Structural transformations and magnetic properties of plastically deformed FeNi-based alloys synthesized from meteoritic matter

Abstract In this work we investigated FeNi-based alloys synthesized from the cosmic matter of Morasko Meteorite (MM). The fall of Morasko iron meteorite is one of the biggest known in Central Europe and it has been dated to take place about five thousand years ago. It is known that iron meteorites contain hard magnetic tetrataenite phase (L10 FeNi), which has not been synthesized on the Earth in significant amount in the bulk form yet, because of very sluggish diffusion of Fe and Ni atoms below the disorder-order temperature of 320°C. Firstly we doped the metallic part of Morasko Meteorite with Ni to obtain nearly equiatomic composition Fe51Ni49, which is the composition of pure tetrataenite. Further synthesis steps (melt-spinning, high pressure torsion, isothermal annealing at Ta = 320°C for τa = 720 h) were undertaken to induce structural disorder and to observe changes in the structure/microstructure of the meteorite melt with nickel and its magnetic behavior. The presence of fcc FeNi with slightly varying lattice constant was confirmed at each step of the treatment. It evolved from the initial value of a = 3.589 ± 0.001 A in the as-quenched state, through 3.591 ± 0.001 A after 10 revolutions in the high pressure torsion process, to the maximum value of 3.593 ± 0.001 A after 30 revolutions of anvils. No significant impact of further application of stress (50 revolutions) was noted, as the lattice parameter changed within the uncertainty limit. After annealing, the lattice parameter decreased to the initial value of the as-quenched sample (a = 3.589 ± 0.001 A). The observed broadening of the peaks in the X-ray diffraction patterns may be attributed to the presence of microstrains and refinement of crystallites. Both contributions were determined using the Williamson-Hall method. In the initial state, the domain sizes were of about 100 ± 17 nm and diminished to 36 ± 11 nm after application of stress. Further isothermal annealing did not change the crystallites size, which was evaluated to be 35 ± 9 nm. Microstrains in the initial state were determined as (0.039 ± 0.017)10-3 and increased significantly to (0.562 ± 0.060)10-3 after 30 revolutions of anvils. Isothermal annealing decreased microstrains to (0.215 ± 0.071)10-3. After application of stress, the coercivity and remanence were enhanced, while subsequent isothermal annealing caused their decrease. Moreover, severe plastic deformation facilitated saturation of magnetization. As-quenched sample reached saturation at a magnetic field of about 2 T, while for high pressure torsion processed alloy this value was significantly reduced.