Abstract

Ti alloys are widely used in aero-engine applications due to their high strength-to weight ratio. In Ti and Ti alloys, the high-temperature solid solution has a body-centered  cubic crystal structure (β-phase), which undergoes the martensitic transformation on  cooling to a hexagonal close-packed structure (either α-phase or ω-phase). This transformation is classified as a reconstructive transformation of the second kind, as the  symmetries of the parent and product phases are related through the symmetry of an  intermediate phase. Using first-principles calculations and group theoretical methods, we  have simulated the experimentally observed martensitic transformations along four  transformation paths. 

Two unstable phonons were identified in the harmonic phonon dispersion relation of the β phase.

The unstable phonon  motion decreases the energy for small displacements. Minimum energy-modulated structures were created. Space groups of the modulated structures are subgroups of the  parent phase space group. Further relaxation of the modulated structure increased the  symmetry of the modulated structure to the symmetry of either the α-phase or the ω-phase.  Hence, the space group of the modulated structure is also a subgroup of the space group of  the α/ω-phase. The relaxation process accommodated the transformation strains and  reduced the energy. An unstable N−4phonon can transform the β-phase to an intermediate  phase with an orthorhombic structure (either Cmcm or Pnnm), which relaxes to the α phase on structure optimization. Similarly, an unstable Λ1 phonon can transform the β phase to an intermediate phase with a trigonal structure (either P3̅m1 or P3m1), which  relaxes to the ω-phase on structure optimization. The four transition paths do not have any  activation energy barrier, and the energy is minimized along the path. 

Bio

Dr. Appala Naidu Gandi received his B.Tech. in Metallurgical Engineering from  the National Institute of Technology, Warangal, in 2005. He completed his M.E. in Materials  Engineering from the Indian Institute of Science, Bangalore, in 2007. He obtained his PhD in  Materials Science and Engineering from Imperial College, London, for his work on  “Martensitic Transformations in TiNi Shape Memory Alloys using First Principles  Calculations.” After that, he completed his postdoctoral studies focusing on thermoelectric  transport in a few semiconductors in the Physical Science and Engineering Division at King  Abdullah University of Science and Technology, Saudi Arabia. In 2017, he joined as an  Assistant Professor in the Department of Metallurgical and Materials Engineering at Indian  Institute of Technology Jodhpur. Presently, he is working as an Associate Professor. His  research group uses the first-principles calculations to study lattice dynamics,  thermoelectric transport, mechanical behavior, Raman and Infrared spectroscopy, and Li ion batteries.