| Body-centered cubic(bcc)?-type titanium alloys exhibit the diversity of thermal-induced metastable phase transformation(?'/?"-martensite,?-phase and O'-phase)and stress-induced deformation mode(?'/?"-martensitic transformation,?-phase transformation,{112}<111>twinning,{332}<113>twinning),which makes them having excellent functional and structural properties,such as shape memory effect,superelasticity,low Young's modulus,transformation-induced plasticity(TRIP)and twinning-induced plasticity(TWIP)effects.Thus,the ?-type titanium alloys have broad application prospect in aerospace,marine engineering,biomedical fields.The key scientific issue in fundamental research fields of titanium alloys is how to establish the criterion of different metastable phases and deformation modes.The difficulty is to find out the correlation among alloy compositions,metastable phases and deformation modes,as well as structural evolution law of metastable phases and transition mechanism of deformation modes.In this study on the basis of first-principles calculations and multi-scale microstructure characterization in the typical ?-type Ti-Mo binary alloys,the correlation between specific orientation modulus softening and thermal/stress-induced atomic shear,shuffle and collapse was established through constructing the cluster structure and cluster-plusstacking fault structure and analyzing the charge density.Crystal structural evolution of not only metastable phase but also ?-phase itself as well as {332}<113>twinning mechanism are also discussed.The main conclusions of the thesis are as follows:To construct the stable cluster structures of ?-phase in Ti-Mo alloy,the occupations of Mo atom in supercells were investigated.The linear-Mo-Ti-Mo-cluster structure with the lowest cohesive energy along<111>? direction was obtained.With the increase of Mo atoms,the-MoTi-Mo-cluster structures along<100>?,<110>? and<111>? directions were used to construct the triangular,tetrahedron,centerless hexahedron,center hexahedron and heptahedron cluster structures.First-principles calculation was used to calculate their total energies and densities of state.With the increase of Mo content,the decrease of total energy and density of state at Fermi level of ?-phase and the increase of number of bonding electrons led to the increase of its stability.Due to the formation of thermal-induced ?"-martensite and ?-phase in Ti-Mo alloy,the calculated values of Young's modulus showed an opposite trend to the experimental values.Based on the stable-Mo-Ti-Mo-cluster structure units,the-Mo-Ti-Mo-linear,triangular and tetrahedral cluster structures of orthorhombic ?"-martensite and hexagonal ?-phase were constructed,respectively.The first-principles calculations,X-ray diffraction and transmission electron microscopy were used to analyse the stability and crystal structure of metastable phases in Ti-Mo alloys with different Mo contents.With the increase of Mo content,the decrease of orthorhombicity and shuffle magnitude of atoms on every other {110}? plane along<110>?direction resulted in the crystal structure of martensite changing from hexagonal close-packed to orthorhombic;the decrease of collapse degree of atoms on {112}? plane along<111>?direction led to the crystal structure of co-phase changing from hexagonal to trigonal.The softening of Young's modulus(E100)and tetragonal shear modulus(C')was favorable for the shear and shuffle components of ?"-martensite,respectively,whereas that of shear modulus(G111)was beneficial to the collapse component of ?-phase.The competition among the specific orientation moduli determined the formation sequence of metastable phases as follows:hexagonal close-packed ?'-martensite,orthorhombic ?"-martensite,hexagonal ?-phase and trigonal ?-phase.The formation and structural evolution of thermal-induced ?-phase in Ti-Mo alloys were investigated by first-principles calculations and high-angle annular dark-field scanning transmission electron microscopy.Trigonal ?-phase formed in the nanoscale Mo-depleted region in the alloy after solution treatment and water quenching.From Mo-enriched to Modepleted regions,the collapse degree of ?-phase changed continuously with Mo content,showing its instabilities of composition and structure.Based on the constructed cluster-plusstacking fault structures,in the Mo-depleted region far away from the cluster structures,the decrease of lattice distortion along<112>p direction and shear modulus(G111)was favorable to formation of ?-phase.In the process of collapse,the increase of collapse degree corresponding to the minimum values of charge density difference and stacking fault energy led to the structure of ?-phase changing from trigonal to hexagonal.The concurrent instabilities of composition and structure of ?-phase were attributed to the coupling effect of cluster structure and stacking fault in terms of electron and atomic basis.The metastable phases before deformation and {332}<113>twin microstructures after deformation(0.2%tensile strain)in Ti-15Mo alloy were characterized by high resolution transmission electron microscopy.The stability and shuffle magnitude of O'-phase were analyzed based on-Mo-Ti-Mo-cluster structure by first-principles calculations.The thermalinduced ?-phase had a trigonal structure before deformation,while the hexagonal ?-phase appeared near {332}<113>twin boundary after deformation and the stress-induced O'-phase distributed along the twin boundary.The total energy and density of state at Fermi level of O'phase were both higher than those of ?-phase and ?-phase,which resulted in the absent thermalinduced O'-phase before deformation.At high Mo content,because Young's modulus(E100)was greater than tetragonal shear modulus(C'),the shear component of stress-induced ?"martensitic transformation occurred difficultly,but the shuffle component of stress-induced O'phase transformation occurred easily.When the shuffle magnitude of O'-phase was larger than 4.8/48,its {130}<310>twinning was preferred to {110}<110>twinning and further transited to {332}<113>twinning of ?-phase.Finally,the {332}<113>twinning mechanism was proposed under the synergistic effect of thermal-induced trigonal ?-phase and stress-induced O'-phase. |
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