Crystallographic features of the approximant H (Mn$_7$Si$_2$V) phase in the Mn-Si-V alloy system

TL;DR

This study investigates the crystallographic features of the approximant H phase in Mn-Si-V alloys, revealing structural disorders and the transformation process from sigma phase to H phase using electron microscopy.

Contribution

It provides detailed crystallographic analysis of the H phase and its formation mechanism, highlighting structural disorders and the role of penetrated units in the approximant phase.

Findings

Structural disorders involve two dodecagonal units

RAPU regions appear before H phase nucleation

Transformation from sigma to H phase involves RAPU regions

Abstract

The intermetallic compound H (Mn$_7$Si$_2$V) phase in the Mn-Si-V alloy system can be regarded as an approximant phase of the dodecagonal quasicrystal as one of the two-dimensional quasicrystals. To understand the features of the approximant H phase, in this study, the crystallographic features of both the H phase and the (\sigma $\, \rightarrow$ H) reaction in Mn-Si-V alloy samples were investigated, mainly by transmission electron microscopy. It was found that, in the H phase, there were characteristic structural disorders with respect to an array of a dodecagonal structural unit consisting of 19 dodecagonal atomic columns. Concretely, penetrated structural units consisting of two dodecagonal structural units were presumed to be typical of such disorders. An interesting feature of the (\sigma $\, \rightarrow$ H) reaction was that regions with a rectangular arrangement of penetrated structural units (RAPU) first appeared in the \sigma $\,$ matrix as the initial state, and H regions were then nucleated in contact with RAPU regions. The subsequent conversion of RAPU regions into H regions eventually resulted in the formation of the approximant H state as the final state. Furthermore, atomic positions in both the H structure and the dodecagonal quasicrystal were examined using a simple plane-wave model with twelve plane waves.