Thermodynamic, structural and magnetic studies of phase transformations in MnAl nanocomposite alloys

Journal Article · Fri Jun 15 00:00:00 EDT 2018 · Materials Characterization

DOI:https://doi.org/10.1016/J.MATCHAR.2018.03.034· OSTI ID:22805000

Crisan, A. D.; Vasiliu, F. ^[1]; Nicula, R. ^[2]; Bartha, C.; Mercioniu, I.

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1. National Institute for Materials Physics, PO Box MG-7, 077125 Magurele (Romania)
2. EMPA, Swiss Federal Laboratories for Materials Science & Technologies, CH-3602 Thun (Switzerland)

Highlights: • Small changes in MnAl stoichiometry (5 at.%) produce different phase structure. • In Mn{sub 60}Al{sub 40} as-cast alloy only hcp ε phase can be found • In Mn{sub 55}Al{sub 45} as-cast alloy, there is a mixture of γ{sub 2} (Al{sub 8}Mn{sub 5}) and ε as well as ferromagnetic MnAl τ-phase. • Energies of phase transformation of about 165 kJ/mol for Mn{sub 60}Al{sub 40} and 290 kJ/mol for Mn{sub 55}Al{sub 45} alloys are found. • hcp ε phase decomposes through the migration of interphase interfaces. - Abstract: We have undertaken a temperature-dependent, investigation of the thermodynamics, structure, morphology and magnetism of two MnAl nanocomposite alloys (Mn{sub 60}Al{sub 40} and Mn{sub 55}Al{sub 45}). Differential scanning calorimetry (DSC) studies allowed the determination of the exo-effects occurring in the structural phase transformation of MnAl, while by using temperature-dependent X-ray diffraction of synchrotron radiation we were able to monitor the phase transformation effects and the evolution with temperature of various structural phases occurring in the samples. We have shown that slight changes in stoichiometry (5 at.%) give rise to different phase structure in the as-cast state. While in Mn{sub 60}Al{sub 40} only hcp ε phase can be found, in Mn{sub 55}Al{sub 45} as-cast alloy, there is a quite complex phase structure with a mixture of γ{sub 2} (Al{sub 8}Mn{sub 5}) and ε as well as ferromagnetic MnAl τ-phase. Activation energies of about 165 kJ/mol and 290 kJ/mol have been calculated from the Ozawa-Flynn-Wall analysis of DSC experimental data. These findings are in good agreement with the results obtained from XRD. For the Mn{sub 55}Al{sub 45} as-cast alloy, we have confirmed by temperature-dependent synchrotron XRD that the hcp ε phase decomposes through the migration of interphase interfaces with the transformation rate controlled by boundary diffusion processes. High-resolution transmission electron microscopy have confirmed the phase structure obtained by XRD, while magnetic properties, obtained for the as-cast alloys are consistent with the multiphase character of the samples and in good agreement with previously reported results. The magnetization does not saturate for the maximum applied field of about 4 × 10{sup 6} A/m and a marked coercivity of about 160 kA/m is obtained for the Mn{sub 55}Al{sub 45} as-cast alloy.

• OSTI ID: 22805000
• Journal Information: Materials Characterization, Vol. 140; Other Information: Copyright (c) 2017 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 1044-5803
• Country of Publication: United States
• Language: English

Related Subjects

77 NANOSCIENCE AND NANOTECHNOLOGY
ACTIVATION ENERGY
ALUMINIUM COMPOUNDS
BINARY ALLOY SYSTEMS
COERCIVE FORCE
DIFFUSION
HCP LATTICES
INTERFACES
MAGNETIC PROPERTIES
MAGNETIZATION
MANGANESE COMPOUNDS
NANOCOMPOSITES
PHASE TRANSFORMATIONS
STOICHIOMETRY
SYNCHROTRON RADIATION
TEMPERATURE DEPENDENCE
THERMODYNAMICS
TRANSMISSION ELECTRON MICROSCOPY
X-RAY DIFFRACTION