Structure and magnetic properties of alnico 8 ribbons

Abstract

Alnico 8 ribbons were prepared by melt-spinning and annealing, and the effects of melt-spinning speeds on the structure and magnetic properties were investigated. The results show that all the as-spun ribbons are composed of the α-Fe(Co)-type α₁ and the AlNi-type α₂. With speed reducing from 40 m/s to 15 m/s, the <100> preferred orientation enhanced, the grain size and the α₂ content increased. The as-spun ribbons show the weak hard magnetic properties. Annealing improves the spinodal structure consisting of the strip-like α₁ and α₂. The annealed ribbons melt-spun at 15 m/s achieve the optimal magnetic properties with the coercivity of 1558.3 Oe and the maximum magnetic polarisation density of 11.3 kGs, which are codetermined by the shape anisotropy of the strip-like α₁ phase and the pinning mechanism of the α₂ phase to the α₁ domains.

Introduction

The spinodal-type Al–Ni–Co permanent magnetic alloy, normally abbreviated as Alnico, has been widely used in the high-temperature devices since Mishima [1] discovered it in 1931 [[2], [3], [4]]. The Alnico alloy first obtained the supersaturated α solid solution with body-centred cubic (BCC) structure by the solid-solution treatment. Then, the spinodal structure formed through the reaction of α→α₁+α₂ by the magnetic heat treatment (MHT) and multi-stage ageing, in which the α₁ phase is Fe–Co-rich ferromagnetic, and the α₂ is Al–Ni-rich weak ferromagnetic [[5], [6], [7], [8], [9]]. The size, shape, distribution and percentage of the α₁ and α₂ phases are the critical microstructure factors to determine the magnetic properties [2,[10], [11], [12], [13], [14]]. Recently, many methods [[15], [16], [17], [18], [19], [20]] have tried to improve the magnetic properties of the Alnico alloys. Ref. [15] reported that the as-cast Alnico 5 plates consisted of Fe₇Co₃-type α₁ and Al_1·1Ni_0.9-type α₂. Li et al. [16] produced the nanoparticles of Alnico 4 and Alnico 5 alloys by hydrogen plasma metal reaction. However, these nanoparticles showed lower coercivity less than 1 kOe even at 4 K. Appling 10 T magnetic field can promote the spinodal decomposition in Alnico 5 alloy, reduce the size of the decomposed particles and increase the volume fraction of the ferromagnetic particles [17]. Zhou et al. [18] concluded that the microstructure of the Alnico is not only sensitive to the doped alloying elements such as Ti and Cu, but also the crystallographic orientation of the parent phase. Sun et al. [21] obtained the high coercivity of 2020 Oe and the high magnetic energy product of 9.5 MG Oe in the Alnico 9 alloy with the ideal columnar crystals. Fan et al. [22] prepared the 32.3Fe-36.9Co-13.5Ni-7.2Al-4.1Ti-2.1Hf-3.0Cu-0.9Nb alloy by the solution treatment at 1250 °C, MHT at 800 °C in a 3 T field, and the tempering at 550–650 °C. They found that the tempering process changed the elemental distribution in the Cu–Ni bridges, but did not alter the morphology and distribution of the Cu-bridges. Zhou et al. [23] ascribed the Hc enhancement in the 32.4Fe-38.1Co-12.9Ni-7.3Al-6.4Ti-3.0Cu alloy to the increased Fe–Co phase (α₁) isolation, the development of Cu-rich spheres/rods/blades and the additional α₁ rod precipitation. Refs. [3,24] made the net shape Alnico magnets using an attractive additive manufacturing (AM) process, these magnets showed the remanence values of up to 9 kGs, which is close to that of the anisotropic cast Alnico 9.

Though AM processing is a promising method to obtain the high-performance Alnico magnets, now all commercial Alnico magnets are prepared by casting or sintering [2,[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. Löwe et al. [20] studied the effect of B addition on the microstructure and magnetic properties of Alnico 5 alloy at two speeds of 5 m/s and 60 m/s. They found that a two-step magnetic field annealing at 600–900 °C can induce the spinodal decomposition. Still, the size of the spinodal structures was not influenced by the wheel speeds but became smaller with the B addition. In this work, we tried to prepare the Alnico 8 alloy by the melt-spinning and simple heat treatment, and no extra magnetic field was applied. It is interesting that the coercivity and remanence of the ribbons strongly depended on the melt-spinning speeds.