The microstructure and mechanical properties of reaction synthesized Ni-Al intermetallic sheets by pure Ni/Al foils with different thickness ratios

Abstract

The Ni-Al intermetallic sheets were prepared by the reaction synthesis of pure Ni and Al foils with different thickness ratios. The first stage was performed at 640 °C/20 MPa/6 h to consume Al layers, and the second stage was performed at 1200 °C/20 MPa/1 h to complete the synthesis of Ni-Al intermetallic sheets. The thickness ratios of original Ni and Al foils strongly affected the microstructure and mechanical properties of Ni-Al intermetallic sheets. When Ni/Al thickness ratio was 1:1.5 (R1:1.5), the microstructure was single-phase NiAl with bimodal grain structure, and it transformed into coarse grains dominated single-phase NiAl structure when Ni/Al thickness ratio was 1:1 (R1:1). The microstructures changed to NiAl+Ni3Al+Ni when the ratios were 1:0.63 (R1:0.63) and 1:0.25 (R1:0.25). At ambient temperature, Samples R1:0.25 and R1:0.63 had high strength, while Samples R1:1 and R1:1.5 showed serious brittleness. At high temperature, Sample R1:1 showed preferable ductility with the elongation of 61.3%, 71.4% and 101.3% at 800 °C, 900 °C and 1000 °C, respectively; Sample R1:0.63 showed the highest tensile strength, with the values of 350.3 MPa, 230.5 MPa and 140.2 MPa at 800 °C, 900 °C and 1000 °C, respectively. For the fracture at ambient temperature, Sample R1:0.25 showed a mixed fracture mode of dimple fracture and quasi-cleavage fracture; the fracture mode of Sample R1:0.63 was mainly cleavage fracture; Sample R1:1 showed a typical brittle cleavage fracture with a large number of river-like patterns; the fracture mode of Sample R1:1.5 was brittle transgranular fracture for coarse grain layers (CGLs) and intergranular fracture for fine grain layers (FGLs). At high temperature, Samples R1:1.5 and R1:1 were mixed fracture modes of dimple fracture and quasi-cleavage fracture; the fracture mode of Sample R1:0.63 was mixed quasi-cleavage and cleavage fracture; the fracture mode of Sample R1:0.25 was single cleavage fracture with microcracks at the NiAl layers.