Deformation and fracture behavior of the novel NiAl alloy sheet with bimodal laminated structure by in-situ reaction synthesis

Due to the intrinsic brittleness of NiAl alloy, its sheets are difficult to fabricate with traditional casting and rolling methods. To solve this problem, a novel NiAl alloy sheet with a special bimodal laminated structure was successfully designed and fabricated via the in-situ solid-state reaction synthesis using pure Ni and Al foils. The mechanical properties of the bimodal laminated NiAl alloy sheet were tested by hot tensile tests at temperatures ranging from 850 degrees C to 1000 degrees C and strain rates ranging from 0.01/s to 0.0005/s. To clarify the deformation mechanism, the deformation microstructure was characterized by scanning electron microscopy and electron backscatter diffraction technology. The fracture behavior was characterized by a three dimensional X-ray microscope. It was found that the sheet with the bimodal laminated structure composed of overlapped coarse-grained layers (CGLs) and fine-grained layers (FGLs) exhibits a significant deformation incompatibility between CGLs and FGLs, leading to a special deformation and fracture behavior. During hot deformation, dislocations mainly accommodate in FGLs at the beginning, and the coarse grains then begin to play a major role in replacing the privilege of FGLs and vital in storing dislocations during further deformation. The bimodal laminated grains are broken up into many fragments and the average grain sizes are refined after deformation due to the transformation of low angle boundaries into high angle boundaries. The nucleation of dynamic recrystallization grains preferentially takes place in FGLs, and DRX will occur in CGLs when sufficient strain and storage energy are accumulated. The fracture surface demonstrates a jagged fracture pattern parallel to the loading direction, which results from the special laminated grain structure. The fracture morphology along the thickness direction experiences dramatic changes related to the non-uniform strain distribution, which presented as the density and size distribution of voids in 3D space. The study is vital to guiding the high temperature forming and service performance of NiAl alloy.