###### Abstract  
Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles was performed to improve the fatigue strength and form a HAp transfer layer on a beta titanium alloy (Ti-22V-4 Al). The surface microstructures of the FPP-treated specimen were characterized using scanning electron microscopy, micro-Vickers hardness testing, energy dispersive X-ray spectrometry, X-ray diffraction, and electron backscattered diffraction. A HAp transfer layer with a thickness of 5.5 \(\mu\)m was formed on the surface of the Ti-22V-4 Al specimen by FPP. In addition, the surface hardness of the Ti-22V-4 Al was increased, and high compressive residual stress was generated on the specimen surface by FPP. Rotating bending fatigue tests were performed at room temperature in laboratory air over a wide cycle-life region (10\({}^{3}\)-10\({}^{9}\) cycles). In the long cycle-life regime, the fatigue strength at 10\({}^{7}\) cycles of the FPP-treated specimen became higher than that of the untreated specimen. This result is attributed to the formation of a work-hardened layer with high compressive residual stress by FPP. However, the fatigue strength was not improved by FPP in the short cycle-life regime, because fatigue cracks were initiated at surface defects formed during the FPP process. The fatigue fracture mode of the FPP-treated specimens shifted from surface-initiated fracture to subsurface-initiated fracture at a stress amplitude level of 600 MPa.  
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titanium alloys. Furthermore, heat treatment gives \(\beta\)-type titanium alloys high static strength. These material properties enable \(\beta\)-type titanium alloys to be used to make individualized implant components with high strength.  
Surface modification processes have been introduced to provide good biological compatibility and strong bonding between bone structures and titanium alloys using hydroxyapatite (HAp) coatings. One modification of the HAp coating process has been introduced for the \(\alpha+\beta\) titanium alloy Ti-6 Al-4V [8; 9] and commercially pure titanium (CP titanium) [10; 11]. However, it has been reported that fatigue fracture often occurs at the interface between the HAp coating layer and the substrate under cyclic loading [11]. Furthermore, thermal energy induced by high-temperature processes, such as plasma spraying, changes the crystalline structure of HAp [12] or the titanium substrate, [13] and this phenomenon has a negative effect on HAp-coated bio-implants.  
Based on this background, it is important to create a HAp coating layer without changing the crystalline structure of HAp. In recent years, it has been reported that FPP treatment can form a surface-modified layer enriched with the colliding particles of carbon steel [14], titanium [15], chromium [16], and carbon-black/steel [17] at room temperature. In our previous studies [18; 19; 20], a HAp transfer layer was formed by FPP using HAp particles on CP titanium, Ti-6 Al-4V alloy, and Ti-29 Nb-13 Ta-4.6 Zr alloy without any change in the crystalline structure of HAp. Furthermore, FPP can modify the microstructure and the fatigue strength of metallic materials [21; 22; 23; 24]; the fatigue strength of CP titanium and the fatigue life of Ti-6 Al-4V were improved by FPP.  
In the present study, FPP using HAp particles was used to form a HAp transfer layer and improve the fatigue strength of a beta titanium alloy (Ti-22V-4 Al). The purpose of this study was to characterize the FPP transfer layer and to experimentally examine the effect of FPP using HAp particles on the fatigue properties of the Ti-22V-4 Al alloy under rotating bending. Furthermore, fatigue fracture mechanisms were identified by fractography using SEM. The effect of FPP using HAp particles on fatigue behavior was discussed in light of the fractography results.