Columnar and nanocrystalline combined microstructure of the nitrided layer by active screen plasma nitriding on surface-nanocrystalline titanium alloy

Abstract

The microstructure and formation mechanism of the nitrided layer by active screen plasma nitriding on surface-nanocrystalline TA17 titanium alloy were studied by TEM. The nitrided layer of both the original and shot-peened TA17 samples was composed of two sublayers: the outer TiN layer formed by deposition of titanium nitride particles and the inner Ti2N layer formed by nitrogen diffusion into the titanium substrate. The Ti2N layer was found to be an Al-depleted zone, which was a proof for its formation mode of nitrogen diffusion. Compared with the TiN layer of the original sample filled with columnar grains, the TiN layer of the shot-peened sample was composed of mainly equiaxed nanograins and a small amount of columnar grains. The large number of high-energy grain boundaries on the shot-peened surface provided numerous nucleation sites, resulting in the formation of equiaxed nanocrystalline TiN. During long term nitriding, most of the nano-scale TiN grains were maintained due to their high thermal stability, while the nano grains of the shot-peened substrate surface grew into microscale due to their low thermal stability. With the thickening of the nitrided layer, some equiaxed TiN nanograins grew into columns perpendicular to the substrate surface due to competitive growth.