P. Bassani, CNR IENI Istituto per l'Energetica e le Interfasi, Lecco, Italy; A. Tuissi, CNR IENI, LECCO, Italy; P. Giuliani, CNR IENI Istituto per l'Energetica e le Interfasi, Milano, Italy; C. Zanotti, Consiglio Nazionale delle Ricerche, Milano, Italy
Self-propagating High temperature Synthesis (SHS) was proposed as a suitable route for the production of porous NiTi alloys that show promising potential in biomedical applications.
Samples obtained from relatively large powders (<150 mm), with total porosity in the range 30-55%, were characterized mainly from a morphological point of view. Total porosity, as well as pore size, shape and distribution were analyzed. Sample microstructure was also investigated indicating that the main phase produced during the SHS reaction is Ti reach NiTi phase, as confirmed by DSC calorimetric analyses.
Moreover, the present of secondary phases, suggested by the low transformation enthalpy, was confirmed by SEM observations. Infact, EDS microanalyses and EBSD mapping, helped in the identification of such secondary phases, such as Ni3Ti, Ti2Niand Ti4Ni2Ox .
Other samples were successively produced starting from the same powders but introducing a different powder compression methodology and operating conditions. In this way, the obtained samples showed higher porosity featured by more uniform size, shape and distribution while from a micro-structural point of view no significant differences were observed.
Mechanical compression tests were carried out at room temperature and selected sample also above Af in order to highlights the influence of pore shape and distribution. Results, obtained at room temperature, show that the mechanical properties decrease with the porosity augmentation. For higher temperatures the sample presented a pseudoelastic behavior.
Dilatometric tests were performed on selected porous samples and the results well indicated the martensite to austenite transformation at the same temperature showed by the DSC analysis.
Summary: Porous NiTi samples were prepared via Self-propagating High temperature Synthesis (SHS), using relatively large powders (<150ƒÝm). Specimens with total porosity in the range 20-55% were obtained: changing operating conditions it was also possible to vary size, shape and distribution of pores.
Specimens were characterized through DSC analyses, SEM observations and dilatometric tests. Mechanical compression tests at different temperatures were also performed on selected samples.
