(1) The first type forms a continuous and mutually soluble phase diagram with both α and β. There are only two Ti-Zr and Ti-Hf systems. Titanium, zirconium, and hafnium are elements of the same family. The atomic outer electron structure is the same, the lattice type is the same, and the atomic radius is similar. These two elements have the same solubility in α-titanium and β-titanium, and have little effect on the stability of α-phase and β-phase. When the temperature is high, the strengthening effect of zirconium is stronger, so zirconium is often used as a component of heat-strength titanium alloy.
(2) The second type β is a continuous solid solution and α is a finite solid solution. There are 4: Ti-V Ti-Nb Ti-TaTi-Mo series. The four metals V, Nb, Ta, and Mo have only one type of body-centered cubic, so they form a continuous solid solution with β-Ti having the same crystal form, and form a limited solid solution with α-Ti closely packed with a hexagonal lattice. V belongs to the element that stabilizes the β phase, and as the concentration increases, it sharply decreases the allotropic transformation temperature of titanium. When the V content is greater than 15%, the β phase can be fixed to room temperature by quenching. For industrial titanium alloys, V has a greater concentration (> 3%) in alpha titanium, so that the advantages of single-phase alpha alloys (good weldability) and the advantages of two-phase alloys (can be strengthened by heat treatment, Better process plasticity than alpha alloy) alloys combined together. There is no eutectoid reaction and metal compounds in Ti-V system. The solubility of Nb in α-titanium is roughly the same as V (about 4%), but the effect as a β stabilizer is much lower. When the Nb content is greater than 37%, it can be quenched into a full β structure. The solubility of Mo in α-titanium does not exceed 1%, while β-stabilizing effect is the largest. When the Mo content is greater than 1%, it can be quenched into a full β structure. The addition of Mo effectively improves the strength at room temperature and high temperature. One disadvantage of Mo at room temperature is its high melting point, and it is not easy to form a uniform alloy with titanium. When Mo is added, it is generally added in the form of Mo-Al master alloy (made by the aluminothermic reduction process of molybdenum oxide).
(3) The third type has limited solubility with α and β, and there is a phase diagram of the envelopment reaction. Ti-Al, Ti-Sn, Ti-Ca, Ti-B, Ti-C, Ti-N, Ti-O, etc. There is an ordered α2 (Ti 3X) phase in the phase area within the 5% to 25% Al concentration range, which will degrade the performance of the alloy. Aluminum equivalent Al * = Al% + 1 / 3Sn% + 1 / 6Zr% + 1 / 2Ga% + 10 [O]% ≤ 8% ～ 9%. As long as the aluminum equivalent is less than 8% to 9%, the α2 phase will not appear. Sn is a relatively weak strengthening agent, but it can significantly improve the thermal strength. When alloyed with tin, its room temperature plasticity does not decrease but the thermal strength increases. A small amount of B can refine the large grains of titanium and its alloys, and Ga can be well dissolved with titanium, and significantly improve the thermal strength of the titanium alloy. Oxygen is a relatively “soft” strengthening agent. When the content is within the allowable range, it can not only ensure the required strength level, but also ensure a sufficiently high plasticity.
(4) The fourth type has limited dissolution with α and β, and has a phase diagram of eutectoid decomposition, including Ti-Cr, Ti-Mn, Ti-Fe, Ti-Co, Ti-Ni, Ti-Cu, Ti -Si, Ti-Bi, Ti-W, Ti-H. In the Ti-Cr system, the formed Ti2Cr compound has two allotropic forms, and its solid solution is represented by δ and γ. Cr is a β-stable element, and its solubility in α-titanium does not exceed 0.5%. When the Cr content is greater than 9%, the β phase can be fixed to room temperature by quenching. Cr can make titanium alloys have good room temperature plasticity and high strength, while ensuring a high heat treatment strengthening effect. In the Ti-W system, a segregation transition occurs: β ′? Α +. The segregation reaction temperature is higher, and the thermal stability of the Ti Ti-W system is much higher than that of the Ti-Cr alloy. The solubility of W in alpha titanium is not high. When the W content is greater than 25%, the β phase can be fixed to room temperature by quenching. Hydrogen lowers the allotropic transformation temperature of titanium to form an eutectoid reaction, which decomposes the β solid solution to form α phase and titanium hydride. The solubility of hydrogen in α titanium at the eutectoid temperature is 0.18%. Hydrogen forms interstitial solid solutions, which are harmful impurities and can cause hydrogen embrittlement of titanium alloys. In unalloyed titanium and single-phase titanium alloys based on the α structure, the main cause of hydrogen embrittlement is the precipitation of brittle hydride phases, which drastically reduces the fracture strength. In the two-phase alloy, no hydride is formed, but a supersaturated solid solution region of hydrogen is formed, which causes brittle fracture during low-speed deformation. In alloys with a small β-phase content, the two have a combined effect. Pure titanium and titanium alloys with near-alpha structure are most sensitive to hydrogen embrittlement. As the β-phase content of the alloy increases, its hydrogen embrittlement sensitivity decreases.
