The Effect of Plasma Nitriding on the Fatigue Behavior of the Ti-6Al-4V Alloy

Abstract

The Ti-6Al-4V alloy is widely used in the manufacture of components that must have low density and high corrosion resistance and fatigue strength. The fatigue strength can be improved by surface modification. The aim of this study was to determine the influence of plasma nitriding on the fatigue behavior of a Ti-6Al-4V alloy with a lamellar microstructure (Widmanstätten type). Nitriding was executed at 720 °C for 4 h in an atmosphere with N2, Ar, and H2. Microstructure characterization of the samples was carried out by X-ray diffraction analysis, optical microscopy, and scanning electron microscopy. The average roughness of the specimens was determined, and fatigue tests were executed in a bending–rotating machine with reverse tension cycles (R = −1). X-ray diffraction analysis of the nitrided alloy revealed the following matrix phases: α, β, ε-Ti2N, and δ-TiN. A nitrogen diffusion layer was formed between the substrate and the titanium nitrides. Plasma nitriding resulted in an increase in low-cycle fatigue strength, whereas at high cycles of 200 MPa, both conditions exhibited similar behaviors. The fracture surface of the fatigue-tested specimens clearly revealed the lamellar microstructure. The fracture mechanism in the non-nitrided specimens appears to be due to cracking at the interface of the α and β phases of the lamellar microstructure.

Hollow cathode plasma nitriding of medical grade Ti6Al4V: A comprehensive study

“…Estimation of lattice strain becomes an issue, as nitrogen fills in the interstitials, distorting cell and forming Ti x N y compounds, thus causing peak broadening and shifting. For the used range of temperature and varying times -including very long treatments, the obtained crystalline structure and layer formation follow what many seminal works have encountered: 16,43,[64][65][66][67][68][69] a thin d-TiN layer occupying the outer layer, followed by e-Ti 2 N and a-Ti in deeper layers. The phase percentage will have a varying value, according to depth.…”

“…63 Both residual compressive stresses and solid solution concentration contribute to higher hardness and, the former, to wear resistance and fatigue improvement, as crack nucleation stage is delayed. 64 In addition, no titanium oxide (TiO 2 ) peaks were observed in the diffractograms. E2 (940,170) , E4 (940,430) , and E6 (1000,300) showed a more pronounced synthesis of stoichiometric TiN.…”

“…Solid solution, along with precipitates and ionic bonds anchor the dislocation movements and, therefore, hardness is increased. 64,72 Scanning electron microscopy (SEM)…”

Special Issue: Probabilistic Mechanical Fatigue and Fracture of Materials

“…Among the latter treatments, the gas nitriding technique has been widely used, providing a poor final surface finish with long duration of time. Therefore, the alternative plasma nitriding was preferred instead, as in the work by de Castro et al [3] with a titanium Ti-6Al-4V alloy. The comparative analysis suggested by the authors provides interesting results: (a) the plasma nitrided specimens exhibit higher low-cycle fatigue strength compared with similar behaviour for high cycles with those non-nitrided; (b) the fracture surfaces in the nitrided specimens show raised regions, characteristic of plastic deformation prior to fracture, while in the untreated specimens large regions with planar propagation are found instead, related to the final rupture.…”

Investigation on Wear Behavior of Cryogenically Treated Ti-6Al-4V Titanium Alloy under Dry and Wet Conditions

“…Titanium alloys are favored in the production of aerospace parts due to their excellent comprehensive properties [1]. The demand and application of these alloys has been increased in other fields such as transportation, the petrochemical industry, biomedicine and daily life, etc.…”