Study On Mesoscopic Characteristic Of Deformation And Its Connection With Damage Nucleation In Advanced High Strength Steels

Faced with the increasingly stringent requirements on energy conservation and emission reduction,the automotive industry has an increasing demand for advanced high strength steel(AHSS)with high strength and excellent formability to realize automotive lightweight.The development of AHSS requires the establishment of the relationship between microstructure and mechanical properties of AHSS,which guides the design of composition and microstructure,preparation and processing of AHSS.Traditional establishment methods,such as Hall-Petch and Taylor formulas,are widely applicable,but have some limitations when applied to materials with complex microstructure and deformation mechanism,such as AHSSs,especially to those whose microstructural parameters show different distribution characteristics on the mesoscale.Through the analysis of the essence of plastic deformationit,it was found that plastic deformation can be simply but efficiently classified into three scales in terms of its characteristic:microscale(atomic),mesoscale(grain),and macroscale(sample).On the microscale(atomic scale),plastic deformation is characterized by dislocation motion,dislocation-dislocation interactions,and dislocation interactions with other crystal defects such as solute atoms,precipitates,and interfaces.On the mesoscale(grain scale),it features a strain and stress inhomogeneity that emerges within and between grains.On the macroscale(sample scale),the plastic deformation features a strain and stress inhomogeneity that emerges between different parts of the sample.At this scale,the entire sample is regarded as a continuous medium with homogeneous properties.Therefore,it is necessary to establish the relationship of microstructure and deformation mechanism with mechanical properties of advanced high strength steel by setting mesoscale characteristics of plastic deformation as a bridge.In present study,based on the mesoscopic features of plastic deformation,the relationships of microstructure and deformation mechanism to mechanical properties were investigated by taking the martensite content in microstructure parameters and transformation and twinning in deformation mechanism as examples.Firstly,a universal law of plastic deformation on mesoscale was revealed,and methods that quantitatively describe the mesoscopic features of deformation were developed.Subsequently,by using the methods developed,the effects of the above mentioned microstructure and deformation mechanism on mesoscopic features of deformation were studied.Meanwhile,the connections of mesoscopic features of deformation with deformation and damage ehavior and further with mechanical properties were studied.The main results and conclusions are as follows:1.By measuring and counting the strain distribution of various materials,it was found that the statistical distribution of mesoscopic strain universally follows a lognormal distribution irrespective of phase content and deformation mechanism.Moreover,this universal law is proved conditional upon the macroscopic homogeneity of deformation on the statistical window scale,equivalent to the equality between the macrostrain calculated from the displacements at the window corners and the average of the local strain.The variance in the parameters of lognormal distribution can quantitatively describe the homogeneity of deformation.Besides,the width and area ratio of deformation band can also characterize the mesocscale features of plasitic deformation.Mesoscopic features of deformation were quantitatively characterized for dual phase(DP)steels with various martensite volume fraction(V_m).The results show that with the increase of V_m,the strain homogeneity decrease first and then increase.This imlies that the strain homogeneity was deteriorated when coupling two phases with large difference in strength,which results in the dramastically decrease in ductility.2.The evolutions of mesoscopic strain were compared among DP,Q&P and TWIP steels with distinct deformation mechanisms.It was found that compared to dislocation slip,transformation and twinning can result in dynamic strain partition and improve the homogeneity of deformation.The improvement in strain homogeneity suppresses damage nucleation and exploits more local work hardening potential,thus enhancing the macro work hardening rate and plasticity.Phase transformation and twinning achieve the same strain homogenization effect through the opposite way.Phase transformation tends to occur in the regions with strain concentration,the subsequent local hardening caused by transformation moderate the strain concentration in these regions.In contrary,twinning prefer to occur in region with low strain where slip was difficult.The local softening caused by twinning stimulate the deformation in this regions.By analyzing the strain features in other methods that enhances the plasticity of materials,such as grain refinement,solid solution and mesoscale structure regulation,it is found that these method universal follow a principle,namely mesoscale strain homogenization.3.The damage nucleation modes and their mesoscopic origin were investigated for dual-phase steels that consist of ferrite and martensite phases,each phase with varying volume fraction and distribution but nearly constant strength.The mesoscopic origin was explored by characterizing,for each mode,the mesoscopic strain and stress distribution using digital image correlation in SEM(?-DIC)and finite element calculations.It is found that with increasing martensite volume fraction(V_m),the primary damage nucleation mode changes from ferrite grain boundary(F/F)decohesion to ferrite/martensite interface(F/M)decohesion and finally to martensite cracking.Although all nucleation modes could be caused by stress concentration on the atomic scale,they are revealed to differ originally on the mesoscale:1)F/F decohesion is mainly caused by high mesoscale strain,which could induce dislocation accumulation and introduce residual dislocations into the grain boundary.2)F/M decohesion is originated from high mesoscale strain gradient,which could also induce the accumulation of dislocations.3)Martensite cracking is stemmed from the high mesoscale stress partitioned in martensite.These results clearly demonstrate that the damage nucleation mode was directly controlled by the features of mesoscopic strain and stress distribution,which in turn is primarily dependent on the microstructure in general and on martensite volume fraction and distribution specifically in this research.Thus,the control of mesoscopic strain and stress distribution should be the pivot for microstructure engineering to suppress damage nucleation and thereby to improve the damage tolerance of composite-like ductile alloys such as dual-phase steels.4.The damage and fracture behaviors were compared for Q&P steels containing retained austenite with different morphology and stability.It was found that in QP980containing equiaxed unstable retained austenite,damage began to nucleate before necking,and its fracture followed the classical damage nucleation and growth process.With respect to QP1180 and QP1500 containing stable nano-lamellar retained austenite,the damage rarely nucleate before and even after serious necking.The fracture mode of the two steels may be that damages nucleate in the principal deformation band and propagate rapidly,leading to instantaneous fracture.The great difference in damage and fracture behaviors between QP980 and Q1500 could be attributed to their difference in phase constitution,morphology of retained austenite and the role the retained austenite play in suppressing the damage nucleation.The above research results highlight the importance of the mesoscopic features of plastic deformation,and are expected to provide theoretical reference for the design of microstructure and deformation mechanism to improve the strength and plasticity of AHSSs.