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

Abstract

The Ti-6Al-4V alloy is widely used in the manufacture of components that should have low density, 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 Ti-6Al-4V alloy with a lamellar microstructure (Widmanstätten type). Nitriding was executed at 720 °C for 4 hours in an atmosphere with N2, Ar and H2. Samples microstructure characterization 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 revealed the matrix phases α and β, and the phases Ɛ-Ti2N and δ-TiN in the nitrided alloy. 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, both conditions exhibit similar behavior. The fracture surface of the fatigue tested specimens clearly revealed the lamellar microstructure. The fracture mechanism appears to be due to cracking at the interface of 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: 16,43,64–69 a thin δ-TiN layer occupying the outer layer, followed by ε-Ti 2 N and α-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…”

“…This intensification in unevenness develops from the ionic blasting of glow discharge that plastically deforms these surfaces. 43,64…”

Additively manufactured titanium alloys and effect of hydroxyapatite coating for biomedical applications: A review

“…29 An increase in low-cycle fatigue strength with improved creep resistance were reported in plasma nitride-treated Ti-6Al-4V alloy. 30,31 Similarly, heat-treated SLM Ti-6Al-4V alloys have uniform, rough and hydrophilic surfaces that promotes easy cell attachment, proliferation and osteointegration. 32 Improving the mechanical properties of laser additive manufactured Ti-6Al-2V-1.5Mo-0.5Zr-0.3Si alloy by tuning basket-weave structure reported 75 MPa reduction in yield strength, 34.2% increase in impact toughness with double the ultimate strength when compared with the normal basket-weave structure.…”