Classification of titanium alloys

Titanium alloys are generally divided into α-type, α+β-type, and β-type titanium alloys. According to the relationship between the phase composition of titanium alloys after quenching from the β phase region and the content of β stabilizing elements, titanium alloys are divided into the following six types:
α-type titanium alloys: including industrial pure titanium and alloys containing only α-stabilizing elements;
Near α-type titanium alloy: an alloy with a β-stabilizing element content less than C1;
Martensitic α+β titanium alloy: an alloy with a β-stabilizing element content ranging from C1 to Ck. This type of alloy can be referred to as α+β titanium alloy;
Near metastable beta titanium alloy: alloy with beta stable element content from Ck to C3. This type of alloy can be referred to as near beta titanium alloy;
Metastable β-type titanium alloy: an alloy with a β-stable element content from C3 to Cβ. This type of alloy can be referred to as β-type titanium alloy;
Stable β-type titanium alloy: An alloy with a β-stabilizing element content exceeding Cβ. This type of alloy can be referred to as a full β-type titanium alloy.

Titanium alloy elements

Pure titanium has high plasticity but low strength, which limits its application in industrial production. In order to meet the requirements of high strength and corrosion resistance in actual production, some alloying elements are added to pure titanium to form titanium alloys. According to the effects of alloying elements and impurities on the β-transition temperature of titanium, they can be divided into α-stable elements, β-stable elements and neutral elements.

alpha stable elements

Elements that increase the β-transition temperature, expand the α-phase region, and increase the stability of the α-phase are called α-stabilizing elements [see Figure 4(a)]. Alpha stable elements mainly include: alloy elements aluminum, gallium, germanium, boron and impurity elements oxygen, nitrogen, carbon, etc. Aluminum is the most commonly used α-stable element in industry. Through substitution and solid solution strengthening, adding an appropriate amount of aluminum element can improve room temperature and high temperature strength and thermal strength. Therefore, appropriate amounts of aluminum are almost added to various types of titanium alloys at home and abroad. However, when the aluminum content exceeds 7wt.%, the brittle Ti3Al phase is easily formed, which should be avoided in alloy design. Gallium and germanium elements are rarely used in actual production. Boron element is called the vitamin of metal materials. Adding a small amount of boron to titanium alloys can refine the grains and improve the properties of the alloy. The impurity elements oxygen and nitrogen can greatly increase the strength of titanium, but also seriously reduce the plasticity of the alloy. Therefore, their content must be strictly limited in actual production. Carbon element has little effect on the strength and plasticity of the alloy, and is easier to control during production.

Isomorphous β-stable elements

Elements that have the same lattice structure and similar atomic radius as titanium, lower the β transition temperature, infinite solid solution in the β phase, expand the β phase area, and increase the stability of the β phase are called isomorphous β stable elements [see Figure 4(b)]. It includes elements such as molybdenum, vanadium, niobium, and tantalum. Among them, molybdenum has the most obvious strengthening effect, which can improve room temperature and high temperature strength, increase hardenability, and improve the thermal stability of alloys containing chromium and iron. Molybdenum and vanadium are the most widely used. The strengthening effect of niobium is weak, but it is often added to titanium alloys, especially to Ti-Al intermetallic compounds to improve plasticity and toughness. Tantalum has the weakest strengthening effect and is dense, so it is only added in small amounts to improve oxidation resistance and corrosion resistance.

Eutectoid β-stable elements

Elements that lower the β-transition temperature, expand the β-phase area, and also cause eutectoid transformation are called eutectoid-type β-stable elements [see Figure 4(c)]. This type of elements covers a wide range, and the eutectoid reaction rates vary greatly. Among them, elements such as chromium, manganese, iron, etc. react with titanium at low eutectoid temperatures and have extremely slow transformation speeds, making it difficult to transform under general heat treatment conditions, so they are called inactive eutectoid elements; conversely, silicon, copper, hydrogen, Elements such as nickel and silver have extremely fast eutectoid transformation speeds and cannot be inhibited by quenching, so they cannot stabilize the β phase to room temperature. They are called active eutectoid elements. Iron is one of the strongest β-stable elements, but it has poor thermal stability and is prone to segregation during smelting, so it is rarely used. Iron can be added to some low-cost titanium alloys to replace expensive vanadium. Chromium is one of the elements that is widely added. Chromium-added alloys have high strength and good plasticity, and can be strengthened by heat treatment. However, under certain conditions, the plasticity will be reduced due to the precipitation of compounds. Manganese is a widely used element in early alloy design to improve strength and plasticity, but it produces eutectoid decomposition under certain conditions and is unstable. Silicon is one of the important trace elements that improves thermal strength and heat resistance. It is added to most high-temperature titanium alloys, but generally does not exceed 0.5%. As a harmful element, hydrogen must be strictly controlled. The main reason is hydrogen embrittlement caused by the precipitation of hydride. Other elements such as copper, nickel, and silver are rarely used.

Neutral elements

Elements that have little effect on the beta transition temperature are called neutral elements, mainly zirconium, hafnium and tin. Zirconium and hafnium have similar properties to titanium, and their atomic sizes are also very close, and they can be infinitely dissolved in the α phase and β phase. The room temperature strengthening effect of zirconium is weak, but the high temperature strengthening effect is strong, and it is usually used in thermally strong titanium alloys; the room temperature strengthening effect of tin is even weaker, and eutectoid reactions will occur, but it can improve the thermal strength.

Effect of impurity elements on titanium and titanium alloys

The impurity content in titanium has a great influence on the mechanical properties of titanium. An increase in the impurity content can increase its strength and reduce its plasticity. Oxygen, carbon, and nitrogen are impurities that often exist in titanium. They can increase the strength of titanium and reduce its plasticity. Among them, nitrogen has the greatest impact and carbon has the smallest impact.
The influence of hydrogen on the mechanical properties of titanium is mainly reflected in hydrogen embrittlement. In titanium, when the hydrogen content reaches a certain value, the sensitivity of titanium to notches will be greatly increased, thereby drastically reducing the impact toughness and other properties of notched samples. It is generally believed that the mass fraction of hydrogen in titanium should be lower than 0.007%~0.008%, and not allowed to be higher than 0.0125%~0.015%, because above this content, hydrides will precipitate on the structure and obvious hydrogen embrittlement will occur. .
In addition to oxygen, carbon, and nitrogen, the elements that have a greater impact on improving the strength of titanium are boron, beryllium, and aluminum. Other elements have a less strong effect on the strength of titanium, in order: chromium, cobalt, niobium, manganese, iron, vanadium and tin.