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Reordering kinetics and magnetic properties of mechanically disordered nanocrystalline Ll2-type Ni3Al + Fe alloys

Authors
Journal
Acta Metallurgica et Materialia
0956-7151
Publisher
Elsevier
Publication Date
Volume
41
Issue
5
Identifiers
DOI: 10.1016/0956-7151(93)90248-q
Disciplines
  • Physics

Abstract

Abstract The paramagnetic disordered high temperature states of Ll 2 or γ′-Type Ni 3AlFe alloys become ferromagnetic at low temperatures if retained in metastable disordered (γ) state. Using mechanical attrition (milling) which also transforms the material into nanocrystalline form, various Ni 3AlFe alloys have been prepared in metaastable-γ form and their magnetisations M( γ) and M( γ′) and Curie temperatures T c( γ) and T c( γ′) measured. Fe atoms possess a giant magnetic moment in these alloys and polarise neighbouring Ni atoms but this polarisation effect is lower in the disordered state due to a lower number of FeNi nearest neighbours resulting in M( γ) < M( γ′) and T c( γ) < T c( γ′). Currently available expressions for magnetic ordering energies based on Bragg-Williams nearest neighbour interactions are found to be inadequate for these giant-moment-type alloys. Isothermal ordering kinetics of the metastable-γ-state which is also nanocrystalline indicate that the ordering reaction cannot propagate across the disordered nanograin boundaries and each grain must obtain its own γ′ nucleus. This results in a Johnson-Mehl-Avrami exponent n ≈ 1.

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