Dynamic observations of compositionally complex alloys by in situ TEM

Abstract text (incl. figure legends and references)

Introduction

The development of multicomponent high entropy alloys (HEAs) has opened the door to sculpt advanced microstructures in the near-infinite compositional space, resulting in materials with unprecedented mechanical and functional properties. Understanding the underlying phase transformation mechanisms during processing and application of these compositionally complex alloys is a key requirement to further advance alloy design strategies. In situtransmission electron microscopy (TEM) techniques nowadays make it possible to probe even compositionally complex alloys under varying temperatures or strains, while being able to dynamically observe microstructure evolution down to the atomic level.

Objectives

The present talk focuses on in situ probing strategies in the TEM to observe plastic deformation and temperature dependent phase transformations in HEAs. I will show a novel mechanism of transformation induced plasticity in an FeMnCoCr alloy revealed by in situ straining in the TEM. In a further example, in situ heating experiments in the TEM will be used to track temperature induced phase transformations and secondary phase formation in a carbon (C) doped FeMnCoCrNi alloy.

Materials & methods

The HEAs are fabricated by vacuum induction melting and investment casting using at least 99.5 wt.% pure metals. Thermo-mechanical treatment is used to homogenize the alloys and adjust the desired starting microstructure. In situ tensile straining experiments are performed using a custom-built straining inset in a model 654 Gatan straining holder. Samples are heated in situ in the TEM using a DENSsolutions Lightning heating/biasing holder, while using both TEM and scanning TEM to observe microstructural changes in Titan Themis (Thermo Fisher Scientific) microscopes.

Results

The dual-phase Fe₅₀Mn₃₀Co₁₀Cr₁₀ (at.%) alloy is primarily deforming via displacive phase transformation from the face-centered cubic (FCC) to the hexagonal close packed (HCP) structure. Our in situ tensile straining reveals that a bi-directional transformation is active, where FCC transforms to HCP and back to FCC upon straining. This strain induced phase transformation leads to dynamic microstructural refinement, which equips the alloy with outstanding work-hardening capabilities.

Such a nanostructure provides excellent room temperature properties, but rapidly coarsens at elevated temperatures leading to a loss in mechanical properties. Interstitial doping with C is a way to stabilize the microstructure and our in situ TEM observations reveal the underlying phase transformation mechanisms. At intermediate temperatures of ~300ºC we observe the formation of nano-twinned FCC originating from an initial HCP grain. Upon further heating to 700ºC, elongated nanoscale carbides are forming along the FCC twin boundaries, which can stabilize the nano-twinned regions up to 900ºC. Atomic scale observations at extreme temperatures at >900ºC reveal the dissolution of the carbide phases and associated de-twinning, providing insights into microstructural evolution in the compositionally complex alloys.

Conclusions

In the present talk, we showed that in situ straining and heating in the TEM provides unprecedented insights into the phase transformation mechanisms in compositionally complex alloy systems. In situ probing is pivotal in observing the underlying transformation dynamics and is able to reveal previously overlooked phase transformation mechanisms.