The Influence of Precipitation of Alpha2 on Properties and Microstructure in TIMETAL 6-4
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INTRODUCTION
THE alloy Ti-6Al-4V is commonly referred to as the workhorse Ti alloy because it is the most widely used Ti alloy. It was developed over 50 years ago and a great deal of research has been carried out aimed at understanding how to optimize the microstructure for specific properties. But, despite that, there are some areas of uncertainty and this paper is focussed mainly on just one aspect—the development of order and/or precipitation of ordered a2 in the alpha phase and its influence on properties. Earlier experimental work[1,2] has shown that the alpha phase in a-stabilized alloys is able to develop ZHIWEI WU, formerly with IRC in Materials, University of Birmingham, Edgbaston B15 2TT, U.K., is now Research Engineer, with the GE Global Research, China Technology Centre, Shanghai, 201203, P.R. China. CHUNLEI QIU, Research Fellow, is with IRC in Materials, University of Birmingham. V. VENKATESH, Staff Scientist, is with Pratt & Whiney Aircraft, 400 Main Street, M/S 114-41, East Hartford, CT. HAMISH L. FRASER, Professor, R.E.A. WILLIAMS, Research Scientist, and G.B. VISWANATHAN, Staff Scientist, are with Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH. MATTHEW THOMAS, Metallurgist, is with Timet U.K., PO BOX 704, Witton, Birmingham, B6 7UR, U.K., S. NAG, Research Fellow, and RAJARSHI BANERJEE, Professor, are with Department of Materials Science and Engineering, University of North Texas, Denton, TX. MICHAEL H. LORETTO, Professor, is with IRC in Materials, University of Birmingham. Contact e-mail: M.H.Loretto@ bham.ac.uk Manuscript submitted April 17, 2012. Article published online November 13, 2012 1706—VOLUME 44A, APRIL 2013
short-range order (SRO); ordering is expected from phase diagram modeling.[3] Other work on deformed Ti6Al[2] had shown that weak fringes were observed in TEM images within the planar slip bands and these were interpreted as being due to the destruction of SRO by the movement of dislocations. In fact, there have been many examples [e.g., References 4 through 8] of the observation of planar slip in Ti alloys with more than 4 pct Al and of fringes in other alloys which could well have been due to the destruction of order. The work presented in this paper has been carried out with the aim of establishing whether ordering or precipitation of a2 occurs in the alloy Ti-6Al-4V, and if so, if there is an influence on the tensile and fatigue properties.
II.
EXPERIMENTAL
The Ti-6Al-4V used in most of this work was prepared from PREP powder that was HIPped (Hot Isostatically Pressed) at 1203 K (930 C) for 4 h at a pressure of 150 Mpa and cooled in the HIP at 5 K/min, and its analysis is shown in Table I. In addition, a limited amount of work was carried out on forged and heat-treated Ti6Al4V. Cylindrical samples of Ti6Al4V, which were large enough to prepare standard tensile and fatigue samples, were cut from these HIPped samples and some of these were used to define the properties and microstructure in this standard condition and some o
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