.The deformationmechanism of the fivefold twins is analyzed.The results indicate that the Young's modulus and yield strength the fivefold-twinned nanowires in tension have been increased compared with that of single crystal nanowires.During the elastic deformation process,five twin boundaries inhibit the partial dislocations nucleation.Moreover,in the plastic deformation stage of the fivefold-twinned nanowires,the formation of secondary fivefold-twin structures make tensile stress of nanowires increase,this is because the formed five twin boundaries(FTBs)block partial dislocations slipping,acting as a resistance to plastic deformation.In addition,during the stretching process of the fivefold-twinned nanowires,the partial dislocations accumulate at the interior of the fivefold-twinned nanowires,resulting in the stress concentration,the fivefold twinned nanowires are break more quickly.Nanoindentation experiments are carried out on the nickel-based alloy C-2000.Before the experiment,Chemical mechanical polishing(CMP)is performed to improve the surface quality of the alloy sample.The surface of the sample is loaded with different peak loads.It is found that when the peak load is 1 N,the hardness of the alloy surface is increased greater.By using transmission electron microscopy(TEM)to observe the the alloy surface after 100 cyclic loading,nanotwin structures are formed in the nickel-based alloy.