Tailoring the Plasticity of Topologically Close‐Packed Phases via the Crystals’ Fundamental Building Blocks

Abstract

Brittle topologically close‐packed precipitates form in many advanced alloys. Due to their complex structures, little is known about their plasticity. Here, a strategy is presented to understand and tailor the deformability of these complex phases by considering the Nb–Co µ‐phase as an archetypal material. The plasticity of the Nb–Co µ‐phase is controlled by the Laves phase building block that forms parts of its unit cell. It is found that between the bulk C15–NbCo2 Laves and Nb–Co µ‐phases, the interplanar spacing and local stiffness of the Laves phase building block change, leading to a strong reduction in hardness and stiffness, as well as a transition from synchroshear to crystallographic slip. Furthermore, as the composition changes from Nb6Co7 to Nb7Co6, the Co atoms in the triple layer are substituted such that the triple layer of the Laves phase building block becomes a slab of pure Nb, resulting in inhomogeneous changes in elasticity and a transition from crystallographic slip to a glide‐and‐shuffle mechanism. These findings open opportunities to purposefully tailor the plasticity of these topologically close‐packed phases in the bulk by manipulating the interplanar spacing and local shear modulus of the fundamental crystal building blocks at the atomic scale.

Thermally activated nature of synchro-Shockley dislocations in Laves phases

“…Synchro-shear is the dominant mechanism for dislocation-mediated plasticity on the basal or {111} plane in Laves phases, which was confirmed by experimental observations of synchro-shear-induced stacking faults and synchro-Shockley dislocation cores in C14 HfCr 2 [6]. Moreover, as demonstrated by ab initio calculations [7,8] and atomistic simulations [9], synchroshear slip is the energetically favorable mechanism compared to another competitive crystallographic slip between kagomé and triple layers for basal or {111} planes in Laves phases.…”

“…When σ yz > 3.52 GPa, a slip between the triple-layer and the adjacent kagomé layer other than the synchro-shear slip was predicted as the first event, see Figure S4. This triple-kagomé slip was demonstrated as a competing mechanism to synchro-shear slip in the Laves crystal building block of µ phases in experiments and ab-initio calculations [8,26]. The triple-kagomé full slip dissociates into two mirror symmetric partial slip events in CaMg 2 , and a stacking fault state with a high fault energy 409 mJ/m 2 compared to the low fault energy of the synchro-shear induced stacking fault (14 mJ/m 2 [13]) was obtained, see Figure S5(a-b).…”

Carbides Formation in MarM247 Directional Solidified during Stress-Rupture Test at High Temperature

“…The eutectic compositions of gamma and gamma prime in the inter-dendritic regions are highly sensitive to incipient melting during the solution heat treatment and are crucial for the final mechanical properties of the superalloy at high temperatures. However, the formation of undesirable phases such as carbides, topologically close-packed (TCP) phases (sigma, µ, and Laves phases), also affects the mechanical properties of the alloys [8].…”