Four types of binary phase diagrams of titanium and titanium alloys

(1) the first type forms a continuous and mutually soluble phase diagram with both alpha and beta.There are only two ti-zr and ti-hf systems.Titanium, zirconium and hafnium are members of the same family, with the same electron structure, lattice type and atomic radius.

     These two elements have the same solubility in alpha titanium and beta titanium and have little influence on the stability of alpha and beta phases.Zirconium has a strong strengthening effect at high temperature, so zirconium is often used as a component of thermo-strong titanium alloy.

(2) the second type beta is a continuous solid solution and alpha is a finite one.There are 4: ti-v ti-nb ti-tati-mo series.The four metals of V, Nb, Ta and Mo have only one kind of body-centered cube, so they form a continuous solid solution with beta Ti with the same crystal shape, and a finite solid solution with alpha Ti with dense hexagonal lattice.V is an element of stable beta, and as the concentration increases, it sharply decreases the allocrystalline transition temperature of titanium.When V content is greater than 15%, the beta phase can be fixed to room temperature by quenching.

     For industrial titanium alloys, V has a higher concentration in alpha titanium (>3%), which allows for the combination of the advantages of single-phase alpha alloys (good weldability) and the advantages of two-phase alloys (heat treatment and hardening, better plasticity than alpha alloys).There were no eutectoid reactions and metal compounds in ti-v series.Nb is about as soluble in alpha titanium as V (about 4%), but has a much lower effect as a beta stabilizer.When Nb content is more than 37%, quenching can perfect the beta structure.The solubility of Mo in alpha titanium is less than 1%, and the beta stabilization effect is the greatest.When Mo content is greater than 1%, it can quench to perfect the beta structure. The addition of Mo effectively improves the strength at room temperature and high temperature.One disadvantage of Mo room temperature is its high melting point, which makes it difficult to form even alloy with titanium.When Mo is added, it is generally in the form of an intermediate alloy of Mo-Al (prepared by thermo – aluminum reduction process of Mo oxide).

(3) the third type is finitely soluble in alpha and beta and has phase diagrams of inclusion reactions.Ti-al, ti-sn, ti-ca, ti-b, ti-c, ti-n, ti-o, etc.The ordered alpha 2 (Ti 3X) phase exists in the phase region within the concentration range of 5% ~ 25% Al, which degrades the properties of the alloy.Aluminum equivalent Al * = Al % + 1/3 sn zr % % + 1/6 + 1/2 ga + 10 [O] % % 8% ~ 9% or less.No alpha 2 phase occurs as long as the aluminum equivalent is less than 8 to 9 percent.Sn is a relatively weak enhancer, but it can significantly improve the thermal strength. When alloyed with tin, its room temperature plasticity does not decrease but the thermal strength increases.Trace B can refine the large grain of titanium and its alloy, Ga can dissolve well with titanium, and improve the thermal strength of titanium alloy significantly.Oxygen is a more “soft” enhancer, which can not only ensure the required strength level, but also ensure a high enough plasticity when the content is allowed.

(4) the fourth type is finite-soluble in alpha and beta, and has phase diagrams of eutectogenic decomposition, such as ti-cr, ti-mn, ti-fe, ti-co, ti-ni, ti-cu, ti-si, ti-bi, ti-w and ti-h.In the Ti2Cr system, there are two allocrystalline forms of Ti2Cr, and the solid solution is delta and gamma.Cr is a beta stable element, and is less than 0.5% soluble in alpha titanium.When Cr content is greater than 9%, the beta phase can be fixed to room temperature by quenching.Cr can make titanium alloy have good room temperature plasticity and high strength, and at the same time, it can ensure high heat treatment strengthening effect.W is not very soluble in alpha titanium.When W content is greater than 25%, the beta phase can be fixed to room temperature by quenching.Hydrogen lowers the allocrystalline transition temperature of titanium to form a eutectoid reaction, which results in the decomposition of the beta solid solution to form alpha phase and titanium hydride. At the eutectoid temperature, the solubility of hydrogen in alpha titanium is 0.18%.Hydrogen is a kind of interstitial solid solution, which belongs to harmful impurity and can cause hydrogen embrittlement of titanium alloy.In unalloyed titanium and single phase titanium alloy based on alpha structure, the main reason of hydrogen embrittlement is the precipitating of brittle hydride phase, which sharply reduces the fracture strength.

     In the two-phase alloy, no hydride is formed, but the susaturated solid solution zone of hydrogen is formed, which causes brittle fracture in the low-speed deformation.In alloys with a small beta content, the two work together.Titanium alloys with pure titanium and near-alpha structure are most sensitive to hydrogen embrittlement.The hydrogen embrittlement sensitivity decreases with the increase of beta content in the alloy.

Editor : Carrie Qi (rowlynti@rowlyn.com)