Mechanical Amorphization and Recrystallization of Mn-Co(Fe)-Ge(Si) Compositions

Antonio Vidal-Crespo,Alejandro Conde,Javier S Blázquez,Jhon J Ipus

doi:10.3390/met9050534

Abstract

Mechanical alloying using a planetary ball mill allowed us to obtain two homogeneous systems formed by units with nanometer size and MnCo0.8Fe0.2Ge1−xSix stoichiometry (x = 0 and 0.5). The phase evolution of the systems with the milling time was analyzed using X-ray diffraction. Thermal stability of the final products was studied using differential scanning calorimetry. Room temperature 57Fe Mössbauer spectroscopy was used to follow the changes in the Fe environments. A paramagnetic Co-based amorphous phase developed in both alloys as milling progressed. However, while the presence of Si stabilized the Mn-type phase, mechanical recrystallization was observed in a Si-free composition leading to the formation of a MnCo(Fe)Ge intermetallic (Pnma space group) with a crystal size of 7 ± 1 nm. Mössbauer results indicate that Fe atoms migrate from the initial bcc phase to the amorphous and intermetallic phases.

Highlights

Half-Heusler MnCoGe alloys can show a martensitic transformation from an orthorhombicTiNiSi-type structure (Pnma space group) to a hexagonal Ni2 In-type structure (P63 /mmc, it can be interpreted as a different orthorrombic Pnma structure with different lattice parameters to those of the TiNiSi-type structure [1])
High entropy alloys (HEAs) are homogeneous solid solutions formed by at least five different elements with atomic fractions between 5 and 35 at. % [6]. In such high entropy alloys (HEAs), bcc and fcc solid solutions as well as amorphous phases can be observed as product phases when produced by rapid quenching [7] or mechanical alloying [8]
Both amorphous and supersaturated solid solutions are very attractive homogeneous precursor systems to develop stoichiometric intermetallic phases, strongly reducing the annealing time required with respect to the samples obtained by conventional methods [9]

Summary

Introduction

TiNiSi-type structure (Pnma space group) to a hexagonal Ni2 In-type structure (P63 /mmc, it can be interpreted as a different orthorrombic Pnma structure with different lattice parameters to those of the TiNiSi-type structure [1]) Coincidence of such a structural transformation with a magnetic one has been proposed to enhance the magnetocaloric effect exhibited by these systems [2], which can be achieved by compositional tailoring with partial substitution of Fe for Co [3]. In such HEAs, bcc and fcc solid solutions as well as amorphous phases can be observed as product phases when produced by rapid quenching [7] or mechanical alloying [8] Both amorphous and supersaturated solid solutions are very attractive homogeneous precursor systems to develop stoichiometric intermetallic phases, strongly reducing the annealing time required with respect to the samples obtained by conventional methods [9]. The development of HEAs from half-Heusler compositions has been recently reported for Ti(NiCo)(SnSb) [10], CoMnSn(Cu) [11] and (TiZr)Ni(SnSb) [12] systems

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