Dolzago, Italy
Dolzago, Italy
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Fare S.,Polytechnic of Milan | Lecis N.,Polytechnic of Milan | Vedani M.,Polytechnic of Milan | Vimercati G.,Polytechnic of Milan | And 3 more authors.
Metallurgia Italiana | Year: 2010

Two titanium alloys subjected to ion nitriding were investigated in the present work with the aim of studying the effects of material composition and of nitriding-process parameters. Experimental analyses have been carried out by microstructural observations, microhadness measurements and by compositional profile analyses using the glow-discharge optical emission spectroscopy technique. The results showed that higher hardness profiles as well as improved enrichment in nitrogen on the surface layers are achieved by prolonged exposure cycles to the active atmosphere or by the alternation of exposure and diffusion cycles, as expected. Nitrogen enrichment occurred to a larger degree in the Ti-6Al-4V (grade 5 Ti) alloy when compared to the commercially pure titanium (grade 2 Ti) treated by the same conditions. Finally, elemental profiles of the Ti-6Al-4V alloy along the sample thickness showed that Al and V accumulated beneath the nitrogen rich surface layer (the TiN/Ti2N layer) thus generating a significant contribution the improvement in hardness of the sub-surfacial layers.

Fare S.,Polytechnic of Milan | Lecis N.,Polytechnic of Milan | Vedani M.,Polytechnic of Milan | Silipigni A.,TAG SrL | Favoino P.,TAG SrL
Surface and Coatings Technology | Year: 2012

An investigation was carried out on commercially pure titanium and on a Ti-6Al-4V alloy plasma nitrided at 730̊C according to different conditions. Diffusion of nitrogen and formation of compound layer were very limited for the shortest processing times of 20h in both materials while the combination of diffusion periods after active nitriding led to a clear improvement of nitriding efficiency only for the Ti-6Al-4V alloy. Extension of nitriding times to 76h generated a significantly thicker compound layer composed of a combination of TiN and Ti 2N phase while TiN became predominant at nitriding times of 156h. Ti 2AlN was also supposed to be present in the outermost layers of the Ti-6Al-4V alloy nitrided in both conditions. Al-enriched and V-enriched regions were detected beneath the above layers. Modifications observed in extension and chemistry of the nitrided layers also resulted in different hardness and scratch resistance properties. The thin TiN layer found in the soft CP titanium nitrided to 20 h clearly cracked due to extensive deformation produced during scratch testing. On the contrary, shallower scratch tracks were generated with no evidence of surface cracking in the harder Ti-6Al-4V alloy. Increasing the nitriding times led to generation of small cracks at root of track, more developed after nitriding for 156h. It was speculated that the different fractions of TiN and Ti 2N found for the two processing times could have an effect on damage sensitivity, promoting cracking where the most brittle TiN was predominant. © 2011 Elsevier B.V.

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