TEM Study of a Layered Composite Structure Produced by Ion-Plasma Treatment of Aluminum Coating on the Ti-6Al-4V Alloy

Abstract

Using the methods of transmission electron microscopy and energy-dispersive spectroscopy, we study the microstructure and phase composition of the coating and modified intermetallic layers obtained in a Ti-6Al-4V alloy by the deposition of the Al coating and subsequent processing in low-pressure non-self-sustained arc discharge plasma (CIPT—complex ion-plasma treatment). The deposition of the aluminum coating on the Ti-6Al-4V alloy is accompanied by the formation of a layered and a gradient microstructure: nanocrystalline near the “coating/substrate” interface and ultrafine-grained in the outer part of the aluminum coating, with α-stabilized region of ≈5 µm thick in the surface layer in base titanium alloy. After the CIPT, the coating and the surface of the base titanium alloy have a layered morphology: each of the layers possesses different grain structure and composition. In the direction from the outer surface of the specimen to the base material, the following phase sequence has been confirmed by diffraction and elemental analysis: TiAl3 → TiAl3 + nc-(Al(Ti) + α-Ti) → nc-(Al(Ti) + α-Ti) → TiAl3 → TiAl3 + TiAl → TiAl → Ti3Al → α-Ti alloy → (α + β)-Ti alloy. The nanocrystalline aluminum layer, which has been formed during the deposition of the aluminum coating, does not undergo phase transformation and recrystallization under the CIPT. Nanocrystalline structure can favor the interdiffusion of the elements between the coating and base material, and stimulate phase transformation in coarser grains situated under and over it.