Unusual solute segregation phenomenon in coherent twin boundaries

Abstract

Interface segregation of solute atoms has a profound effect on properties of engineering alloys. The occurrence of solute segregation in coherent twin boundaries (CTBs) in Mg alloys is commonly considered to be induced by atomic size effect where solute atoms larger than Mg take extension sites and those smaller ones take compression sites in CTBs. Here we report an unusual solute segregation phenomenon in a group of Mg alloys—solute atoms larger than Mg unexpectedly segregate to compression sites of {10$$\overline 1$$ 1 ¯ 1} fully coherent twin boundary and do not segregate to the extension or compression site of {10$$\overline 1$$ 1 ¯ 2} fully coherent twin boundary. We propose that such segregation is dominated by chemical bonding (coordination and solute electronic configuration) rather than elastic strain minimization. We further demonstrate that the chemical bonding factor can also predict the solute segregation phenomena reported previously. Our findings advance the atomic-level understanding of the role of electronic structure in solute segregation in fully coherent twin boundaries, and more broadly grain boundaries, in Mg alloys. They are likely to provide insights into interface boundaries in other metals and alloys of different structures.

Massive transformation in FeNi nanopowders with nanotwin-assisted nitridation

“…In contrast, coherent TBs are dislocation-free and have perfectly aligned atomic stacks, resulting in low interfacial energies compared to grain boundaries; thus, strong solute segregation is not initially expected at coherent TBs. However, several recent reports on different alloy systems have shown solute segregation along TBs and proved it is associated to the migrating heterophase interface boundary, driven by minimizing the intrinsic stacking fault, elastic strain, and/or total energy in the system 20 – 22 . On the other hand, it is also worth notice that solutes accumulation at the TBs can also act as Cottrell clouds surrounding the twins and thereby restrict further slip of the dislocations which was well discussed as the strength enhancement mechanism for the twinned soft materials 23 – 25 .…”

Nanotwin-assisted nitridation in FeNi nanopowders for synthesis of rare-earth-free magnets

“…In contrast, coherent TBs are dislocation-free and have perfectly aligned atomic stacks, resulting in low interfacial energies compared to grain boundaries; thus, strong solute segregation is not initially expected at coherent TBs. However, several recent reports on different alloy systems have shown solute segregation along TBs and proved it is associated to the migrating heterophase interface boundary, driven by minimizing the intrinsic stacking fault, elastic strain, and/or total energy in the system [20][21][22] . On the other hand, it is also worth notice that solutes accumulation at the TBs can also act as Cottrell clouds surrounding the twins and thereby restrict further slip of the dislocations which was well discussed as the strength enhancement mechanism for the twinned soft materials [23][24][25] .…”

Twin Boundary Superstructures Assembled by Periodic Segregation of Solute Atoms

“…21,23−25 size larger than the matrix atom tends to take the extension site and the smaller one segregates favorably to the compression site, 21,34−37 and it can thus be understood as the larger/smaller solutes replacing the matrix atoms at the extension/ compressive sites, respectively, can minimize the strain in the lattice. However, recent results showed that larger size solutes segregated to the compression sites of {101̅ 1} TBs in Mg−Bi and Mg−Pb binary systems, 27 meaning that chemical bonding also played an indispensable role in determining the segregation behaviors. Additionally, what should be emphasized here is that the elastic strain minimization principle dominated by the atomic size effect requires that all the alternately distributed extension/compression sites in the coherent TBs should be occupied by larger/smaller solutes in an orderly and continuous manner, and all the TB 1g) and the corresponding EDS mapping images related to the elements Mg (Figure 1h), Nd (Figure 1i), and Mn (Figure 1j).…”

“…Interfaces, including grain boundaries (GBs), phase boundaries, domain boundaries, twin boundaries (TBs), etc. , are ubiquitous crystal planar defects in materials and play a decisive role in their properties. − Generally, in materials containing more than one element, the segregation of impurities/solutes at such interfaces driven by the reduction of interfacial energy can significantly alter the performances, such as cohesion, mobility, thermostability, and strength. ,− A typical example is the catastrophic brittle intergranular failure of metals, which is caused by the diffusion of certain elements into the boundary core. ,, The segregation behaviors at interfaces were described by the early Langmuir–McLean segregation theorem, , and it was assumed that one-layer/sub-one-layer coverage of an interface, without any solute interactions or structural transformations, nor multilayer segregation or additional solute enrichment within the bulk, e.g., precipitations, which had been demonstrated in many special interfaces, including artificial bicrystal boundaries ,− and coherent TBs. − …”

“…Twinning is a prevalent deformation mechanism in metals usually with a limited number of active slip systems. , TBs generated by twinning during plastic deformation are special dislocation-free high-angle tilt GBs, and with a lower interfacial energy than general GBs . In hexagonal close-packed (hcp) metals, such as Mg, Ti, Zn, and Zr, there are three main types of TBs commonly observed experimentally, i.e., {101̅1}, {101̅2}, and {101̅3}. − The segregation of solutes in these coherent TBs is not considered possible due to low interfacial energy characteristic, until such TB segregation phenomenon was first found in Mg alloys by Nie et al TB segregation is commonly considered to be effective in enhancing the strength of materials and the thermal stability of interfaces, etc . , …”