SFB270 Kolloquium talk by Prof. Dr. Kaustubh R.S. Priolkar

Non-ergodicity, shell ferromagnetism, etc., are caused due to impurity doping in martensitic alloys. Dopants form nanoscopic point defects within the host matrix and introduce new interactions, often breaking the ergodic behaviour. An increase in the concentration of the dopants, or temper annealing, facilitates the segregation of these point defects and leads to a phase separation. However, the segregation process of these defect phases is not clearly understood. We attempt to understand these behaviours by studying the local structures of the host atoms and impurities doped in some martensitic alloys using X-ray absorption fine structure (XAFS) spectroscopy. In Ni50+xTi50-x alloys, we show that the ergodicity is interrupted by the formation of bcc Ni defects within the martensitic B19’ NiTi. With the increase in Ni concentration, the defect phase segregates and phase separates as fcc Ni. A similar observation is noted in the martensitic Ni0.50Mn0.375In0.125 doped with Fe at the Mn site. Here, the long-range order of the elastic strain vector is obstructed by the formation of the γ-FeNi phase.
On the other hand, in indium-doped NiMn, the In atoms are accommodated within the lattice by a structural ordering and a conversion from B2 to L2¬1 structure. Using XAFS, we demonstrate that point defects are segregated even in such structurally single-phase alloys, in which phases separate upon temper annealing. Further extension of this work to other Mn-rich Heuslers will also be discussed.