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Rare-earth-free permanent magnet materials based on Mn show great promise for applications in electric motors and devices. The metastable ferromagnetic τ phase of the Mn-Al system has magnetic properties between those of the high-performance Nd-Fe-B magnets and the lower-performance ferrite magnets. However, the hybrid displacive-diffusional pathway of τ formation, from the parent ε phase through the intermediary ε’ phase, is still not fully understood. This phase transformation progression was studied in-situ using diffractive, calorimetric, and magnetometric techniques to show that the progression from ε to τ in Mn54Al46 at <450 ◦C involves the ordering of ε into ε’. Density functional theory calculations were performed on each phase and confirmed the experimental observation that the ε to ε’ to τ pathway is energetically favorable. Isothermal annealing of quenched-in ε at 350 ◦C demonstrated that ε’ is ferromagnetic, also in agreement with theoretical results, with a moderate coercivity of at least 50 kA/m. The τ phase was observed to nucleate along the prior ε phase grain boundaries and grow into the ε’ phase regions. A boundary front of ε’ was observed between the τ and ε phases. Both Kissinger and Flynn-Wall-Ozawa methods were used to determine the activation energies for the ε’ and τ phase transformations with values of ~140 kJ/mol obtained for both phases. Therefore, the ordering transformation to ε’ and the hybrid displacive-diffusional transformation to τ were shown to overcome the same magnitude energy barrier. Both activation energies were less than previous τ phase activation energies measured on Mn55Al45 in the absence of a significant ε’ ordering exotherm, providing a kinetic benefit to the ε to ε’ to τ pathway at 350 ◦C. The results of this study give insight into the phase transformation of L10 binary materials as well as materials that undergo a disorder–order transformation followed by displacive shear. 

The present work is the first to undertake systematicin situobservations of the microstructural changes on samples taken at ∼10‐m intervals along the length of a 80‐m firn core, extracted at Summit, Greenland (72°35’ N, 38°25’ W) in June, 2017, under interrupted load and at a strain rate of ∼8 × 10⁻⁵s⁻¹at −10°C, using a X‐ray micro‐computed tomograph. Several noteworthy features of the densification were found: the ice particle size increases, while the specific surface area, the total porosity, the pore size, and the structure model index (a measure of convexity/concavity of ice surface) decreases. The results were used to formulate semi‐empirical models (valid in the stress range of ∼0.05–2.15 MPa) that can be used to quantitatively assess the relative contributions of lattice diffusion (LD) and grain boundary diffusion (GBD) to the densification of polar firn. We found that 0.28 and 2.15 MPa are two critical stresses, which represent the start and end of LD as the dominant deformation mechanism to the densification of polar firn under the interrupted increasing loads. This bimodality when LD dominates implies that stress is not the only factor governing the densification of polar firn. On the other hand, GBD dominates the densification of polar firn both for stresses lower than 0.28 MPa and greater than 2.15 MPa. At stresses greater than 2.41 MPa, the firn specimens either fractured or other deformation mechanisms dominated, e.g., grain boundary sliding or power‐law dislocation creep.

 

Abstract. The flow of carbon through terrestrial ecosystems and the response toclimate are critical but highly uncertain processes in the global carboncycle. However, with a rapidly expanding array of in situ and satellitedata, there is an opportunity to improve our mechanistic understanding ofthe carbon (C) cycle's response to land use and climate change. Uncertaintyin temperature limitation on productivity poses a significant challenge topredicting the response of ecosystem carbon fluxes to a changing climate.Here we diagnose and quantitatively resolve environmental limitations onthe growing-season onset of gross primary production (GPP) using nearly 2 decades of meteorological and C flux data (2000–2018) at a subalpineevergreen forest in Colorado, USA. We implement the CARbonDAta-MOdel fraMework (CARDAMOM) model–datafusion network to resolve the temperature sensitivity of spring GPP. Tocapture a GPP temperature limitation – a critical component of the integratedsensitivity of GPP to temperature – we introduced a cold-temperature scalingfunction in CARDAMOM to regulate photosynthetic productivity. We found thatGPP was gradually inhibited at temperatures below 6.0 ∘C (±2.6 ∘C) and completely inhibited below −7.1 ∘C(±1.1 ∘C). The addition of this scaling factor improvedthe model's ability to replicate spring GPP at interannual and decadal timescales (r=0.88), relative to the nominal CARDAMOM configuration (r=0.47), and improved spring GPP model predictability outside of the dataassimilation training period (r=0.88). While cold-temperaturelimitation has an important influence on spring GPP, it does not have asignificant impact on integrated growing-season GPP, revealing that otherenvironmental controls, such as precipitation, play a more important role inannual productivity. This study highlights growing-season onset temperatureas a key limiting factor for spring growth in winter-dormant evergreenforests, which is critical in understanding future responses to climatechange.