| Ti-1023 alloys is a typical near?titanium alloy in Ti-Al-V series.The main features of this alloy are high specific strength,good fracture toughness,low forging temperature and strong resistance to stress corrosion.It is suitable for the manufacture of high strength titanium forgings.In recent years,with the vigorous development of aviation and aerospace industry,the research on new high strength-toughness titanium alloys with independent intellectual property rights,which can replace ultra-high-strength steel and applied to large-scale structural parts of aviation and aerospace,has been paid attention to by all countries in the world.Studies have shown that the gradient structure can increase the strength of the material while maintaining a high level of plasticity.Based on the above situation,stress-induced martensitic transformation(SIMT)in metastable Ti-1023 alloy(?????)is the main line.The relationship between strain metallurgy preparation,microstructure and mechanical properties of gradient structure titanium alloy,as well as the critical factors affecting SIMT,are the emphasis of this research.The Ti-1023 alloy composed of?phases was varied by the?-solution treatment.The influences of?-solution treatment temperature,loading rate and indenter load on the micromechanics properties were investigated.The results showed that both the micro-hardness and Young's modulus of?phase declined drastically with increasing indenter load,while it remained the same level when the indenter load was above 300 m N.The plastic energy dissipation,which was significantly influenced by the?-solution treatment temperature and loading rate,increased with the growth of indenter load exerted on the specimens and can be expressed in a power-law function of indenter load for?phase.The indentation size effect(ISE)was systematically explored as a function of loading rate and indenter load.It was found that there is a strong ISE on the hardness of(?+?)phase in Ti-1023 alloy when the peak-load is less than 3500 m N,regardless of the variation of loading rates.Besides,as indenter loading rates increased,the degree of ISE also reduced considerably,which is related to the decrease of the internal indentation length scale ranging from 23.08?m to 6.80?m.Furthermore,a positive strain rate sensitivity of the hardness in the whole peak-load range was found in(?+?)phase,which indicates that the strain rates have less influence on the microhardness at small indentation load.The underlying mechanism is well explained by the variation of geometrically necessary dislocation(GNDs)and statistically stored dislocations(SSDs)in(?+?)phase.Indentation creep behavior at room temperature and its mechanism of(?+?)phase in Ti-1023 alloy were investigated.The effect of indenter load on creep behavior and creep parameters,like creep rate,creep strain rate,indentation stress and creep stress exponent,were analyzed at the steady-state creep.The results revealed that creep parameters exhibited significantly indentation depth dependent.At the secondary stage of creep,creep strain rate and creep rate increased with increase of maximum indenter load,while creep stress and creep stress exponent exhibited an opposite trend.Especially,creep stress exponent of 7.65±1.25 in power-law creep behavior of Ti-1023 alloy,which was consistent with the dislocation process,indicated that the second stage of creep may be dominated by dislocation climb.Based on the study of the martensitic transformation mechanism during the tensile deformation of metastable Ti-1023 alloy,the influence of strain rate and grain size was analyzed.The experimental results indicated that the SIMT occurs in the?-solution treated Ti-1023 alloy at the strain rate ranging from 10-5 s-1 to 10-1 s-1.The trigger stress rose continuously with the increase of strain rate,which could be explained by the free energy change associated with the SIMT.However,both the ultimate tensile strength(UTS)and the uniform elongation(?)were found to increase with decrease of strain rate,which exhibited the negative strain rate sensitivity(NSRS).The high work hardening rate and three-stage work hardening behavior were closely related to the SIMT serving as obstacles of dislocation motion.It is found that the dynamic Hall-Patch effect caused by the formation of thin?"plates dividing?grain into smaller domains,which was responded to the improvement of mechanical properties.In addition,the trigger stress of SIMT increases with increase of?grain size.A systematic study on the formation mechanism of torsional deformation-induced gradient martensite was carried out.The gradient?"martensite structure was introduced along the radius of the cross-section in the cylindrical specimen by torsional deformation,which simultaneously improved strength and ductility of the Ti-1023 alloy.Multi-scale structural characterization showed that the torsional strain gradient produced martensite gradient with increasing density and decreasing thickness from center to surface.?"martensite had parallel and V-shaped morphologies,which not only divided coarse?grains into finer?blocks but also blocked dislocation slip.In addition, the synergistic effect of dislocation slips in the?"martensite and?blocks led to grain refinement,namely simultaneously forming a gradient?"martensite and gradient?block structure on the cross section.The formation of GNDs caused by the mechanical incompatibility and the increasing shear stress required for martensitic transformation contributed to high strain hardening.Further research on the preparation of gradient phase structure induced by cyclic forward-reverse torsion(CFRT)was carried out,and the effects of reverse strain and cycle number on gradient microstructure evolution and mechanical properties were analyzed.The results revealed that CFRT with strain reversal accumulated lower dislocation density and less martensitic transformation in comparison with monotonic torsion(MT)under the same total accumulative strain.SIMT was retarded by strain reversal and this suppression of martensitic transformation is more pronounced with the decreasing amplitude of strain reversal.It was further observed that CFRT samples with a gradient?"martensite have higher mechanical properties both in strength and ductility than MT samples,which is due to the synergy effect between?"martensite and dislocation slip,especially the apparent effect caused by the appearance of kink?"martensite.In order to expand the application scope of the research work,and explore the applicability of the preparation process of the torsional deformation induced-gradient structure material,the CFRT process was used to produce the industrial application of the ultrafine grain Ti-6Al-4V alloy.The hardening analysis function of the cross-section of Ti-6Al-4V alloy bar as a function of gradient strain was established.By designing the magnitude of the reverse strain during the CFRT process,the overall mechanical properties of the material can be further controlled to enhance the material performance potential. |
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