The internal friction, Young's modulus and heat flow were measured in Ni50.8Ti49.2 as a function of temperature after aging, solutionizing and water-quenching. The internal friction of aged material at kHz frequencies exhibited, as well as hardly detectable peaks which were associated with the A ↔ R and R ↔ M transitions, 2 additional non-thermally activated peaks which appeared to be due to stress-induced hysteretic motions of twin boundaries within the martensite and the R-phase, respectively. In the solutionized state, a large peak which corresponded to the A ↔ M transition was observed as well as a thermally activated relaxation peak which occurred at about 100K (at a frequency of 1.2kHz). The activation energy and limit time of this peak were 0.11eV and 10-10s, respectively. One of the non-thermally activated peaks was not observed in the solutionized state. The thermally activated relaxation peak, which was found to increase with the number of thermal cycles through the transition region, was attributed to the stress-induced motion of dislocations around their equilibrium positions within the B19' structure of the martensite. The absence of a twin boundary peak in the martensite which formed from solutionized austenite, as well as a large difference in the strength of the transformation relaxations in the aged and solutionized states of the material, were accounted for in terms of the differing natures of the predominant twin boundaries in the 2 different states of the material.

Mechanical Spectroscopy and Twin Boundary Properties in a Ni50.8Ti49.2 Alloy. B.Coluzzi, A.Biscarini, R.Campanella, L.Trotta, G.Mazzolai, A.Tuissi, F.M.Mazzolai: Acta Materialia, 1999, 47[6], 1965-76