Local lattice distortions and chemical short-range order in MoNbTaW.

Extended X-ray absorption fine structure (EXAFS) conducted on an equiatomic MoNbTaW bcc medium-entropy alloy that was annealed at 2273 K reveals unexpectedly small 1^st and 2^nd shell element-specific lattice distortions. An experimental size-mismatch parameter, $ \delta _{exp} $ δ exp , is determined to be ca. 50% lower than the corresponding calculated value. Around W, short-range order (SRO) preferring 4d elements in the 1^st and 2^nd shells persists. A Nb-W ordering is found, which is reminiscent of ordering emerging at lower temperatures in the B2(Mo,W;Ta,Nb)- and B32(Nb,W)-phases. With high-temperature ordering preferences in fcc also foreshadowing low-temperature phase, these findings suggest a general feature of high-temperature SRO. A medium-entropy MoNbTaW bcc alloy annealed at 2273 K shows still signs of short-range ordering which relate to its segregational behavior at lower temperatures, corroborating observations on fcc alloys. [ABSTRACT FROM AUTHOR]