Microstructure formation and mechanical properties of isothermally-solidified titanium alloy joints brazed by a Ti - Zr - Cu - Ni - Be amorphous alloy foil

Abstract

Two titanium alloys, OT4 and VT6-c, with a pseudo α and α + β structure, respectively, were brazed using transient liquid phase (TLP) bonding. To obtain high strength joints an amorphous foil (Ti - 12Zr - 22Cu - 12Ni - 1.5 Be - 0.8V wt.%) was used. Based on microstructural studies and analysis of two- and three-component phase diagrams, the mechanism of the microstructural evolution of the brazed seams of titanium alloys OT4 and VT6-c is described. Brazing at 800 oC with exposure for 0.5 h leads to the formation of a heterogeneous structure consisting of Widmanstätten, eutectoid, and eutectic. Brazed OT4 and VT6-c joints with the presence of a eutectic layer in the centre show low mechanical properties; their ultimate strength lies in a range from 200 to 550 MPa. Increasing the brazing temperature to 840 oC and the exposure time to 2 h, leads to the disappearance of the brittle eutectic component from the seam. This structure typically consists of Widmanstätten with a small number of eutectoid fractions. Joints with the absence of a eutectic layer in the brazed seam demonstrate a strength equal to the base titanium alloys. In this case, failure occurs in the base metal. For brazed samples from the OT4 alloy, the tensile strength value is σb = 750 ± 3 MPa, and for samples from VT6-c, σb = 905 ± 3 MPa.