Knowledge of titanium alloy

Titanium is an isomer with a melting point of 1720 ℃. It has a close packed hexagonal lattice structure below 882 ℃, which is called α titanium. It has a body centered cubic structure above 882 ℃, which is called β titanium. According to the different characteristics of the above two structures of titanium, titanium alloys with different microstructures can be obtained by adding appropriate alloying elements to gradually change the phase transformation temperature and phase content. At room temperature, titanium alloy has three kinds of matrix structure, and titanium alloy can be divided into the following three categories: α alloy, (α + β) alloy and β alloy. China is represented by TA, TC and TB respectively.

Alpha titanium alloy

It is a single-phase alloy composed of α - phase solid solution. It is α - phase no matter in general temperature or in higher practical application temperature. It has stable structure, higher wear resistance than pure titanium and strong oxidation resistance. However, it can not be strengthened by heat treatment, and the strength at room temperature is not high.

Beta titanium alloy

It is a single-phase alloy composed of β - phase solid solution. It has high strength without heat treatment. After quenching and aging, the alloy is further strengthened, and the strength at room temperature can reach 1372-1666mpa. However, it has poor thermal stability and is not suitable for use at high temperature.

α + β titanium alloy

It is a two-phase alloy with good comprehensive properties, good microstructure stability, good toughness, plasticity and high temperature deformation properties. It can be well processed by hot pressure and strengthened by quenching and aging. The strength after heat treatment is about 50% ～ 100% higher than that of annealed state. It has high high temperature strength and can work at 400 ℃～ 500 ℃ for a long time. Its thermal stability is inferior to that of α titanium alloy.

The most commonly used of the three titanium alloys are α titanium alloy and α + β titanium alloy; the machinability of α titanium alloy is the best, followed by α + β titanium alloy, and β titanium alloy is the worst. The code of α titanium alloy is Ta, the code of β titanium alloy is TB, and the code of α + β titanium alloy is TC.

Titanium alloys can be divided into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (Ti Mo, Ti PD alloys, etc.), low-temperature alloys and special functional alloys (Ti Fe hydrogen storage materials and Ti Ni shape memory alloys), etc. The composition and properties of typical alloys are shown in the table.

Different phase composition and microstructure can be obtained by adjusting the heat treatment process. It is generally believed that fine equiaxed structure has better plasticity, thermal stability and fatigue strength; acicular structure has higher endurance strength, creep strength and fracture toughness; mixed equiaxed and acicular structure has better comprehensive properties.