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Mechanochemical synthesis of Compositionally Complex Alloys

Mon, 11 October, 2021

The FORGE project is exploring the world of Compositionally Complex Alloys deploying diverse theoretical, computational and experimental techniques. The aim is to find compositions for coating that address the performance targets of resistance to wear, hydrogen embrittlement and corrosion.

These alloys are characterized by the presence of 5 or more principal elements and exhibit unique and extensively adjustable properties compared to the traditional alloys. Indeed, depending on the application, the composition can be tailored to exhibit excellent toughness at low temperatures via the mechanism known as twinning-induced plasticity (TWIP), a combination of high strength and ductility via the deformation mode known as transformation-induced plasticity (TRIP) or enhanced room temperature as well as high temperatures mechanical properties with the dispersion of coherent intermetallic precipitates.

Since their formalization in 2004, various techniques have been applied to synthesize CCA, such as rapid solidification, PVD coatings, induction melting, vacuum arc melting and mechanical alloying. In these processes, segregations were observed especially with the increase of the alloy complexity and thus with the formation of unwanted phases. This is not occurring, or is very limited, using Mechanical Alloying, and this is one of the main reasons why FORGE chosen this technique to move from the selected compositions to a powder material processable by HVOF, HVAF, CGS and Laser Cladding.

Mechanical Alloying has also the advantage to have virtually no limits in the composition, and element with very different melting points or reactivity can be mixed together directly obtaining a powder that can be further refined by thermal treatments, toward the desired alloy phase.

FORGE is progressing in the selection and definition of compositions for each performance target, but the activities on the Mechanical Alloying process has started, in preparation to the main powder material production phase for the coating plants.

Three different equiatomic CCA has been produced in order to identify the most effective production methodology and the synthesis strategies: a Cantor Alloy (CoCrFeMnNi) to be used as reference internally and externally, an alloy with ductile/low melting elements (AlCrCuFeNi) and an alloy with refractory elements (CrMnNbSiW). These alloys has been obtained to gatherer information to manage the synthesis approaches for the future FORGE compositions, and at the same time provide reference material to support the experimental activities that are ongoing.

The Mechanical Alloying technology used by MBN is based on a proprietary high energy ball mill process design, the Mechanomade®, consistently scaled from lab to pilot to industrial plants. This process allows to produce materials characterized by an ultrafine and homogeneous chemical distribution of elements and a refined crystalline structure. For the synthesis of CCA powders, MBN set up a methodology that combines its mechanical alloying process with thermal treatments and multiple classification steps in order to deliver powders that are suitable both morphologically (for the processability) and compositionally (for the final performances).

Trials on the production of Cantor alloy highlighted the role of raw materials selection and processes energy in the element dispersion. In particular, different combinations of commodity powders has been tested as raw materials, confirming the viability and the effectiveness of this approach for a competitive powder metallurgy production route[1].

Thermal Treatments resulted to be, in most of the cases, a crucial step to achieve homogeneous powder and CCA with a stress-free matrix, that is a critical feature for a good processability especially in Cold Gas Spraying. The Thermal Treatments in the powders also trigger the formation solid solutions that corresponds to the predicted ones. The trials on the production of CCA with ductile and refractory elements show different trends and requirements based to the elements features and this sets the bases for the understating of the processing route for the production of the final powder in FORGE.

The powder produced so far by MBN are currently being utilised in FORGE as references and base material to advance on other lines of activities, such as the development of smart coatings and the correlation between different experimental setups.

To find out more about the project, join us for our webinar on "How to FORGE a new class of coatings" on 14th October @ 13:00 BST.

Article Courtesy: MBN Nanomaterialia (MBN)

References:

[1] Torralba, J. M., & Kumarán, S. V. (2021). Development of competitive high-entropy alloys using commodity powders. Materials Letters, 130202.