Microstructure and mechanical behaviors of electron beam welded NiTi shape memory alloys

The vacuum electron beam welding (EBW) technique was employed to weld Ni50.8Ti49.2 shape memory alloy sheets, and the microstructure, transformation behaviors and mechanical behaviors of the welding joints were investigated systematically. The microstructure observation showed that the weld seam was composed of coarse columnar crystals at the center and relatively fine columnar crystals near the fusion line. The abnormal high intensity of B2(2 0 0) peak in XRD patterns and preferred orientation in EBSD indicated that the grains in the weld seam have grown preferentially along the < 100 > crystal orientation. Differential scanning calorimetry (DSC) curves exhibited an increase of the martensite start temperature (M-s) of the weld seam, which led to the mixed microstructure of martensite and austenite at room temperature. As a result, the ultimate tensile strength of the welding joint was 85% as high as that of the base metal at room temperature, while it could reach 93% at 223 K when both the weld seam and the base metal were in pure martensitic state. (C) 2013 Elsevier Ltd. All rights reserved.