Hexagonal OsB2: Sintering, microstructure and mechanical properties

Abstract

The metastable high pressure ReB 2-type hexagonal OsB 2 bulk ceramics was produced by spark plasma sintering. The phase composition, microstructure, and mechanical behavior of the sintered OsB 2 were studied by X-ray diffraction, optical microscopy, TEM, SEM, EDS, and nanoindentation. The produced ceramics was rather porous and contained a mixture of hexagonal (~80 wt%) and orthorhombic (~20 wt%) phases as identified by X-ray diffraction and EBSD analysis. Two boron-rich phases, which do not contain Os, were also identified by TEM and SEM/EDS analysis.

Effects of Re addition on phase stability and mechanical properties of hexagonal OsB2

“…The nano‐hardness and Young's modulus of OsB 2 , 36 ± 2 and 625 ± 57 GPa similar with those values reported by Xie et al within the standard experimental error under the same applied load (8 mN), which are 31 ± 9 and 574 ± 112 GPa, respectively. The smaller standard deviation might be due to the high relative density of the samples prepared in the present work, which is about 91%, compared with about 73% of the sample prepared Xie et al.…”

“…The moral ratio of (Os + Re):B = 1:3. The detailed powder synthesis process could be found in our previously published work . The acquired powders were dry‐pressed at 256 MPa followed by iso‐statically cold‐pressed at 200 MPa to form disks of 10 mm in diameter and 3‐4 mm in thickness.…”

“…Hardness and Young's modulus were determined from the load‐displacement curves following the method developed by Olive and Pharr . The Possion's ratio of the material was taken as 0.18 for calculation of the nano‐hardness and Young's modulus values, according to Xie et al …”

“…Theoretical study suggested that OsB 2 could have three crystal structures: RuB 2 ‐type orthorhombic, ReB 2 ‐type hexagonal, and AlB 2 ‐type hexagonal structures . ReB 2 ‐type hexagonal OsB 2 was reported to be synthesized successfully for the first time by Xie et al at 2014 . Previously all experimentally synthesized OsB 2 were orthorhombic phase.…”

“…Thereby, it is expected that the mechanical properties of hexagonal OsB 2 could be higher than orthorhombic OsB 2 . However, Xie et al found that part of the ReB 2 ‐type hexagonal OsB 2 transformed into the orthorhombic phase during the subsequent sintering process at a temperature of 1500°C . As a result, the produced ceramic contained a mixture of hexagonal and orthorhombic OsB 2 phases.…”