Multicomponent equi-molar materials (MEMs) are a fairly new design for alloys where the material is composed of many elements in equal proportions. They differs from the most commonly used alloys such as steal and nickel superalloys because they are not based on a single element. This idea was explored separately in 2004 by Cantor et al and Yeh et al which started a growing trend in the field. Ranganathan discusses these new alloys and similar ideas which break the conventional alloying techniques in his article on multimaterial cocktails. The phrase “High Entropy Alloy” (HEA), also used to describe MEMs, was defined by Yeh et al in 2004 as an alloy composed of five or more constituents in equi-molar or near equi-molar ratios. The name is not due to the entropy of the material, which is generally not that high, but rather their high entropy of mixing. With the enormous amount of possible combinations, these materials can have a wide range of useful microstructural and mechanical properties. Cold spray additive manufacturing is a potential method for producing MEM near net shape structures and coatings.
Cold spray additive manufacturing is a process in which powder materials are deposited onto a solid substrate, producing thick, wrought coatings up to 50mm. The particles are accelerated through a deLaval nozzle using a high pressure inert gas, often helium or nitrogen. The cold spray process is very similar to the thermal spray technique; however the powder is deposited without melting, thus in solid state form. The solid particles must achieve a certain material and temperature dependant velocity threshold, known as critical velocity, to plastically deform and adhere on impact. Areas of research within cold spray include process modelling, gas dynamics, nozzle design, particle bonding, coating properties and development of applications.
The aim of this research is to further understand the cold spray process by investigating the effect of process and material parameters on the coatings and the behaviour of cold sprayed MEMs. Powder production and properties of MEMs are to be explored to observe the potential changes from bulk material to coatings. A significant area of research is the study of individual cold sprayed particles which have bonded to the surface, named splats. These provide information on the microstructural behaviour of the powdered materials after impact, which can provide insight into the bonding mechanisms in cold spray.