Materials Science and Engineering A 527 (2010) 6157–6165 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea High temperature deformation processing maps for boron modified Ti–6Al–4V alloys Indrani Sen a,∗ , Ravi Sankar Kottada b , U. Ramamurty a a Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India b Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Madras, Chennai 600036, India article info Article history: Received 27 January 2010 Received in revised form 24 April 2010 Accepted 16 June 2010 Keywords: Titanium alloys Electron microscopy Thermomechanical processing Recrystallization abstract The alloy, Ti–6Al–4V is an ˛ + ˇ Ti alloy that has large prior ˇ grain size (∼2 mm) in the as cast state. Minor addition of B (about 0.1 wt.%) to it refines the grain size significantly as well as produces in-situ TiB needles. The role played by these microstructural modifications on high temperature deformation processing maps of B-modified Ti64 alloys is examined in this paper. Power dissipation efficiency and instability maps have been generated within the temperature range of 750–1000 ◦ C and strain rate range of 10 -3 –10 +1 s -1 . Various deformation mechanisms, which operate in different temperature–strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate four distinct deformation domains within the range of experimen- tal conditions examined, with the combination of 900–1000 ◦ C and 10 -3 –10 -2 s -1 being the optimum for hot working. In that zone, dynamic globularization of ˛ laths is the principle deformation mecha- nism. The marked reduction in the prior ˇ grain size, achieved with the addition of B, does not appear to alter this domain markedly. The other domains, with negative values of instability parameter, show undesirable microstructural features such as extensive kinking/bending of ˛ laths and breaking of ˇ laths for Ti64–0.0B as well as generation of voids and cracks in the matrix and TiB needles in the B-modified alloys. © 2010 Elsevier B.V. All rights reserved. 1. Introduction and background Ti–6Al–4V (also referred as Ti64), an ˛ + ˇ titanium alloy is an important engineering alloy that is extensively used particularly in aerospace industry. This is due to its low density combined with high strength and toughness as well as outstanding corrosion resistance. An additional benefit associated with Ti alloys, in gen- eral, is that their properties are relatively temperature-insensitive between cryogenic temperature and ∼500 ◦ C. In the as-cast state, Ti64 exhibits the classical Widmanstätten microstructure of (hcp) ˛ and (bcc) ˇ phases. However, Ti alloys – and Ti64 is no excep- tion – typically suffer from large prior ˇ grain size, which tends be in the order of a few mm. Therefore it becomes necessary to break this coarse microstructure down, through several thermo- mechanical steps. Typically, this involves upset forging in the ˇ regime, i.e. above 1000 ◦ C. This not only adds considerably to the cost of the final product, but also brings in additional complexities. For example, the oxide layer that forms on the surface during forg- ing has to be machined out at each step, causing loss of material as well as adding to the manufacturing cost, as it otherwise could get ∗ Corresponding author. Fax: +91 80 2360 0472. E-mail address: indrani.indrup@gmail.com (I. Sen). included in the material leading to low fatigue performance. Thus, the necessity to break the coarse as-cast structure makes the fin- ished Ti alloy products considerably expensive vis-à-vis competing alloys. Recently, it was discovered that the addition of B in minor amounts (within the hypo-eutectic range) to Ti64 reduces the grain size significantly (by an order of magnitude) [1,2]. This circum- vents the need for processing steps such as ˇ upset forging and hence makes Ti alloys relatively more affordable. As a result, there has been considerable interest in understanding the mechanical behavior of these alloys. It has been shown that the microstruc- tural refinement leads to anomalous increase in elastic modulus, moderate enhancement in yield and ultimate strengths, and signif- icant benefit in terms of the unnotched fatigue performance of the Ti64 [3–7]. Although the addition of B leads to a markedly reduced grain size, it does not completely eliminate the need for some thermo- mechanical processing steps subsequent to casting of the alloy. These are necessary for, at least, two reasons. The first is to impart the desired shape to the alloy and the second is to close any as-cast porosity, which is otherwise detrimental to the fatigue perfor- mance of the alloy. Therefore, it is essential to determine the hot working conditions, i.e., optimum temperature and strain rate com- bination, for deformation that will yield required microstructures 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.06.044