Electron beam welding of Nb-8Hf-2Ti high temperature alloy to Ni dissimilar joint: The microstructural analysis, mechanical properties, and fracture characterization

In this work, the electron-beam welding (EBW) behavior of Ni/C-103 Nb-based refractory alloy was investigated, in terms of fusion-zone characteristics, texture evolution, mechanical properties, and joint efficiency. Compared to Nb, the higher concentration of Ni in weld zone indicated that the Ni side participates more in welding than the C-103 one. A continuous intermetallic layer was formed at the joint/C-103 interface composed of NbNi3, Nb7Ni6 and Hf3Ni7 phases. Continuous curved-shaped layers composed of columnar NbNi3 and eutectic Nb7Ni6+NbNi3 phases were also observed in the central and lower regions of the weld. These have been peeled off from the C-103 upper walls lateral to the weld and directed to the bottom of the weld pool due to the Marangoni flow. Solidification microcracks internal to the continuous intermetallic layers are also initiated and grown at the final stage of eutectic Nb7Ni6+NbNi3 phase formation leading to the brittle cleavage failure within this layer. The average microhardness of the joint was measured as 562 HV, which was higher than the base metals due to the presence of intermetallic compounds. Tensile strength of the joint was measured as 119.5 MPa, which is approximately one third of the value for the bead on plate weld of C-103 plate using EBW method with a tensile strength of 317.5 MPa. Nb7Ni6 and NbNi3 intermetallics were dominant phases in the fracture area.