Mechanisms for manufacturing NiAl thin-walled spherical shell by combining superplastic forming of Ni/Ni₂Al₃ heterogeneous bimodal-grained composite sheet with subsequent in-situ reaction

As a novel lightweight and heat-resisting alloy, NiAl intermetallic compound has become the best candidate for replacing Ni-based high-temperature alloy to manufacture thin-walled component. However, NiAl intermetallic compound is characterized by intrinsic brittleness, so it fails to be made into sheet to be directly used for forming complex thin-walled component. A novel approach of “fabrication-forming-reaction” is proposed to NiAl thin-walled spherical shell in the present work. Firstly, Ni/Ni₂Al₃ composite sheet with heterogeneous bimodal grains containing coarse-grained Ni layer and fine-grained Ni₂Al₃ layer is fabricated by first-order reaction between Ni foil and Al foil and it exhibits a certain superplasticity at the temperatures of 700–850 ℃ in the case of 1 × 10⁻³ s⁻¹, where the maximum elongation to fracture achieves to 348 %. Then, Ni/Ni₂Al₃ composite sheet is made into thin-walled spherical shell by superplastic forming at 750 ℃. Finally, NiAl thin-walled spherical shell is manufactured by subsequent in-situ reaction of Ni/Ni₂Al₃ thin-walled spherical shell and it is completely composed of homogeneous bimodal grains. The current work contributes to the fundamental advancement in superplastic deformation mechanism of Ni/Ni₂Al₃ composite sheet with heterogeneous bimodal grains as well as manufacturing NiAl thin-walled components with complex shape.