Soft magnetic properties of severely drawn pearlitic-ferritic wires with nanocrystalline microstructure

Abstract

Recently, pearlitic wires, which experienced large drawing strains awoke the interest of material scientists due to their astonishing high strength of up to 7 GPa [1]. Because of the large amount of deformation applied during wire drawing, the grain size refines into the nanocrystalline regime and microstructural sizes of 10 nm and below are reported. The present study focusses on the capabilities to magnetically soften pearlitic steel by this “top-down”, conventional metal-forming process, as these length scales are already below the ferromagnetic exchange length of pure Fe. To do so, volume saturation magnetization and temperature dependent coercivities of wires experiencing high drawing strains (ε > 5) were investigated. The results are compared to the ones of the initial wire, having more than twenty times the diameter and consequently a much coarser microstructure.To investigate the magnetic softening upon wire drawing, SQUID magnetometry and VSM measurements were performed. For the accurate determination of the volume saturation magnetization, the precise determination of the volume of the small samples is necessary. A confocal laser scanning microscope was used – the results allow the 3D reconstruction of the surfaces of the samples, for an example see inlay in Figure 1. From the surface reconstruction the volume can be calculated. It was found that the volume saturation magnetization is higher for the very thin wires in comparison to the initial state. While the thick, non-deformed wire shows a saturation magnetization very close to the one of pure pearlite, the saturation values for the thin wires are close to the one of pure body centred cubic (bcc) Fe (Figure 1). The slightly higher values, even in comparison to pure Fe, can be explained by C being supersaturated in the Fe matrix. The C necessary for supersaturation originates from the dissolution of cementite (Fe3C) during the wire drawing process [1].Regarding the potential of wire drawing for magnetic softening, a decrease in coercivity was indeed found. The measured coercivities of the thin wires, determined with VSM, are smaller (24µm: 520 A/m, 36µm: 610 A/m) compared to the thick wire (1510 A/m). It is evident from magnetic measurements that there is a small but no substantial further refinement in grain size upon drawing the wire from 36 µm to 24 µm, although this constitutes a large degree of further deformation. Restoration mechanisms, as they are also known from other techniques involving severe deformation, seem to keep the grain size almost constant.For the thin wires, the grain size is calculated from the room temperature coercivity measurements. The grain size is found to be below the ferromagnetic exchange length of pure Fe, thus the random anisotropy model is applied. Using magnetic constants of pure Fe yields a grain size of 11 nm. This is in excellent agreement with atom probe tomography studies on the same material, where a subgrain size of 10 nm and below was found [2].To further confirm the finding of dissolving cementite, the coercivity was determined as a function of temperature. Figure 2 includes calculated coercivities based on temperature dependent magnetocrystalline anisotropies for pure bcc Fe from [3–5]. Comparison reveals a very good match of measured and literature data; thus it can be safely assumed that the cementite has largely dissolved.Residual stresses, which might prevail from the drawing process would lead to an increased coercivity. Thus, a further softening of the drawn wires can be expected after thermal treatment, as it was the case for severe plastically deformed material, presented in [6]. One of the thin wires was annealed at 150°C for 30 min, which resulted in a decrease in coercivity by about 76 A/m or 12%. This proves the temperature stability of the microstructure at that temperature, as an increase in grain size would result in an increase in coercivity. The decrease in coercivity can be explained by reduced magneto-elastic effects, as residual stresses after wire drawing increase the effective anisotropy.In summary, pearlitic material was drawn to very high strains, resulting in very thin wires with nanocrystalline microstructure. Results of magnetic measurements indicate grain sizes close to 10 nm, the dissolution of cementite and a supersaturation of bcc Fe with C. It was possible to magnetically soften the coarse grained pearlitic steel from ~1500A/m to ~500A/m by this wire drawing process. However, the almost constant coercivity and grain size for very high drawing strains demonstrates that the magnetic softening seems to be maxed out.This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant No. 757333). **