Mechanical behavior and impact toughness of ultrafine-grained Grade 5 Ti alloy processed by ECAP

Ultrafine-grained metals or alloys manufactured through a number of methods of severe plastic deformation exhibit unique physical and mechanical attributes. Ductility, strength, and fracture toughness are the most important mechanical attributes for an adequate mechanical design of structural elements. Unfortunately, ultrafine-grained metals and alloys generally have lower ductility when compared to coarse-grained metals and this restricts their implementation as structural members in the industry. Ductility-related issues relate to the attributes including the total as well as uniform tensile elongation, fracture toughness, and impact toughness that tend to decrease in metals as well as alloys exposed to severe plastic deformation.

It has been observed that ultrafine-grained structure formation in titanium as well as titanium alloy Ti-6Al-4V leads to high values of strength, superplasticity, fatigue endurance limit, and many more. In addition, a reduction in the α-grain sizes in titanium alloys results in a decline of ductile attributes as well as impact toughness. For this reason, identifying new ways of enhancing the ductility-related attributes, and specifically the impact toughness, of the ultrafine-grained titanium-based materials for a number of engineering applications is a topical problem.

Currently more attention is being given to enhancing the ductility of ultrafine-grained metals and alloys by using different methods for ultrafine-grained structure formation in metals. For instance it is possible to realize a combination of high strength and ductility via the formation of a bimodal structure composed of nanometer-sized grains and a fraction of micrometer-sized grains where high strength is as a result of ultrafine grains while high ductility is as a result of the coarse grains.

Irina Semenova, Ruslan Valiev and their colleagues at Ufa State Aviation Technical University in collaboration with Terence Langdon at University of Southampton studied the relationship between microstructure, impact toughness, and mechanical behavior of the ultrafine-grained Grade 5 titanium alloy. The collaborative research team produced two forms of ultrafine-grain structure through extrusion and equal-channel angular pressing with subsequent deformation and thermal treatment. Their work is published in Materials Science & Engineering A.

The authors investigated extensively the mechanical attributes and the impact toughness of the grade 5 titanium alloy in the course-grained and ultrafine-grained states, which was produced by equal-channel angular pressing and subsequent deformation and thermal treatment through warm upsetting in isothermal conditions and extrusion.

From the results of their study, the authors demonstrated that the formation of the ultrafine-grained structure with a 0.35µm α-grain led to a high strength of 1435MPa. At the same time, there was a reduction in the total as well as uniform elongation when compared to the same attributes of the coarse-grained alloy.

The increase in the yield strength and the drop in the uniform elongation in the ultrafine-grained alloy reduced the impact toughness of the selected samples with varying notch sharpness by approximately 2 times.

An ultrafine-grained structure composed of equiaxed α-gains with a mean size of 0.8µm, characterized by high-angle misorientations and a reduced dislocation density owing to recovery processes as well as dynamic recrystallization during isothermal upsetting, led to an improvement in the total and uniform elongation. It also led to an increase in the impact toughness.