Deformation and Strain Storage Mechanisms during High-Temperature Compression of a Powder Metallurgy Nickel-Base Superal
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INTRODUCTION
TURBINE disks are among the most critical components in aircraft and power generation turbines. Due to the requirement for their operation within high stress and temperature environments, turbine disks are often made of powder-consolidated nickel-base superalloys. Fatigue is one of the most critical properties for turbine disk alloys, and because superalloy disk materials are mostly free of any extrinsic defects, the grain structure has a significant influence on fatigue properties.[1–8] However, there is a lack of understanding of how the grain structure evolves throughout the complicated thermomechanical processing of superalloys. Powder metallurgy (P/M) processing of superalloys requires consolidation, usually by extrusion, followed by subsequent isothermal forging.[9] Under isothermal forging conditions, the forming process is nominally superplastic, with the advantage of low forging stresses.[9] Furthermore, superplastic forging allows the material to accept large strains without cracking, WEN J. TU, Postdoctoral Candidate, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] TRESA M. POLLOCK, Professor, is with the Department of Materials Science and Engineering, University of California–Santa Barbara, Santa Barbara, CA 93106. Manuscript submitted September 16, 2009. Article published online June 2, 2010 2002—VOLUME 41A, AUGUST 2010
making forming of large disks with uniform material properties possible. Following isothermal forging, the superalloy material undergoes several heat treatments to reach a desired operating microstructure.[10] There have been many studies of the evolution of grain size during processing of Ni-based superalloys and some attempts at modeling microstructural evolution.[11–13] Though these studies are very detailed in their analysis of grain growth during heat treatments, there is still an incomplete understanding of the effects of the deformation processing operations (extrusion, forging) on the microstructure and grain growth during subsequent heat treatments. The objective for this research is to experimentally examine strain storage at the grain scale and grain structure evolution during high-temperature compression across a range of strain rate and temperature. The focus of the present study is on superplastic deformation in order to evaluate the microstructural consequences of this mode of deformation processing and its ultimate influence on grain structure and mechanical properties.
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MATERIALS AND EXPERIMENTAL PROCEDURES
Experiments were conducted on an as-extruded alloy designated Rene´ 88DT with nominal composition listed in Table I. METALLURGICAL AND MATERIALS TRANSACTIONS A
Table I.
Rene´ 88DT Alloy Composition in Weight Percent[14]
Al
Ti
Cr
Co
Zr
Nb
Mo
W
C
B
Ni
2.1
3.7
16
13
0.04
0.7
4
4
0.07
0.015
bal
A. Compression Tests Cylindrical compression samples measuring 7 mm in height with a 4-mm diameter were machined using wire electrical discharge
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