Shape Memory Properties of an Iron Modified Nickel Aluminide Alloy
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cast material could be produced with room temperature ductility and SME. 9-10 Alloying NiAl with Fe and B results in a fabricable alloy with a room temperature ductility in excess of 7%. '0 In the binary alloy, phase separation occurs during slow cooling, producing B2 and a metastable L10 phase. The L1 0 phase can transform to Ni5 A13 upon annealing below 700*C. Quenching suppresses the phase separation and results in a fully martensitic L10 structure. In the NiAl alloys with iron additions, annealing temperatures and quenching technique were optimized to produce a two-phase alloy consisting of approximately 5% L1 2 phase and the remainder an L 10 martensite.10 Yang and Wayman" found that in specimens of Ni-25 Al-15Fe (at. %) annealed at 400*C, Ni 5AI3 formation occurred more rapidly (one day) when the alloy was in the martensitic state than when it was in the austenitic state (one year). EXPERIMENTAL PROCEDURE Baseline properties were measured on a reference alloy, SMA-23, with a composition of Ni25.5 AI-16 Fe-0.14 B (at. %). Previous reports concerned the microstructure and crystallography 0 of the phase transitions,12 alloying effects and processing conditions to produce a ductile alloy,' temperatures.13 transformation and Al/Fe ratios vs. The alloy was arc melted as •-400 g buttons in an argon atmosphere and drop cast into rectangular copper chill molds. The ingots were clad in stainless steel and hot rolled at 10001200"C at 15-20% reduction in thickness per pass to a final thickness of 1.3-1.9 mm. The sheets were annealed in flowing argon at 1300TC for one half hour and then oil quenched to room temperature. Sheet tensile specimens were electrodischarge machined. The microstructures were examined by light microscopy, x-ray diffraction (XRD), and 549 Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
transmission electron microscopy (TEM). XRD was carried out with Cu Ka X-rays and a graphite diffracted-beam monochromator. TEM was carried out with a Philips CM30 operated at 300 kV with a GATAN heating stage. Heating to 200'C typically required only a few minutes to reach a point where the transformation was complete. Ductility measurements were made in a screw-driven Instron testing machine at a strain rate of 3.3 x 10-3 s-1. Shape memory recoverable strain measurements were made with a constant tensile load in a modified creep frame using an LVDT for length measurement. A current of up to 12 amps was passed through the predeformed sheet tensile specimens for heating. Typical dimensions for the specimens were 3.17 x 0.7 x 25 mm. A chromel-alumel thermocouple spot welded to the center of the gage length was used with a standard temperature controller. Cycle times of 40 s were typically used to cycle from 100 to 200 and back to 100°C. The main disadvantage of this arrangement was the grip end cooling effects; when the center of the specimen was at 200"C, at some point near the grip ends the temperature was such that some transformation would still be in progress. While this effect widens the appa
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