Anomalies of Thermal Expansion/Contraction of Martensite Crystal Lattices in Ti–Ni and Ti–Nb–Zr Alloys

The time-temperature changes of martensite lattice parameters (MLPs) of Ti–50.26Ni and Ti–18Zr–14Nb (at%) shape memory alloys have been investigated using X-ray diffraction methods both in situ and ex situ in order to verify the existence of the time dependence of MLPs and to ascertain the extent to which the correct syngony of the martensite lattice is preserved when its parameters change during heating and cooling at temperatures ranging from –180°C to ≥ A_s. The reversibility of MLP alterations was observed across the entirety of the investigated temperature range, when subjected to disparate combinations of heating and cooling rates (ranging from 0.03 to > 50°C/s). The MLP values remain constant regardless of the duration of X-ray diffraction imaging or holding at a given temperature within the martensite existence interval. The width of the X-ray lines of B19' and α" martensite remains constant regardless of the heating and cooling rates and holding times, which suggests the absence of martensite lattice distortions akin to those observed in premartensitic materials, leading to reversible broadening of X-ray lines of austenite with approaching the М_s point in the area of formation of nanodomains of intermediate shear structure. The Fisher criterion (F), nowhere exceeding its critical value, in conjunction with the unaltered width of the X-ray lines, suggests that the undistorted lattice syngony of unconverted martensite crystals has been preserved and that the lattice as a whole has undergone gradual homogeneous shear as the reverse transformation was approached.