Effect of the precipitates on the thermodynamics of the martensitic transformations in T i -rich N i - T i - C o thin films

ISOLA, L.M.; MALVASIO, B.F.; GIORDANA, M.F.; MALARRIA, J.

JOURNAL OF ALLOYS AND COMPOUNDS

ELSEVIER SCIENCE SA

Año: 2020 vol. 818

0925-8388

The wide-spread multipletransformation in Ti -rich Ni-Ti shape memory alloys produced by Ti2Ni nano-precipitates was investigated. For thispurpose, a series of amorphous Ti-Ni-Co thin films with the same composition were subjected todifferent annealing treatments. The characteristic temperatures and heat of thetransformations were measured by four-point probe resistivity and differentialscanning calorimetry. A critical review of the thermodynamics ofthermally-induced thermoelastic martensitic transformations is presented. Thevalues of the frictional work and the stored elastic energy associated with thethermoelastic transformations were assessed from the transformationtemperatures, and related with the transformation heats measured in the samplesannealed at different temperatures. It was observed that the stored elasticenergy decreases as the annealing temperature rises due to a change in the distributionof the Ti2Ni precipitates, in concordance with an increase of thetransformation heat measured by the calorimeter. Nevertheless, the values ofthe frictional work were independent of the annealing temperature. Theinfluence of the dissipation of the stored elastic energy on the transformation temperatures and thermal hysteresisis discussed. Finally, a better understanding of the wide-spread multipletransformation, as an effect of the local stresses introduced by Ti2Ni nano-precipitatesin the matrix, is proposed.