Ni-Free Shape Memory Alloys (SMA) for Biomedical Application

Abstract

Shape memory alloy (SMA) as its name has proposed, is an alloy which remembers its original shape after deformation. It is done by the reversible- martensitic transformation from its parent phase. By different recovery methods, two different properties can be classified. One is the Shape Memory Effect (SME) and the other is Superelasticity (SE). The recovery of shape by SME is to heat the sample to a temperature higher than the Reverse Martenistic transformation finish temperature, also name as austenitic finish temperature. For the recovery of SE is by the release of externally imposed load. Biomedical application is one of the applications using the properties of SMA. Examples of biomedical application include the self-expanding stents, orthodontic archwires and bone implant. The most common SMA for biomedical application nowadays is the Nitinol. However Nickel hypersensitivity, cytotoxic and long term effect had led to some concern about the more extensive use of Nitinol. Hence Nickel free SMA had attracted particular interest in the past decade. The Cold rolled Titanium-Niobium-Tin (Ti-Nb-Sn) SMA with 3 different compositions; Ti-16at%Nb-1at%Sn, Ti-12at%Nb-4at%Sn and Ti-16at%Nb-8at%Sn were studied. This investigation focused on the phase transformation and hardness evolution with respect to rolling. α-phase was found in all three compositions and β-phase only in the Ti-16at%Nb-8at%Sn alloy using X-ray Diffraction. From the hardness test and the degree of cracking during the cold rolling, addition of Tin was found have effectively increased the hardness of alloys. The composition of Ti-16at%Nb-1at%Sn was taken for further studies with various heat treatments. β-phase nucleated at temperatures above 850oC. Hardness can be increased by heating at room temperature higher than 950oC. This may be due to the formation of β-phase which acts as precipitation hardening. The samples of first heated by 850oC for 30 minutes were used in subsequent investigation concerning the effect of second heat treatment, with maximum hardness obtained at 550oC for 5 minutes. The Ti-16at%Nb-1%Sn cold rolled sample and first heat treated by 950oC for 1 hour after cold rolled sample were taken for the further characterized by Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA) and 3-point cyclic bending test. The characteristic (transformation) temperature cannot be found in the range of -100oC to 500oC for heat treated Ti-16at%Nb-1at%Sn sample using DSC and DMA. A very small amount of SE, 0.5% was observed at room temperature in the 3-point cyclic bending test for both cold rolled and heat treated samples. The yield strain of the cold rolled sample is higher than most common metals and alloys. Heat treatment was found to have effectively increased the elastic modulus and decrease the yield point, causing a loss of elasticity. Ti-16at%Nb-1at%Sn was found not appropriate for biomedical application due to its high characteristic temperature and too small the amount of recovery by SE at room temperature. Higher atomic percentage of Niobium or Tin should be used to stabilize the β phase. Although this alloy is not suitable for biomedical application such as bond replacement, it can be used in other the application as a high temperature SMA.