Intermetallic Compound Formation in Ni-Ti-Fe Powder Mixtures
Bildning av intermetalliska föreningar i Ni-Ti-Fe pulverblandningar
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Summary, in English
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.
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.
Summary, in English
Smart materials are hugely useful. If their properties can be adjusted for the specific application. The shape memory metal nitinol can be created out of nickel and titanium. Its properties can be altered by also adding iron. The problem is that the knowledge of how iron affects the system is limited.
When a mixture of different elements are heated to sufficiently high temperatures, different chemical compounds can form. Depending on both the relative amount of each element, and the temperature, different compounds are more stable than others.
This applies to the powder mixtures of equal amount of nickel (Ni) and titanium (Ti) that was the starting point for this project. In this mixture several compounds can form. At temperatures up to about 940 °C NiTi2, NiTi and Ni3Ti were formed through diffusion. At 946 °C a strong reaction between the remaining titanium and NiTi2 takes place, it leads to the formation of more NiTi. At 1111 °C one more reaction starts, this time between NiTi and Ni3Ti. By adding iron (Fe) to the powder mixture you can affect the two reactions.
In this project up to 20 % iron was added to equal amounts of nickel and titanium. The higher the iron content, the weaker the first reaction between the free titanium and NiTi2 became. Also, the melting of NiTi2 was separated from the first reaction. The second reaction on the other hand grew in strength as the iron content was increased. Of great interest was the connection that emerged between the temperatures for the first reaction and for the melting of NiTi2. These temperatures increased linearly with the iron content. The first reaction, which in the sample without any iron happened at 946 °C, occurred in the sample with 20 % iron at 999 °C. An increase of over 50 °C!
It was made clear that the iron did not form any compounds with nickel or titanium. Instead the iron could replace the nickel in NiTi2 and in NiTi. Therefore the first reaction decreased in strength since there was less titanium left over when both iron and nickel could form compounds with it. In a similar way it is also the reason as to why the second reaction increased in strength since there was extra nickel left over to form more Ni3Ti.
For the interested, more information is available in Intermetallic Compound Formation in Ni-Ti-Fe Powder Mixtures.
When a mixture of different elements are heated to sufficiently high temperatures, different chemical compounds can form. Depending on both the relative amount of each element, and the temperature, different compounds are more stable than others.
This applies to the powder mixtures of equal amount of nickel (Ni) and titanium (Ti) that was the starting point for this project. In this mixture several compounds can form. At temperatures up to about 940 °C NiTi2, NiTi and Ni3Ti were formed through diffusion. At 946 °C a strong reaction between the remaining titanium and NiTi2 takes place, it leads to the formation of more NiTi. At 1111 °C one more reaction starts, this time between NiTi and Ni3Ti. By adding iron (Fe) to the powder mixture you can affect the two reactions.
In this project up to 20 % iron was added to equal amounts of nickel and titanium. The higher the iron content, the weaker the first reaction between the free titanium and NiTi2 became. Also, the melting of NiTi2 was separated from the first reaction. The second reaction on the other hand grew in strength as the iron content was increased. Of great interest was the connection that emerged between the temperatures for the first reaction and for the melting of NiTi2. These temperatures increased linearly with the iron content. The first reaction, which in the sample without any iron happened at 946 °C, occurred in the sample with 20 % iron at 999 °C. An increase of over 50 °C!
It was made clear that the iron did not form any compounds with nickel or titanium. Instead the iron could replace the nickel in NiTi2 and in NiTi. Therefore the first reaction decreased in strength since there was less titanium left over when both iron and nickel could form compounds with it. In a similar way it is also the reason as to why the second reaction increased in strength since there was extra nickel left over to form more Ni3Ti.
For the interested, more information is available in Intermetallic Compound Formation in Ni-Ti-Fe Powder Mixtures.
Department/s
- Materials Engineering
Publishing year
2018
Language
English
Publication/Series
Master's Thesis
Full text
- Available as PDF - 11 MB
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Document type
Student publication for Master's degree (two years)
Topic
- Technology and Engineering
Keywords
- Phase Evolution
- Ni-Ti-Fe System
- Intermetallics
- Powder Metallurgy
- Reactive Sintering
- DSC
- SEM
- EDS
- XRD
Supervisor
- Hossein Sina
ISBN/ISSN/Other
- ISRN LUTFD2/TFMT--18/5058--SE