High-temperature strength-coercivity balance in a FeCo-based soft magnetic alloy via magnetic nanoprecipitates

Highlights

• Magnetic FeCo-based nanoprecipitates were introduced in FeCo-2V soft magnetic alloy by a small addition of Cr, Mo elements.

• Magnetic nanoprecipitates increase the high-temperature strength while maintaining a low coercivity in FeCo-based alloy.

• The characteristic of magnetic nanoprecipitates was revealed by TEM observation.

Abstract

Precipitation strengthening is an effective approach to enhance the strength of soft magnetic alloys for applications at high temperatures, while inevitably results in deterioration in coercivity due to the pinning effect on the domain wall movement. Here, we realize a good combination of high-temperature strength and ductility (ultimate tensile strength of 564 MPa and elongation of $\sim$ 20 %, respectively) as well as low coercivity (6.97 Oe) of FeCo-2V-0.3Cr-0.2Mo soft magnetic alloy through introducing high-density magnetic nanoprecipitates. The magnetic nanoprecipitates are characterized by FeCo-based phase with disordered body-centered cubic structure, which enables the alloy to have a low coercivity. In addition, these nanoprecipitates can impede the dislocation motion and suppress the brittle fracture, which lead to a high tensile strength and ductility. This work provides a guideline to enhance strength and ductility while maintaining low coercivity in soft magnetic alloys via magnetic nanoprecipitates.

Introduction

The near equiatomic FeCo-based soft magnetic alloys have been attracted considerable attention due to their excellent soft magnetic properties, such as high saturation, high Curie temperature, low coercivity and low core loss [[1], [2], [3], [4], [5]]. The applications of soft magnetic alloys in many fields, such as rotor or stator for power generation working at 500−600 °C [1,2,6], require not only the excellent soft magnetic properties but also good mechanical properties at high temperatures. Unfortunately, most conventional strengthening approaches that enhance strength will inevitably lead to a substantial increase in coercivity [2], namely strength-coercivity trade-off. Therefore, it is required to develop a soft magnetic material with high strength and low coercivity at high temperatures.

It is well known that the addition of 2 at.% V can improve the cold workability of disordered FeCo alloys while remaining good soft magnetic properties [[7], [8], [9], [10]]. Recently, in addition to conventional preparation processing, laser-based additive manufacturing was used to process brittle FeCo alloys, which introduces multiscale microstructures that simultaneously promote strengthening and enhance ductility to a level previously unobtainable in the FeCo alloy [11]. However, the FeCo alloy lacks the required mechanical properties (high yield strength, good ductility and creep resistance) for their high temperature applications [1]. In this respect, studies have focused on improving the mechanical properties of the alloy by using severe plastic deformation and specific heat treatment to obtain ultrafine-grained microstructure, leading to high tensile strength [3,[12], [13], [14], [15], [16], [17]]. For instance, Duckham et al. significantly improved the high temperature yield strength of FeCo-2 V alloy by obtaining ultrafine-grained FeCo-2 V alloy [3]. But the magnetic materials with ultrafine grain have a high coercivity due to grain boundaries acting as pinning points for magnetic domain wall movement [18]. In addition, the ultrafine-grained alloys usually exhibit poor thermal stability at high temperatures due to the grain coarsening [19], which limits their applications. Micro-alloying and precipitation hardening are also effective ways to enhance mechanical properties of FeCo alloys [[20], [21], [22]], but it simultaneously deteriorates the soft magnetic properties due to the pinning effect on the domain wall movement [[23], [24], [25]]. Extensive development efforts over the years have resulted in significant improvements in mechanical properties while maintaining good magnetic properties at room temperature [[26], [27], [28], [29]]. But a systematic research on balancing high-temperature mechanical properties and soft magnetic properties of FeCo-2 V alloys has been rarely reported so far. The existing literature has focused on either high-temperature mechanical properties or high-temperature soft magnetic properties for FeCo-based alloys [3,12,[30], [31], [32]]. The investigation of balancing between high-temperature mechanical and soft magnetic properties is still lacking. One possible reason is that various approaches for enhancing high-temperature mechanical properties, such as precipitation hardening by nonmagnetic precipitates, will inevitably lead to a significant loss of soft magnetic properties [1].

In this work, we achieve a good combination of high tensile strength and ductility as well as low coercivity at 600 °C in FeCo-2V-0.3Cr-0.2Mo soft magnetic alloy by appropriate heat treatments. Annealing at 800−850 °C leads to the formation of high-density magnetic nanoprecipitates that are identified as FeCo-based phase with disordered body-centered cubic (bcc) structure. The bcc nanoprecipitates exhibit similar soft magnetic properties with B2 FeCo matrix and thus enable the alloy to have a low coercivity. Meanwhile, these nanoprecipitates can impede the dislocation motion and suppress the brittle fracture, resulting in a high tensile strength and ductility. This work provides a guideline to design high strength, high ductility and low coercivity of soft magnetic alloys for applications at high temperatures via soft magnetic nanoprecipitates.