Local lattice distortions and chemical short-range order in MoNbTaW
Andrea Fantin, Anna M. Manzoni, Hauke Springer, Reza Darvishi Kamachali, R. Maaß
Abstract
Extended X-ray absorption fine structure (EXAFS) conducted on an equiatomic MoNbTaW <i>bcc</i> medium-entropy alloy that was annealed at 2273 K reveals unexpectedly small 1<sup>st</sup> and 2<sup>nd</sup> shell element-specific lattice distortions. An experimental size-mismatch parameter, δexp, is determined to be ca. 50% lower than the corresponding calculated value. Around W, short-range order (SRO) preferring 4<i>d</i> elements in the 1<sup>st</sup> and 2<sup>nd</sup> 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 <i>fcc</i> also foreshadowing low-temperature phase, these findings suggest a general feature of high-temperature SRO. A medium-entropy MoNbTaW <i>bcc</i> alloy annealed at 2273 K shows still signs of short-range ordering which relate to its segregational behavior at lower temperatures, corroborating observations on <i>fcc</i> alloys.