Origin of high strength in the CoCrFeNiPd high-entropy alloy

Abstract

Recent experiments show that the CoCrFeNiPd high-entropy alloy (HEA) is significantly stronger than CoCrFeNi and with nanoscale composition fluctuations beyond those expected for random alloys. These fluctuations were suggested to be responsible for strengthening. Here, a recent parameterfree theory for initial yield strength in fcc random alloys is shown to predict the strength of CoCr-FeNiPd in good agreement with experiments. The strengthening is due mainly to the large misfit volume of Pd in CoCrFeNi, indicating that effects of the non-random composition fluctuations are secondary. Analyses of strength variations and strengthening-associated length scales helps rationalize why dislocation motion is insensitive to such fluctuations. These findings point to the value of theory for understanding mechanical behavior of HEAs. IMPACT STATEMENT Experiments and theory are highlighting chemical ordering in high-entropy alloys (HEAs) as important for mechanical properties but the high strength in CoCrFeNiPd is predicted here to be achievable in the random alloy due to the large misfit volume of Pd, in spite of observed ordering. Ordering is thus not essential for attaining high-strength HEAs.

Yield strength and misfit volumes of NiCoCr and implications for short-range-order

“…Solute strengthening in NiCoCr: theory versus experiment. With the experimental misfit volumes and two recent literature values for the room temperature single-crystal elastic constants 30,31 , we can predict the initial yield strength of NiCoCr within the framework of an existing solute strengthening theory for random alloys 32,33 , which has previously shown quantitative success for a number of HEAs 25,[32][33][34][35][36] . We can then compare the random alloy strength prediction against experiments.…”

Simultaneously enhancing the ultimate strength and ductility of high-entropy alloys via short-range ordering

“…The idea was that SRO tends to create resistance to deformation/slip, leading to larger dislocation glide resistance in the more ordered CoCrFeNiPd than in CoCrFeNiMn HEAs. Theoretical work by Yin et al 22 suggested that the strengthening was due mainly to the large atomic/misfit volume of Pd in CoCrFeNi. Ma et al 23 identified a number of unusual features associated with dislocations/slips in HEAs that were related to lattice distortion and/or local chemical order.…”

Microelasticity model of random alloys. Part I: mean square displacements and stresses

“…This approach relies on the evaluation of the interaction energy between dislocations and solutes of different sizes. The model can be parameterized from DFT calculations but in its simplest elastic version, it relates the yield stress at 0 K denoted τ y0 to the misfit parameter δ as [17,18,19]:…”