| The failure of fatigue,frictional wear,corrosion etc.usually originate from the surface of materials in their service process,so optimizing the surface properties of structural materials could prolong their service life.The materials with gradient structure on the surface,which were prepared by mechanical induced plastic severe deformation,always show high resistance to high and low cycle fatigue and frictional wear because of their good strength-ductility synergy.At present,the stress-controlled high cycle fatigue(HCF)performance has been studied mainly,while the study on the strain-controlled low cycle fatigue(LCF)performance has been reported little.Therefore,systematical studying the stress-controlled high cycle fatigue behavior,strain-controlled low cycle fatigue behavior and frictional wear performance of gradient structural materials to clarify their mechanism has important scientific significance and engineering application value.The 7075 high strength aluminum alloy,which is widely used in aerospace field,was selected as the research object.The newly developed ultrasonic surface rolling processing(USRP)method was used to prepare laboratory samples of 7075 aluminum alloy with surface gradient structure.Their microstructural evolution mechanism,mechanical properties,high and low cycle fatigue behavior and the failure mechanism of frictional wear of the gradient structure deformation layer were studied.The effects of surface treatment quality,microstructural evolution and residual stress on the crack initiation and propagation,fatigue life and failure mechanism of gradient structure aluminum alloy were contrastively investigated.The failure characteristics of frictional wear and relevant mechanism of 7075 aluminum alloy with gradient structure were discussed.The following main research contents and conclusions can be drawn from the study:1.The microstructural evolution mechanism of deformed layer on the surface of7075 aluminum alloy with gradient structure was studied.The microstructural characteristics of the surface layer of 7075 aluminum alloy with gradient structure was analyzed by XRD,SEM and TEM.The results show that dislocation slip dominates the grain refinement process.It shows that the main mechanisms of grain refinement should be the continuous cutting and refinement of dislocation cells,subgrains and the fragmentation of the second precipitates.Based on it,the evolution process of grain refinement on the surface layer of 7075 aluminum alloy with gradient structure was clarified,which provided a theoretical support for the surface grain refinement and nanocrystallization of the same type of precipitation-strengthened aluminum alloy.2.The static tensile mechanical behavior of 7075 aluminum alloy with gradient structure was studied.The experimental results show that the 7075 aluminum alloy with gradient structure possesses relatively higher tensile strength and yield strength while has less plastic loss,which indicates the good strength-ductility synergy between the gradient deformation layer and the internal coarse-grained matrix.The tensile plastic deformation behavior of Al with gradient structure was simulated by ABAQUS software.It is found that the yield process of the gradient structure material follows the gradual change from internal coarse grains to external fine grains under different tensile strain loading process,and also,clarifies that the plastic strain gradually accumulates from the inner coarse-grained matrix with better toughness to the gradient structure surface with higher strength.The above mentioned deformation mechanism can effectively delay the prematurely failure of gradient structure material caused by strain localization in the deformation process.3.The influences of USRP treatment on HCF failure behavior of 7075 aluminum alloy were studied.The fatigue crack initiation mechanism of the interface between the second phase precipitated particles and the aluminum matrix in the subsurface deformation hardening layer of 7075 aluminum alloy with gradient structure were found,and the essential reason of increasing the HCF life of 7075 aluminum alloy with gradient structure was revealed.The results show that the fatigue strength and fatigue ratio of 7075 aluminum alloy after USRP treatment are significantly increased,and the fatigue limit is increased by 89%.The improvement reasons of the HCF life of the GNS 7075 aluminum alloy are attributed to the synergy of the following two influence factors: on the one hand,the fatigue crack initiation was suppressed by the nanocrystalline surface layer with high strength and better surface quality from the material surface and the strain localization of nanostructured surface layer suppressed the GNS deformation layer with better strength-ductility synergy in the cyclic deformation process.In addition,after USRP treatment,the second phase particle fragmentation and residual compressive stress on the material surface also contributed to delay surface crack initiation,leading to that the interface between the second phase precipitated particles and the aluminum matrix is the favorable location for crack nucleation in the subsurface deformation hardening layer.On the other hand,the high density dislocation configurations(high density dislocation net,dislocation tangles and dislocation wall),formed in subsurface grain and grain boundary,effectively reduce the crack propagation rate,while delays the crack growth rate to a certain extent.4.The influences of USRP treatment on LCF properties of 7075 aluminum alloy were studied.It revealed that the shear band deformation was restrained in the gradient deformation layer during LCF cyclic loading,which effectively delayed the initiation of fatigue crack at the shear band interface.The high density dislocation wall,dislocation tangles and subgrains formed in the surface deformation layer by USRP treatment can reduce the fatigue crack growth rate to a certain extent,which improved the LCF life of 7075 aluminum alloy with gradient structure.The results of quantitative study showed that the cyclic hardening rate and LCF life of 7075 aluminum alloy with gradient structure are higher than those of original sample under the same cyclic strain amplitude.The qualitative analysis showed that the LCF cracks of 7075 aluminum alloy with gradient structure mainly originated at the grain boundary with high density dislocation configuration and the interface of the second phase particles on the subsurface.After nucleation,the crack propagated to the coarse grain region inside the material,rather than along the surface gradient deformation layer.5.After treated by USRP method,the failure mechanism of the friction and wear properties of 7075 aluminum alloy was studied.The results show that USRP treatment could improve the friction and wear properties of 7075 aluminum alloy.The dry friction and wear mechanism of USRP samples could be expressed as follows: abrasive wear was the main wear mechanism in the early stage,supplemented by a small part of adhesive wear.With the generation of friction heat during friction and wear process,adhesive wear intensified,and oxides appeared on the surface of wear marks,and the wear mechanism changed to adhesive wear and oxidation wear.Once the friction energy maked the wear particles reacting with oxygen in the air,the wear changed into abrasive wear + oxidation wear + adhesion wear. |
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