TEM study of microstructural characteristic and evaluation of mechanical performance for the hastelloy N/Ti/Hastelloy N superalloy joint brazed for diverse soaking time

The purpose of this work was to describe and discuss the fundamental issues involved in brazing of Hastelloy N to Hastelloy N superalloy using pure Ti foil at 1333 K for different soaking time (1∼90 min). A detailed electron microscopy examination (transmission electron microscopy and high-resolution transmission electron microscopy) was performed to study the microstructure of the joint. The mechanical performance of the joint was evaluated by a shear test. Results show that the brazed joint obtained could be described as Hastelloy N/Zone 1/Zone 2/Zone 1/Hastelloy N. The Zone 1 whose constitution was hardly affected through modifying the soaking time was produced because of the eutectic melting of Cr9Mo21Ni20 and Ni3Ti intermetallics. The constitution of Zone 2, however, was very sensitive to the joining condition. Among the soaking time under investigation, the highest joint average shear strength obtained was 350.4 MPa. The strength of the joint, as revealed, was closely related with the microstructure in the Zone 2. When the Hastelloy N superalloy was brazed for different soaking time, the microstructural evolution in the joint was clarified and its relationship with mechanical performance was built. The work will contribute to understand the brazeability of the Hastelloy series alloys and then broaden their applications.