High temperature low cycle fatigue in beta processed Ti-5Al-5Sn-2Zr-2Mo-0.25Si
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IN recent years the aircraft industry has expanded the use of titanium alloys into the 500 ~ temperature range to produce lighter rotating jet engine components and lighter airframe elements. High temperature titanium alloys in current use are defined as near alpha alloys with relatively low content of beta stabilizing elements. Alloys such as Ti-6A1-2Sn-4Zr-2Mo, ~Ti-1 l,: IMI-6853 and Ti-5A1-5Sn-2Zr-2Mo 4 were developed to operate under prolonged exposure at 500 ~ The alloy Ti5A1-5Sn-2Zr-2Mo-0.25Si (Ti-5522S), the subject of this work, was developed through an effort sponsored by the Air Force Materials Laboratory.4-6 Microstructures resulting from cooling through the fl-transus temperature proved, in most cases, to have best creep resistance of this alloy group. 2~,7 These microstructures typically consist of packets of a-platelets, similarly aligned and crystallographically oriented, separated by films of fl-phase. In wrought material, these microstructures can be obtained either by solution treatment or work in the fl-phase field; this super transus working offers the advantage of lower press load requirements 8,9 and better shape definition. Recent work by Bania ~~showed that strain control high temperature low cycle fatigue (HTLCF) strength of fl-processed Ti-5522S is inferior to a + fl processed material. The objective of this work was to study in more detail the behavior of the microstructures resulting from fl-processing and determine the mechanisms responsible for their poor HTLCF properties. EXPERIMENTAL PROCEDURE Material The Ti-5522S tested in this investigation and in the previous work, I~was taken from a single heat produced by RMI and supplied in 75 mm diam round bars. The
D. EYLON and M. E. ROSENBLUM are with Metcut-Materials Research Group, Wright-Patterson AFB, OH 45433. T. L. BARTEL, formerly with the Metals and Ceramics Division, AFML, W-PAFB, OH 45433, is now with National-Standard Co., Niles, MI 49120. Manuscript submitted April 9, 1979.
fl-transus temperature (Ta) of this heat was determined to be 986 _ 2 ~ A small amount of Y203 (Yttria) was added to the alloy (approximately 250 ppm) to improve hot work-ability; chemical composition of the asreceived material is given in Table I. Processing and Microstructure To produce the desired microstructures, two combinations of processing and heat treatment in the a + fl or f l - phase fields were used. The material was worked in a two-step process; a 6:1 ratio extrusion (from 76 mm to 32 mm diam) followed by a 3:1 ratio hot swage (from 32 mm to 19 mm diam) with an air cool. Processing temperatures of the examined conditions are summarized in Table II and Figs. l(a) and (b); fl-solutioning time was 1 h and stabilizing treatment time was 2 h, both followed by an air cool. The a-grain structure, as well as the prior beta grain (PBG) structure of the two conditions, can be seen at low and high magnifications in Figs. l(c) through (f). Specimens were etched with Kroll's reagent and all photomicrographs were taken with polarized light and Nomarski diffe
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