High Plasticity of an Iron Aluminide-based Material at Low Temperatures

Aims: To compare surprisingly high plasticity in compression at low temperature of high-quality compacts prepared by spark plasma sintering from atomized Fe-30.8Al-0.35Zr-0.11B (at%) powder with tensile tests at the same conditions.

Study Design: Compressive tests and tensile tests at room temperature and at 77 K, scanning and transmission electron microscopy, measurements of Young’s and shear moduli data of the sintered material from room temperature to 80 K.

Place and Duration of Study: Faculty of Mathematics and Physics, Institute of Plasma Physics, Institute of Thermomechanics, Department of Metals and Corrosion Engineering, between October 2015 and November 2017.

Methodology: The feedstock powder was prepared using atomization in argon and consolidated by spark plasma sintering method. The microstructure and phase composition of the sintered samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD) and by transmission electron microscopy (TEM). Mechanical properties of the feedstock powder were characterized by microhardness data, the compacts were in addition tested in compression and in tension. The elastic properties (Young’s and shear moduli) of the examined material were measured by a combination of two ultrasonic methods: the pulse-echo method and the resonant ultrasound spectroscopy.

Results: High plasticity (plastic strain more than 30% without failure) was observed in compressive tests at room temperature and at 77 K. Electron microscopy observations revealed the dominating role of dislocation motion in compression at low temperatures. The ductility measured at tensile tests, on the other hand, was only about 1% with a typical brittle failure.

Conclusion: The TEM observations confirm that dislocations enable the plastic flow in compression at low temperatures. The poor ductility in tension is not an intrinsic behavior of the alloy, but it results from the nucleation and opening of nano/microcracks between sintered powder particles and/or cavities in partly hollow atomized particles.