Physical metallurgy of Ti–Ni-based shape memory alloys

Amorphous ribbons of composition Fe/sub 74.5-x/Cu/sub x/Nb/sub 3/Si/sub 13.5/B/sub 9/ (x=0, 1 at.%) have been annealed between about 500 degrees C and 900 degrees C. This produced a series of crystallized samples with grain sizes between about 10 nm and 300 nm and with coercivities H/sub c/ and initial permeabilities mu /sub i/ varying over several orders of magnitude. The best soft magnetic properties (H/sub c/ approximately=0.01 A/cm and mu /sub i/ approximately=80*10/sup 3/) were observed for the smallest grain sized of about 10 nm. With increasing grain size D, coercivity steeply increases following a D/sup 6/-power law (up to D approximately=50 nm). H/sub c/ then runs through a maximum of H/sub c/ approximately=30 A/cm and decreases again for grain sizes above 150 nm according to the well-known 1/D law for polycrystalline magnets. The initial permeability was found to vary in a similar manner, essentially being inversely proportional to coercivity. The variation of the soft magnetic properties with the average grain size is discussed and compared with the predictions of the random anisotropy model and other theories for the magnetization reversal. >

Grain size dependence of coercivity and permeability in nanocrystalline ferromagnets

Understanding the correlation between magnetic properties and nanostructure involves collaborative efforts between chemists, physicists, and materials scientists to study both fundamental properties and potential applications. This article introduces a classification of nanostructure morphology according to the mechanism responsible for the magnetic properties. The fundamental magnetic properties of interest and the theoretical frameworks developed to model these properties are summarized. Common chemical and physical techniques for the fabrication of magnetic nanostructures are surveyed, followed by some examples of recent investigations of magnetic systems with structure on the nanometer scale. The article concludes with a brief discussion of some promising experimental techniques in synthesis and measurements.

Magnetic Properties of Nanostructured Materials

https://www.andrew.cmu.edu/user/dl0p/laughlin/pdf/PMS.pdf

Amorphous and nanocrystalline materials for applications as soft magnets

Amorphous ribbons of composition Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 13.5/B/sub 9/ have been annealed above their crystallization temperature, which produces a homogeneous, ultrafine grain structure of alpha FeSi with typical grain diameters of 10-20 nm. The temperature dependence of saturation magnetization reveals two different magnetic phases which correspond to the alpha -FeSi grains and the grain boundary phase, respectively. The nanocrystalline material exhibits excellent soft magnetic properties which are discussed within the framework of the random anisotropy model. >

Grain structure and magnetism of nanocrystalline ferromagnets

The magnetic properties of Fe‐Si‐B‐M (M: additives) alloys prepared by annealing amorphous alloys made by the single roller method over their crystallization temperature have been investigated for development of new Fe‐based soft magnetic alloys. Excellent soft magnetic properties were obtained by adding the two elements Cu and Nb to Fe‐Si‐B alloys. It was found that these new alloys, called ‘‘FINEMET,’’ have an ultrafine grain structure composed of bcc Fe solid solution. They are suitable for many kinds of magnetic components such as saturable reactors, choke coils, and transformers, because they have superior soft magnetic properties and a high saturation flux density, and because different types of B‐H hysteresis loops are obtained by magnetic field annealing.

New Fe-based soft magnetic alloys composed of ultrafine grain structure

Fe-Based Soft Magnetic Alloys Composed of Ultrafine Grain Structure

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Fe Based Soft Magnetic Alloys Composed of Ultrafine Grain Structure

Volume and grain boundary diffusion of chromium in Ni-16mass%Cr-7mass%Fe alloys containing 0.004, 0.015 and 0.07 mass% of carbon have been measured using radioactive tracer 51 Cr over the temperature range of 858-1424K by serial radio-frequency sputter-microsectioning technique. While the volume diffusion coefficients are largely unaffected by the presence of carbon, the increase in carbon content markedly reduces the mobility of chromium atoms along the grain boundaries. As a result, the difference between the activation energy for lattice and grain boundary diffusion decreases with increase in carbon content. In fact, for 0.07 at% carbon, the activation energies for volume and the grain boundary diffusion are nearly same. Among all three alloys, the difference between the volume diffusion coefficient, D v , as well as δD gb (δ = grain boundary width; D gb = grain boundary diffusion coefficient) decreases gradually in a regular manner with the rise of temperature. Grain bounadry energy is reduced by addition of carbon and enhanced by temperature. At higher temperatures, there is no difference between the grain boundary energies of the three alloys.

Volume and Grain Boundary Diffusion of Chromium in Ni-Base Ni-Cr-Fe Alloys

It has been demonstrated with systematic experiments that the appearance and disappearance of multistage martensitic transformation in aged Ni-rich Ti-Ni alloys depend on the heat treatment atmosphere. No multistage transformation occurs when the evaporation of Ti and Ni and/or the preferential oxidation of Ti in the specimen are prevented and the purification of heat treatment atmosphere in an evacuated quartz tube is achieved. The heterogeneity in precipitation morphology of Ti3Ni4 phase which is responsible for the multistage transformation can be suppressed with the regulation of heat treatment atmosphere as mentioned above. We have concluded that the multistage martensitic transformation in aged Ni-rich Ti-Ni alloys is an extrinsic nature, i.e., a kind of artifact during the heat treatment.

Kiyoshi Yamauchi

Papers

New Fe-based soft magnetic alloys composed of ultrafine grain structure

Fe-Based Soft Magnetic Alloys Composed of Ultrafine Grain Structure

Fe Based Soft Magnetic Alloys Composed of Ultrafine Grain Structure

Volume and Grain Boundary Diffusion of Chromium in Ni-Base Ni-Cr-Fe Alloys

Experimental consideration of multistage martensitic transformation and precipitation behavior in aged Ni-rich Ti-Ni shape memory alloys