Study Of Iron Aluminides Intermetallic-Tetragonal Zirconia Ceramic Matrix Composite

Tetragonal ZrO₂ (TZP) possesses the most excellent mechanical properties among ceramics. However, The intrinsic low fracture toughness of ceramic, mechanical properties dramatically decrease with increasing temperature due to the t-ZrO₂ to m-ZrO₂ transformation toughening effect decreasing at higher temperature with increasing of t-ZrO₂ phase stability, poor thermal shock resistance induced by high thermal expansion coefficient and low thermal conductivity as well as the severe strength degradation after exposure to low temperature, have weaken the predominance competing with metal, and thus largely limited the actual application of TZP ceramics. Various methods have been applied to improve the properties, and composite technique that secondary reinforcing phase with high modules and strength such as whiskers, particles, platelets are dispersed in matrix, have improved the mid and high temperature properties, unfortunately, the room temperature properties declined due to the high stress in matrix producing by the thermal mismatch. Exploiting a new approach to improve the properties of TZP is significant for widening its actual application.Fe₃Al, an intermetallic comparing to 'semi-ceramic material', with more excellent high temperature strength, high temperature creep resistance and oxidation resistance compared with most metal, and with higher fracture toughness compared with ceramics. Its higher fracture toughness and thermal conductivity, similar thermal expansion coefficient compared with monolith ZrO₂ make it possible to improve the fracture toughness as well as the thermal shock resistance. According to above characteristic of Fe₃Al, a novel composite—ZrO₂ (3Y)/Fe₃Al has been suggested in this thesis, and the preparation technology, microstructure, mechanical properties and toughening mechanism were investigated. The following works haven been developed.The chemically compatibility of iron aluminide intermetallic with zirconia matrix were analyzed by thermodynamic theory to predict the chemical reaction between matrix and toughening phase, and thus to provide the academic evidence depend on which to choose the appropriate Fe-Al intermetallic and preparation technology to depress baneful interface reaction. The results show that the chemical interaction between iron aluminide intermetallic and zirconia matrix wouldn't take place when Al <42at%, and interface reaction production, Al₂O₃ and ZrAl₂ will produced when Al>42at%. From this point, Fe₃Al is appropriate toughened phase for ZrO₂ matrix.Nano-Fe₃Al powders were prepared by mechanical alloying technology. The structural evolution of Fe-Al elemental powders during mechanical alloying process under the protection of argon atmosphere and low temperature annealing process, and the microstructure and mechanical properties of bulk Fe₃Al have been investigated by means of XRD, SEM, TEM and DSC. The results show that disordered a-Fe solid solution formed during milling. The reduction of crystal size and the increase of microstrain exist simultaneity, and the microstrain increased with the increasing of mill time. The disorder a-Fe solid solution milled for 50h translates to order DO₃ structure through Al atom order rearrangement and removing of APS domains during annealing at 800 ℃. The bulk Fe₃Al were prepared by hot-pressed vacuum sintering. The room compressive yield strength, compressive yield strain and hardness (HRC)were 1900 MPa, 14% and 61 respectively. The room bending strength and fracture toughness were 1300 MPa and 49 MPam1/2. The elevated room mechanical properties result from the effects of fine grain and homogeneous structure effects. The successful preparing of powder and bulk are important for the preparing of ZrO2 (3 Y)/Fe3Al composite.From both toughening and reinforcing viewpoint, sub-micron ZrO2 (3Y) and micron Fe3Al are selected as matrix and toughen phase respectively. ZrO2 (3Y)/Fe3Al composites were prepared by hot-pressed vacuum sintering. Densification behavior of ZrO2(3Y)/Fe3Al composites were investigated under hot pressing. The results show that the grain growth rate of Fe3Al is higher than that of ZrC>2. The apparent activity energies of grain growth are 212KJ/mol and 250KJ/mol for Fe3Al and ZrO2 respectively. In the stage of hot pressing, the mass and quick deformation of specimens occur in 20min from the beginning of putted on pressure, after that, deformation rate increase slightly with the increasing of holding pressure time. The quick densification of specimens occur when the liquid formed at 1340°C. The slippage between particles become easy due to the decreasing viscosity, and thus accelerates the process of densification. The relatively density of composites reach nearly 100% under hot pressing sintering at 1350°C, holding 30min.The fracture toughness and bending strength of ZrO2(3Y)/Fe3Al composites increase with the increasing of Fe3Al content. The au KiC and hardness are 1321MPa, 36MPa ? m1/2 and 88HRA respectively when Fe3Al content is 40vol%. The K\c value of composite is 2.6 times as that of ZrO2(3Y), and o( increase 29% compared with ZrO2(3Y). The hardness of composites decline slightly with the increasing of Fe3Al content due to the pin effect of interface and reinforcement effect brought about by intragranular ZrO2.The influence of Fe3Al content on the phase stability of t-ZrO2 was investigated by XRD. Due to the compatible effect of dropped elastic module by addition Fe3Al and the thermal mismatch between matrix and toughening phase, which induced radial compressive stress of ZrO2, the phase stability of t-ZrO2 decrease with the increasing of Fe3Al content. The degressive degree of the phase stability of t-ZrO2 is not obviously, the t-ZrO2 content decrease from 91% for ZrO2 to 85% for composites.The microstructures of composites were investigated by SEM, TEM, EDS and HREM. Most of the ZrO2 and Fe3Al grains had an equiaxed morphology and that part of the Fe3Al was clubbed. The average grain sizes of the ZrO2 and the Fe3Al were 350 nm and lum, respectively. ZrO2 and A12O3 particles were observed in Fe3Al grain. The order degree of Fe-Al intermetallics is not uniform, most of Fe-Al intermetallic with DO3 structure, where part with B2 structure. The HREM image of ZrO2-Fe3Al interface along the [001] Fe3Ai//[0H] Z102projection shows that the interfaces are very clean, and no reaction phase existed at the interface. The atom arrangement beside interface between Fe3Al and ZrO2 is partly corresponding, existing at semicoherent state. The mismatch degree 5 of spacing between planes in crystal is 30%, and dislocation induced by mismatch is observed at interface. Due to the discrepancy of thermal expansion coefficient between ZrO2 and Fe3Al is small, accordingly, the residual thermal stress in interface is small, and the residual strains layer in interface, which always observed in mostmetal-ceramics composites, are not observed in ZrO2 (3Y)/Fe3Al composites, and this is help for the obtain of high strength. Radial dislocations produced by thermal mismatch were observed around intragranular ZrO2 particles in Fe3Al. The HREM image indicates that intragranular ZrO2 have not orientation dependence with Fe3Al, the interface of intragranular ZrO2-Fe3Al is non-coherent interface.Base on known theories on phase transformation toughening and bridging toughening, the R-curve of ZrO2 (3Y)/Fe3Al composites was obtained by using indentation crack growth method, and the crack growth resistance behavior were quantificationally investigated through measuring the phase transformation zone height (h) and monoclinic amount in fracture surfaces (Vf) by Raman spectra and XRD respectively. The results show that phase transformation zone height (h) and monoclinic amount in fracture surfaces (Vf) increase with the increasing of Fe3Al content, indicating the elevated phase transformation toughening effect. Due to the compatible effect of stress-induced transformation toughening and crack bridging, ZrO2(3Y)/Fe3Al composites have more obvious R curve behavior than monolith ZrO2. The figure of R curve of ZrO2 (3Y)/Fe3Al composites is steeper, and the X is bigger than those of monolith ZrO2, indicating that ZrO2 (3Y)/Fe3Al composites process higher security in actual application than monolith ZrO2. From the results of the quantificationally analysis of toughening effects, electron density calculation as well as observation of interface and fracture surface by HREM and SEM, the following conclusions can be drawn: the appropriate and favourable interface bonding is the main reason for the consistent between theoretical predict and measured values.Thermal shock resistance behavior of monoclinic ZrO2(3Y)and ZrO2 (3 YyFe^l composites have been investigated by residual strength method and indentation-quench technique respectivly. The interrelation between thermal shock resistance, property parameter and R-curve behavior have been investigated by the calculating of thermal shock resistance parameter (/?> R'and R"" ) . The pattern of indentation crack growth under thermal shock was also investigated. The results show that the critical temperature difference (ATc) increased from 220°C for ZrO2(3Y) to 450°C for ZrO2(3Y)/30vol%Fe3Al composite. The higher fracture toughness, thermal conductivity and lower elastic modulus and Poisson's ratio of ZrO2 (3Y)/Fe3Al composite compared with monolith ZrO2 can relax the thermal stress, and absorb more elastic strain energy without fracture, and then the thermal shock fracture resistance improved .The higher fracture toughness will increase the barriers of crack propagation, and then the thermal shock damage resistance improved.ZrO2 (3Y)/Fe3Al exhibit an improved mechanical strength at mid and high temperature. The bending strength of ZrO2(3Y)/30vol%Fe3Al composite up to 670MPa at 800°C.This should be due to not only the retained crack bridging effect of Fe3Al before 800 °C,which offsetting the decline of phase transformation at increasing temperature, but also the decreasing of restricted stress suppressing phase transformation induced by releasing of residual thermal redial compress stress effect at ZrO2 before 800 °C.Summarizing the above results, the preparation, mechanical property and microstructure of the ZrO2 (3Y)/Fe3Al composite are first reported in this paper internationally. Above studies also provide the firmly theoretical foundation for the practical application of it. The ZrO2 (3Y)/Fe3Al composite can be...