Molecular Dynamics Simulation Of The Effect Of Twinning On ?(?)? Transformation In Fe Nanoscale System

The phase transitions in metallic materials,especially in steels,have been intensively studied for many years.The main proposal of the studies is to clarify the influence of the microstructure change on the macroscopic properties,for instance the mechanical properties and further to design the reasonable warm treatment technique.Here,defects,which are the sites with energetic and structural positions,play a critical role in controlling the properties of the metallic materials.On the other hand,the defects also magnificently influence the transition mechanism as the nucleation sites.In recent years,nanoscale materials become the focus of the research due to their unique properties.Many studies show that the phase transition theories in bulk may be not suitable in nanoscale systems.Thus,it is essential to further study the transition mechanism in nanoscale systems.This thesis studied the phase transitions induced by temperature and strain change in pure iron(Fe)thin films containing twin boundaries(TBs)with various twin spacing and orientation.In addition,the effects of the film thickness and surface orientation on the transition mechanism have been investigated.The main results can be summarized as following.(1)Both the austenitic and martensitic phase transitions induced by temperature change in Fe thin films have been studied.The influence of the TB fraction and film thickness on the transition mechanism has been discussed.The result show that for the austenitic transition,there exists only weak relationship between the film thickness and transition temperature,when the surface does not attend the nucleation.Serving the surface as nucleation site,the austenitic transition temperature obviously decreases with thickness decrease.The TB fraction does not has influence on the transition temperature.The body centered cubic(bcc)to hexagonal close packed(hcp)transition obeys the Burgers relationship.In the martensitic transition,the martensitic transition temperature decreases with film thickness increase.Similar to the austenitic transition,the TB fraction also does not have obvious influence on the martensitic transition temperature.Both the free surface and TB assist the transition as nucleation position.Both the Kuddjumov-Sachs(K-S)and Nishiyama-Wassermann(N-W)transition relationships coexist during the martensitic transition.(2)The phase transitions induced by strain have been studied in Fe thin films containing TBs with various fraction and orientation.Hall-Petch relationship has been observed during the biaxial straining applied on the films.In the film group with parallel TB-surface orientation relationship,the new phase nucleated mainly at three positions,namely the intersections between the surface and TB,the free surface and the TB,i.e.,both the TB and the free surface assist the phase transition as nucleation sites.Increasing the TB fraction,the new phase preferred to nucleate at the TBs rather than the free surface.The critical strain,at which the phase transition takes place,decreases with TB fraction decrease.Comparing with the parallel group,the TB fraction directly influences the nucleation in the film group with perpendicular TB-surface orientation relationship.Increasing the TB fraction,the surface lost its role as nucleation sites.This observation has not been found in the parallel film group.In addition,the transition dynamics has been discussed in both film groups.The results of this paper play a certain role in understanding the influence mechanism of intrinsic defects of materials on phase transition in nanoscale systems,and can be used as a supplement to the existing phase transition theory.Meanwhile,it also provides some theoretical basis and new ideas for the design of Fe thin films.