ION NITRIDING OF TITANIUM ALUMINIDES WITH 25-53 AT.PERCENT AL .1. NITRIDING PARAMETERS AND MICROSTRUCTURE CHARACTERIZATION

Titanium aluminides with Al contents of 25, 40, 50 and 53 at.% were io n nitrided to modify the surface conditions. The effects of four proce ss parameters, namely nitriding temperature, nitriding time, working p ressure and [N-2]/[H-2] ratio, were investigated by the orthogonal tes t. The phases and microstructures of the nitrided surface were studied by X-ray diffraction (XRD), electron probe microanalysis, optical mic roscopy and scanning electron microscopy. Experimental results indicat e that nitriding temperature and time are the primary parameters respo nsible for improving surface hardness. The nitrided compound layer con sists of TiN and Ti2AlN phases. As the Al content in titanium aluminid es increases, the XRD intensity of Ti2AlN phase becomes stronger. The diffusion layer in all the cases has little nitrogen, allowing this la yer to be easily attacked by the etching reagent. A mechanism to expla in the formation of the ion-nitrided layer in titanium aluminides is a lso proposed in this study.