Grain Refinement Mechanism And Fretting Wear Behavior Of Ti-6Al-4V Alloy Subjected To Shot Peening Process

Ti-6Al-4V alloy is the key material for aeroengine blades due to the advantages of high specific strength,good fatigue resistance,strong corrosion resistance,etc.During service process,blades and discs are prone to failure resulted from fretting wear.The fretting wear not only dramatically reduces the service life of aeroengine blades,but also poses a potential safety hazard for aircraft service safety.How to improve the fretting wear resistance of Ti-6Al-4V alloy has been an urgent and tough problem to be solved.Tailoring interfacial grain size is an important method to improve the fretting wear of metal materials.However,the grain refinement mechanism of Ti-6Al-4V alloy and the effect of mechanical properties of interfacial grains on fretting wear behavior are still not clear.It is of great theoretical and engineering significance for improving the comprehensive performance of titanium alloy blade to study the grain refinement process,mechanical properties change and its influence on fretting wear behavior of Ti-6Al-4V alloy subjected to shot peening process.In this paper,Ti-6Al-4V alloy with an initial{112（?）0}〈101（?）0〉rolling texture was used as experimental modeling material,and surface microstructure with gradient nanostructures in alloy was obtained by shot peening process.Firstly,the effects of texture evolution in surface layer of gradient nanostructures on the slip modes and grain refinement mechanism were studied.Secondly,the relationship between mechanical properties and plastic deformation of coarse grains（CGs）and nanograins（NGs）Ti-6Al-4V alloys was investigated by micropillar compression and notch microcantilever test methods at the micron scale,which provided theoretical guidance for studying the effect of interfacial grain size on the fretting wear behavior.Finally,Si₄N₃/Ti-6Al-4V were selected as friction pairs to study the effect of interfacial grain size on the fretting wear resistance;Ti-6Al-4V/Ti-6Al-4V were selected as friction pairs to study the effect of interfacial grain size on friction modes of the fretting wear.Main research results are shown as follows:（1）For the surface microstructure with gradient nanostructures in Ti-6Al-4V alloy obtained by shot peening process,evolutions of microstructure under strain control,such as texture,slip modes and grain boundaries,were systematically studied,the grain refinement mechanism of Ti-6Al-4V alloy was revealed,and the role of pyramidal<c+a>dislocations in the grain refinement was discussed.The results showed that the dominant crystal orientation perpendicular to the direction of shot peening process firstly changed from prismatic{112（?）0}planes to pyramidal{101（?）0}planes,and then from pyramidal{101（?）0}planes to basal{0001}planes with the increasing strain.The change of the crystal orientation caused that the dominant slip modes first changes from pyramidal<c+a>slip to basal<a>slip,and then from basal<a>slip to pyramidal<c+a>slip.Based on slip modes and texture evolutions,the grain refinement process of the Ti-6Al-4V alloy with the initial{112（?）0}〈101（?）0〉rolling texture was divided into four stages:（I）formation of dislocation walls with pyramidal<c+a>dislocations in CGs;（II）CGs refined to ultra-fine grains（UFGs）by basal<a>dislocations intersecting with non-basal dislocations;（III）UFGs refined to NGs by pyramidal<c+a>dislocations intersecting with basal<a>dislocations;and（IV）formation of NGs with high angle grain boundaries by grain rotation.In the grain refinement process,pyramidal<c+a>dislocations played two important roles:coordinating the large c-axis strain,thereby achieving generalized plastic flow,especially in nanograins;forming of the unique low-angle grain boundaries with basal-pyramidal dislocation locks（prismatic<c>and prismatic<c+a>dislocations）by interacting with basal<a>dislocations.This unique low-energy boundary greatly enhances the stability of the strain-induced grain boundary and dislocation density(～6.6×10¹⁵m^-2 in NGs).（2）Strength,toughness,work hardening capacity,and plastic deformation mechanism of in CGs and NGs Ti-6Al-4V alloys were studied by micropillar compression and notched microcantilever test methods,and the effect of grain size on plastic deformation mechanism was revealed.The results indicated that when CGs were refined into NGs,the plastic deformation changed from the dislocation mediated plasticity（<a>slip and pyramidal<c+a>slip）to the grain boundary mediated plasticity（grain boundary migration）.Grain boundary migration led to the strain localization,which resulted in failure by forming shear band under micropillar compression;additionally,it was difficult to release the stress concentration at the crack tip under notched microcantilever,thereby contributing to the rapid expansion of the crack tip.Therefore,when CGs were refined into NGs,the yield strength_0.7 increased from991±230 MPa to 1489±25 MPa,the work hardening rateΘdecreased from 8.57±3.31 to3.68±0.39,and the fracture toughnessdecreased to 9.72±0.97 MPa m^1/2.（3）Si₄N₃/Ti-6Al-4V were selected as friction pairs to investigate surface microstructure evolution mechanism of fretting wear scars in CGs and NGs Ti-6Al-4V alloys under different loads,and the relationship between the surface microstructure evolution mechanism and the fretting wear resistance was revealed.The results showed that under different load force,the friction modes of CGs and NGs were both slip regime（slip dominated）,and oxidation-delamination wear occurred on the surface.The nucleation and propagation of cracks mainly occurred in the oxidized amorphous layer and the interface between the oxidized amorphous layer and the extremely fine NGs layer.The fretting wear resistance of Ti-6Al-4V alloy was related to the surface microstructure evolution under elastic-plastic deformation.Under 5 N load force,the elastic deformation occurred in the matrix of NGs,and the grains did not change significantly;while the plastic deformation occurred in the CGs,and formed extremely fine NGs and amorphous structure,resulting in that the wear resistance of NGs was higher than that of CGs under 5 N load force.Under 10 N,20 N and 50 N load forces,NGs occur the plastic deformation,and form extremely fine NGs and amorphous;while CGs occur the plastic deformation,are first refined into NGs,and then form extremely fine NGs and amorphous,resulting in that the wear resistance of NGs is lower than CGs under 10 N,20 N and 50 N load forces.（4）Ti-6Al-4V/Ti-6Al-4V were selected as friction pairs to study friction modes of CGs,UFGs,NGs and heterogeneous grain structure Ti-6Al-4V alloys under stick-slip motion.The results demonstrated that the interfacial plasticity affected by grain size was the key to maintain the contact interface stick.Based on the effect of grain size on the plastic deformation mechanism and friction modes,we suggest to control friction modes by tailoring plastic deformation mechanism via interfacial grain size:（I）mixed regime（stick-slip competed）achieved by dislocation slip in interfacial CGs and UFGs;（II）slip regime（slip dominated）realized by grain boundary migration in interfacial NGs;（III）partial slip regime（stick dominated）achieved by dislocation slip in interfacial heterogeneous grain structure.