Revealing the effect of rapid annealing on nano-crystallization behavior and soft magnetic properties of Fe–Co–B amorphous alloy

Rapid annealing (RA), or the application of a high temperature for a short duration, is an efficient strategy to promote the nano-crystallization of Fe-based amorphous alloys and improve their soft magnetic properties. Herein, the ternary Fe–Co–B amorphous alloy is investigated as a typical system to reveal the effect of RA on crystallization behavior and soft magnetic properties. It is found that the large superheating generated during RA significantly increases the homogenous nucleation rate of nanocrystals, while the short duration greatly inhibits the growth kinetics, yielding fine and dense nanograins in the amorphous matrix. The Fe–Co–B alloy shows optimized properties of low coercivity (5.6 A/m) and high saturation magnetization (1.87 T) after RA at 813 K for 3 s. In contrast, all superheating and long duration used in conventional annealing produce large and sparse grains with single crystal morphology and large coercivity above 30 A/m. This work reveals the thermodynamic and kinetic effects of RA on the nano-crystallization behavior of Fe-based amorphous alloys, showing that the RA treatment (high annealing temperature and short annealing time) can induce in-situ homogeneous nucleation and growth, which may help guide the design of Cu-free amorphous/nanocrystalline alloys with desirable soft magnetic properties

Effect of W Addition on Fe-P-C-B Soft-Magnetic Amorphous Alloy

In this work, the thermal behavior, soft magnetic properties, and structure of Fe86−xP11C2B1Wx (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1, 2, and 4) amorphous alloys were researched by several experimental methods and ab initio molecular dynamics. The addition of W improved the thermal stability of the alloy system when the first onset crystallization temperature (Tx1) increased from 655 K to 711 K, significantly reduced the coercivity Hc, and decreased the saturation magnetization Bs. The Fe85.6P11C2B1W0.4 alloy showed optimal soft magnetic performance, with low Hc of 1.4 A/m and relatively good Bs of 1.52 T. The simulation results suggested that W atoms increased the distance of the neighboring Fe-Fe pair, reduced the coordination number, narrowed the gap between the spin-up and spin-down electrons of each atom, and decreased the average magnetic moment of the Fe atoms. This work demonstrates a micro-alloying strategy to greatly reduce Hc while maintaining high Bs

On the ε → τ phase transformation and twinning in L10−MnAl alloys

Formation of twins have been recognized as the bottleneck that limit the high-performance of L10-type Mn-Al permanent magnets. Although it is known that twinning occurs as a consequence of ε → τ phase transition, the detailed formation mechanism is still unclear. We studied systematically the phase transformation processes of ε → τ by transmission electron microscopy. The two-step transformation includes first a diffusion-controlled ordering transformation from disordered A3-type ε-phase to ordered B19-type ε’-variants, and second, a shearing transformation from ε’-phase to L10-type τ-phase via the stepwise atomic displacement with the vector of x/3ε’. The two τ-variants generated from the same ε’-variant constitute the true twins with included angle of ∼76° The combination of τ-variants which are generated from different ε’-variant produces the order twins and pseudo twins with the included angles of ∼85° and ∼48°, respectively. The effects of true twins and order twins on coercivity are compared by studying the domain structures during demagnetization. The order-twin boundary has a more significantly effect on promoting the nucleation and propagation of the reversal domain, giving rise to a severer degradation effect on coercivity compared with true-twin boundary. The combination of directional solidification and hot-deformation is identified to effectively manipulate the ε → τ phase transition based on the proposed mechanism, as demonstrated by good magnetic properties of ∼400 mT in coercivity and ∼0.67 in remanence ratio in L10−MnAl magnets. Our work explains the mechanism on the formation of various twins in L10−MnAl, and offers guidelines of fabricating high-performance twins-containing MnAl permanent magnets