Ductility and Fracture of FeAl: Effects of Composition and Environment
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Dog-bone specimens with two different gage sections were utilized for the tensile tests in UHV and hydrogen. The first set of specimens, with gage sections 3.2 x 1.6 x 0.5 (mm), were electro-discharge machined (EDM) from the extruded ingots of B-doped Fe-40A1 and B-free Fe-45A1 with their long axes parallel to the extrusion direction. Each specimen then had -0.08 mm removed from their EDM surfaces by grinding prior to annealing in vacuum for lh at either 1158K (B-doped Fe-40AI) or 1203 K (B-free Fe-45AI), followed by 5d at 673 K for both alloys. After annealing, all the specimens were hand polished to reduce surface defects, finishing with 4000 grit SiC paper. Another set of specimens, having a somewhat larger gage section of 7.4 x 1.7 x 0.5 (mm), were EDM from the remaining alloys (i.e., B-free Fe-37A1, Fe-40A1, Fe-48A1; and B-doped Fe-40AI, Fe-45A1, Fe-48AI). These specimens were also polished to remove surface defects. The specimens in the second set were given a grainsize anneal of 1073 K for lh followed by a vacancy-removal anneal at 673 K for 5d. The specimens were tensile tested at room temperature in an ultrahigh vacuum (UHV) chamber with a base pressure less than 1 X 10-7 Pa. Dry high purity H2 was leaked into the UHV chamber until the desired pressure was reached. Additional details of the experimental set-up have been described previously [ 19]. The two different gage lengths yielded strain rates of 5 x 10 or 1.2 x 10-3 s-1, both of which are in the range where the room-temperature ductility of FeAl is relatively insensitive to strain rate [20,211. The fracture surfaces of the tensile tested specimens were examined in a scanning electron microscope (SEM). A PHI 590 scanning Auger microprobe was utilized to study the segregation of boron to the grain boundaries of FeAl. Notched specimens were machined from the gage sections of tensile tested specimens, loaded into the UHV of the Auger microprobe, and fractured in-situ by striking with a sharp hammer blow. The Auger analyses were carried out in the pulse counting mode with a 5 KV primary electron beam and beam currents of -8 nA. Approximately 25 grain boundaries were analyzed in each specimen. RESULTS AND DISCUSSION The effects of stoichiometry and B-doping on the ductility and fracture behavior of FeA1 in UHV are summarized in Fig. 1.As the aluminum concentration increases beyond 37% Al, the ductil-
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Figure 1. Comparison of the effects of alloy stoichiometry and B-doping on the ductility and fracture mode of FeAI alloys in UHV. KK4.7.2
ity of B-free FeAl drops off precipitously. Associated with this drop in ductility is a change in the fracture mode from transgranular to intergranular. Since tests conducted in UHV presumably measure the "intrinsic" ductility, the present results show that FeAI becomes intrinsically brittle when its composition approaches the stoichiometric
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