The role of boron in ductilizing Ni 3 Al
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I.
INTRODUCTION
THEaluminide Ni3AI has been receiving increasing attention lately because of its potential for engineering applications at elevated temperatures. One of the attractive features of this alloy is that its flow stress increases with temperature. The limitation of room temperature brittleness of polycrystalline Ni3AI has been recently overcome through research at Oak Ridge National Laboratories which has shown that B-doped polycrystalline Ni3AI has significant room temperature ductility. Boron is believed to modify grain boundary cohesive energy, thereby ductilizing polycrystalline Ni3AI at room temperature. An extensive study of the effect of boron on grain boundaries in Ni3A1 has been recently reported. ~ In addition, several other investigators have also studied the mechanical behavior of Ni3A1 and B-doped Ni3A1.2-9 This paper reports the results of an investigation of the grain size effects in B-doped, extruded Ni3A1, made by powder metallurgy processing. These results point to an interesting aspect of the role of boron which has not been emphasized before.
II. EXPERIMENTAL PROCEDURE AND RESULTS
Ni3A1 powder prealloyed with boron and hafnium was received from Oak Ridge National Laboratory, Oak Ridge, TN. The composition has been analyzed to be 0.16 at. pct boron, 0.38 at. pct Hf, and 24.04 at. pct A1, with the balance nickel. The powder was canned in a mild steel container and extruded at 16:1 area reduction ratio at 1400 K. Extruded specimens were heat treated in flowing argon to obtain a range of grain sizes. Tensile specimens were machined from the extruded as well as the heat treated rods and tested at room temperature on an Instron testing machine. Only one specimen per grain size was utilized for lack of more material. All the tensile tests were conducted in air. The details of the heat treatments, resulting grain sizes, tensile specimen dimensions, and tensile properties obtained are listed in Table I. P.S. KHADKIKAR, Graduate Student, and K. VEDULA, Associate Professor of Metallurgy, are with the Department of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, OH 44106. B. S. SHABEL is Scientific Associate, Alloy Technology Division, Alcoa Laboratories, Alcoa Center, PA 15069. Manuscript submitted June 13, 1986. METALLURGICALTRANSACTIONS A
Optical as well as scanning electron microscopy was employed to examine the microstructure. Grain size was estimated using the linear intercept method. Fracture surfaces of the tensile test specimens were observed using a JEOL 35CF scanning electron microscope. The yield strength is plotted as a function of (grain size) -~/2 in Figure 1. A good straight line fit is observed, indicating agreement with Hall-Petch behavior. Data for room temperature tensile yield strength of ductile B-doped Ni3AI alloys and brittle undoped Ni3AI alloys, compiled from literature 1,2,4,s,9along with data from the present study, are plotted as a function of (grain size) -'2 in Figure 2. The details of composition, grain size, tensile test specimen
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