Processing soft ferromagnetic metallic glasses: on novel cooling strategies in gas atomization, hydrogen enhancement, and consolidation

Abstract

Processing soft ferromagnetic glass-forming alloys through gas atomization and consolidation is the most effective technique to produce bulk samples. The commercial viability of these materials depends on commercial purity feedstock. However, crystallization in commercial purity feedstock is several orders of magnitude faster than in high purity materials. The production of amorphous powders with commercial purity requires high cooling rates, which can only be achieved by extending the common process window in conventional gas atomization. The development of novel cooling strategies during molten metal gas atomization on two model alloys ({(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05}₉₆Nb₄ and Fe₇₆B₁₀Si₉P₅) is reported. Hydrogen inducement during liquid quenching significantly improved the glass-forming ability and soft magnetic properties of {(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05}₉₆Nb₄ powders. Spark plasma sintering experiments verified that amorphous rings could be produced regardless of the cooling strategies used. While the saturation magnetization was almost unaffected by consolidation, the coercivity increased slightly and permeability decreased significantly. The magnetic properties of the final bulk samples were independent of feedstock quality. The developed cooling strategies provide a great opportunity for the commercialization of soft ferromagnetic glass-forming alloys with commercial purity.

Introduction

Soft magnetic amorphous and nano-crystalline materials have attracted considerable interest for their energy-saving potential due to their high magnetic performance [1,2]. Today, silicon steels are predominately used for various magnetic applications, but have the drawback of high core losses $(\sim$ hysteresis losses + eddy current losses). Research has shifted from improving the magnetic properties of conventional materials to ferromagnetic amorphous alloys, and today, towards nano-crystalline materials [3,4]. Nano-crystalline soft ferromagnetic alloys were introduced 30 years ago and show excellent soft magnetic properties through precisely controlled nano-grains embedded in an amorphous matrix [[5], [6], [7], [8], [9]]. These particular materials are expected to substitute Fe-Si alloys or ferrites for high performance power transformers and electric motors [10]. Soft ferromagnetic glass-forming alloys produced with high purity exhibit a high glass-forming ability, a high degree of magnetic softness (low coercivity, high effective permeability, and low core losses), as well as a high saturation magnetization, mechanical strength, and corrosion resistance [11].

The quality of amorphous materials depends on alloy composition and cooling conditions during solidification. The melt must be cooled quickly to circumvent crystallization. The required cooling rates limit the sample geometry and size, as cooling or quenching is often governed by time-dependent heat conduction that causes crystallization. Melt spinning is the most commonly used technique to produce amorphous ribbons. Amorphous ribbons are subsequently heat-treated to adjust their soft magnetic properties. These ribbons are usually wound around transformer rings, but cannot be manufactured in complex shapes while having a low packing density, leading to reduced soft magnetic properties. These limitations can be overcome by alternative techniques such as the combined use of powder synthesis and consolidation to introduce soft ferromagnetic glass-forming alloys to a broad commercial market [12]. Despite rapid development of soft ferromagnetic glass-forming alloys in a few industrial applications, they are still limited to only a few sectors, as they depend on expensive high purity materials and clean laboratory conditions to avoid crystallization. From an economic and ecological point of view, compositions with commercial purity such as ferrous metals like iron, nickel, and chromium are preferred [13]. However, present impurities or oxygen in commercial purity materials promote heterogeneous crystal nucleation, causing a shift in the crystallization time [14]. Crystallization for soft ferromagnetic glass-forming alloys is orders of magnitudes faster than in many common polymers, silicate glass-forming liquids, and even in robust metallic glasses [15]. Therefore, a high cooling rate is inevitable during solidification in molten metal gas atomization to avoid kinetically related crystal growth.

The droplet solidification time depends on the droplet size, the temperature gradient between the melt droplet and the surrounding cooling gas, as well as the convective heat transfer coefficient. Increasing the heat transfer coefficient in conventional gas atomization is very challenging [16], though it is desired to increase the amorphous powder fraction. The atomization of soft ferromagnetic glass-forming alloys with commercial purity on the commercial scale has been rarely studied, whereas the production of soft ferromagnetic glass-forming alloys with high purity has been successfully demonstrated [2,17,18].

The aim of this paper is therefore the production of soft ferromagnetic glass-forming powders with commercial purity through the development of novel cooling strategies. These cooling strategies increase the cooling rate, creating new process windows that are inaccessible in conventional gas atomization. However, these cooling strategies require water as a cooling medium, promoting the formation of an oxide layer around the resulting particles. The oxide layer may decrease material properties as well as alter powder consolidation. In order to investigate the powder consolidation ability, Spark plasma sintering experiments were performed. The subsequent powder consolidation enables the determination of soft magnetic properties on the final bulk samples.

Another critical aspect is the formation of hydrogen during liquid quenching. The positive effect of hydrogen on metallic glasses is rarely discussed with respect to transport processes, structural changes, glass formation, and material properties [[19], [20], [21], [22], [23], [24], [25]]. Thus, hydrogen and oxygen content in the powders were measured in the powders and correlated with the GFA and final soft magnetic properties.