Intermetallic Compound Formation in Ni-Ti-Fe Powder Mixtures

Abstract: The Ni-Ti system has a broad range of applications, in areas as diverse as biomedicine and robotics. This is in large due to the NiTi intermetallic compound, which exhibits the shape memory effect. In addition to this property, the intermetallic compound is also biocompatible, has good mechanical properties and good corrosion resistance. Studies on NiTi has shown that the temperatures at which the shape memory effect occurs are highly sensitive to the exact composition of the non-stoichiometric NiTi. One method of controlling this sensitivity is to add iron as a third element to the system. The ternary Ni-Ti-Fe system has mainly been investigated for low iron contents, as the focus has been on the NiTi shape memory effect, and how to improve it. The effect of iron on the other intermetallic compounds present in the Ni Ti system and the possible formation of intermetallic compounds containing iron has not been widely investigated. In this project the phase evolution in the ternary Ni-Ti-Fe system has been investigated. Elemental powder mixtures of nickel, titanium and iron were prepared by adding 0-20 at.% Fe to equiatomic Ni-Ti. The powders were compacted into discs and sintered. Differential scanning calorimetry was used to study the sintering process, samples of all compositions were heated to 1200 °C and then cooled back down to room temperature. Two compositions, containing 6 at.% and 10 at.% Fe, were chosen for further study of the phase evolution, and samples of these two compositions were heated to several temperatures below 1200 °C. The microstructure of the samples were studied using scanning electron microscopy. Energy-dispersive spectroscopy was used in conjunction with the microscopy to study the distribution of the nickel, titanium and iron in the samples. X-ray diffraction was used to identify the phases present in each sample. Iron was shown to affect the intermetallic compound formation in the samples by acting as a substitute for nickel in (Fe,Ni)Ti2 and (Fe,Ni)Ti. Therefore, allowing for an increased amount of Ni rich compounds to form in the samples with higher iron content. These compounds were Ni3Ti and the metastable Ni4Ti3. The onset temperatures for two exothermic peaks in the DSC curves showed linear dependence on the iron content in the samples. The temperatures associated with the β Ti + (Fe,Ni)Ti2 → L and (Fe,Ni)Ti2 → (Fe,Ni)Ti + L reactions are proposed to depend on the ratio of iron to nickel in (Fe,Ni)Ti2.