Premartensitic Phenomena in Ti₄₉Ni₅₁ Single Crystal

¹ Institute of Strength Physics and Materials Science, SB RAS, 2/1 Academichesky Av., Tomsk 634021,
² Institute of Metal Physics, Ural Division, RAS, 18 Kovalevskoi Str., Ekaterinburg 620219, Russia

Abstract

The evolution of microstructure in the bulk of a massive Ti4 9 Ni51 single crystal has been studied above the start temperature of martensitic transformations (MT) B2→R and B2→B19' (T and Ms, correspondingly).The intermediate shear of a first type (ISS-1) forms as a result of the second kind phase transformation. It is characterized by the appearance of transverse atomic displacement waves with incommensurate wave vectors q̅'₁ =(1/3±δ)<110>2π/a and q̅''₁= (l/3±δ)<lll>2π/a in the parent crystal lattice within the temperature range 55 degrees before T . ISS-1 becomes a commensurate phase at cooling and 25 degrees before T . The stretched diffuse extra-reflections of (1/2±δ)<110> type appear on (110)_B2 neutron diffraction patterns before the beginning of MT B2→B19' (Ms = 180 K) at cooling of samples below 420 K. It is caused by the development of a short-range order and the long-range order of transverse atomic displacement waves with incommensurate wave vectors q̅₂=(l/2±δ)<110>2π/a. In this case atomic displacement waves with q̅'₁, and q̅''₁ are also developing but it is observed at more lower temperatures.