Evaluation of the Fatigue Properties of Ti-6Al-4V Alloy with Harmonic Structure in 4-Points Bending

Abstract

Titanium alloy (Ti-6Al-4V) having a bimodal "harmonic structure", which consists of the coarse-grained structure surrounded by the network structure with fine grains, was fabricated by mechanical milling (MM) and spark plasma sintering (SPS) to achieve high strength and good plasticity. The microstructure of the MM-processed powder and the sintered compacts were characterized using a micro-Vickers hardness tester, scanning electron microscope (SEM) and an electron backscatter diffraction technique (EBSD). Harmonic structure was created in the sintered Ti-6Al-4V compacts prepared from the MM-processed powders having fine grains at its surface. The Ti-6Al-4V alloy with harmonic structure exhibited high tensile strength and good plasticity. The effects of the harmonic structure on the 4-points bending fatigue properties of Ti-6Al-4V alloy was investigated under the stress ratio R = 0.1 in ambient air without any controls of the temperature. The compacts with harmonic structure exhibited higher fatigue strength compared to the conventional coarse-grained material prepared from as-received initial powders. This was because the Ti-6Al-4V alloy with harmonic structure had higher tensile strength and hardness. Moreover, fatigue fracture mechanism of the Ti-6Al-4V alloy with harmonic structure was discussed from viewpoints of fractography and crystallography. As results of observing and analyzing the fracture surfaces, the Ti-6Al-4V alloy with harmonic structure failed from the coarse grain in the harmonic structure in the surface fracture mode.

Evaluation of near-threshold fatigue crack propagation in Ti-6Al-4V Alloy with harmonic structure created by Mechanical Milling and Spark Plasma Sintering

“…Fig. 1 shows the image quality (IQ) maps obtained by EBSD for the (a) IP and (b) Harmonic series [14]. The IP series has a coarse acicular microstructure, whereas the Harmonic series two different microstructures; fine-grained and coarsegrained structures.…”

“…The author's group [7][8][9][10][11][12][13] has developed an exquisite microstructural design, called "harmonic structure", which consists of coarse-grained structure surrounded by a network structure of fine grains. In the previous studies, metallic materials with harmonic structure, such as pure copper [7], stainless steel [8][9][10], Co-Cr-Mo alloy [11], commercially pure titanium [12] and Ti-6Al-4V alloy [13,14] exhibited high strength and high ductility compared to their homogeneous counterparts. The authors have focused on the fatigue properties of the Ti-6Al-4V alloy with harmonic structure and reported that a fatigue crack was initiated from the coarse-grained structure in the harmonic structure in 4-points bending [14].…”

“…In the previous studies, metallic materials with harmonic structure, such as pure copper [7], stainless steel [8][9][10], Co-Cr-Mo alloy [11], commercially pure titanium [12] and Ti-6Al-4V alloy [13,14] exhibited high strength and high ductility compared to their homogeneous counterparts. The authors have focused on the fatigue properties of the Ti-6Al-4V alloy with harmonic structure and reported that a fatigue crack was initiated from the coarse-grained structure in the harmonic structure in 4-points bending [14]. However, fatigue crack growth changes depending on the stress ratio and grain size of materials [15][16][17] so that the fatigue crack propagation of the material with a bimodal harmonic structure should be examined to achieve the practical use of it.…”

Effect of bimodal grain size distribution on fatigue properties of Ti-6Al-4V alloy with harmonic structure under four-point bending

“…However, a homogeneous fine-grained structure formed by severe plastic deformation generally leads to a decrease in the ductility of metallic materials [3][4][5][6][7][8] due to the initiation of plastic instability in the early stage. Therefore, various microstructural designs have been proposed to improve both the strength and ductility of these materials [4][5][6][7][8][9][10][11][12][13][14][15][16].…”

Effect of the Nitrogen Diffusion Layer Formed by Gas Blow Induction Heating Nitriding on Wear Resistance and Fatigue Properties of Titanium Alloy

“…The disk-shaped specimens that had undergone each treatment were machined to the configuration shown in Figure 2, by precision cutting and polishing using emery paper (#120), to carry out four-point bending fatigue tests [7]. Fatigue tests were performed at room temperature in air, under a stress ratio of 0.1 and a stress cycling frequency of 10 Hz.…”