Microstructure of precipitates and magnetic domain structure in an annealed Co38Ni33Al29 shape memory alloy

TL;DR

This study investigates the microstructure and magnetic domain behavior of a Co38Ni33Al29 shape memory alloy using advanced microscopy techniques, revealing precipitate structures, phase transformations, and domain evolution during cooling.

Contribution

It provides detailed insights into the microstructure and magnetic domain relationships in CoNiAl FSMA, including precipitate characterization and in-situ domain evolution analysis.

Findings

Presence of 10-60 nm hcp ε-Co precipitates confirmed by HRTEM.

Transformation from B2 austenite to L10 martensite involves domain nucleation and narrowing.

Magnetic domains are aligned with crystallographic directions, with complex relationships in martensite.

Abstract

The microstructure of a CoNiAl FSMA was determined by conventional transmission electron microscopy, electron diffraction studies together with advanced microscopy techniques and in-situ Lorentz microscopy. 10 to 60 nm sized rod-like precipitates of hcp $\epsilon$-Co were confirmed to be present by HRTEM. The orientation relationship between the precipitates and B2 matrix is described by the Burgers orientation relationship. The crystal structure of the martensite obtained after cooling is tetragonal L10 with a (1-11) twinning plane. The magnetic domain structure was determined during an in-situ cooling experiment using the Fresnel mode of Lorentz microscopy. While transformation proceeds from B2 austenite to L10 martensite, new domains are nucleated leading to a decrease in domain width, with the magnetization lying predominantly along a single direction. It was possible to completely describe the relationship between magnetic domains and crystallographic directions in the austenite phase though complications existed for the martensite phase.