Fabrication And Properties Of Carbon Nanotube/Iron Aluminides Intermetallic Matrix Composite

Carbon nanotubes (CNT) with the special hollow tube structure, extraordinary properties and potential application in nano-device and high performance nanophase materials are interesting to the researchers since it was discovered. They have a perfect structure and a very high aspect ratio (length-to-diameter ratio), as well as excellent mechanical property and good electrical property, so, they are considered to be the utmost type of fiber-like reinforcement, applications that benefit from the simultaneously exploitation of their numerous asset are very attractive. Carbon nanotube composites are an important branch of the application research of CNTs. In recent times, a significant research activity has been conducted on exploiting the strengthening and toughening ability of carbon nanotubes in polymer, ceramics matrix. To our known knowledge, very few works have been done considering intermetallics matrix. So it is creative for the investigation on CNT/intermetallics composites.We provide a short presentation of the progress on polymer; metal; ceramics reinforced by carbon nanotube, which focus on the currently fertile fields in CNT research science and technology. To takes full advantage of the high elastic modulus, high strength and excellent physical properties, and expand the potential of CNTs for applications in nanocomposite, the novel Fe₃Al composite reinforced by carbon nanotube is designed and fabricated. Carbon nanotubes are introduced into intermetallics, the formed composites would have high strength and toughness. In other hand, due to the good electrical property and magnetic property, the new composite maybe possess some functional properties. According to the philosophy mentioned above, the application of CNTs as reinforcement in intermetallic matrix with the aim of exploring the possible toughening ability of the CNTs is conducted. We systematic study the fabrication process, physical properties (mechanical properties, electrical properties and magnetic properties), microstructure, and strengthening mechanism of CNT/Fe₃Al composite.First the interface structure of CNT/Fe₃Al composite is designed theoretically; the crystallographic model of carbon nanotube is set up. According to the method provided by Empirical Electron Theory of Solid and Molecules, the valence electron structure is analyzed. Based on the model of interface electron structure, the interfacial electron structure is studied; the deviation continuning of electron density means the strengthening and toughening at high stress level. The chemically compatibility of iron aluminides with CNT are analysis by differential scanning calorimeter, no reactant is found. The ingredient on the interface is analyzed with the help of X-ray photoelectric Spectroscopy; the results show that when iron bonds aluminum, the inner shell bonding energy of iron decreases, the inner shell bonding energy of aluminum increases.Strain of iron element and aluminum element caused by mechanical alloying is calculated by two-atom model based on TFDC theory, which were 0.1142 and -0.1961, respectively. It illustrates that iron expands, aluminum shrinks. It agrees with the result of XRD well. This extends the two-atom model application in mechanical alloying fields.The structure, morphology and defects of CNTs are observed recurring to the SEM, TEM, and HREM. The results indicated that CNTs have high purity and aspect ratio (MOO). Its diameter is about 40⁶0nm and length about tens of micron. The CNTs are not straight and have some defects such kinks, bent, fracture and dislocation.Different fabrication technics such as hot press sintering on vacuum and spark plasma sintering are used to form the densely CNT/Fe₃Al composites. Densification behavior of CNT/Fe₃Al composites are studied in the spark plasma sintering process, the sintering curve shifts up due to the addition of CNT, the sintered velocity increased, namely the addition of CNT promotes the transfer mass in the densification process, the efficient increased. The apparent activation energy of grain growth is 169 kJ/mol, lower than that of hot press sintering process. This is attributed to the plasma effects on the surface of particle, decreased the apparent activation energy, facilitated diffusion, the growth velocity of grain is higher than that of common hot press sintering. The addition of carbon nanotube in iron aluminides matrix facilitates the sintering process, decreases sintering temperature of composites due to dual effects of the iron alumides powder spark and tip electrical effects of carbon nanotube.Based on the experimental results, the mechanical properties of composites reveal the largely enhanced toughness and yield strength by adding carbon nanotubes. The fracture toughness of composites increases with the increasing content of carbon nanotube. The toughness of the composites is 40 MPa·m^1/2 when the content of carbon nanotube is 3 vol%, 1.6 times higher than that of pure iron aluminides. This value is relatively high for structural materials; the increased range is foremost in the reported documents. Compared to the fracture toughness, the compressive yield strength of the composites also increase with the content of carbon nanotube, especially from the range of lvol% to 3vol%. Although the yield strength is decreased when the content of carbon nanotube is exceeded 3vol%, the yield strength is higher than that of pure iron aluminides. Observation and analysis on the microstructure of composites, it can be found that the dispersion of CNTs in composites is not homogeneous, which reduces the validity of reinforcement of CNTs to some extensive. Besides carbon nanotube, Al₂O₃ particles dispersed in the matrix due to the oxygen contamination. This is attributed to pullout and bridging effects of carbon nanotube in the composites. The matrix contains DO₃ structure of Fe₃Al primarily, exists the little B2 structure simultaneously. Carbon nanotube survived in the sintering process with hollow tube and multi-walled structure, resulting in the effective strengthening. The result shows that the good interfacial bonding is the intrinsic factor for improving the strength of composite. The possible toughness mechanisms are pull-out, bridging of carbon nanotube, crack deflect and cross-ply structure of carbon nanotube in composite.Further studies on magnetic properties of composites measured by alternating gradient force magnetometer (AGM) reveal the soft magnetic properties. The saturation magnetization and coercivity of 3% CNT/Fe₃Al composites is 103.59 emu/g, 24.32 Oe, respectively. The coercivity follows the cubic equation with the content of carbon nanotube. Based on the classical hysteresis theory, a new inclusion theory with the cylinder morphology is developed, combined with stress theory the expression of coercivity is given. The synergetic effect of inclusion theory and stress theory is the major hysteresis mechanism of CNT/Fe₃Al composite.The electrical properties of composites are investigated at room temperature. The conductivity is decreased with the content of carbon nanotube. The difference of work function existed in Fe₃Al and carbon nanotube, the grain boundary energy obstacle formed in the grain interface. It has negative effects on the transfer of carrier, resulting in the low conductivity. Additionally, the defects (bending or buckling) of carbon nanotube, the dispersion manners and the contact resistivity of carbon nanotube and so on had effects on the electron diffraction. Based on F-S theory and dual fluid model, the conductivity of composites is simulated. When the diffraction coefficient of grain boundary r goes to zero, the experimental results agree with simulated results well. It shows that grain boundary diffraction has the little effect on the resistivity of composites.Based on the results mentioned above, the fabrication process, physical property (mechanical, electrical, magnetic) and microstructure of CNT/Fe₃Al composites are investigated firstly; it will provide the firmly theoretical foundation for the practical application. The structural intermetallics with better magnetic property are significant and promising for use in devices working under severe conditions such as printing and dyeing industry as electric thermal alloys; communication, electric technology as soft magnetic materials with excellent physical properties.