Metastable High Entropy Alloys

What is a metastable high entropy alloy? 

Metastability alloy design is an important approach to equip high entropy alloys  with high strain hardening response and damage tolerance.

Most materials are thermodynamically metastable at some stage during  synthesis, processing and service. However, purposeful Metastability Alloy Design refers to approaches where metastable phases are not coincidentally inherited from processing, but rather are engineered to allow triggering displacive transformation phenomena upon mechanical loading.

How can phase metastability be designed in High Entropy Alloys?

Metastability in high and medium entropy alloys can be realized through compositional (partitioning), thermal (kinetics), and microstructure (size effects and spatial confinement) tuning of metastable phases so that they can trigger athermal transformation effects when mechanically, thermally, or electromagnetically loaded.

Such a concept works both at the bulk scale and also at a spatially confined 
microstructure scale, such as at lattice defects. In the latter case, local stability tuning works primarily through elemental partitioning to dislocation cores, stacking faults, interfaces, and precipitates. Depending on stability, spatial confinement, misfit, and dispersion, both bulk and local load-driven athermal transformations can equip alloys with substantial gain in strength, ductility, and damage tolerance. Examples include self-organized metastable nanolaminates, austenite reversion steels, metastable medium- and high-entropy alloys, 
as well as steels and titanium alloys with martensitic phase transformation and twinning-induced plasticity effects. 

What is the purpose of metastable High Entropy Alloys? 

Rendering phases thermodynamically metastable is an ideal approach to create complex microstructures. Most materials are in a thermodynamically metastable state in some stages during synthesis, processing, and service. Microstructure is actually defined as the collective ensemble of all features in a material that are not in thermodynamic equilibrium (i.e., interfaces, dislocations, stacking faults, composition gradients, and dispersed precipitates). These defects, though not in thermodynamic equilibrium, are often retained in materials due to their local mechanical stability and slow relaxation and annihilation kinetics. As these microstructure ingredients endow most materials (e.g., metallic alloys) with their characteristic good load-bearing properties, thermodynamic metastability is a desired material state and the main target behind practically all processing steps that follow primary synthesis. In contrast, alloys in thermodynamic equilibrium 

are a rare exception with little relevance for applications. 

Bulk composition tuning for achieving well-defined phase metastability is a well-established design target e.g., for transformation induced plasticity High Entropy Alloys [TRIP HEAs], twinning induced plasticity High Entropy Alloys [TWIP HEAs] and metastable duplex HEAs.

How to design Bidirectional Transformation in Nanolaminate Dual-Phase High-Entropy Alloys?

Microstructural length-scale refinement is among the most efficient approaches to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, 
compromising the material’s ductility. Here, a fundamentally new approach 
that allows load-driven formation and permanent refinement of a hierarchical 
nanolaminate structure in a novel high-entropy alloy containing multiple principal elements is reported. 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.

How to design metastable nanostructured bulk high-entropy  alloys ?

We developed a metastability alloy design strategy which we used to develop nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit 
from phase stabilization through mixing entropy maximization. Motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution 
strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity 
that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-

engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.

How to design Crystal–Glass High-Entropy Nanocomposites 

High-entropy alloys (HEAs) and metallic glasses (MGs) are two material classes based on the massive mixing of multiple-principal elements. HEAs are single or multiphase crystalline solid solutions with high ductility. MGs with amorphous structure have superior strength but usually poor ductility. Here, the stacking fault energy in the high-entropy nanotwinned crystalline phase and the glass-forming-ability in the MG phase of the same material are controlled, realizing a novel nanocomposite with near theoretical yield strength (G/24, where G is the shear modulus of a material) and homogeneous plastic strain above 45% in compression. The mutually compatible flow behavior of the MG phase and the dislocation flux in the crystals enable homogeneous plastic co-deformation of the two regions. This crystal–glass high-entropy nanocomposite design concept provides a new approach to developing advanced materials with an outstanding combination of strength and ductility.

What is an interstitial TRIP-TWIP High-Entropy Alloy?

We developed an approach of utilizing a hierarchical microstructure design to improve the mechanical properties of an interstitial carbon doped high-entropy alloy (HEA) by cold rolling and subsequent tempering and annealing. Bimodal microstructures were produced in the tempered specimens consisting of nano-grains (~50 nm) in the vicinity of shear bands and recovered parent grains (10-35 mm) with pre-existing nano-twins. Upon annealing, partial recrystallization led to trimodal microstructures characterized by small recrystallized grains (<1 mm) associated with shear bands, medium-sized grains (1^-6 mm) recrystallized through subgrain rotation or coalescence of parent grains and retained large un-
recrystallized grains. To reveal the influence of these hierarchical microstructures on the strength-ductility synergy, the underlying deformation mechanisms and the resultant strain hardening were investigated. A superior yield strength of 1.3 GPa was achieved in the bimodal microstructure, more than two times higher than that of the fully recrystallized microstructure, owing to the presence of nano-sized grains and nano-twins. The ductility was dramatically improved from 14% to 60% in the trimodal structure compared to the bimodal structure due to the appearance of a multi-stage work hardening behavior. This important strain hardening sequence was attributed to the sequential activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects as a result of the wide variation in phase stability promoted by the grain size hierarchy. These findings open a broader window for achieving a wide spectrum of mechanical properties for HEAs, making better use of not only compositional variations but also microstructure and phase stability tuning.

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