Formation of martensitic microstructure in epitaxial Ni-Mn-Ga films after fast cooling

TL;DR

This study investigates how rapid cooling via flash lamp pulses influences the formation of complex martensitic microstructures in epitaxial Ni-Mn-Ga films, revealing differences from slow cooling due to limited formation time and thermal stress.

Contribution

It provides new insights into microstructure formation under rapid thermal cycling in Ni-Mn-Ga films, relevant for high-frequency shape memory applications.

Findings

Rapid cooling alters microstructure compared to slow cooling.

Thermal stress impacts microstructure formation.

Limited time affects hierarchical microstructure development.

Abstract

Shape memory alloys have a wide range of applications, including high stroke actuation, energy-efficient ferroic cooling, and energy harvesting. These applications are based on a reversible martensitic transformation, which results in a complex microstructure in the martensitic state. Understanding the formation of this microstructure after fast heating and cooling is crucial, as a high cycle frequency is essential for achieving high power density in the mentioned applications. This article uses an epitaxial Ni-Mn-Ga film as a model system to study the formation of the martensitic microstructure, since the high surface-to-volume ratio of thin films enables rapid heating and cooling, and since the formation of a multi-level hierarchical microstructure during slow cooling of this material system is already well understood. We apply a millisecond flash lamp pulse of varying energy density on the Ni-Mn-Ga film and analyse the hierarchical martensitic microstructure afterwards. Substantial differences compared to slow cooling occur, which we attribute to the limited time available for the microstructure to form and to the thermal stress between film and substrate during rapid temperature changes.