CPFEM Study Of Nanoindentation And High Cycle Fatigue Behavior For Ti-6Al-4V Alloy

Due to its high specific strength and specific stiffness,good ductility and other fine features,Ti-6Al-4V titanium alloy has been widely used in manufacturing aerospace structures.For aerospace structures to withstand alternating loads,high cycle fatigue(HCF)failure is one of the main failure modes.Although researchers have conducted a lot of researches on the HCF behavior of Ti-6Al-4V alloy,there are still controversial opinions on the effects of microstructure and cyclic loading conditions on the HCF behavior.The mechanism of fatigue damage is still not deeply understood.In this paper,numerical modeling on the meso-scale,combined with theoretical analysis and experimental validation,was adopted to thoroughly study the nanoindentation and HCF behavior of Ti-6Al-4V alloy.The main contents and some main achievements obtained are as follows.Equiaxed,duplex,and full lamellar microstructure models in 2D and 3D were created based on the curvature driven grain growth cellular automata model and the subdivision Voronoi grain method,respectively.Furthermore,a microstructure modeling software for titanium alloy based on the ABAQUS platform were developed.By introducing the nonlocal geometrically necessary dislocation(GND)density in the rate-dependent crystal plasticity constitutive model,and adopting a modified Armstrong-Frederick nonlinear kinematic hardening equation to account for cyclic softening effect,a nonlocal cyclic crystal plasticity constitutive model for titanium has been developed.By conducting load controlled and displacement controlled nanoindentation on Ti-6Al-4V alloy with equiaxed and duplex microstructure,coupled with local and nonlocal CPFE simulations,the factors that affecting nanoindentaiton behavior have been investigated,and the formation mechanism of pileup has also been analyzed.The results show that grain orientation significantly affects the nanoindentation behavior of ? phase.Grain boundaries have effect on the nanoindentation behavior,and the effect is greater if the indenter tip is nearer the grain boundaries.The Berkovich indenter direction slightly affects the pileup morphology.The pileup of ? phase during nanoindentation is mainly caused by prismatic slip.The pileup morphology evolves with the loading and unloading process of nanoindentation,and the change in pileup height due to the elastic recovery at unloading stage is significant.Cyclic nanoindentation experiments indicate that,? phase exhibits a cyclic softening phenomenon,and the degrees of cyclic softening vary for different grain orientations.Macroscopical cyclic deformation experiments show that both equiaxed and duplex microstructure exhibit cyclic softening,and the softening amplitude for equiaxed microstructure are greater than that for duplex microstructure.The influences of microstructure factors(volume fraction of primary ? phase,? lamella width,grain orientation distribution)and cyclic loading conditions(stress level,stress ratio,frequency)on the microplasticity behavior of Ti-6Al-4V alloy during HCF have been investigated through CPFE simulations.Results show that,at the early stage of cyclic deformation,the plastic strain continues to accumulate,and the GND density continues to increase with the increase of number of cycles.Plastic strain mainly concentrates on the grain with soft orientation,and the GND density mainly concentrates on the grain boundaries.At the same cyclic loading conditions,the average equivalent plastic strain increases with the increase of the volume fraction of primary ? phase and the ? lamella width,while the average GND density shows a opposite tendency;Crystal orientation distribution significantly affect the distribution of plastic strain,while it has slight effect on GND density distribution;For the same microstructure(duplex microstructure),the average equivalent plastic strain and GND density increase with the increase of the stress level and stress ratio,decrease with the increase of loading frequency.The influences of microstructure factors and cyclic loading conditions on HCF behavior of Ti-6Al-4V alloy have been investigated by combining CPFE simulations and extreme value statistics.Results show that the greater the grain sizes in equiaxed microstructure,the greater the maximum value of Fatemi-Socie fatigue indicator parameter(FS-FIP),indicating easier formation of fatigue cracks.Grains oriented for easy basal slip(with Schmid factors between 0.35 and 0.5 for basal slip,while that between 0.25 and 0.4 for prismatic slip)are more prevalent at the locations of extreme value of FS-FIP.The smaller the volume fraction of primary ? phase,the smaller FS-FIP characteristic maximum value,and indicating higher resistance to fatigue crack initiation.The characteristic maximum value of FS-FIP for primary ? phase is larger than that for lamellar grain,indicating the fatigue cracks are more likely to initiate from primary ? grain.Microstructures with smaller ? lamella width tend to have smaller characteristic maximum value of FS-FIP,which will have better fatigue resistance.For equiaxed microstructure,there will be larger characteristic maximum value of FS-FIP with higher stress level,higher stress ratio and lower frequency,which is easier for cracks initiation.The extreme value of FS-FIP shows larger dispersion with lower stress level,lower stress ratio and lower frequency,indicating that the fatigue cracks will be more sensitive to the microstructure at these conditions.The GND density extreme value shows opposite tendency with that of FS-FIP when it is used in analyzing the effects of microstructure factors,while the tendency is consistent when it is used in analyzing the effects of cyclic loading conditions.The inconsistent phenomenon should be ascribed to the size related nature of GND.So it can be concluded that GND could be used as a FIP when studying the effects of cyclic loading conditions on HCF.