An experimental investigation of the structure of some internally oxidised ferromagnetic alloys

Results are presented of optical, scanning electron and transmission electron microscope observations on some internally oxidised ferromagnetic alloys. The alloys investigated were of nominal composition 0.058 wt % Si in Ni, 0.48 wt % Si in Ni and 0.17 wt % Si in Co 66.2%-Ni 33.7%. The alloys were in polycrystalline form and in addition single crystals of 0.058 wt % Si in Ni were examined. The diffusion and oxidation rate constants are calculated and the state of the oxide product as a function of (i) depth in the alloy, (ii) oxidising temperature and (iii) alloy composition is considered. The possibility of using such systems as device material and as test material for theories of coercivity and approach to magnetic saturation is considered.     

The morphology and stability of precipitates in internally oxidised silicon-bearing nickel and cobalt alloys

The results of an experimental investigation into the morphology of silica precipitates in internally oxidised silicon-bearing nickel and cobalt alloys are compared with the predictions of theories that concern the growth of spheroidal and extended precipitates and dendritic phases. Some degree of agreement is obtained between theory and experiment and a fairly detailed dependence of morphology on oxidation conditions and diffusion and concentration factors is outlined. Some observations on the stability of internal oxide precipitates are discussed and some conclusions, which are believed, in the main, to be general and applicable to any internally oxidised system, are made as to the dependence of morphology on the conditions of experiment.     

Coercivity and domain structure of nanograined Fe–C alloys after high-pressure torsion

The microstructure and magnetic properties of binary hypo- and hyper-eutectoid Fe–C alloys were studied. The investigations have been carried out on the samples in the as-cast state, after a long annealing at 725 °C and on the specimens after the high-pressure torsion (HPT). The deformation was carried out at the ambient temperature and the pressure of 5 GPa. The grain size after HPT is in the nanometer range. Long annealing leads to a drastic decrease of the coercivity in comparison with the as-cast alloys. In all alloys the coercivity H _c increases with increasing carbon content. The distance L between pinning points for domain walls decreases with increasing carbon content. Increase of the coercivity and decrease of L are more pronounced below the eutectoid concentration. The coercivity of the nanostructured samples is higher than that of the as-cast alloys. Due to the pinning of domain walls by the cementite particles, the hysteresis loop in the coarse-grained alloys both in as-cast and annealed states has a narrowing near the zero magnetization.     

Parameters affecting oxide adherence in nickel based alloys and techniques allowing estimation of adherence

Abstract

It is currently accepted that oxide/substrate adherence is related to the stresses developed in both parts of the oxidised system and many parameters have been suggested as being responsible for the oxidation stresses and for scale breakdown. Among these parameters, it is shown, on nickel based alloys, that predictions using the Pilling–Bedworth ratio are not always verified. However, the difference between the oxide and substrate expansion coefficients seems to be a very important factor contributing to the stress level in both oxide and substrate and the lifetime of oxidised Ni–Co–Cr–Al alloys. Furthermore, it is shown that various other parameters (strain rate differences between the substrate and its oxide, internal oxidation, impurities, etc.) can also contribute to adherence loss. The techniques most frequently used to estimate adherence and oxidation stresses and strains are described and illustrated by examples on oxidised nickel based alloys.

MST/861     

The role of the compensation temperature in thermomagneticswitching

Amorphous rare-earth-transition-metal alloys with different compensation temperatures, Tcom_p, exhibit pronounced differences in the temperature characteristics of the coercivity in the high-temperature range. This is of primary importance for the thermomagnetic switching process in which the domain radius, domain regularity, or domain jitter must be controlled. The influence of the coercivity profile on these parameters has been investigated for alloys with Tcom_p>100 K. The radial dependences of the driving fields acting on the domain wall are calculated using experimental data for the saturation magnetization, uniaxial anisotropy, and coercivity     

高性能软磁合金的研究进展EI北大核心CSCD

软磁材料是一种极为重要且应用十分广泛的能源材料,近年来,随着磁性元件的日益高频化和小型化,以及节能环保的号召,开发和研究高性能软磁材料具有重要意义。本工作概述了软磁合金的发展历史,重点归纳出各类软磁合金(包括传统软磁合金、非晶/纳米晶软磁合金、高熵软磁合金)的成分、微观组织、磁性能以及应用范围,并总结出不同软磁合金的优、缺点;指出典型合金的微观组织对合金软磁性能(尤其矫顽力)具有关键性的主导作用,进而探讨了影响软磁合金矫顽力的因素及其微观机制,发现控制晶粒尺寸(或纳米粒子尺寸)是获得低矫顽力的关键,并描述了矫顽力的微观影响机制在高熵软磁合金中的发展;最后,展望了高熵软磁合金因多主元混合的成分特性带来的组织多样化,更有利于实现对合金性能的调控,并有望作为新一代高温软磁体材料。     

Sm(CO,Fe,Cu,Zr)z(6.5≤z≤8.5)高温永磁合金的组织结构与性能

介绍了Sm（Co,Fe,Cu,Zr）z永磁合金的显微组织结构特点以及热处理工艺参数对组织结构的影响,讨论了Sm含量及合金元素对微观组织与性能的影响规律,并对合金的矫顽力随温度的反常变化现象、矫顽力增强机理和热稳定性研究进行了总结和讨论．     

Technical note Fabrication of alumina dispersion strengthened copper strips by internal oxidation and hot roll bonding

Abstract

Alumina dispersion strengthened copper strips were fabricated by internal oxidation and hot roll bonding of Cu–Al alloy strips. Cu–Al alloy strips were internally oxidised without using any oxidant powders by a surface oxidation method. Several of the internally oxidised alloy strips were stacked and bonded by rolling at high temperatures. The bonded strip was cold rolled to achieve tensile strengths of 484–539 MPa and yield strengths of 472–522 MPa with thermally stable mechanical properties.     

钕铁硼永磁合金的晶粒相互作用和矫顽力

钕铁硼永磁合金的矫顽力由单个晶粒的矫顽力和晶粒之间的相互作用决定。晶粒的矫顽力及其相互作用与晶粒取向有关。按照不同的矫顽力机制,晶粒的矫顽力及其角度关系有不同的表达式。晶粒相互作用可分为长程静磁相互作用和近邻晶粒的交换耦合相互作用。烧结磁体的交换作用影响很小,静磁相互作用影响较大,使晶粒混乱取向磁体的矫矫顽力大于晶粒理想取向磁体的矫顽力。综合考虑单个晶粒的矫顽力和不同取向晶粒之间的相互作用的影响。     

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

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 ～ 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,whichenables 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.     

Structure and coercivity of nanocrystalline Fe-Si-B-Nb-Cu alloys

Crystallization behaviour and magnetic properties of melt-spun Fe-Si-B-Nb-Cu alloys have been investigated. It is found that the primary phase changes from α-Fe(Si) to Fe₃Si (DO₃) on increasing the Si content The coercivity of the alloys containing the Fe₃Si phase is significantly lower as compared to the alloy containing α-Fe(Si) phase. A heat treatment temperature-time-coercivity map has been obtained for optimization of the coercivity.     

Magnetic properties of R-Nd-Fe-B (R = Tb, Dy) and Nd-Fe-Co-B Alloys in the Range -80 to 250°C

The remanence, coercivity, and maximum energy product of R-Nd-Fe-B (R = Tb, Dy) and NdFe-Co-B alloys were studied in the range-80 to 250°C. Doping of Nd-Fe-B alloys with Tb and Dy was shown to increase coercivity and, accordingly, the temperature coefficient of magnetic induction. The introduction of 7-12 wt % Co raises the Curie temperature of the alloys, thereby improving their thermal stability.¹ Ural State University, Yekaterinburg.     

Oxide particle densities in internally oxidised Ag-Cd alloys: Part 2 Alloys with ternary additions

Abstract

The densities of oxide particles are studied as a function of depth below the surface of internally oxidised Ag–9·5 at.-%Cd alloys with additions of ~0·2 or 2 at.-%Al, Mn, Ni, Sn, or Zn. Because of its low diffusivity and low free energy of oxide formation, Ni has no influence on particle density. Tin forms needlelike oxides and reduces the particle density as a result of geometric effects. Zinc has a marked effect on particle shape with platelike precipitates which coalesce during subsequent precipitation of CdO, but does not greatly affect particle density. Both Al and Mn increase particle density up to two orders of magnitude. Planar size distributions are presented which indicate that two types of nucleation site are present in the Mn containing alloys. In the alloy containing 0·15 at.-%Al the Al₂O₃ particles are too fine to act as nuclei for CdO precipitation; two types of band both with a narrow size distribution of CdO particles, but with slightly different size and largely different density are formed. Heterogeneous nucleation and depletion of the centre region of sample sheets, both of which result from the counterdiffusion of the oxidising elements, are shown to have a great influence on particle density, resulting in a more homogeneous distribution of particle densities over the sample cross-section than that predicted by available theories.

MST/900b     

Influence of thin platinum layer on the magnetic properties of multiple layers of CVD cobalt thin films

This paper presents the results obtained on the multiple layers of cobalt (Co)/platinum (Pt) and cobalt (Co)/platinum (Pt)/cobalt (Co) on the oxidised silicon substrate. The cobalt layers were deposited by metal-organic chemical vapour deposition on oxidised-silicon substrates at 450?°C, in H₂ ambient with 2-torr processing pressure. The platinum layers were deposited by E-beam evaporation in a separate vacuum system. The magnetic properties of Co/Pt/Co and Co/Pt multilayer were compared with the single cobalt layers of similar thicknesses on the oxidised silicon substrate. From the hysteresis loops it was observed that thin cobalt layers on oxidised-silicon substrate shows hard magnetic property with coercivity H _c values of 360 Oe and 500 Oe respectively for the 30- and 15-nm cobalt layers. The multiple layers of Co/Pt/Co and Co/Pt of cobalt thickness 15- and 30-nm with platinum 1.5-nm spacer-layer show significant change in magnetic properties (i.e. coercivity H _c and magnetisation M _s ) and, gave soft magnetic properties with H _c values 51 and 49 Oe respectively, which are significantly less than the H _c values of single cobalt layers on oxidised silicon. Also, single and multiple layers cobalt with platinum were annealed and compared with the as-deposited layer structures. From the microstructure analysis by SEM, and AFM it was found that the single and multilayer had similar roughness. Magnetic images were observed by MFM and analyzed in terms of domain structure.     

Oxide scale spallation behaviour of cast chromia-forming TaC-strengthened superalloys

Six {Ni,Co}-based alloys containing TaC were oxidised at 1250°C. The mass variations files were exploited to follow the progress of the spallation of the oxide scales during cooling. The oxides formed on surface were characterised. On average, spallation started after a 300?K cooling, at higher temperature for the nickel-richest alloys than for the others. These results are discussed by considering the effect of the base element and the natures and thicknesses of the external oxides. Tantalum has a negative influence on the scales adherence. The thicker scales formed on the cobalt-richest alloys are less affected by spallation. The CrTaO₄ oxide is more deleterious for the adherence of the external oxide scale when it is gathered at the interface.     

Phase transformation and magnetic properties of annealed Fe52.2Co46V1.8 alloy

In this work, FeCoV alloys were annealed in vacuum at various temperatures for different time, with and without applying an external magnetic field, respectively. The effect of the annealing on their magnetic properties has been investigated in regard with microstructural characterizations. X-ray diffraction analysis shows that the alloy is characterized of α-Fe bcc structure. It has been found that an order-disorder phase transformation took place locally in the surface around 993 K. In addition, α-γ phase transformation occurs at the same temperature at which the Curie point is observed in the TG curve. Coercivity is decreased with increasing annealing temperatures, and changed from about 3.2 kA/m to 78 A/m when the annealing temperatures are increased to 1173 K. Microstructural observations show that the decrease of coercivity after annealing is contributed mainly by grain growth. The coercivity of the alloy is further decreased by 32 A/m after magnetic field annealing at 1033 K. Hysteresis loops of the alloys after field annealing at 1033 K featured more rectangular shape with smaller coercivity, compared with those after vacuum annealing.     

Improving Accuracy of Intrinsic Coercivity Measurement for Magnetically Soft Materials

This paper considers problems associated with an established coercivity measurement procedure, the “pull out” method, used at the Magnet Centre for many years. Such measurements are an important quality control tool for magnetically soft materials. The main emphasis of this paper is on low carbon relay steels, but other materials have been investigated. Relays and actuators play an increasingly important role in automotive and aerospace technology. Here we review the procedure in detail, but our findings are relevant to other “open circuit” methods of intrinsic coercivity measurement. We have found that errors are caused by rapid changes in applied magnetic field strength at a specific part of the hysteresis cycle, between saturation and the open circuit remanent flux density. We have devoted much attention to the geometry of the samples, with particular emphasis on thickness and dimension ratio, as these are major factors in intrinsic coercivity determination. We propose a modification of the relevant international standard for intrinsic coercivity measurement.     

Magnetic properties of melt spunSmFe11Ti-Sm2TM17 pseudobinary alloys

Several alloys in the SmFe₁₁Ti-Sm₂TM₁₇ (TM=Co/Fe/Cu/Zr) pseudobinary system containing O-90 wt.% Sm₂TM₁₇ were prepared by the melt spinning technique. A maximum as-spun coercivity of 4.6 kOe was obtained for the alloy containing 10 wt.% Sm₂TM₁₇ at a substrate velocity of 15 m/s. The coercivity is enhanced to 5.3 kOe after annealing at 800°C for 1 h. XRD (X-ray diffractometry) and TEM (transmission electron microscopy) were used to investigate the microstructure of the alloys. It is suggested that the grain refinement, doping of the 1-12 phase with Sm and other elements, and reduced free iron might be the main causes coercivity enhancement     

Preparation of nanosized particles of FeNi and FeCo alloy in solution

FeNi and FeCo alloys of different compositions are prepared at room temperature in solution by surface catalysis or high-pressure inducement. Their sizes and morphologies are analyzed with X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM) and X-ray photoelectron spectroscopy (XPS). The phase transformations of these alloys are similar to the reported phase diagrams of FeNi and FeCo alloys. HRTEM image of FeCo₄ alloy shows the presence of rods. Their magnetic properties are studied. Surprisingly, the coercivity of FeCo₄ alloy reaches as high as 718 Oe.     

Predicting the thermal conductivity of aluminium alloys in the cryogenic to room temperature range

Aluminium alloys are being used increasingly in cryogenic systems. However, cryogenic thermal conductivity measurements have been made on only a few of the many types in general use. This paper describes a method of predicting the thermal conductivity of any aluminium alloy between the superconducting transition temperature (approximately 1 K) and room temperature, based on a measurement of the thermal conductivity or electrical resistivity at a single temperature. Where predictions are based on low temperature measurements (approximately 4 K and below), the accuracy is generally better than 10%. Useful predictions can also be made from room temperature measurements for most alloys, but with reduced accuracy. This method permits aluminium alloys to be used in situations where the thermal conductivity is important without having to make (or find) direct measurements over the entire temperature range of interest. There is therefore greater scope to choose alloys based on mechanical properties and availability, rather than on whether cryogenic thermal conductivity measurements have been made. Recommended thermal conductivity values are presented for aluminium 6082 (based on a new measurement), and for 1000 series, and types 2014, 2024, 2219, 3003, 5052, 5083, 5086, 5154, 6061, 6063, 6082, 7039 and 7075 (based on low temperature measurements in the literature).