Mechanical Behavior And Deformation Mechanism Of Lamellar Nanostructure

Regardless of the homogeneous or heterogeneous structure,the ultimate goal is to achieve the excellent mechanical properties of materials.According to the Hall-Petch law,the nanocrystalline materials are with greatly high yield strength but extremely low plasticity,which results from quick plastic instability.Coarse grains have good plasticity,but are at the expense of strength.To be compatible with high yield strength and good plasticity needs to improve the work hardening ability.Therefor,studies have reported a variety of strengthening and plasticizing strategies.For instance,the strain hardening ability of gradient nanostructured materials whose characteristic is the gradient distribution of strain has been increased greatly,which resulting in plastic strain incompatibility.However,the meticulous research of gradient nanostructure is not very mature and not suitable for large-scale industrial production.Some researchers have prepared lamella nanostructured material that enhanced work hardening to achieve better strength and ductility in recent years.It not only contains lamella and nano structure,but also has great difference of hardness,even several orders.Although having proposed the concept and the characterizing method of back stress,researchers not only have a considerable difference of understanding and characterizing back stress,and have not thorough research of relationship between strain hardening mechanism and microstructure for lamella nanostructure,but also have no systemic understanding of back stress.From the perspective of microstructure,gradient nanostructure corresponds to consist of so many layers of lamella nanostructure,and its mechanism of microstructure is mutual.Consequently,to study and reveal the mechanical behavior,strain hardening mechanism of lamella nanostructure becomes a scientific research of greatly academic significance and applied value.This article selects 10# steel and multi-layer steel as the model materials.Lamellar structure which contains an asymmetrical distribution of soft-hard field and organization of diverse grain size can be obtained through heat treatment of lamellar and ultra-fine granular structure achieved by means of rolling.Then Studying correlation of material properties by employing quasi-static state tensile tests and micro hardness tests,back stress hardening and forest dislocation hardening by employing Loading-Unloading-Reloading(LUR)tests and stress relaxation tests,and evolution of local strain with strain field tests,conduced to research mechanical behavior and strain hardening mechanism of lamella nanostructure by employing optical microscope(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The conclusions of this paper include such as:(1)Soft and hard micro-field exists in the samples of lamella nanostructure of 10 # steel with different annealing process,what is more,aggrandizing the difference and asymmetrical distribution of hardness indicates the more obvious plastic strain incompatibility of the lamella nanostructure results in higher back stress,ultimately,back stress hardening greatly magnifies the improvement of strain hardening ability.The local strain of specimen surface was heterogeneous during the quasi-static tensile deformatiom of lamella nanostructure.In other words,the existence of strain field and strain rate field was obvious.In the previous state of plastic deformation,local strain of sample surface centralized and strain rate was larger than surrounding,but the instability of plastic deformation was held up,which was rising from the heterogeneous structure of soft and hard micro-field.(2)The essence of transient strain hardening of lamella nanostructure of 10 # steel is the plastic strain incompatibility of soft and hard micro-field during deformation,and it leads to the generation of back stress which makes the Kernal average misorientation of grains increase,namely,the augment of density of geometrically necessary dislocation results in the remarkable enhancement of back stress hardening.Meanwhile,the density of mobile dislocationswent up,activation volume decreased and motion of dislocations blocked lead to cross-slip and multi-slip,and then make the walls and interfaces of dislocations formed in the grians.Consequently,the strengthened interaction of dislocations during deformation brings about the forest dislocation hardening.In a word,back stress hardening and forest dislocation hardening influenced the process of work hardening during deformation.(3)Mechanical behaviors of the large-scale and heterogeneous multi-layer steel is in terms of the hardening mechanism of lamella nanostructured 10 # steel.In other words,when difference in hardness reaches to maximum value and structure is more asymmetrical,the ability of strain hardening may be remarkably enhanced.Thereby,it may bring about better strength and ductility.The gradient distribution of grain crystal size exists in the interface of multi-layer steel,and the crystal grains near the interface are larger,the heart grains are smaller.The interface of multi-layer steel is so stable that it may have a considerable effect on mechanical properties.