Self-organized multigrain patterning with special grain boundaries produced by phase transformation cycling

In crystalline solids, grain boundaries (GBs) play a significant role in determining a large number of material properties. The design and synthesis of special GBs has been a long-standing challenge in materials science and engineering. Here we demonstrate a mechanism to produce special GBs. Unique multigrain structures can be obtained through cyclic, diffusionless phase transformations under external fields, with all GBs being coherent special GBs. The crystallographic character of the GBs produced in this way is dictated by the broken symmetry during the phase transformations, while the topology of the GB network is determined by the geometric compatibility and self-organization of the multigrain structures. Such a mechanism not only suggests an alternative method of GB engineering, but also reveals a fundamental relationship between special GBs and phase transformations from a crystallographic point of view.