Effect of thermal processing on the microstructure of Ti-26Al-11Nb: Applications to fusion welding

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I.

INTRODUCTION

ADVANCED titantium-aluminum intermetallic alloys have certain properties which make them potentially attractive as aerospace materials, t~j Successful joining of these materials will increase their utility in engineering applications. Virtually all commercial joining operations (welding, brazing, diffusion bonding, etc.) involve a hightemperature thermal excursion where properties meticulously imparted by prior thermomechanical processing may be lost. In particular, fusion welding operations necessarily destroy all remnants of prior processing, at least in the fusion zone. Cooling of these welded parts to room temperature often results in the evolution of a microstructure in both the fusion zone and HAZ which bears little resemblance to the preprocessed microstructure. Nonequilibrium transformations are common in steels and conventional Ti alloys, t2-6] Very little work has appeared in the open literature which deals with either joiningt7,8~ or continuously cooled, transformed microstructures t9.1~ in highly alloyed Ti aluminides. No comprehensive study has been reported which examined the effect of cooling rate on the microstructure developed in these Ti aluminides. The primary purpose of this work was to develop an understanding of microstructural evolution during solidification and subsequent solid-state phase transformation in Ti-26Al-11Nb with the intent of anticipating the response of this material to fusion weld processing. The conclusions drawn, though, are quite general and can be

M.J. CIESLAK, Senior Member of Technical Staff, Process Metallurgy Division 1833, and T.J. HEADLEY, Senior Member of Technical Staff, Electron Optics Division 1822, are with Sandia National Laboratories, Albuquerque, NM 87185. W.A. BAESLACK HI, Professor, is with the Department of Welding Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted July 24, 1989. METALLURGICAL TRANSACTIONS A

applied to a wide variety of industrial operations involving continuous cooling processing, from rapid water quenching to extremely slow furnace cooling. II.

EXPERIMENTAL PROCEDURE

The heat of Ti-26Al-11Nb used in this study has the composition given in Table I. The material, in the form of 1.7-mm sheet stock, was processed to give a twophase microstructure of matrix a2 (ordered hexagonal, D019 ) with/3 (bcc) at the a 2 grain boundaries and triple points. Figure 1 shows the as-received microstructure of this alloy. Figures 2(a) and (b) are a backscattered electron image and X-ray map (Nb L~ radiation) pair for the as-received material showing the segregation of Nb to the/3 phase. Figure 3 is a transmission electron micrograph of the as-received structure showing a typical region which includes both grain boundary/3 and matrix ot2. Thermal processing of this material involved two types of experiments. The first set of experiments involved continuously cooling samples from the single-phase /3 region through the transformation temperature range at various rates. These experiments employed a Gleeble 1500

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