Modificação de Superfície por implantação de nitrogênio na superliga de níquel inconel 718

Abstract

The insertion of nitrogen into Fe-Cr-Ni alloys results in the formation of the expanded austenite γN, a metastable phase supersaturated with nitrogen. The UNS N07718 alloy (Inconel 718) was selected for the present study to gain insights into the control of the expanded austenite formation with a plasma immersion ion implantation system (PIII), by exploring the relationship between the energy density per pulse (E̅) and ion fluence (Γ). One part of the study consisted in analyzing these treatment parameters, managed through implantation frequency (0,7-1,8 kHz), pulse width (12-30 μs) and ion current (0,64-0,91 A), while employing the same 350 °C temperature to restrict thermal diffusion effects. In a second approach, the temperature effect on the layer formation was investigated at the 300- 400 oC range. In all cases, the treatment time was 3 hours. Inversely proportional effects arise from the implantation energy and ion fluence parameters, affecting the layer thicknesses formed under the 350 °C temperature. The lower E̅ and higher  values produce a layer with ~3,5 μm in thickness, which is 9% thicker than the thinnest one obtained at the same temperature. The extreme E̅ (75 ± 10-5 J/cm2 ) conditions produce the thinnest layers, whereas extreme  result in the thickest ones. These results are understood in terms of the nitrogen concentration gradient, found at the surface during the treatments, in association with the sputtering rate, vacancies formation and nitrides growth in the modified layer. Such phenomena rule the solute diffusion in the matrix. The lower temperatures resulted in ~1.9 μm thick layers, while significantly larger ~6 μm layers ensue from the highest employed temperature, 400 °C, a condition that holds the most significant retained nitrogen dose in the entire modified region, but the lowest concentration among the studied cases (28 at. % versus 32 at. %). Considerable amount of CrN and Cr2N exist on the top of the surface prepared at 400 °C; however, the expanded austenite N is the prevailing phase found on all the modified layers. In some layers, possible nickel nitride (Ni3N) are considered as well, since the PIII technique provides the necessary requirements for this phase ́s precipitation in the Inconel 718 alloy. The stability of expanded austenite was tested at a temperature of 350 o C; the changes comprise the possible γN depletion into chromium nitrides and a Cr-poor substructure. The modified layers display improved mechanical properties, with hardness three times higher and elastic modulus similar to the substrate values. The E̅ and  parameters correlate inversely with respect to hardness – in the 350 °C treatments, it increases with E̅. It follows from the analysis of the TiN and Nb2O5 precipitates, originally present in the alloy, that they are not affected by the ion implantation process. These results highlight the influence of E̅ and  parameters, implemented jointly and in association with temperature, to achieve a fine tune control on the modified nickel alloys surfaces.