Phase transitions in some nickel-rich nickel-boron-titanium hard alloys

A family of alloys in the nickel-rich region of the Ni-B-Ti phase diagram have been examined by differential thermal analysis (DTA), X-ray and electron diffractions, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A ternary phase with chemical formula Ni21Ti3B6(τ) was observed and characterized in these alloys. Solid-state eutectoid transformation of the ternary phase τ was observed in alloys with low titanium contents. However, alloys with high titanium contents did not present this transformation due to the large amount of titanium which enhanced the formation of hard boride phases such as TiB2. In addition, crystallographic orientation relationships between nickel and the boride phases were also reported with the lattice matching [1 1 2]τ//[0 3 1]Ni dominating the interface.