*Article* **Morphology and Growth Mechanism of** β**-Rhombohedral Boron and Pentagonal Twins in Cu Alloy**

**Junqing Han 1,†, Wentao Yuan 1,†, Yihan Wen 1, Zuoshan Wei 2, Tong Gao 1, Yuying Wu 1,\* and Xiangfa Liu <sup>1</sup>**


**Abstract:** In this work, boron particles with β-rhombohedral structure were prepared in Cu-4B alloy. The morphology and growth mechanism of β-B and pentagonal twins were analyzed. Results show that boron crystals possessed an approximate octahedral structure which consisted of two planes belonging to {001} facet and a rhombohedron formed by {101} planes. The morphology of the boron crystal was determined by the position and size of {001} planes. During growth, parts of boron crystal formed twins to reduce surface energy. Five particular single crystals can shape a pentagonal twin. The morphological distinction between pentagonal twins mainly came from the difference in morphology of single crystal. When the {001} exposed planes were large and showed a hexagonal shape, the boron crystal often formed parallel groupings and polysynthetic twins to reduce surface energy.

**Keywords:** β-rhombohedral boron; twins; pentagonal twin; growth mechanism
