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.

Keywords: Ti-6Al-4V alloy; fatigue; plasma nitriding.

Figure 1

The Ti-6Al-4V alloy in the as-received annealed condition with equiaxial microstructure (a) and after heat treatment to obtain the Widmanstätten-type microstructure (b).

Figure 2

Scanning electron micrograph of the Ti-6Al-4V alloy after plasma nitriding. The arrows indicate the titanium nitride layer and the nitrogen diffusion layer in the matrix.

Figure 3

X-ray diffraction of the non-nitrided and nitrided Ti-6Al-4V alloy. Top: plasma nitrided alloy. Bottom: non-nitrided alloy.

Figure 4

Graphic of stress versus number of cycles obtained from the fatigue tests on the non-nitrided and nitrided Ti-6Al-4V alloy. Arrows indicate samples that did not fail.

Figure 5

Fracture surfaces of specimens after the tensile test. These images were obtained using a stereoscopic microscope. Conditions: (a) non-nitrided specimen tested at 304 MPa and 123,700 cycles to failure, (b) non-nitrided specimen tested at 406 MPa and 68,700 cycles to failure, (c) non-nitrided specimen tested at 610 MPa and 6700 cycles to failure, (d) nitrided specimen tested at 304 MPa and 233,000 cycles to failure, (e) nitrided specimen tested at 406 MPa and 90,800 cycles to failure, (f) nitrided specimen tested at 610 MPa and 35,000 cycles to failure.

Figure 6

Scanning electron micrographs of the fracture surfaces of nitrided specimens after fatigue tests at 304 MPa and 233,000 cycles to failure. (a) A planar region where the crack originated. (b) Microcracks perpendicular to the propagation direction. (c) Striations. (d) Lamellar microstructure consisting of α and β phases.

Figure 7

Scanning electron micrograph of the fracture surface of non-nitrided specimen after a fatigue test at 610 MPa and 6700 cycles to failure.

Figure 8

Scanning electron micrograph of the fracture surface of the plasma nitrided specimen that was fatigue tested at 610 MPa and 233,000 cycles to failure.

Figure 9

Scanning electron micrograph of the fracture surface of the plasma nitrided specimen that was fatigue tested at 610 MPa and 233,000 cycles to failure.

Figure 10

Scanning electron micrograph of the fracture surface of the plasma nitrided specimen that was fatigue tested at 406 MPa and 90,800 cycles to failure.

Figure 11

Scanning electron micrographs of the final rupture region on the fracture surface of the plasma nitrided specimen that was fatigue tested at 610 MPa and 90,800 cycles to failure. (a) Dimples. (b) Detail of the dimples.