Fatigue Behaviors Of Nanotwinned 316L Austenitic Stainless Steels

316L austenitic stainless steels(316L SS)were widely used in nuclear power and medical materials due to their excellent corrosion resistance,good plasticity and processing properties.However,its strength and fatigue limits are extremely low,which limit their technological application in the harsh environment.Traditional strengthening strategies such as second phase strengthening and dispersion strengthening have limited improvement in the fatigue strength of austenitic stainless steels.Moreover,this kind of micro structure design often leads to an early failure due to the preferential occurrence of interface cracking between hard phases and the soft matrix,especially in materials undergoing cyclic loading during service.This phenomenon originates from the elastic inhomogeneity and plastic deformation incompatibility among the reinforcing stiff phases and the soft,elastically more compliant matrix.How to improve the fatigue performance of austenitic stainless steels is still in challenging.In the present work,we produced a novel type of nano-twinned 316L austenitic stainless steel by using dynamic plastic deformation technology(DPD).The fatigue properties,damage behaviors and deformation mechanism of this nano-twinned 316L austenitic stainless steel were systematically studied.The main results are as follows:A duplex micro structural nanotwinned/nanograined 3 16L stainless steel composed of nanotwinned austenitic grains(56 vol.%),nanograins and dislocation structures was prepared by dynamic plastic deformation technique.We investigated the stress-controlled push-pull fatigue behavior of this nanotwinned/nanograined 3 16L stainless steel.It is found that the nanotwins exhibit good stability,and therefore the generation of persistent-slip-bands-like shear bands and crack initiation in nanotwins were effectively suppressed.The fatigue limit of this nanotwinned/nanograined 316L stainless steel is as high as 425±25 MPa,which is twice that of coarse grained counterpart.We successfully prepared large nanotwined/nanograined 316L stainless steel samples(19 x 7 0 x 3mm)by using DPD technique to investigate the low cycle fatigue behavior of this nanotwined/nanograined 3 16L stainless steel samples under push-pull constant plastic strain-controlled fatigue tests.The nanotwinned/nanograined 316L stainless steel samples exhibit obvious cyclic softening.The cyclic softening ratio continuously increased with the increasing of plastic strain amplitudes till to ??pl/2=2×10-3.When plastic strain amplitudes are higher than ??pl/2=2 × 10-3,the softening ratio remains almost constant with a value of 0.23.At small plastic strain amplitudes(plastic stain amplitude ??pl/2=5×10-4),cyclic softening was mainly induced by dynamic recovery of dislocation structures,while both nanotwins and nanograins keep stable.When plastic strain amplitude increasing to ??pl/2=2×10-3,significant coarsening occurred both in nanograins and dislocation structures.Most of nanotwins are still stable,but detwinning occurred within some shear bands in nanotwins.A duplex nanotwinned/recrystallized(SRX)micro-sized grained 316L stainless steel composed of hard nanotwinned grains(26±5 vol%),dislocation structures and static recrystallized(SRX)micro-sized grains(67 vol.%)was prepared by dynamic plastic deformation technique and subsequent annealing treatment.We investigated the stress-controlled push-pull fatigue behavior of this nanotwin/SRX grained 316L stainless steel.The nanotwinned/SRX grain interface crack susceptibility significantly decreases in comparison with conventional phase boundaries where most cracks initiate and propagate in hard phase reinforced materials.More than half of the cracks(57%in number fraction)are found in recrystallized grains while a small fraction(11%)is observed at the interfaces between nanotwinned and recrystallized grains.This is ascribed to the elastic homogeneity and cyclic deformation compatibility between nanotwinned and recrystallized grains.At small cumulative cyclic strains(below 4000 cycles at sa 1/4%450 MPa),nanotwinned grains deform compatibly with the recrystallized grains without noticeable strain localization at their interfaces.Nanotwins can accommodate cyclic plastic strains by interaction of dislocations with twin boundaries,especially through the motion of the well-ordered threading dislocations inside the twin lamellae.At large cumulative strains,a moderate strain gradient is developed in recrystallized grains surrounding nanotwinned grains as a function of distance from the interfaces due to the occurrence of localized deformation in nanotwinned grains.The nanotwinned grains show high microstructural stability without notable de-twinnning,thus retarding crack initiation and propagation.Therefore,improved fatigue property with high fatigue limit of?350 MPa and high fatigue ratio of?0.45 is achieved in the nanotwin strengthened stainless steel,which is better than that of conventional second phase reinforced steels with comparable strength.