Issue34

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 34 (2015) 261 - 270; DOI: 10.3221/IGF-ESIS.34.28 269 C ONCLUSIONS n the present study, titanium alloy (Ti-6Al-4V) having a bimodal “harmonic structure”, which consists of coarse- grained structure surrounded by a network structure of fine grains, was fabricated by mechanical milling (MM) and spark plasma sintering (SPS), and its near-threshold fatigue crack propagation was investigated. The following conclusions were reached: 1. Threshold stress intensity range,  K th , of the material with harmonic structure decreases with increasing stress ratio, R , whereas the effective stress intensity range,  K eff , shows constant value for R lower than 0.5. The influence of the stress ratio, R , on  K th of Ti-6Al-4V with harmonic structure can be concluded to be that on crack closure. 2.  K th value of the material with harmonic structure is low compared to the compact prepared from as-received powders with coarse acicular microstructure due to the existence of the fine-grained structure. This is because the topography of fracture surfaces is smooth in the material with harmonic structure, which results in decreasing the closure stress intensity, K cl . 3. A crack profile is influenced by the harmonic structure. The crack propagation behavior of the material with harmonic structure is determined by that of fine-grained structure in the harmonic structure. 4. The  K th value of the material with harmonic structure can be estimated based on the crack propagation behavior of the bulk homogeneous material. A CKNOWLEDGEMENT he authors would like to acknowledge JSPS KAKENHI Grant Number 15K05677 and the Hattori Hokokai Foundation for the support. R EFERENCES [1] Hall, E.O., The deformation and ageing of mild steel: III discussion of results, Proc. Phys. Soc. B, 64 (1951) 273– 280. [2] Petch, N.J., The cleavage strength of polycrystals, J. Iron and Steel Inst., 174 (1953) 25–28. [3] Wang, Y., Chen, M., Zhou, F., Ma, E., High tensile ductility in a nanostructured metal, Nature, 419 (2002) 912–915. DOI: 10.1038/nature01133 [4] Fang, T.H., Li, W.L., Tao, N.R., Lu, K., Revealing extraordinary intrinsic tensile plasticity in gradient nano-grained copper, Science, 331 (2011) 1587–1590. DOI: 10.1126/science.1200177 [5] Kim, C.P., Oh, Y.S., Lee, S., Kim N.J., Realization of high tensile ductility in a bulk metallic glass composite by the utilization of deformation-induced martensitic transformation, Scripta Mater., 65 (2011) 304–307. DOI: 10.1016/j.scriptamat.2011.04.037 [6] Kondoh, K., Nakanishi, N., Mimoto, T., Umeda, J., High strength and ductility mechanism of pure titanium materials with oxygen solid solution via powder metallurgy route, Proc. 8th Int. Forum Adv. Mater. Sci. Tech., USB (2012) 3P-OS03-01. [7] Sawangrat, C., Kato, S., Orlov, D., Ameyama, K., Harmonic-structured copper: performance and proof of fabrication concept based on severe plastic deformation of powders, J. Mater. Sci., 49 (2014) 6579–6585. DOI: 10.1007/s10853- 014-8258-4 [8] Ueno, A., Fujiwara, H., Rifai, M., Zhang, Z., Ameyama, K., Fractographical analysis on fracture mechanism of stainless steel having harmonic microstructure, J. Soc. Mater. Sci., Jpn., 61 (2012) 686–691. DOI: 10.2472/jsms.61.686 [9] Ciuca, O.P., Ota, M., Deng, S., Ameyama, K., Harmonic structure design of a SUS329J1 two phase stainless steel and its mechanical properties, Mater. Trans., 54 (2013) 1629–1633. DOI: 10.2320/matertrans.MH201321 [10] Zhang, Z., Vajpai, S.K., Orlov, D., Ameyama, K., Improvement of mechanical properties in SUS304L steel through the control of bimodal microstructure characteristics, Mater. Sci. Eng. A, 598 (2014) 106–113. DOI: 10.1016/j.msea.2014.01.023 I T