Microstructural characterisation and mechanical response of laminated Ni-intermetallic composites synthesised using Ni sheets and Al foils

The laminated Ni–(Ni2Al3 + NiAl3) and Ni–(Ni3Al + NiAl) composites have been successfully fabricated by reaction synthesis in vacuum using 400 μm thick Ni sheets and 150 μm thick Al foils. The aluminium layers were completely consumed due to the formation of intermetallics. The final microstructures consisted of alternating layers of intermetallic phases and unreacted nickel could be designed easily because the structures of the composites depend on the treating time and temperature. Reaction synthesis at 620 °C for 2 h resulted in a microstructure consisting of Ni and Ni2Al3 + NiAl3 layers. At room temperature the Ni–(Ni2Al3 + NiAl3) laminated composites had an ultimate tensile strength of 615 MPa and an elongation of 10%. The fracture behaviour of the composites exhibited a mixture of brittle fracture of Ni2Al3 + NiAl3 intermetallics and ductile one of Ni layers. Delamination that occurred in the middle of the intermetallic layers was caused by the presence of continuous Al2O3 inclusions. The next thermal aging cycle was employed to change the microstructure of intermetallic layers and to improve mechanical properties of the laminated composites. Heat treatment at 1150 °C for 4 h resulted in a composite comprising Ni and Ni3Al + NiAl layers. The Ni–(Ni3Al + NiAl) laminated composites had an ultimate tensile strength of 875 MPa and an elongation of 24%. The dislocations in the Ni layers could easily slide through the coherent Ni/Ni3Al interfaces, so Ni3Al layers co-operatively deformed with the Ni layers during tensile testing. Debonded Ni3Al/NiAl interfaces containing spherical Al2O3 inclusions further encouraged the capability of the Ni3Al layers for plastic deformation. As a consequence, the Ni–(Ni3Al + NiAl) laminated composites at room temperature exhibited significant strain-hardening, a good tensile strength and a high ductility. The effect of temperature on the tensile properties and deformation behaviour of the laminated Ni–(Ni2Al3 + NiAl3) and Ni–(Ni3Al + NiAl) composites have been also investigated. With increasing tensile test temperature from room to 700 °C, the ultimate tensile strength decreased gradually for both types of composites. The yield strength of the Ni–(Ni2Al3 + NiAl3) laminated composites decreased and elongation increased with increasing temperature. The abnormal strengthening of the Ni3Al phase in the Ni–(Ni3Al + NiAl) composites led to a gradual increase of the yield strength and the fracture elongation decrease with increasing the tensile test temperature.