Phase transformations and modulated microstructures in Ti-Al-Nb alloys
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I.
INTRODUCTION
IN recent years increasing attention has been drawn to certain anomalies observed during decomposition of the ordered/3(B2, DO3) phases in a wide variety of alloy systems. In the first extensive attempt to characterize systematically these effects Delaey, Perkins, and Massalski I observed that many quenched and aged beta-brass type alloys when examined through the use of transmission electron microscopy displayed contrast such as "mottling, striations, crosshatching, etc." and diffuse streaking and extra maxima in certain positions of electron diffraction patterns. Some of these effects were attributed to the appearance of transformation products commonly observed during decomposition of the disordered/3 phase in many Ti and Zr alloys. In years following similar-type phenomena have been observed in a wide variety of ordered/3 phases occurring in such diverse alloy systems as Hume-Rothery alloys, shapememory alloys, and various aluminide systems such as NiA1. In many cases the exact nature of the decomposition products and their mechanism of formation is still the subject of considerable controversy. Recently a new family of Ti alloys based roughly on the composition Ti3A1 with ternary additions of Nb has been developed. In these alloys it is possible, as will be shown, to produce a B2(CsC1) type structure over a wide range of alloy compositions. In the present study we have investigated the decomposition modes of this ordered/3 phase, in light of some of the issues raised above, through the use of transmission electron microscopy and selected area electron diffraction.
R. STRYCHOR, Graduate Student, and J.C. WILLIAMS, Professor and Dean, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Schenley Park, Pittsburgh, PA 15213. W. A. SOFFA is Professor, Department of Metallurgy and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261. This paper is based on a presentation made in the symposium "Pretransformation Behavior Related to Displacive Transformations in Alloys" presented at the 1986 annual AIME meeting in New Orleans, March 2-6, 1986, under the auspices of the ASM-MSD Structures Committee.
METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL
Ti-A1-Nb alloys containing --25 pct A1 and - 5 to 17 at. pct Nb were cut into thin ( - 0 . 7 5 mm thick) slices and then sealed into evacuated quartz ampoules. These capsules were then solutionized in the single phase/3 field at 1250 ~ for 0.5 hour and rapidly quenched. Aging of selected alloys was performed for various times over temperatures of 400 ~ to 500 ~ Following heat treatment thin foils were prepared and the microstructure of these alloys was examined through the use of transmission electron microscopy and selected area electron diffraction using a JEOL 120CX electron microscope.
III.
RESULTS
An equilibrium ternary section for the Ti3A1 + Nb system with varying percentages of Nb which has been proposed by several investigators 2,3 is shown in Figure 1. The solid lines represent su
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