Deformation behavior of materials at high temperature is controlled primarily by thermally activated processes. That is to say, the deformation of materials is strongly dependent on strain rate, temperature, and structrual variables such as grain size, elastic modulus, and stacking fault energy.

For description of deformation behavior at high temperature, the constitutive equations were established by Frank R.N. Nabarro, Conyers Herring, Robert L. Coble, John E. Dorn, Oleg D. Sherby, Johannes Weertman, Farghalli A. Mohamed, and Terence G. Langdon. Using constitutive equations, we have developed the deformation mechanism maps and processing maps which is useful guide for hot deformation and hot working conditions.

We have investigated the deformation mechanism, especially, grain boundary sliding which leads the superplastic flow and dislocation glide creep for Magnesium alloys, Aluminum alloys, High entropy alloys, and Composites. Also, we have tried to upgrade the constitutive equations for better prediction of deformation behavior at high temperature.

Severe plastic deformation (SPD) such as equal-channel angular pressure (ECAP), high-pressure torsion (HPT), and accumulative rolling bonding (ARB) is effective method to obtain the ultrafine-grained materials. However, SPD methods have a poor process efficiency and produce a small dimensions of products.

To overcome this problems, we have used the high-ratio differential speed rolling (HRDSR) technique to produce the ultrafine-grained materials. Empirically, applying HRDSR technique on various materials including Magnesium alloys, Aluminum alloys, Titanium alloys, High Entropy Alloys, and Composites, ultrafine-grained microstructure (grain size less than 1 micrometer) could be produced.

The detail of HRDSR is introduced in below review paper

A. Bahmani and W.J. Kim, Effect of grain refinement and dispersion of particles and reinforcements on mechanical properties of metals and metal matrix composites through high-ratio differential speed rolling, Materials 13 (2020) 4159. [Link]

Superelasticity is a reversible response to an applied stress due to the phase transformation between austenitic and martensitic phases in shape-memory alloys. Many researchers have tried to improve the superelasticity of shape-memory alloys by increasing the strength through the formation of nano-grained microstructure.

We have obtained nano-grained shape-memory alloys by using HRDSR technique and continuously tried to modify the compositoin of shape-memory alloys and manufacturing to enhance the superelasticity of shape-memory alloys. Also, we have investigated the correlation between mechanism of superelasticity and microstructure of shape-memory alloys.