Phase Evolution by Annealing of Mechanically Activated Ni, Mn, and Sn Elemental Powders Mixture with the Ni<sub>2</sub>MnSn Heusler Compound Ratio.

Abstract

A Ni₂MnSn Heusler alloy composition of elemental powders was high-energy milled for a short time for powder activation. The milling times were chosen to be 1 and 4 h to study how mechanical mixing triggers the phase formation in the Ni-Mn-Sn system. After milling, the samples were analyzed by differential scanning calorimetry and the thermal events of Ni₂MnSn L₂₁ phase formation were investigated. The milled samples were compacted at 700 MPa and annealed in a vacuum for 10 min at different temperatures (230 °C, 330 °C, and 600 °C). The annealing temperatures were chosen to emphasize the activated powders' behavior before and after Sn melting on L₂₁ Structure formation. Using X-ray diffraction and Rietveld analysis, the phase quantity was computed, showing that the largest L₂₁ phase (63%) can be obtained from the elemental powder mixture due to Sn melting during the annealing. For milled samples, a Ni₃Sn₄ phase was obtained by milling, and by annealing this phase, along with the remaining element, it reacts to form a Ni₂MnSn L₂₁ phase and a Ni₃Sn₂ phase. The microstructural evolution of the phase was illustrated by backscattering electron microscopy for milled and subsequent annealed samples, and, by image analysis, a correlation of the phase's amount was performed. The results of the image analysis were correlated with the X-ray diffraction patterns.