Anharmonic Incommensurate Structure Modulation in Ni-Mn-Ga Martensite Exhibiting Highly Mobile Twin Boundaries

TL;DR

This study reveals that Ni-Mn-Ga martensite exhibits incommensurate and anharmonic modulation functions, leading to nanodomain formation and nanotwinning, which are key to understanding its high twin boundary mobility and structural transitions.

Contribution

The paper identifies the incommensurate anharmonic modulation function in Ni-Mn-Ga martensite and links it to nanotwinning and structural phase transitions, providing new insights into its crystal structure.

Findings

Incommensurate and anharmonic modulation functions explain diffraction patterns.

Nanodomains are formed due to divergent periodicity in modulation.

Specific low-temperature orthorhombic nanotwinned structures are identified.

Abstract

Understanding the crystal structure of magnetic shape memory alloys is crucial for insights into their unique properties, such as the high mobility of twin boundaries and magnetic field functionality. The complex neutron diffraction patterns betweeen 10-300 K indicate an incommensurate and simultaneously anharmonic modulation function (AMF) in Ni50.0Mn27.7Ga22.3 10M martensite. Our identification of the dominant Fourier components in the AMF allowed for a comparison between calculated diffraction patterns and experiments. The AMF explains the appearance of peculiar small-intensity diffraction peaks when nearly commensurate AMF at 300 K turns to incommensurate AMF upon cooling. Further analysis reveals that divergent periodicity between the incommensurate modulation and the lattice leads to the formation of additional nanodomains. Interpreting the modulation displacements within the nanodomains in the terms of the (2-3)2 stacking sequence of basal (110) planes enables to understand that these nanodomains are emerging a/b nanotwins. The nanotwinning interlinked with incommensurate modulation also explains the transition from the tetragonal to the orthorhombic symmetry upon cooling. We identify specific low-temperature orthorhombic structures, like 34O, 24O, 14O, which are significant as their unit cell simultaneously represents an a/b-nanotwin. The ab initio calculations confirm the stability and low and comparable energy of all found nanotwinned structures. Based on the presumed stability of the nanotwinned state and literature comparisons, we propose that the low temperature state of the martensite nominally marked as five-layered modulated, 5M or 10M, is one of the specific a/b-nanotwinned configurations such as 34O, 24O, 14O. The exact choice depends on the composition, but 14O resulting from q = 3/7 is the ultimate limit with a corresponding smallest twin domain size of 3.5 nm.