Characteristics Of Tini Shape Memory Alloy Fracture

Shape Memory Alloys have been recognized as one of the most popular structural and functional materials in recent years, they have been used for various applications because of their unique shape memory effect and superelasticity. Near-equiatom Ti-Ni alloys have been widely used for medical applications after Fe-Cr-Ni, Co-Cr, Ti-6Al~4V alloys because of their good wear resistance, corrosion resistance, excellent bio-hemocompa -tibility and high damping capability. There have some papers on wear resistance, corrosion resistance and bio-hemocompatibility of Ti-Ni SMA, hut only rarely investigations on fracture of Ti-Ni SMA have been reported up to now. In order to improve the structure design and quality of products, the fracture characterization occurred in manufacturing and application of Ti-Ni SMA as medical devices have been studied in this paper. Wear resistance and corrosion resistance of Ti-Ni alloys and quenching and tempering steel 45 have been tested. The testing results show that Ti-Ni SMA have good wear resistance, this is different from common metals. They have good corrosion resistance in air, sea-water (Cl), oxidaz -ing acid to compare with quenching steel 45. Although their corrosion resistance is lower than quenching steel 45 in sulphuric acid because of bad reductive acid resistance. In the final analysis, their corrosion resistance is better than quenching steel 45 in many media.Sensibility on notch, fracture toughness, rupture tendency and impact processing with energy absorbing of nitinol have been investigated also. The testing results show:the Ti-Ni alloy sheets are 1.68mm in thickness, the tensile strength of Ti-Ni alloys is 74% and the elongation of Ti-Ni alloys is 52. 5% of no notch when the notched radius is 2. Omm, the tensile strength of Ti-Ni alloys is only 42% and the elongation is 18.5% of no notch when the notched radius is reduced to 0. 5mm;The notched radiusis 2.0mm, the tensile strength of Ti-Ni alloys is 84% and the elongation is 56% of no notch when the thickness of the Ti-Ni alloy sheets is 1.68mm, the tensile strength is 74% and the elongation is 52.5% of no notch when the thickness is 2. 16mm, the tensile strength is only 51% and the elongation is 38% of no notch when the thickness is 3. 18mm. Therefore, Ti-Ni alloys are very sensible to notched radius,the tensile strength and the elongation of Ti-Ni alloys are evidently reduced whether the thickness increases or the notched radius reduces. They show low fracture toughness (K1cmzx= 52.6MPa .m1/2 ) whether it is in parent phase or in martensite phase.They also show low impact toughness of notched specimen(5. 24J~B. 99J), the calculated critical dimension of the crack(ac) i s about: 0.7340mm(σb = 800MPa) ~0. 4698 mm(σb = 1000MPa), Therefore, some fracture phenomena in manufacturing and working process of products may be explained, some improved measures have been given on the basis of the results. For example, the surface finish of the products should be improved , the transition contour radius of the products should be enlarged to avoid sharp transition and so on. It will direct to improve the structure design and quality of products.