The microstructure and properties of industrial pure titanium subjected to hydrostatic extrusion

By refining the microstructure, the mechanical properties of metal materials can be significantly improved. As the grain size decreases, the strength, fatigue resistance, and superplasticity of the material can all be enhanced. Ultrafine-grained materials can be obtained through strong plastic deformation. For pure metals, the grain size obtained through strong plastic deformation is usually in the range of 100 to 500 nm, and the resulting microstructure is called an amorphous microstructure. Strong plastic deformation includes techniques such as high-pressure torsion, channeling angle extrusion, and torsional extrusion. The advantages of these methods are their ability to accumulate deformation without changing the original shape and size of the workpiece. Repeated application of conventional deformation methods, such as rolling or compression, can also achieve an amorphous microstructure.

However, multi-pass deformation processing is costly and labor-intensive. Therefore, a processing method that achieves a large strain in one deformation cycle is highly attractive. Static hydrostatic extrusion is such a method: through a mold, high-pressure liquid exerts force on the billet, and high pressure can reduce the transition temperature between brittleness and plasticity, and the high-pressure medium can reduce the friction during the extrusion process. Static hydrostatic extrusion is suitable for titanium processing because the room-temperature plasticity of titanium materials is relatively low. Studies have shown that after 20 extrusion passes on titanium, the microstructure grain size can be refined to 47 nm, and the material strength can be significantly increased to 1320 MPa. Static hydrostatic extrusion can also achieve a large strain with one or two passes, which is more attractive than multi-pass extrusion. However, the strain accumulated in one pass depends on the material strength and is also limited by the capacity of the extrusion equipment. The strain accumulation degree increases with the material temperature, but the influence of the elevated temperature on the microstructure and properties of titanium materials is not well understood during the static hydrostatic extrusion process.

Researchers from the Belgorod State University of Science and Technology in Russia studied the effect of static hydrostatic extrusion on the microstructure and properties of industrial pure titanium under conditions of 20°C, 350°C, and 450°C, with a true strain of up to e = 2. At 20°C and e = 1.8, a lamellar microstructure with a width of 100 to 500 nm was formed. At higher deformation temperatures of 350 and 450°C, a mixed microstructure of lamellar and fine-grained zones was formed. In terms of performance, static hydrostatic extrusion significantly increases the material strength, with the ultimate tensile strength at 20°C and 350°C being 1080 and 765 MPa, respectively. Higher temperatures reduce the material strength but increase plasticity, and the strength and elongation at fracture can be controlled by changing the temperature of the extrusion billet.