Tuning martensitic transformations via coherent second phases in nanolaminates using free energy landscape engineering

TL;DR

This study demonstrates how coherent second phases in nanolaminates can be used to precisely tune martensitic transformation temperatures and microstructures by modifying the free energy landscape, with implications for designing advanced shape memory alloys.

Contribution

It introduces a molecular dynamics simulation approach to systematically engineer martensitic transformations through nanoscale second phases, revealing their impact on transformation temperatures and microstructure.

Findings

Transformation temperatures can be tuned by ±200% with second phases.

Coherency stresses influence martensitic variants and can induce phase transformations.

Reducing second phase stiffness increases transformation strain and microstructural volume fraction.

Abstract

We explore the possibilities and limitations of using a coherent second phase to engineer the thermo-mechanical properties of a martensitic alloy by modifying the underlying free energy landscape that controls the transformation. We use molecular dynamics simulations of a model atomistic system where the properties of a coherent, nanoscale second phase can be varied systematically. With a base martensitic material that undergoes a temperature-induced transformation from a cubic austenite to a monoclinic martensite, the simulations show a significant ability to engineer the transformation temperatures, from a ~50% reduction to a ~200% increase, with 50 at. % of the cubic second phase. We establish correlations between the properties of the second phase and the transformation characteristics and microstructure, via the free energy landscape of the two-phase systems. Coherency stresses have a strong influence on the martensitic variants observed and can even cause the non-martensitic second phase to undergo a transformation. Reducing the stiffness of second phase increases the transformation strain and modifies the martensitic microstructure, increasing the volume fraction of the transformed material. This increase in transformation strain is accompanied by a significant increase in the Af and thermal hysteresis, while the Ms remains unaltered. Our findings on the tunability of martensitic transformations can be used for informed searches of second phases to achieve desired material properties, such as achieving room temperature, lightweight shape memory alloys.