Effect of thermomechanical treatments on the room-temperature mechanical behavior of iron aluminide Fe 3 AI
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I.
INTRODUCTION
ORDERED iron aluminide intermetallics of composition Fe3A1 and FeAI possess attractive properties for application as structural materials at elevated temperahares in aggressive environments.I~z3~ However, their poor roomtemperature ductility limits their use as engineering materials as they are difficult to process into useful shapes such as plates and tubes. In recent years, efforts have been intensified to identify both the intrinsic as well as the extrinsic factors governing their room-temperature brittle fracture. [41 Several studies have established that the room-temperature ductility is caused mainly by an extrinsic effect--environmental embrittlement due to hydrogen.Is,6.n Several methods have been proposed to minimize hydrogen embrittlement (HE) in iron aluminides. Most of the methods that have been suggested to curb HE aim to restrict entry of hydrogen into the lattice by providing a passive film on the iron aluminide surface. Oxide coatings have been found to be beneficial in increasing the ductility of iron aluminides.t81 Another method for improving the ductility of iron aluminides is by the addition of chromium. Even small chromium additions are effective in minimizing HE and providing ductility.tgI The role of chromium in affecting HE and causing ductility enhancement has been elucidated from an electrochemical viewpoint.rio] It has been recently shown that thermomechanical treatments (TMT) also play a crucial role in affecting the mechanical properties of Fe25A1-1 B intermetallic alloy,vq The present investigation aims at understanding the ef-
ARVIND AGARWAL, Graduate Student, R. BALASUBRAMANIAM, Assistant Professor, and S. BHARGAVA, Professor, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India. Manuscript submitted February 21, 1995. METALLURGICALAND MATERIALSTRANSACTIONS A
fect of various TMT on the mechanical behavior of Fe-25AI iron aluminide.
II.
EXPERIMENTAL PROCEDURE
The starting intermetallic alloy was of composition Fe25A1 and was supplied by the Defence Metallurgical Research Laboratory (DMRL) (Hyderabad) in the form of a cast pancake of about 120-mm diameter and 15-mm height. The composition of the alloy was analyzed by a JEOL* *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
electron probe microanalyzer and was found to be 74.72Fe25.28A1 (_+ 0.25). Rectangular pieces of 10-mm thickness were cut from this pancake and subsequently homogenized at 1000 ~ for 4 hours prior to their rolling via different thermomechanical schedules. Various thermomechanical schedules followed in the present study are schematically shown in Figure 1. These schedules basically consisted of deformation of the Fe-25AI alloy in three different phase fields, namely, (1) in the disordered a phase field at 1000 ~ (2) in the ordered B2 phase field at 800 ~ and (3) in the ordered D O 3 phase field at 500 ~ Prior to rolling, the samples were soaked in a furnace in an air atmosphere at their respective rolling temperatur
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