Processing, microstructure and properties of laminated Ni-intermetallic composites synthesised using Ni sheets and Al foils

Abstract

The laminated Ni-(NiAl3H-Ni2Al3) and Ni-Ni2Al3 intermetallic composites were fabricated by reaction synthesis in vacuum using Ni sheets and Al foils. The aluminium layers were completely consumed due to the formation of intermetallic phases. The Ni-Al reaction at 62O°C was studied by interrupting in steps the reaction process to observe the microstructural changes. The final microstructure consisted of alternating layers of intermetallic phases and unreacted nickel can be designed easily because the stable structures of the composites depend only on the treating time. Microstructural examinations using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray microprobe analysis (EDX) demonstrated that after I h of treatment Ni2Al 3 is the predominant intermetallic phase. The formation of the Ni2Al3 phase is thermodynamically favoured over the formation of the other phases and can be understood from the steps occurred through a series of solid state reactions. The tensile strength of the laminated composites increases with an increase of the volume fraction of the intermetallic products. However, it decreases after long heat treatment because the Ni2Al3/Ni2AI3 interfaces can very easily delaminate due to a very weak bonding caused by continuous Al 2O3 inclusions. Observations show that the laminated composites exhibit a mixture of brittle fracture of intermetallics and ductile one of residual Ni layers.