High-Entropy Alloys with Bidirectional Transformation Induced Plasticity

What is a Bidirectional Transformation ?

We observed a new mechanism that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure in a novel high-entropy alloy containing multiple principal elements.

This is achieved by triggering both, dynamic forward transformation from a faced-centered-cubic γ matrix into a hexagonal-close packed ε nanolaminate structure and the dynamic reverse transformation from ε into γ. This new mechanism is referred to as the “bidirectional transformation induced plasticity” (B-TRIP) effect, which is enabled through a near-zero yet positive stacking fault energy of γ. Modulation of directionality in the transformation is triggered by local dissipative heating and local micromechanical fields. The simple thermodynamic and kinetic foundations for the B-TRIP effect render this approach generally suited for designing metastable strong and ductile bulk materials with hierarchical nanolaminate substructures.

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Bidirectional Transformation in a Nanolaminate Dual-Phase High-Entropy Alloy
Adv. Mater. 2018, 1804727
Bidirectional TRIP effect High Entropy A[...]
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Strength and ductility profiles of different types of metallic materials. The TRIP-assisted dual-phase HEA shows the simultaneously increased strength and ductility by grain refinement, which contracts to the strength-ductility trade-off by grain refineme Strength and ductility profiles of different types of metallic materials. The TRIP-assisted dual-phase HEA shows the simultaneously increased strength and ductility by grain refinement, which contracts to the strength-ductility trade-off by grain refineme

 

 

How can bidirectional martensitic transformation lead to hierarchical nanolaminates in high-entropy alloys ?

The refinement of microstructural length scales is a very efficient approach to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, often reducing the material’s ductility.

In a high entropy alloy we now observed a novel phenomenon that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure. This is achieved by triggering both, dynamic forward transformation from a faced-centered cubic (FCC) γ matrix into a hexagonal close-packed (HCP) ε nanolaminate structure and the dynamic reverse transformation from ε into γ.

We refer to this new mechanism as Bidirectional Transformation Induced Plasticity effect (B-TRIP), which is enabled through a near-zero yet positive stacking fault energy of γ. Modulation of directionality in the transformation is triggered by local dissipative heating and local micromechanical fields. The simple thermodynamic and kinetic foundations for the B-TRIP effect render this approach generally suited for designing metastable strong and ductile bulk materials with hierarchical nanolaminate substructures. 

 

TEM/STEM analysis of the phase interface in the as-quenched HEA prior to deformation. (a) Low magnification bright-field TEM and (b) the corresponding dark-field TEM images of the dual-phase structure containing FCC γ block in green and HCP ε block in red TEM/STEM analysis of the phase interface in the as-quenched HEA prior to deformation. (a) Low magnification bright-field TEM and (b) the corresponding dark-field TEM images of the dual-phase structure containing FCC γ block in green and HCP ε block in red
TEM analysis of an HCP ε block with a local strain of 70%. (a) A typical bright-field TEM image, (b) SADP and (c) simulated diffraction patterns of the deformed HCP ε block. (d), (e) and (f) are the dark-field TEM images highlighted by the ε, γ and γ-twin TEM analysis of an HCP ε block with a local strain of 70%. (a) A typical bright-field TEM image, (b) SADP and (c) simulated diffraction patterns of the deformed HCP ε block. (d), (e) and (f) are the dark-field TEM images highlighted by the ε, γ and γ-twin
In-situ deformation study conducted in the low angle annular dark-field scanning transmission electron microscope (LAADF-STEM). (a) Schematic illustration of the in-situ deformation set-up: an in-house custom-made Cu tensile holder (in purple) with TEM. In-situ deformation study conducted in the low angle annular dark-field scanning transmission electron microscope (LAADF-STEM). (a) Schematic illustration of the in-situ deformation set-up: an in-house custom-made Cu tensile holder (in purple) with TEM.