L1₀ FeNi Tetrataenite in Iron Meteorites: An Atom Probe Tomography Investigation

Metal matrix in siderites are iron based alloys containing up to 35at%Ni as well as other minor elements such as Co and P, at levels lower than 1at%, the microstructure of which keeps records of their thermal and mechanical history. The initial metal alloy was melted in the heart of planetesimals in the early days of the solar system. After formation, the planetesimal slowly cooled down, leading to the solidification of the metal alloy as FCC austenite. Due to the extremely slow cooling rate (between 1 and 100K/10⁶ years), austenite grains could reach sizes up to several meters. During cooling, austenite experienced several solid-state transformations, starting with the precipitation of Widmanstätten ferrite, resulting in the development of the iconic microstructures of iron meteorites [1]. As it grows, ferrite rejects nickel, creating a Ni composition gradient in austenite, that extends over several hundred microns. This gradient, starts at more than 50% Ni at the interface with the ferrite, down to the initial Ni content, usually close to 7at%Ni. This naturally graded material makes it possible to explore the Fe rich part of the low temperature FeNi phase diagram [2]. In addition, due to the small cooling rate it possible to explore the reach full equilibrium conditions, at least for temperatures higher than approx. 300°C.<br/>In this work, we used atom probe tomography (APT), combined with electron backscattered diffraction (EBSD) and transmission Kikushi diffraction (TKD) on focused ion beam (FIB) lift outs for site specific specimen preparation, allowing investigation of the compositional and nanostructural features of this gradient material. The fine scale complexity of the final microstructures revealed in octaedrites and ataxites, two types of Ni rich iron meteorites, is discussed in the framework of the FeNi phase diagram. Of particular technological interest is Fe-36Ni alloy, also known as INVAR®, for which equilibrium phases can be described, providing an alternative investigation route to electron irradiation. Another technologically important phase present in iron meteorites is tetrataenite, an equiatomic FeNi L1₀ ordered variant of austenite. This phase, only present in meteoritic irons, is an extremely promising substitute to rare earth element (REE) based permanent magnet, as it exhibits very similar magnetic properties. Being REE free, and based on low cost and widely available elements, it would reduce dependence on toxic, environmentally harmful rare earth mines, and on China’s multi-billion rare earth element monopoly. Understanding the formation path of tetrataenite is becoming a global geo-strategic issue of the next decade. Complex nanostructures containing tetrataenite are analysed by atom probe tomography, and some will be shown, in particular in the Santa Catharina meteorite.<br/><br/>References:<br/>[1] Buchwald V.F., Handbook of Iron Meteorites - Vol. 1 (1975) Univ. of California Press.<br/>[2] Goldstein J.I., Short J.M., Geochim. Cosmochim. Acta 31 (1967) 1733-1770