Effect of duplex aging on microstructure and mechanical behavior of beta titanium alloy Ti–15V–3Cr–3Al–3Sn under unidirectional and cyclic loading conditions R. Santhosh a , M. Geetha b , V.K. Saxena c , M. Nageswara Rao b,⇑ a School of Advanced Sciences, VIT University, Vellore 632 014, India b School of Mechanical and Building Sciences, VIT University, Vellore 632 014, India c Defence Metallurgical Research Laboratory, Hyderabad 500 058, India article info Article history: Received 28 May 2014 Received in revised form 27 November 2014 Accepted 3 December 2014 Available online 16 December 2014 Keywords: Beta titanium alloy Duplex aging Microstructure Work hardening rate HCF behavior abstract The microstructure and mechanical behavior of beta titanium alloy Ti–15V–3Cr–3Al–3Sn (Ti15-3) under unidirectional and high cycle fatigue (HCF) loading conditions were studied after single aging (SA) and duplex aging (DA). After SA, well-developed grain boundary a and micro precipitate free zones (micro PFZs) were present in the microstructure. Whereas after DA, grain boundary a was poorly developed and micro PFZs were absent. DA resulted in smaller size, higher density and volume fraction of alpha par- ticles. DA led to higher work hardening rate, better strength-ductility combination and higher HCF life. Improved mechanical behavior after DA is explained based on microstructural observations. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Beta-titanium (b-Ti) alloys have received increasing emphasis over the years, thanks to their high strength levels, high hardena- bility, excellent hot and cold workability, etc. [1–3]. They have found several applications; for example, near b-Ti alloy 10-2-3 is being used for manufacture of landing gears for Boeing aircrafts [2–4]. b-Ti alloy Ti–15V–3Cr–3Al–3Sn (Ti15-3) is another grade which has come to prominence. It can be produced in sheet form due to its superior hot and cold working characteristics and fur- ther, it can be heat treated to high strength levels [4–7]. For several of the applications, fatigue resistance, particularly the life under high cycle fatigue (HCF) loading conditions, is an important design consideration [1]. The HCF life of Ti15-3, among other b-Ti alloys, has been the subject of investigation over the years. It was reported that highly b-stabilized or solute rich alloys like Beta-C or Ti15-3 have lower fatigue strength than the solute lean alloys such as Ti10-2-3 or SP 700 [2]. The trend for inhomoge- neous precipitation of alpha (a) in richer alloys was cited as a pos- sible reason for their low endurance limit [2]. There have been several publications bringing out that duplex aging (DA) compared to single aging (SA) of b-Ti alloys improves the homogeneity of a precipitation and exerts beneficial effect on the behavior of mate- rial under monotonic and cyclic loading conditions. For example, studies by Krugmann and Gregory [8] and Wagner and Gregory [9] demonstrated that DA of the metastable b-Ti alloy Ti-38-644 (b-C) leads to a more homogeneous distribution of a precipitates. Schmidt et al. [10] showed that DA treatments lead to a finer and homogeneous a precipitation in b-C alloy without any precipitate free zones (PFZs) in the microstructure and resulted in an enhance- ment of fatigue life. The study of Furuhara et al. [11] brought out that DA of Ti15-3 alloy resulted in a more uniform and finer distri- bution of a precipitates compared to SA. These authors have also shown that higher hardness levels were obtained by resorting to DA. Ivasishin et al. [12] subjected the Ti15-3 alloy to preaging at 300 °C for 8 h followed by aging at 450 °C and 538 °C and obtained an improvement in proof strength (PS) and ultimate tensile strength (UTS) coupled with an improvement in % elongation (% El) and % reduction in area (% RA) values. Santhosh et al. [13] reported on the effect of DA on the microstructural and mechanical properties of Ti15-3. They observed that DA led to an increased volume fraction of a phase, finer a-particle size and a four to five fold improvement in fatigue life under rotating bending conditions, compared to SA. Boyer et al. [14] reported an improvement in UTS and fatigue life of Ti15-3 on subjecting to DA. However, unlike in http://dx.doi.org/10.1016/j.ijfatigue.2014.12.005 0142-1123/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: School of Mechanical and Building Sciences, VIT University, Vellore 632 014, Tamil Nadu, India. Tel.: +91 416 2202297; fax: +91 416 2240411. E-mail address: m.nageswararao@vit.ac.in (M. Nageswara Rao). International Journal of Fatigue 73 (2015) 88–97 Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue