| Commercially pure titaniums have outstanding advantages such as high specific strength,light weight,high heat resistance and excellent corrosion resistance,which make them become a high-quality light corrosion-resistant structural material and an important biomedical material.Thus,how to obtain titanium with high strength and good plasticity is a hot issue for material scientists.The initial coarse grain size of traditional metallic materials can be refined to ultra-fine or even nano size after severe plastic deformation(SPD)processing,so that the strength,hardness and fatigue properties of the materials can be significantly improved without changing the element distribution of the original materials.According to the classic Hall-Petch relationship,the increase of average grain size will reduce the strength and hardness,and may also lead to the loss of corrosion resistance or other special properties of the material.In recent years,most of the researches on the grain stability of nano and ultra-fine grained metallic materials focus on the high-temperature annealing induced grain growth,but there are few reports on the fatigue induced grain growth phenomenon.It is difficult to decoupling the contribution of temperature and stress to the grain growth of nano and ultra-fine grained metallic materials and the mechanisms are still controversial.In addition,due to the low crystal symmetry and relatively few slip systems,the mechanisms of deformation,especially the machanisums of texture evolution under different temperature and stress conditions are still unclear.Based on the above problems,two typical severe plastic deformation processing methods,Equal channel angular pressing(ECAP)and Multi-directional forging(MDF),are used to prepare ultra-fine grained CP-Ti in this paper.The microstructure and mechanical properties of the materials after different processing conditions are characterized and analyzed.Through the innovative neutron diffraction experimental design,decoupling the contribution and effect of temperature and mechanical stress on the grain growth during fatigue loading.The grain stability and recrystallization mechanisms of ultra-fine grained CP-Ti under different temperature and stress conditions were investigated by high temperature annealing,monotonic tensile and fatigue experiments.The grain growth kinetics was quantitatively studied,and the mechanisms of texture evolution and lattice rotation of hcp-structrued pure titanium were revealed.The research in this paper is of great significance to optimize the physical and metallurgical parameters of nano and ultra-fine grained titanium materials,guide the microstructure tailoring methods and lay a theoretical data foundation for the practical application of ultra-fine grained CP-Ti.The main conclusions are as follows:(1)Ultra-fined grained CP-Ti were prepared by using ECAP and MDF respectively,the degree of the grain refinement is more than one order of magnitude,and the grain size distributions basically obey the lognormal distribution.The average grain size on cross section plane of 3-cycle MDFed sample can reach about 253 nm,which is less than 12-pass ECAPed sample(about 669 nm).The tensile properties and microhardness of ultra-fine grained CP-Ti at room temperature are significantly higher than those of the as-received coarse-grained CP-Ti.The relationship between yield strength,microhardness and grain size are in accordance with the classic Hall-Petch relationship.(2)The grain refinement of CP-Ti during 450? large volume ECAP was dominated by dynamic recovery and by few partially dynamic recrystallization at the iron impurities region.However,in the MDF process at 450?500?,due to the high strain rate and the inherent low thermal conductivity of pure titanium,the grain refinement was dominated by dynamic recrystallization mechanism.(3)The thermally activated(diffusional)process should play a significant role in triggering grain growth during HCF fatigue.For the MDF CP-Ti,the UFG microstructure is rather unstable and undergoes a continuous grain growth as a function of room temperature HCF fatigue cycles.Continuous dynamic recrystallization is responsible for the grain coarsening in response to room-temperature HCF fatigue.This process involve a violent dynamic recovery process which is driven by the release of storage energy produced by SPD processing.This process involves the interaction of temperature and stress and there was no evidence to suggest that the grain growth in CP-Ti with UFG microstructure in response to HCF fatigue loading is only a result of dislocation slip or twinning process,i.e.the so-called stress induced grain growth.(4)As-MDFed ultra-fine grained CP-Ti was thermally stable below 500?.The time exponent n=0.3 and the activation energy Q=241.7 kJ/mol were calculated from the annealing tests of the temperature ranging from 600? to 800?.For the MDFed CP-Ti,annealing above 600? was dominated by static recrystallization,the high-cycle fatigue at 280 MPa under 14 Hz was dominated by dynamic recrystallization.However,there was no evidence to suggest that any recrystallization and subsequent grain growth could occur in quasi-static monotonic tensile loading at the strain rate ranging from 0.0001 s-2 to 0.01 s-2 under room temperature.(5)For the MDFed CP-Ti,the high temperature annealing induced recrystallization texture can enhance the original texture intensity but there was no significant change in the texture component.The formation of basal texture(c-axis perpendicular to stress axis)after high-cycle fatigue tests could attribute to the gradually lattice rotation by the activation of prismatic<a>({10-10}<11-20>)slips along LD and the activation of basal slip system perpendicular to LD during fatigue loading.The grain coarsening dominated by dynamic recrystallization may also serve as an auxiliary mechanism for the slip-induced grain rotation.(6)Dynamic or static recrystallization mechanisms can be clearly identified and distinguished by the combination of in-grain misorientation profile(IGMP)and in-grain average misorientation angle(IAMA)method which is of great importance to the microstructure control of CP-Ti during industrial processing... |
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