Effect Of Hierarchical Microstructures Of Low Carbon Martensitic Steel On Ftigue Damage Behavior

The past tens of years have witnessed significant progress in science and technology,leading to the increasingly higher requirements for excellent properties of materials,e.g.,strength,toughness,fatigue and fracture properties.Hierarchical microstructures are frequently observed in most engineering materials,including titanium alloys,intermetallic compounds,martensitic/bainitic steels,high-entropy alloys and other advanced metal materials.For example,low-carbon martensitic steel,widely used in the production of structural parts such as gears and bearings,is consisting of hierarchical microstructures including block,packet and lath.Thus,it is of significant importance to reveal its fatigue damage mechanism under cyclic loading for scientific research and engineering application.In this paper,low-carbon 20CrNi2Mo steel is selected is selected for obtaining hierarchical structures of martensite,to comprehensively investigate the underlying mechanism of crack initiation and early propagation during fatigue,and to reveal the particular microstructure responsible for the strength,plasticity,fatigue threshold?Kth,crack initiation and propagation behavior,respectively.These results can be considered as a useful guideline for microstructure design,property optimization and reliability assessment.The hierarchical microstructures are systematically characterized by OM,SEM,EBSD and TEM,confirming the linear relationship between martensite block/packet and prior austenite grain.Neighboring matensite blocks are mainly separated by high angle boundaries?HBs?,whose fraction decreases with the decrease of austenitizing temperature.More HBs lead to an increasing strength in martensitic steel,and it is inferred using Hall-Petch equation that the martensite block is the effective control unit of the strength.In contrast,the plasticity is dependent on the crystal plasticity and interface plasticity,which are respectively controlled by dislocation activities and deformation compatibility among hierarchical microstructures,involved with their rotation,bending and interface slipping.Therefore,the plasticity of martensitic steel is controlled by the martensitic lath with an overwhelming proportion of boundaries.The crack growth behavior of the near-threshold investigated shows that martensite block is the effective control unit of threshold value?Kth and transition behavior of growth rate.A higher?Kth and lower crack growth rate are obtained in coarse-grained sample with obvious transition behavior of growth rate.In the near-threshold zone,crack propagates along certain crystallographic plane.The fracture morphology under SEM displays the cumulative damage behavior during propagation,while the typical fatigue strips are observed in the steady-state extension zone.The crack path observed is affected by the dislocation activities,i.e.,the transition behavior of growth rate is observed as the single-to multi-slip mode occurs.Also,a comparative investigation on the relationship between microstructural parameters and?Kth or the radius?Ry?of plastic zone ahead of crack tip to reveal the role of martensite block in crack deflection behavior.Furthermore,the crystallographic characteristics of crack path under EBSD show that crack propagates along the favorable slip plane of the crystal.However,the statistical results indicate that average angle of crack deflection is related to the misorientation between hierarchical structures,and more significant deflection tends to occur at the block boundaries with larger misorientation.Crack closure behavior of a lath martensitic steel is investigated using a closing coefficient U.For the as-quenched microstructure,higher values of U are obtained in both coarse-and fine-grained samples,ranged from 0.6 to 0.7 and from 0.7 to 0.8,respectively.This indicates that lath martensite exhibits the excellent growth resistance in essence,and stronger closure effect is achieved in coarse-grained sample.which possesses significant roughness-induced closure?RICC?effect,⁷0%relative contribution to total closure effect,stronger than its fine-grained microstructure,⁵0%relative contribution.The crack tip and the wake cooperatively provide the resistance against crack propagation,as calculated using plastic deformation work ahead of the crack tip.That is,coarse-grained structure has higher intrinsic growth resistance and closure resistance.The length-to-width ratio of lath is an significant parameter for assessment of crack propagation behavior.The crack propagates along a boundary of lath with a higher ratio,and soon rotates to the main propagation direction.More laths are sheared during such deflection processing,leading to the more tortuous path and the significant increase in plastic deformation work.Moreover,more zigzag path means higher surface roughness after fracture and stronger closure effect for propagation.The very high cycle fatigue tests show martensitic lath is controlling unit for crack initiation and initial growth life.Crack initiation occurs at surface and boundaries of internal non-inclusion/inclusion for tested sample,while inclusions farther from the surface are observed for its nucleation in fine-grained sample with a bigger FIE size.A linear relationship between inclusion size and fisheye radius is observed,and an increasing size leads to the decreased fatigue life.The fatigue life is proportional to the size of GBF.The formaiton of GBF zone is cooperatively determined by the local deformation ahead of crack tip and the extrusion of crack sides,leading to the strain accumulation.The stress intensity factor at inclusion?K_INC is lower than the effective threshold vale of fatigue crack?K_th-eff,causing the formation of GBF zone with a size related to the lath size.Dislocation pileup model shows that the fatigue life of GBF zone is affected by yield/tensile strength and characteristic size at micro-scale,including block and lath sizes in martensite.