Effect of germanium and silicon additions on the mechanical properties of a near-α titanium alloy T. Kitashima n , K.S. Suresh, Y. Yamabe-Mitarai High Temperature Materials Unit, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan article info Article history: Received 9 September 2013 Received in revised form 26 December 2013 Accepted 31 December 2013 Available online 8 January 2014 Keywords: Titanium alloys Phase transformation Precipitation Mechanical property Germanium abstract The effect of Germanium (Ge) addition and simultaneous addition of Ge and Silicon (Si) on the tensile properties, impact toughness, and fracture behavior of the near-α alloy Ti–5Al–2Sn–4Zr–2Mo has been investigated at room temperature and at 650 1C. Addition of Ge monotonically increases the 0.2% yield stress (YS) and ultimate tensile stress (UTS) at both temperatures, and increases the rupture elongation at 650 1C; however, the Charpy impact toughness at both temperatures decreased. At 650 1C, this alloy ruptured in a hybrid manner, showing cracking at the germanide precipitates and plastic deformation in the surrounding matrix forming equiaxed dimples. The effect of simultaneous addition of 1 wt% Ge and 0.1 wt% Si on the mechanical properties is also discussed in relation to the effect of precipitate volume fraction and solid solution strengthening. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Near-α titanium (Ti) alloys have been developed for their application as compressor components in jet engines because of their high creep resistance and good mechanical property reten- tion at high temperatures. Conventional near-α Ti alloys such as IMI 834 are strengthened by fine precipitates of α 2 phase with DO 19 structure and intermetallic silicides [1–3]. However, during long exposure at high temperatures, these precipitates grow, coarsen, and cause embrittlement. The empirical aluminum (Al) equivalent formula given by Al þ 1/3Sn þ 1/6Zr þ 10O in wt% has been used to predict the formation of α 2 phase or ordering associated with the amounts of the α-stabilizing elements [4]. Silicon (Si) addition has been suppressed to less than or compar- able to 0.45 wt%, because the solubility of Si in the α phase is low and large amounts of silicide formation lead to embrittlement. Many of the earlier studies have focused on the mechanical properties of near-α Ti alloys with co-precipitation [5,6], whereas there are only a few reports on the effect of exclusive precipitates in near-α alloys [7–10]. It is well known that Si addition is effective in increasing high temperature tensile and creep strength of α and α–β Ti alloys [11– 14]. This is partly achieved by solid solution strengthening and precipitation strengthening by silicides. As mentioned before, the solubility of Si in the α phase is low. However, even with minor Si addition in the solution, Si atoms interact with dislocations to increase the energy barriers for slip and cross-slip [14], causing the increase of the tensile strength that can be maintained even at high temperatures [12,13]. Near-α alloys such as IMI 834 and Ti- 1100 include relatively high Si content to enhance the formation of silicides. These silicides precipitate at prior-β grain boundaries, α plate boundaries and/or on dislocations formed within the α plates, depending on the composition and heat treatment condition [3,5,8,9–11,15,16]. These silicides retard grain sliding and impede the dislocation movement by pinning, resulting in an increase of the tensile and creep strength [8,9,11]. However, it has been reported that silicide precipitation decreases the ductility [7,9,10,12] because fracture occurs due to the linkage of the cracks that are initiated in the regions where the slip bands intersect with silicide particles [5]. In this study, the effect of germanium (Ge), which has several similar aspects to Si, on the mechanical properties of a near-α Ti alloy is investigated. For example, Ge and Si are completely soluble, Ge can form Ti 5 Ge 3 germanide having the same crystal structure as Ti 5 Si 3 silicide, and Ti 5 Ge 3 and Ti 5 Si 3 are completely soluble in a ternary Ti–Ge–Si system [17]. Moreover, it has been shown that the lattice parameters a and c increase as the Ge amount increases in Ti 5 (Si 1 x Ge x ) 3 [17], in which the lattice misfit between the precipitate and α and β phases could change. More- over, the solubility of Ge in the α phase is higher than that of Si [18,19]. Thus, Ge has the potential to increase high temperature strength and improve ductility. However, there have been no studies about the effect of Ge addition on the mechanical proper- ties of near-α Ti alloys. This paper reports on the effect of Ge Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.12.099 n Corresponding author. Tel.: þ81 29 859 2428; fax: þ81 29 859 2501. E-mail address: Kitashima.Tomonori@nims.go.jp (T. Kitashima). Materials Science & Engineering A 597 (2014) 212–218