The Invar problem

doi:10.1016/0304-8853(91)90828-X

Journal of Magnetism and Magnetic Materials

Volume 100, Issues 1–3, November 1991, Pages 346-362

https://doi.org/10.1016/0304-8853(91)90828-X Get rights and content

Abstract

After briefly reviewing the history and practical applications of Invar, we present results of recent total energy calculations as a function of the magnetic moment and atomic volume E(M, V). Based on these results, we analyze Invar-relevant physical properties for a broad variety of fcc ferromagnetic and antiferromagnetic 3d-alloy systems as a function of the electron concentration. The observed systematics can principally be understood within the framework of the band calculations mentioned. We also give an answer to the question, why close to the Invar range martensitic fcc-bcc transitions occur. Finally we show that a deeper understanding of all the observed anomalies results from an analysis of available data as a function of the lattice constant. Almost quantitative agreement between experiment and theory allows the conclusion that the moment-volume instabilities are undoubtedly responsible for the magnetic, magnetovolume and structural properties exhibited by 3d-alloy systems.

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The microstructure and the hysteresis properties of Fe-Pd nanowire arrays were studied in the as-electrodeposited condition at 300 K, and as functions of temperature after a thermal cycle between 300 K and 950 K. Fe₃₆Pd₆₄ and Fe₆₅Pd₃₅ nanowires, 200 nm in diameter and 7–10 μm long, were electrodeposited into commercial 26 % porosity alumina templates. Initially, the main magnetic phase in both arrays is the metastable γ-Pd(Fe) disordered phase. After a thermal cycle up to 950 K the microstructures transform into a majority ferromagnetic ordered phase (FePd₃ in Fe₃₆Pd₆₄ and Fe₃Pd in Fe₆₅Pd₃₅) embedding γ-Pd(Fe) remaining grains. Curie temperatures result 500 K, 550 K and 900 K for Fe₃Pd, FePd₃ and γ-Pd(Fe), respectively. The temperature dependence of the magnetic properties exhibits two stages: one below 500–550 K, when the two phases are ferromagnetic and another one above this temperature when only the minority γ-Pd(Fe) is ferromagnetic. Coercivity and remanence exhibit a maximum near the Curie temperature of the corresponding ordered phase. The effective magnetic anisotropy K_eff remains high up to the magnetic transition (ferro to paramagnetic) of the ordered phase, due to the contribution of the magnetocrystalline anisotropy of these relatively hard phases. At higher temperature, the effective anisotropy mainly arises from the magnetostatic (shape and inter-wire dipolar interactions) contribution of the minority phase. The polarization reversal mechanism in the γ-Pd(Fe) phase is found to be the nucleation of inverse magnetic domains and the further expansion of the domain walls into the ferromagnetic grains, surrounded by the paramagnetic ordered phase.
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In this work, a binary metastable β-type Ti-41 Nb (wt. %) with a multi-directional Invar effect (i.e., near-zero thermal expansion) was fabricated through the cross-rolling (Cro-R), with an aim to elucidate the relationship between martensitic phase transformation and multi-directional Invar effect. It was found that the Cro-R TiNb alloy shows the Invar effect along the transverse direction (TD) and rolling direction (RD) across a wide temperature range (∼180 K), and the coefficient of thermal expansion (CTE) along the TD and RD are α_TD = +0.64 × 10⁻⁶ K⁻¹ and α_RD = −0.016 × 10⁻⁶ K⁻¹, respectively. Further experimental results show that the randomly oriented β parent phase and α″ martensitic phase are distributed in the tiny grains with a high density of dislocations. This high density of dislocations can effectively impede martensitic phase transformation from β to α″ that appears during the subsequent cooling, resulting in the martensitic phase transformation occurring over a wide temperature range. In such cases, the multi-directional Invar effect was achieved via the compensation between the negative thermal expansion (NTE) caused by martensitic phase transformation and the positive thermal expansion (PTE) due to the TiNb matrix lattice vibration during cooling. These experimental results act as a foundation for the design and development of the metastable β-type Ti alloy with a multi-directional Invar effect.
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The Invar problem

Abstract

J. Appl. Phys.

Metall

Phys. Scripta

Physica B

Physica B

Acta Phys. Polon. A

Phys. Rev. B

Scandia Nat. Lab. Report SAND 89-8504

Phys. Rev. B

Phys. Rev. B

Physica B

Proc. Roy. Phys. Soc. (London)

Phys. Rev. B

Phys. Rev. B

Phys. Rev. B

Phys. Rev. B

J. de Phys.

Physica B

J. Magn. Magn. Mater.