The Fabrication Of Iron And Nickel-based Metal-Intermetallic-Laminate Composites And Study On Microstructure And Properties Of Intermetallic Layer

Metal-Intermetallic-Laminate (MIL) is designed to mimic the mollusk shells structure, which optimizes the unique properties and benefits of the constituent components by incorporating the high strength and stiffness of the intermetallic phase with the high toughness of the residual metal. Multi-layer effects along with the micro-laminate structure generate a zone of bridging ligaments, which restrict crack opening and growth by generating closure tractions in the crack wake, and utilize the work of plastic deformation in the ductile metal phase to increase fracture resistance of the composite The lower density, higher specific strength and modules of the MIL composites make them an attractive material for protection structural applications, such as protective materials of high-speed collision in airspace or ballistic armor for tanks.The most widely researched MIL composites are based on the AI-Ti system. There is only one brittle intermetallic phase Al3Ti formed in Ti-based MIL composites which are harmful to restrict the growth of the cracks and always causes locally stress concentration at the interface area of the metal/intermetallic layer. It is therefore desirable to fabricate similar MIL composites with multiple intermetallic layers and second phases for enhanced toughening. The SEM, XRD, EBSD and microhardness tests are applied to study the microstrucrure evolution, phase identification, growth kinetics and microhardness distribution of the iron-based and Ni-based MIL composites.The main conclusions are listed below:1. The intermetallic layer formed in the Al/430-SS reaction are mainly composed of Fe2Al5 phase. The eutectic structure with good toughness is formed during the semisolid reactions. The Cr-rich particles precipitate along the uniform/eutectic layer toughens the matrix of the uniform layer. There are Cr-rich and Fe-rich primary phases precipitate in the Al melt of the solid-liquid reaction of Al/430-SS, which will separate the continuity microstrucrure of centerline area of the 430-SS MIL. The semisolid reaction is proved to be the perfect processing to fabricate the iron-based MIL composites.2. Iron-based MIL (pure Fe-based,430-SS based,304-SS based) are unique in that they have multilayer structure along with lower cost of the initial foils. The main phase formed in the Al/Fe reaction is Fe2Al5. The addition of Cr and Ni can extend the volume fraction of the eutectic layer of the total thickness of the intermetallic layer and contribute to forming the precipitations and solid solution area. A single-phase intermetallic layer, with parabolic growth kinetics, is shown in the pure Ni/Al reaction, while a complex multi-phase layer (eutectic and uniform layer) with mixed growth kinetic mechanisms are identified in the Invar/Al reaction by EDS and EBSD analysis. The growth activation energies of Fe/Al. 430-SS/Al,304-SS/Al reactions are 150 KJ/mol.200 KJ/mol and 220 KJ/mol, respectively.3. A multilayer structure (namely Al-rich layer, transition layer and B2 phase layer) with a complex mix of intermetallic phases (FeAl2. Cr5Alg and FeAl) form in the. FeAl phase with B2 structure formed in the intermetallic layer of high temperature annealed 430-SS MIL composites was proved to smooth the micro-hardness distribution across the intermetallic/metal layer. The growth mechanism of the B2 phase layer is diffusion controlled with the effective diffusion constant of 3.8×10-13 m2/s which reflects the average diffusion rate across the concentration between 30 at.% to 50 at.%. The numerical calculation based on the concentration dependent diffusion coefficient is useful for estimating the growth kinetics of the intermetallic layers during high temperature annealing process and can precisely predict the Al concentration profile at the region where Fe-rich intermetallic phases formed.4. Multi-structure of intermetallic layer are obtained in the pure Ni, Invar and Inconel based MIL composites. The phases formed in the intermetallic layer are changed from the single Al3Ni, Al3Ni2 phase in pure Ni MIL to the multi-layer structure in Invar and Inconel alloy with eutectic layer and uniform layer, which is affected by the alloying of Cr and Fe in the Ni alloy. The eutectic layer perform good toughness which combined with the multi-interface effects are efficiently benefit for restricting the cracks grow in the Ni-based MIL. The Invar-based MIL composites are proved to have the best mechanical properties of the three studied composites.5. EHF theory along with a simple prediction rules based on solubility of the components are proved to be valid for predicting the phase formation in the eutectic layer of ternary or quaternary reactions for Ni-based and Fe-based MIL composites. The intermetallic yield of Ni-based and Fe-based MIL composites are calculated based on the phase identification results, which are shown to be reliable for predicting the final intermetallic layer thickness in these MIL composites. A microhardness map has been established based on the view of relating the process-microstructure-property of the MIL composites and can be used for predicting and tailoring the microstructure and microhardness distribution across the metal/intermetallic layer of 430-SS MIL composites.