Diffusion couples of aluminum nitride (AlN) and Ti were annealed under an argon atmosphere at temperatures ranging from 1300° to 1500°C for 0.5–36 h. The morphologies, crystal structures, and chemical compositions of the reaction zones at AlN/Ti interfaces were characterized using analytical scanning electron microscopy and analytical transmission electron microscopy. An interfacial reaction zone, consisting of TiN, τ2‐Ti2AlN, τ1‐Ti3AlN, α2‐Ti3Al, and a two‐phase (α2‐Ti3Al+α‐Ti) region in sequence, was observed in between AlN and Ti after annealing at 1300°C. The α2‐Ti3Al region revealed equiaxed and elongated morphologies with
and
. In the two‐phase (α2‐Ti3Al+α‐Ti) region, α2‐Ti3Al and α‐Ti were found to satisfy the following orientation relationship:and. The γ‐TiAl and a lamellar two‐phase (γ‐TiAl+α2‐Ti3Al) structure, instead of τ1‐Ti3AlN, were found in between τ2‐Ti2AlN and α2‐Ti3Al after annealing at 1400°C. The orientation relationship of γ‐TiAl and α2‐Ti3Al in the lamellar structure was identified to be as follows:and. Compared with the reaction zone after annealing at 1400°C, the γ‐TiAl was not found at the interface after annealing at 1500°C. The microstructural development resulting from isothermal diffusion at 1300°C and subsequent cooling at the interface are explained with the aid of the Ti–Al–N ternary phase diagram and a modified Ti–Al binary phase diagram.
were formed during cooling. Further diffusion of N atoms into the reaction zone precipitates a chopped fiber-like ␣ 2 -Ti 2 AlN in the matrix of ␥-TiAl, with [110] ␥−TiAl //[1120] 2 −Ti 2 AlN and (111) ␥−TiAl //(1103) 2 −Ti 2 AlN , by substituting N atoms for one-half Al atoms after annealing at 1400°C for 1 h. The released Al atoms, due to the precipitation of 2 -Ti 2 AlN, resulted in an ordered Al-rich ␥-TiAl or Ti 3 Al 5 . Furthermore, the ␣-Ti (Al, N) was nitridized into a lamellar layer (␦-TiN + ␣-Ti) with [110] ␦−TiN //[1120] ␣−Ti and (111) ␦−TiN //(0001) ␣−Ti .
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