Development of Refractory High Entropy Alloys for Additive Manufacturing

High Entropy Alloys (HEAs) are a class of alloys consisting of multiple primary elements with similar content, exhibiting many promising properties, therefore garnering interest in many sectors such as nuclear and aerospace. HEAs are frequently reported as having good mechanical properties, particularly when manufactured by additive manufacturing (AM). In AM, components are built up layer-by-layer using a moving heat source to sequentially melt feedstock to build up a geometrically optimised 3D part with a controlled microstructure. AM is especially useful in manufacturing refractory HEAs (RHEAs), due to the high melting temperatures involved and the difficulties associated with their manufacture via conventional methods such as casting and machining. AM enables the manufacture of these types of high temperature alloys with complex part geometries, suitable for highly specialised applications in the pursuit of increasing efficiency and reducing material waste.

This thesis concerns development of new HEAs suitable for AM, more specifically for the AM process laser powder bed fusion (PBF-LB/M). In-situ alloying (ISA), a technique where powders are blended prior to the AM process and alloyed using the motion of the laser and the melt pool, is explored. The impact of different elemental additions on alloying is assessed, including the benefit of minimising melting point differences and the tendency of that elemental addition to segregate, impacting the miscibility of the addition in the base alloy. The impact of elemental powder size, shape and contamination on the ISA process is also explored, showing that although defects can form due to low powder flowability, representative homogeneous microstructures can be obtained, comparable to equivalent samples manufactured using pre-alloyed powder.

An alloy design procedure is also proposed, using empirical parameters to design solid solution HEAs suitable for high temperatures. The top ranked HEAs are then manufactured via arc-melting, resulting in 4 solid solution RHEAs. The AM processability of these alloys is then assessed using melt tracks, providing information on their crack susceptibility. Consequently, a new solid-state cracking indicator is proposed based on considering the impact of bond energy on ductility, which is used along with a solidification cracking indicator and 12 conventional alloys from literature to map AM crack susceptibility for new HEAs. The AM processable Mo5Nb35Ti30V30 RHEA is then manufactured via ISA and pre-alloyed powder comparing the results. The pre-alloyed samples are heat-treated, initiating the formation of a secondary TiCN phase on cell and grain boundaries, which coarsens with heat treatment time due to atmospheric infiltration. The removal of the cellular microstructure, interstitial strengthening and coarsening of the TiCN results in an improvement in alloy mechanical properties compared to the as-built condition. High throughput alloy design and optimisation is important to facilitate rapidly evolving industrial requirements for high performance materials. The results of this thesis aim to streamline and accelerate the alloy screening process, while also minimising time and costs associated with alloy development.