Enhancing Surface Hardness of Titanium Through Ni-Ti Intermetallic Microstructures Formed In Situ During Weld Deposition of Nickel

Attempts to improve tribological properties of titanium generally employ laser cladding with ceramic reinforcements. Here, we present an alternative weld deposition-based technique for surface hardening of titanium substrates through hard intermetallic compounds (IMCs) formed in situ. Using pure nickel as a filler in a gas metal arc welding (GMAW) setup, we produced Ti-Ni IMC reinforcements in near-surface regions of the substrate with concomitant increase in hardness. The alloyed region exhibited considerable microstructural heterogeneity that could be traced back to solidification events taking place in a compositionally and thermally inhomogeneous melt. Fusion interface in the substrate is characterized by consecutive bands of Ti2Ni and NiTi. Two different morphologies of the NiTi + Ni3Ti eutectic, along with some isolated regions of primary NiTi and primary Ni3Ti, constitute rest of the alloyed zone. The eutectic morphologies differ significantly in their length scale originating from non-reciprocal heterogeneous nucleation found in faceted/non-faceted eutectic systems. We constructed a hardness map of the alloyed zone that highlighted a wide hardness variation that correlated well with the observed microstructural heterogeneity. The interaction of macroscopic transport phenomena with the system thermodynamics is shown to have a profound influence on solidification of inhomogeneous melts, resulting in unusual solidification modes and microstructures.