L Journal of Alloys and Compounds 345 (2002) 221–227 www.elsevier.com / locate / jallcom Superior superplastic behavior in fine-grained Ti–6Al–4V sheet b, a a c c c * S.N. Patankar , J.P. Escobedo , D.P. Field , G. Salishchev , R.M. Galeyev , O.R. Valiakhmetov , b F.H. (Sam) Froes a School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA b IMAP, Mines Building, University of Idaho, Moscow, ID 83844-3026, USA c Institute for Metals Superplasticity Problems, Khalturin Str. 39, Ufa, Bashkortostan Republic, 450001 Russia Received 7 January 2002; received in revised form 22 February 2002; accepted 22 February 2002 Abstract The superplastic behavior of extremely fine-grained (300 nm) Ti–6Al–4V was studied by performing elevated temperature uniaxial 25 21 21 tensile tests in the temperature range of 700–900 8C. The strain rate was varied from 10 to 10 s to estimate the value of the strain rate sensitivity coefficient, m. The ductility of the fine-grained Ti–6Al–4V specimens was compared with that of a control set of conventional superplastic Ti–6Al–4V specimens with an average grain size of 3 mm. The results obtained indicate that the fine grain sized materials should see application in commercial use, with the caveat that a 1-mm grain size is considered optimum in terms of the superplastic forming temperature and subsequent creep behavior at use temperature.  2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal alloys; Microstructure; Mechanical properties 1. Introduction higher strain rates. Both these features are advantageous. The optimum strain rate for conventional superplasticity is 25 23 21 Superplasticity is the capability to deform crystalline in the range of 10 –10 s , which is quite slow for solids in tension to unusually large plastic strains. Pre- large scale forming of components. Therefore, a shift in 21 23 21 requisites for superplasticity include a large neck free optimum superplastic strain rates to 10 –10 s is tensile elongation, a low flow stress and high strain rate desirable. The objective of this paper is to report on a sensitivity. This mode of plastic deformation is normally study of the superplastic behavior of sub-micron Ti–6Al– observed at low strain rates and high homologous tempera- 4V (300-nm grain size) and compare it with the per- tures in material with thermally stable, fine and equiaxed formance of conventional superplastic Ti–6Al–4V with an grains [1–3]. Superplasticity in titanium alloys is of average grain size of 3 mm. interest commercially since these alloys are difficult to shape by conventional methods. In particular, the Ti–6Al– 4V alloy has been found to be superplastic and this has led 2. Experimental procedure to its widespread use in the manufacturing of aerospace components. The extensive use of this alloy in the aero- Ti–6Al–4V alloy sheet with two different grain sizes space industry has generated interest in further improving was obtained for this study: a fine-grained sheet of 2-mm the superplasticity of this alloy. thickness, and commercially available sheets of 1.36-mm Nanocrystalline or sub-micron grained materials provide thickness. The processing of the fine-grained Ti–6Al–4V an opportunity to investigate deformation mechanisms at involved multi stage forging in the temperature range of an extremely fine microstructural scale [4–6]. Because 600–700 8C [7]. Tensile specimens with cross-sections of superplasticity is a grain size dependent phenomenon, it is 5 mm and sheet thickness and gage lengths of 15 mm were expected that nanocrystalline / sub-micron grained materials machined from these sheets. Tensile tests were carried out would exhibit superplasticity at lower temperatures or on a screw driven tensile testing machine equipped with a three-zone split type resistance furnace. The tensile tests were carried out in an atmosphere of flowing Ar. The tests *Corresponding author. E-mail address: pata1630@uidaho.edu (S.N. Patankar). were performed at constant strain rates with the crosshead 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(02)00406-1