Microstructure Evolution in a Fast and Ultrafast Sintered Non-Equiatomic Al/Cu HEA

Abstract

One of the attractive characteristics of high entropy alloys (HEAs) is the ability to tailor their composition to obtain specific microstructures and properties by adjusting the stoichiometry to obtain a body-centered cubic (BCC) or face-centered cubic (FCC) structure. Thus, in this work, the target composition of an alloy of the FeCrCoNi family has been modified by adjusting the Al/Cu ratio in order to obtain a BCC crystalline structure. However, processing conditions always play a key role in the final microstructure and, therefore, in this work, the microstructure evolution of FeCrCoNiAl1.8Cu0.5 HEA sintered by different powder metallurgy (PM) techniques has been investigated. The techniques used range from the conventional PM sintering route, that uses high heating rates and sintering times, going through a fast sintering technique such as spark plasma sintering (SPS) to the novel and promising ultrafast sintering technique electrical resistance sintering (ERS). Results show that the increase in the processing time favours the separation of phases and the segregation of elements, which is reflected in a substantial change in the hardness of the alloy. In conclusion, the ERS technique is presented as a very promising consolidation technique for HEA.

Effect of Al Content on Phase Compositions of FeNiCoCrMo0.5Alx High Entropy Alloy

“…The unique features attributed to HEA such as solid solution strengthening due to severe lattice-distortion, sluggish diffusion, and the cocktail effect [5][6][7], were supposed to be caused by the multiprincipal solid solution effect, so in this context, the term HEA should refer to single-phase multiprincipal alloys [8]. However, the majority of HEA considered nowadays in the equilibrium state at an ambient temperature has several phases [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25], even those that were considered to be single-phase in early research [16,17]. It is likely that during solidification, the different atoms distribute in accordance with their affinity, leading to liquation, namely, the appearance of regions of different compositions not satisfying the criteria of solid solution formation.…”

“…Precipitation behavior in HEA is complex and abstruse and may induce the formation of useful properties as well as their deterioration and instability. Currently, methods are being developed to predict the formation and stability of HEA taking into account the formation enthalpies of competing phases [13,26]. To date, among all the others that ever proposed HEA, the alloys based on an AlCoCrFeNi system have been the most studied [2,5,[9][10][11][12][13][14][15][19][20][21][22][23][24][25].…”

“…Currently, methods are being developed to predict the formation and stability of HEA taking into account the formation enthalpies of competing phases [13,26]. To date, among all the others that ever proposed HEA, the alloys based on an AlCoCrFeNi system have been the most studied [2,5,[9][10][11][12][13][14][15][19][20][21][22][23][24][25]. The reason for this are the outstanding mechanical properties and phase formation phenomena.…”

“…It was demonstrated that due to strongly attractive interactions between Al and other elements, partially disordered structures (L12, DO3, and B2) became more stable than the disordered solid solutions. As a result, the elements segregate, forming Ni-Al-rich and Fe-Cr-rich areas [9,13,15,22]. Cr and Fe stabilize the BCC structure [11,12,28], while Ni and Co stabilize the FCC solid solution [10,22,28].…”