Quantitative Research On Micro-plastic Deformation Mechanism And Microstructure Envolution Of Polycrystal-dual Phase Titanium Alloy

Dual-phase materials usually possess excellent comprehensive mechanicalproperties and plastic formability because the disadvantages can be weakened theadvantage can be promoted by the interatction of two composed phases.Polycrystaline materials which are compsed of single grain with differentmorphologies and random crystal orientation, usually present isotropic mechanismproperties at macroscal and are widely used as engineering structure materials.However, becaues of the different crystal structures and different physical ormechanical properities of composed phase and the interaction of composed phasesor grains with different crystal orienation, morphologies and sizes, the micro-plasticdeformation mechanisms and microstucture evolutions are relatively complex andhave not been systematicly studied.TA15is a typical near α titanium alloy. It is is widely used for aerospaceapplications due to its high specific strength, thermal stability, excellent creepresistance and welding properity. However, the effect of β phase on the micro-plastic deformation mechanisms are usually neglected in the previous studies. Thequantitative characterization on microstrucures and systematic researches on themechanism of microstrucure evolution are also insufficient in the conducted studies.In the current study, we took TA15titanium alloy as an example to study themicro-plastic deformation mechanisms and the mechanisms of compatibledeformation of dual-phase polycrystalline materials by combination of experimentaltechniques including nano-indentation, FIB-EBSD, SEM(Scanning ElectronMicroscopy)/EBSD and TEM(Transmission Electron Microscopy) with numericalsimulation methods including the calculation of stress/strain field on grain-scale,calculation of geometrically necessary dislocation (GND) density and crystalplasticity finite element method(CP-FEM). The evolutions of grain morphology,size, distribution and crystal orientation during plastic deformation process havebeen quantitatively characterized. The mechanisms of microstructure evolution alsohave been analyzed based on quantitatively experimental observation.The mechanical parameters of α and β phase have been measured by nano-indentaion test, respectively. The stress/strain and GND fields around the indentaionare also revealed. The results indicate that both the elastic modulus and hardness ofα phase are significantly higher than the values of β phase. stress/strain and GNDfields around the indentaion are significantly influenced by the neighboring β phase.The3D-microstructures have been presented by FIB-EBSD technique and theresults shows that the β phase is a continuous network structure and α lamellae are embedded in the continuous β phase.Quasi-in situ tensile tests are performed on TA15titanium alloy; the slipsystem activation, crystal orientation evolution, stress/strain fields and GNDdistribution under different tensile strains have been quantitatively characterized.The micro-plastic deformation mechanisms and the mechanisms of compatibledeformation between the two phases or polycrystalline grains have been investigatedbased on the experimental results. The results indicated that dislocation slipping isthe main deformation mechanism for TA15titanium alloy under low strain rate(~1×10-4s-1) with small tensile strain. The slip activation is focused on basal andprismatic systems; the pyramidal slip system has also been gradually activated withincreasing tensile strain.During the plastic deformation process, the compatible deformation betweenthe two phases or among polycrystalline grains expressed as the slip model changedfrom single slip to multi-slip and slip transmission across phase or grain boundaries.The research demonstrates that both the crystal orientation relationship (COR)between α/β phases and the thickness of β layers have a significant influence on theslip transmission and compatible deformation behavior. Furthermore, the grainmorphology, grain size, grain distribution and the interaction effect of neighboringgrains have a significant influence on the slip system activation, stress/strain fields,GND distribution and the compatible deformation mechanism among polycrystallinegrains. It is obvious that the Schmid law is restrictive when it was used to predict theslip system activation in polycrystalline materials. The evolutions of dislocationmorphologies and styles have been revealed based on TEM observation andinvisibility criterion analysis; then the the micro-plastic deformation mechanismsare further validated.Isothermal tensile deformation has been carried out on TA15titanium alloy inthe dual-phase region(750-850°C). The plastic deformation mechanism, deformationbehavior of polycrystalline grains, and the mechanism of microstructure evolutionhave been studied basing on thermal activation analysis and the microstructurequantitative characterization using EBSD. It shows that the deformation mechanismgradually transformed from dislocation slip to diffusion controlled creep and thecompatible dafomation capability is promoted with increasing deformationtemperature. Furthermore, the recrystallization mechanism changed from continuousdynamic recrystallization(CDRX) at the relatively low temperature (750°C) toconventional discontinuous dynamic recrystallization(DDRX) at the temperatures of850°C. The globularization mechanism of α lamellae also changared from thedislocation accumulation or subgrain boundaries sliding to recrystallization.Combination of EBSD measurement, the compatible deformation behavior andthe microstructure evolution of TA15titanium alloys during compression deformation in near β and β single phase regions have been investigated. The resultsindicated that the near β deformation (980°C) is important to refining grain size andpromoting the microstructure homogeneity. The deformation strain, strain rate andsubsequent cooling rate have no significant influence on the Burgers COR betweenα variants and β phase during βâ†'α phase transformation after compressiondeformation in β phase region. However, the factors of strain rate and cooling ratehave significant influence on the morphology and size of α variants. The CP-FEMmodel of TA15titanium alloy uniaxial compression in β phase region has been builtand the grain-scale stress/strain distributions under different deformation strain andstrain rate conditions have been studied. The simulation results demonstrate that thecompression strain and strain on the prior β grain have no significant influence onthe heterogeneous distribution of stress/strain fields at grain-scale. The simulationresults perfectly explained the experimental results on Burgers COR obeying duringβâ†'α phase transformation after compression deformation in β phase region.Finally, a series of heat treatment tests have been preformed on TA15tiatniumin near β and single β phase regions. The microstructure characters have beenquantitatively characterized and the mechanisms of microstructure evolution havebeen investigated. The relationships between the microstructure characteristic andthe tensile properties have been revealed. The results indicate that the secondα grains first increase in length and then in thickness during heat treatment process.For TA15titanium alloy, the tri-modal microstructure which possesses wellcomprehensive mechanical properties, can be obtained by double heat treatment:970°C/20min/water cooling+940°C/1×60min./air cooling.