Nucleation of R-phase as studied by dynamic Young's modulus in the Ni₄₉Ti₅₁, aged alloy

Abstract
The Young's modulus E and the internal friction Q^-1 have been measured during thermal cycles carried out at temperatures higher than the start temperature R_s (R_s = 335 K) of the B2 → R martensitic transition in the Ni₄₉Ti₅₁ alloy, prior cold-worked (ε = 32 %) and then aged at 673 K for 30 minutes. The cooling/heating E(T) and Q^-1(T) branches of cycles extending down to the temperatures 350, 340 and 336 K exhibit progressively increasing thermal hysteresis, while those referring to a cycle only extending down to 369 K does not. This thermal hysteresis is attributed to the formation of stable nuclei of R martensite at dislocation or at some other type of lattice defects.