Issue34

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 34 (2015) 261 - 270; DOI: 10.3221/IGF-ESIS.34.28 262 I NTRODUCTION i-6Al-4V has been widely used for structural applications such as bio-implants and aerospace components due to its high specific strength and excellent corrosion resistance. Recently, improving mechanical properties of materials are required to increase structural reliability and reduce the size and weight of various components. Since mechanical properties of metallic materials are determined by their microstructural factors, the grain-refinement process is effective to improve their yield strength [1, 2] and fatigue strength. However, a homogeneous fine-grained structure generally leads to decrease the ductility of materials due to their plastic instability. Previous investigations have reported the microstructural design which improves both of strength and ductility of materials [3-6]. Wang et al. [3] reported that pure copper with a bimodal grain size distribution created by a thermo- mechanical treatment exhibited stable tensile deformation leading to a high tensile ductility. The author’s group [7-13] 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 [8-10], 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 [15-17] so that the fatigue crack propagation of the material with a bimodal harmonic structure should be examined to achieve the practical use of it. The present work deals with the evaluation of the near-threshold fatigue crack propagation of the Ti-6Al-4V alloy with a bimodal harmonic structure, which exhibits superior mechanical properties. Furthermore, the effects of the stress ratio and the grain size on the fatigue crack propagation of the material with harmonic structure were investigated on the basis of the crack closure concept, and its mechanism is discussed from viewpoints of fractography and crystallography. E XPERIMENTAL PROCEDURES Material he material used in the present study was a Ti-6Al-4V alloy with the chemical composition shown in Tab. 1. This Ti-6Al-4V powder (186  m diameter) was produced using the plasma rotating electrode process (PREP). The powders were mechanically milled (MM) in a Fritch P-5 planetary ball mill with tungsten carbide vial and SUJ2 steel balls in the argon gas atmosphere at room temperature. Mechanical milling was performed at a rotational speed of 200 rpm for 90 ks under the condition of the ball-to-powder mass ratio 1.8 : 1. After mechanical milling, the powders were consolidated by a spark plasma sintering (SPS) at 1123 K for 1.8 ks under vacuum and 50 MPa applied stress using a graphite die with 25 mm internal diameters (Harmonic series). In addition, the compact prepared from the as-received initial (IP) powders were also prepared as a coarse-grained material (IP series). The Harmonic series exhibits higher strength and ductility compared to the IP series as shown in Tab. 2 [13]. 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 coarse- grained structures. The fine-grained structure formed a network structure and the coarse-grained structure was surrounded by the fine-grained structure network. Minimum grain size in the Harmonic is about 1.1  m [13]. Al V Fe H N O C Ti 6.51 4.26 0.17 0.0023 0.003 0.18 0.01 Bal. Table 1 : Chemical composition of Ti-6Al-4V alloy powder (mass%). Microstructure Yield strength, MPa Ultimate tensile strength, MPa Strain to fracture, % IP series 802 864 20.9 Harmonic series 915 956 22.0 Table 2 : Mechanical properties of the sintered compacts [13]. T T