Superior shear strength subject to the regulation of plastic toughness in K4169 alloy/TiAl intermetallic joints vacuum brazed with gradient composite amorphous filler metals

The concept of gradient composite amorphous filler metal (GAFM) was utilized to solve the scientific problem of low strength caused by excessive Ti-containing brittle-hard intermetallic compounds (IMCs) generated in Ni/TiAl brazed joints through interfacial hysteresis reaction. Based on the cluster-plus-glue-atom model, the GAFMs (Zr25Ti21.25Ni25Cu18.75/Zr31.25-XVXCu50Ni18.75) were designed for vacuum brazing of K4169 alloy with TiAl intermetallic. And the shear strength of the joint brazed with (Ti21.25Zr25Ni25Cu18.75/Zr25V6.25Cu50Ni18.75) GAFM reached 344 MPa. The relation between grain boundary, solution, dislocation and strength was established. The cracks initiated from the (Ti,Zr)(Ni,Cu)+(Cr,Fe,Ni) brittle-hard phase and the plastic-tough phase (Zr,Ti)(Ni,Cu) interface in Zone II with (Ti21.25Zr25Ni25Cu18.75/Zr31.25-XVXCu50Ni18.75) GAFMs at 1040 °C/10 min, and then extended to the (Ti,Zr)(Al,Ni,Cu)3[010]/(Ti,Zr)(Ni,Cu,Al)3[0-10] non-coherent interface. To regulate the distribution of the plastic-tough phase in Zone II, raising brazing temperature could promote the dissolution of Zone I into Zone II. The addition of V to the GAFMs, εG and εα mismatches increased and prompted the Niss[00-1]+TiAl[010] phase, (Cr,Fe,Ni)ss[00-1] phase with a-value lattice distortion of 21.38%, (Ni,Cr,Fe) and ZrNi3[0-2-1] grains refinement in Zone II, where KIC*/KIC>1, the brazed joint strengthening effect was enhanced. The columnar grains were transformed into grains with reticulated cladding characteristics. The percentage of substructured grains increased from 46.6% to 56.3%, and the percentage of HAGBs rose to 84.7%, which inhibited dislocation migration. Therefore, the ZrNi3 plastic-tough phase, prohibited the expansion of the major crack generated from the (Cr,Fe,Ni) brittle-hard phase. The major crack extended to the Ti(Ni,Cu,Al)3 [1–10]/Ti(Al,Cu,Ni)2[0-10] semi-coherent interface and then to the TiAl substrate, resulting in a zigzag crack expansion.