Identification of the Interface Phase in Titanium Alloys
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Table I.
D. BANERJEE
Titanium alloys typically consist of mixtures of the o~ (hcp) and/3 (bcc) phases in which the a phase normally precipitates in a /3 matrix with the classic Burgers Relationship: (110) 8 II (0001)+, [1]'1]~ II [1120]~. mwide variety of such alloys exhibit the presence of a thin layer of an 'interface phase', as shown schematically in Figure 1. The interface phase has been identified as having either an fcc structure, 1 or an hcp structure (Type 2(~) which is twin related on {1012} to the Burgers a.2 These two structures appear to exist independently for a given alloy composition and heat treatment (Table I). While both types of interface phase exhibit a complex crystallography, there are similarities in their diffraction patterns 11 as well as in their morphology (see Figure 3(b), for instance). The purpose of this note, therefore, is to analyze the diffraction effects from each phase type and point out ways of clearly distinguishing between the two. The crystallography of the fcc phase in relation to Burgers oz and /3 is shown in Figure 2. Two distinct orientation relationships are observed: 3'4'12 (1T0)fcc II (1T00)t ;[001]fr162II [0001]~ and(111)fcr II (0001);[ll0]fcc II [1120]~, and are designated Type 1 fcc and Type 2 fcc, respectively. Of the three possible variants of the Type 1 f c c / a relationship, only one is observed at the a//3 interface, and this variant is indicated i n Figure 2 as variant A. Due to the asymmetry of the Burgers relationship, it is possible to distinguish between the three (1120)+ zone axes
Observations of Interface Phase in Titanium Alloys (Compositions in Wt Pct)
Alloy Class Composition Near t~ Ti-6A1-5Zr-0.5Mo-0.25Si (IMI-685) Ti-6Al-l.5Zr-lMo-0.35Si0.1Si (Ti-ll) Ti-6Al-lZr-3Mo-0.25Si (BT 9) Ti-6A1-2Sn-4Zr-2Mo a + /3 Ti-6A1-4V Ti-6AI-4V Ti-8Mn Ti-4.5A1-5Mo-I.5Cr (CORONA 5) Metastable Ti-3A1-8V-6Cr-4Mo-4Zr /3 (Beta-C) Ti-ll.6Mo Ti-14Mo-6A1
Type of Interface Phase fcc
Reference 3,4
unknown
5
fcc
3
fcc fcc type 2c+ fcc type 2t~ unknown
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type 2a type 2t~
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INTERFACEPHASE
A SCHEMATICREPRESENTATIONOF INTERFACE PHASE IN A TRANFORMEDfl STRUCTURE
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Fig. 1 - A schematic representation of the interface phase in the transformed/3 structure of titanium alloys.
Fig. 2--A stereographic projection showing the relationship between Burgers a, Type 1 fcc and Type 2 fcc phases. The (334)B habit plane for Burgers a and {10T2}habit planes for Type 2a are also indicated.
D. BANERJEE is Group Leader, Electron Microscopy, at Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad500258, India. Manuscript submittedMarch 18, 1981.
of Burgers a in relation to the/3 or fcc phases. The diffrac
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