Abstract
Understanding the liquid phases and solidification behaviors of multicomponent alloy systems becomes difficult as modern engineering alloys grow more complex, especially with the discovery of high-entropy alloys (HEAs) in 2004. Information about their liquid state behavior is scarce, and potentially quite complex due to the presence of perhaps five or more elements in equimolar ratios. These alloys are showing promise as high strength materials, many composed of solid-solution phases containing equiatomic CoCrCu, which itself does not form a ternary solid solution. Instead, this compound solidifies into highly phase separated regions, and the liquid phase separation that occurs in the alloy also leads to phase separation in systems in which Co, Cr, and Cu are present. The present study demonstrates that in-situ neutron imaging of the liquid phase separation in CoCrCu can be observed. The neutron imaging of the solidification process may resolve questions about phase separation that occurs in these alloys and those that contain Cu. These results show that neutron imaging can be utilized as a characterization technique for solidification research with the potential for imaging the liquid phases of more complex alloys, such as the HEAs which have very little published data about their liquid phases. This imaging technique could potentially allow for observation of immiscible liquid phases becoming miscible at specific temperatures, which cannot be observed with ex-situ analysis of solidified structures.
Highlights
The continuous search for better engineering alloys has given rise to the superalloys, bulk metallic glasses, and more recently, the high-entropy alloys (HEAs), referred to as complex concentrated alloys (CCAs), and multiprincipal element alloys (MPEAs)
The amount of electrons that are scattered back to the detector is proportional to the atomic number of the material, the brighter region at the top is a Cu-rich phase (2.8% Co, 1.2% Cr, and 96.0% Cu), while the bottom region is Cu-lean, containing ≥90% CoCr (45.6% Co, 46.6% Cr, and 7.8% Cu), confirmed by energy dispersive X-ray spectroscopy (EDS)
There is currently no literature for this ternary system regarding the miscibility gap past 1627 o C detailing the presence of a single phase liquid, with higher temperatures it may be possible to image approximately where this demixing occurs with cold neutrons
Summary
The continuous search for better engineering alloys has given rise to the superalloys, bulk metallic glasses, and more recently, the high-entropy alloys (HEAs), referred to as complex concentrated alloys (CCAs), and multiprincipal element alloys (MPEAs). Alloy was first considered to be an equiatomic combination of 3 to 5 elements that form a single-phase solid solution, with a large configurational entropy of mixing [1,2,3,4,5,6]. The definition has since been relaxed somewhat, as researchers have been synthesizing these alloys with many components in non-equiatomic combinations to form dual-phase materials and intermetallic compounds. J. Imaging 2018, 4, 5; doi:10.3390/jimaging4010005 www.mdpi.com/journal/jimaging.
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