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The native metals
Published in R. F. Tylecote, The Prehistory of Metallurgy in the British Isles, 2017
Therefore, if the nickel content is over 4% there is a good chance of meteoric origin. Most of the meteorites with less than about 6% Ni are known as hexahedrites and show little structure and therefore may be difficult to distinguish microscopically from pure irons.49 Meteorites with a nickel content between 7 and 13% are mainly octahedrites which have a very characteristic Widmanstätten structure consisting of a network of bands of a low nickel constituent known as kamacite, bordered by a higher nickel constituent taenite. These bands intersect in three or four directions. Filling the angular interstices are fields containing plessite, a mixture of the above two constituents. There is a third group which is almost structureless known as ataxites. These fall into two compositional groups: up to 7% and between about 12 and 20%.
A review of processing techniques for Fe-Ni soft magnetic materials
Published in Materials and Manufacturing Processes, 2019
Powder metallurgy is a near-net shape technique to produce mini parts and complex shape soft magnetic components. The powder metallurgy technique has both design flexibility and economic benefits for electromagnetic applications.[116,117,118] The magnetic materials produced by powder metallurgy are categorized into two generations. The first-generation materials are produced by normal density technology (NDT) or high-density technology (HDT). The second-generation materials are comprised of insulated powders, amorphous and nanocrystalline materials.[119–121] The NDT uses conventional Press-Sinter process involving 600–800 MPa compaction pressure, 1120–1250°C sintering temperature and gives 92–94% sintered density of the wrought material. As mentioned earlier that saturation induction is a linear function of density. Thus, HDT is a relavent process that involves compaction pressure exceeding 800 MPa, 1250–1300°C sintering temperature and gives 95–98% sintered density of the wrought material.[119,122] Hanejko et al.[32] analyzed the effect of processing parameters on magnetic performance of Fe-Ni alloys. The green samples were compacted uni-axially and sintered at 1260°C in dissociated ammonia. In order to achieve high magnetic performance, the sintered parts were also annealed at 1070°C followed by low cooling rate. The maximum permeability of 8910 was reported in the samples having the highest density of 91%. Rodrigues et al.[123] analyzed the effect of compaction on density and microstructure. The green compacts having density of 6.5 g/cc were produced by the uniaxial press. After sintering in vacuum at 1220°C, XRD revealed the complete austenite phase formation. The sintering temperature was increased to 1350°C to study its effect on densification.[124] After repressing and heat treatment process, the density was improved up to 7.65 g/cc and it was analyzed that the acquired dilatometer data were in accordance with the models of densification and grain growth.[125] Sing et al. fabricated the Fe-Ni alloys with varying Ni content from 10,20,30,40, to 50 wt% by powder metallurgy.[126] The temperature and composition effect on the evolution of α-kamacite phase, γ-taenite phase, and microstructure was investigated. The Ni content from 10% to 50% enhanced the taenite phase formation that is a soft phase as compare to kamacite. Higher sintering temperature increased the taenite phase formation which leads to enhanced densification. The highest wear resistance was achieved in Fe-30Ni alloy due to the presence of both taenite and kamacite phases. Neera et al.[127] analyzed the effect of sintering temperature on various properties of Fe-30Ni alloys for advanced industrial applications. The green specimens were sintered at 1000°C, 1200°C, and 1250°C with 1 h dwell time. The sintering temperature of 1200°C was optimized for good combination of hardness, wear resistance and densification.