Typical physical and mechanical properties of difficult-to-machine materials
3.2.1 Physical and mechanical properties of titanium alloy materials
Titanium is an allotrope with a melting point of 1668℃. When the temperature is below 882℃, it is a close-packed hexagonal lattice structure α phase titanium; when it is above 882℃, it is a body-centered cubic lattice structure B phase titanium. Based on these two different structures of titanium, other alloying elements are added to gradually change its phase transition temperature and phase fraction, thereby obtaining titanium alloy materials with different structures. At room temperature, titanium alloys can be divided into three categories: α phase titanium alloy, β phase titanium alloy and (α+β) phase titanium alloy, which are TA, TB and TC in national standards. Table 3.2 shows the chemical composition and physical and mechanical properties of common titanium alloys.
(1) α phase titanium alloy is a single-phase alloy. Whether at room temperature or at higher cutting temperatures, its structure is stable and its wear resistance is better than that of pure titanium materials. It can still maintain high strength and creep resistance at 500~600℃ and has strong oxidation resistance. However, its strength at room temperature is not high and heat treatment cannot be performed to enhance the material properties. It mainly contains α-stabilizing elements and is commonly used in the petrochemical industry. It includes industrial pure titanium (TA0~TA3), TA9 and TA10.
(2) β-phase titanium alloy is a single-phase alloy. The strength can reach 1327~1666MPa at room temperature without heat treatment. It can be further strengthened after quenching and other treatments, but its thermal stability is poor and it is not suitable for use at high temperatures. It contains enough β-stabilizing elements. At an appropriate cooling rate, the room temperature structure is all β phase. It can usually be divided into treatable B-phase titanium alloy (metastable β-phase titanium alloy) and thermally stable β-phase titanium alloy. Heat-treatable β-phase titanium alloy has very strong process plasticity under quenching conditions, can be cold-formed into plates, and can obtain room temperature tensile strength of up to 1300~1400MPa through aging treatment.
(3) (α+β) phase titanium alloy is a two-phase alloy with good comprehensive performance. Its thermal stability is second only to that of α phase titanium alloy. It can be used for a long time at 400~500℃, has good high temperature strength, good toughness, plasticity and high temperature deformation ability, and can be strengthened by quenching, aging and other processing. It contains a large amount of β stabilizing elements. Under the stable state at room temperature, the alloy composed of α and B phases is mainly used in the fields of petroleum and petrochemical, marine engineering, shipbuilding and aerospace. Commonly used ones are TC10 and TC18.
1. Physical and mechanical properties of TA10
TA10 (Ti-0.3Mo-0.8Ni) belongs to α phase titanium alloy and is the development of α phase titanium alloy. Its tensile strength at 300℃ is about twice that of industrial pure titanium; it has strong resistance to reducing medium corrosion and will not corrode in chlorides at 150~200℃.
TA10 material does not contain rare metals and has relatively good processing performance. TA10 has excellent processing properties and can be forged and rolled within a temperature range of 850-950°C. Hot working at the lowest possible temperature minimizes surface contamination. To prevent hydrogen embrittlement caused by hydrogen absorption, heating in an oxidizing environment is required. It exhibits excellent plasticity and bendability, and forming at 250°C minimizes springback. Argon-shielded tungsten arc welding is typically used, but special protection is required at welding temperatures above 430°C to prevent contamination from carbon, nitrogen, hydrogen, and oxygen.
TA10 has a bulk density of p = 4.54 g/cm. The main chemical composition of TA10 is shown in Table 3.3, with molybdenum and nickel being the primary alloying elements. Its thermal conductivity is shown in Figure 3.3.

Figure 3.3 and Table 3.3 show that the thermal conductivity of TA10 titanium alloy is negatively correlated with temperature before 400°C. After 400°C, the thermal conductivity curve shows a slight increase with increasing temperature. The mechanical properties of TA10 are shown in Table 3.4.

2. Physical and Mechanical Properties of TC4
TC4 (Ti-6A1-4V) is an (α+β) phase titanium alloy, combining the advantages of both α and B phases. Its chemical composition is shown in Table 3.5. Al solid-solubilizes in the α phase, strengthening it, while V stabilizes the β phase. As the temperature rises, the α phase content decreases and the β phase content increases. At approximately 1000°C, the α phase completely transforms into the β phase. The opposite transformation occurs during cooling. This alloy exhibits excellent physical, mechanical, and processability.

TC4's yield strength is close to its tensile strength, with a high yield strength ratio. Its elastic modulus is low, approximately 54% of that of steel. It maintains its strength at high temperatures.