Mechanical Response and Fracture Behavior of the TiAl/Steel Brazed Joint

TiAl-based alloy may be considered as a potential material for high-temperature applications, such as rotating components of advanced ground and space vehicles. In order to manufacture large or complex structural components, it is necessary to bond TiAl-based alloy to other materials. In particular, the bonding technology of TiAl-based alloy to structural steel using brazing method has been carried out. The mechanical response of the reaction phases of the brazed joint is rarely studied.

In this study TiAl and steel were brazed with the Ag-Cu/Ti/Ag-Cu sandwich filler metal. The joint zone between the mating surfaces features a complex structure: Al-Cu-Ti ternary intermetallic reaction layer, Ag-Cu residue filler, AlCu₂Ti intermetallics dispersed in the residue braze and TiC layer. Mechanical properties of the reaction phases and base metals were studied by using the continuous instrumented micro indentation and conventional Vickers hardness testing.

The hardness of the tested phases demonstrates a descending order as follows: (i) the Al-Cu-Ti ternary intermetallic reaction layer adjacent to TiAl substrate (ii) the AlCu₂Ti intermetallics in the residue Ag-Cu filler (iii) TiAl base metal (iv) steel (v) Ag-Cu residue filler. The mechanical responses of the tested phases, in terms of the indentation depth, are consistent with the results of hardness tests: the phases with smaller hardness are always associated with a larger indentation depth. Under the same testing conditions the Young's modulus of the TiAl substrate, Al-Cu-Ti ternary intermetallics layer, AlCu₂Ti intermetallics in the residue Ag-Cu filler, Ag-Cu residue phase, and the steel were determined. The fracture behavior study (in situ observation by scanning electron microscopy) indicates that the fracture in most cases can be associated with the zone of the Al-Cu-Ti ternary intermetallics layer adjacent to the TiAl substrate.