Recrystallization during Thermomechanical Processing of IMI834
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INTRODUCTION
THE near-a titanium alloy IMI834 (Ti-5.8Al-4Sn3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C) is employed in the aerospace industry, due to its creep resistance and mechanical property retention at operating temperatures up to ~600 C.[1] For IMI834 compressor disk applications, thermomechanical processing (TMP) is employed to produce a bimodal a + b microstructure of equiaxed primary a in a fine transformed b matrix, to balance the superior creep resistance of a lamellar morphology with the high-cycle fatigue resistance of an equiaxed structure.[2] Accordingly, deformation and heat-treatment stages during TMP are performed at temperatures above and below the b fi a + b transformation temperature (b transus). The accompanying allotropic a/b phase transformation produces a wide variation in the scale and distribution of phases that may feature lamellar, equiaxed, and bimodal morphologies. Consequently, the characterization of the microstructure development has received significant interest as an avenue of continuous improvement in the forging P. VO, Graduate Student, and S. YUE, Professor, are with the Department of Mining and Materials Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. M. JAHAZI, Senior Research Officer and Program Manager, is with the National Research Council Canada, Institute for Aerospace Research, Aerospace Manufacturing Technology Center, Montreal, PQ, Canada H3T 2B2. Contact e-mail: [email protected] Manuscript submitted April 2, 2008. Article published online September 27, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
process and has been the subject of a number of general reviews.[2–5] A basic forging process, shown schematically in Figure 1, involves primary processing (ingot breakdown) stages, for the conversion of a cast ingot to a forged billet, and secondary processing (closed die forging), for production of the final product form. Typical forging operations are performed at temperatures near the b transus with b forging (e.g., supertransus), generally less than ~150 C above the transus, and with a + b forging (subtransus), greater than ~50 C below the transus.[6] Primary processing employs a b forging stage, to homogenize and refine the as-cast structure through b recrystallization. An a + b forging stage is then employed, to obtain the bimodal a + b microstructure, through a globularization of the lamellar a microstructure produced following the previous b forging stage. During secondary processing, forging may be performed in either temperature regime, although a + b forging is more common, in order to retain the bimodal a + b microstructure. In an initial bimodal a + b microstructure, equiaxed a grains act as hard inclusions and deformation is generally restricted to the softer b matrix.[7] Consequently, the development of b phase characteristics such as grain size and morphology are significant at temperatures in the a + b phase field as well as the b phase field. Referring to Figure 1, the characterization of microstructure development through b phase evolution is thu
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