Silicide Formation in Ti-3Al-8V-6Cr-4Zr-4Mo
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I.
INTRODUCTION
TI-3A1-8V-6Cr-4Zr-4Mo (Beta-C) is a beta titanium alloy. Beta titanium alloys are heat treatable and deep hardenable and they have attractive properties including good corrosion resistance and high strength to weight ratio. Due to these properties, their importance and utilization in the aerospace and other industrial markets is ever increasing. A number of phase transformations take place in beta titanium alloys. ~'2 Among other phases, silicide of the (Ti, Zr)5 Si3 type having a hexagonal crystal structure can form in Beta-C. 3'4 No further information, such as space group, is available on this silicide. This type of silicide can also form in other titanium alloys which contain zirconium as an alloying element. So far, solvus of this silicide is not known. In addition, there is no information available as to the nature and distribution of silicide particles as a function of thermomechanical treatments. The aim of this study was (1) to further characterize the silicide, (2) to determine its solvus, (3)to study its morphology and distribution as a function of heat treatments and thermomechanical treatments, and (4) to determine its effect on properties. It was felt that such an understanding is essential in designing manufacturing process resulting in optimum mechanical properties.
II.
EXPERIMENTAL
The chemical composition of Beta-C alloys used in most of this study is given in Table I. The/3-transus of this alloy is about 1066 K. The alloy was thermomechanically processed to 1.6 cm bar from a production ingot and /3-annealed for 1 hour at 1144 K, AC. All the subsequent heat treatments were in addition to the initial/3-anneal. Heat treatments, which were carried out in air, were terminated by either air cooling or water quenching as indicated. The chemical composition of the Beta-C alloy used for tensile property evaluation is similar to that given in Table I. S. ANKEM is Assistant Professor, Engineering Materials Program, Department of Chemical and Nuclear Engineering, University of Maryland, College Park, MD 20742. D. BANERJEE is Senior Scientific Officer, Defense Metallurgical Research Laboratory, Hyderabad, India. D.J. McNEISH, Supervisor, Metallurgical Research Services, and S.R. SEAGLE, Vice President, are with RMI Company, Niles, OH. J C WILLIAMS is Dean of Carnegie Institute of Technology, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted December 17, 1986. METALLURGICALTRANSACTIONS A
An Amray 1600 scanning electron microscope was used for all SEM work. The specimens for SEM study were electropolished with an electrolyte consisting of 60 ml perchloric acid, 600 ml methanol, and 360 ml ethylene glycol. It is important to note that the specimens should not be etched for SEM examination because the etchant dissolves the silicide particles. As a result, pits can be found in place of silicide particles. Thin foils for TEM were prepared by polishing in a solution of sulfuric acid and methanol. 5 However, the second phase particles separated from the matrix during this th
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