Tensile and impact properties of directionally solidified Fe-40AI intermetallic
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K.-M. CHANG, Staff Scientist, is with the Physical Metallurgy Laboratory, General Electric Corporate Research & Development (GE-CRD), Schenectady, NY 12301. Manuscript submitted March 15, 1990. METALLURGICAL TRANSACTIONS A
above 600 ~ Similar yield behavior has been reported in equiaxed FeA1 of various grain sizes. 0'2'31 Two data points from EX specimens tested at room temperature and 800 ~ are also shown in Figure 1. The yield strengths are equivalent for EX and DS structures. Yield strength exhibits a maximum at 600 ~ as seen in Figure 1. To confirm that this strength maximum was not due to scatter, a special tensile test, called the smallscale yielding test, was devised. Starting from 700 ~ one specimen (DS#) was loaded until yielding occurred and was unloaded at a plastic strain of about 2 pct. The specimen was then cooled down by 100 ~ and tested again for small-scale yielding. This process was repeated down to a testing temperature of 100 ~ where the specimen was strained to failure. The results of this smallscale yielding test are plotted in Figure 1 for comparison. The yield strength peak from this test was at 600 ~ confirming that the maximum is real. The stress-strain curves exhibited a smooth yield point at temperatures around the peak strength, i.e., at 600 ~ and 700 ~ This yield point behavior has been reported also in NiaAl-base single crystals. I8,9,1~ Figure 2 shows the tensile elongation and the Charpy impact energy as functions of temperature in DS Fe-40A1. Surprisingly, tensile ductility and impact toughness exhibit opposite dependences with temperature. Charpy impact energy decreases monotonically with temperature, while the elongation to failure in the tension test increases substantially as temperature increases. At 800 ~ DS Fe-40A1 has an elongation of 81 pct and an impact energy of 2.7 J. In contrast, room-temperature elongation is 3.2 pct, and impact energy is 76 J. Such an unusual correlation of ductility and toughness is believed to be associated with the transition in slip systems. Surface slip trace analysis and transmission electron microscopy both have confirmed that two slip systems operate in FeA1 intermetallics: (111) {110} at low temperatures and (100) {110} at high temperatures. [11.12]The transition in slip systems in iron-rich Fe-40A1 was estimated to occur near 700 ~ If the cubic slip is the cause of lower impact energy, the transition in slip direction is likely to occur over a temperature range rather than a certain point. Fracture toughness specimens loaded at different strain rates will be employed in the future to further clarify the situation. The EX structure reveals a temperature dependence of tensile elongation similar to that for the DS structure: 1.1 pct at room temperature and 32 pct at 800 ~ In comparison, DS offers better tensile ductility at all temperatures, although the strengths remain equal in the two structures. In summary, the mechanical properties of a DS Fe-40A1 intermetallic compound have been evaluated by tensile and impact tests from room temperature
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