Shixue Guan’s research while affiliated with Beijing Information Science & Technology University and other places

The application of compositionally complex carbides under extreme conditions has garnered significant attention. The phase diagram compositionally complex transition metal carbide (Ta0.2Nb0.2Hf0.2Zr0.2V0.2)C for synthesis under high‐pressure and high‐temperature has been systematically investigated for the first time, and a pressure‐induced decrease in the compositionally complex carbides phase formation temperature was observed. The asymptotic Vickers hardness and bulk modulus of (Ta0.2Nb0.2Hf0.2Zr0.2V0.2)C reached 24.0 GPa and 311.3 GPa. The bulk modulus demonstrates an approximate 30% improvement compared to the average values of individual carbides, indicating that it possesses relatively competitive mechanical properties within the compositionally complex carbides group. The Claperon equation has been utilized to predict the lattice contraction of compositionally complex carbides during phase formation, and the physical mechanism of the high‐pressure strengthening has been proposed accordingly. In summary, the research of pressure–temperature phase diagram and the high‐pressure strengthening mechanism lay a valuable theoretical basis for the structural design and performance optimization of novel compositionally complex carbides.

Is the inverse Hall-Petch relation in ceramic systems the same as that in metal systems? The premise to explore this subject is the synthesis of a dense bulk nanocrystalline material with clean grain boundaries. By using the reciprocating pressure-induced phase transition (RPPT) technique, compact bulk nanocrystalline indium arsenide (InAs) has been synthesized from a single crystal in a single step, while its grain size is controlled by thermal annealing. The influence of macroscopic stress or surface states on the mechanical characterization has been successfully excluded by combining first-principles calculations and experiments. Unexpectedly, nanoindentation tests show a potential inverse Hall-Petch relation in the bulk InAs with a critical grain size (Dcri) of 35.93 nm in the experimental scope. Further molecular dynamics investigation confirms the existence of the inverse Hall-Petch relation in the bulk nanocrystalline InAs with Dcri = 20.14 nm for the defective polycrystalline structure, with its Dcri significantly affected by the intragranular-defect density. The experimental and theoretical conclusions comprehensively reveal the great potential of RPPT in the synthesis and characterization of compact bulk nanocrystalline materials, which provides a novel window to rediscover their intrinsic mechanical properties, for instance, the inverse Hall-Petch relation of bulk nanocrystalline InAs.

High-strain-induced microstructural refinement and dislocations are critical for the strengthening of metallic materials; however, this is difficult to achieve in ceramic materials due to their unique bonding characteristics and electronic structure. Here, a series of β-Mo2C bulk ceramics are consolidated by the high pressure and high temperature (HPHT) strategy. Our results demonstrate that high strain-induced grain plastic deformation at high pressure produces a lamellar sub-grain structure with high-density dislocations, and that the dislocations at the lath-like grain boundaries eventually evolve into low-angle grain boundaries. The mechanical properties, superconducting behavior, and onset of oxidation of the specimens are investigated. It is found that superconductivity and hardness arise from the high density of states at the Fermi level, while the high intrinsic hardness is attributed to the strong hybridization between Mo-4d orbitals and C-2p orbitals. Furthermore, strain-induced defect structure (dislocations and low-angle grain boundaries) mainly mainly enhances the intrinsic structure of β-Mo2C.

High-purity (Ta0.2Nb0.2Hf0.2Zr0.2Ti0.2)C high-entropy ceramics are successfully synthesized using high-pressure solid-phase reactions. The high-entropy phase formation route and the effect of pressure tuning on the structural and mechanical properties of the ceramics are systematically investigated. An interesting process is observed whereby a double phase high-entropy carbide is formed, followed by the defusing of one phase into another, and the eventual formation of a high-entropy phase. Moreover, an abnormal, pressure-induced enhancement of the bulk modulus and strength is observed, resulting from the crystal plane distortion in high-entropy carbides. These results reflect the unusual lattice distortion effects of continuous, pressure-induced tuning on the structure and properties of high-entropy carbides.

Reciprocating pressure-induced phase transition (RPPT) has been proposed as a new approach to synthesize nanostructured bulk materials with clean grain boundary interfaces for structures that undergo reversible pressure-induced phase transitions. The modulation effects on grain size under different cycle numbers of RPPT for InAs were investigated and the initial single-crystal bulk, with a dimensional size of about 30 μm, was transformed into a nanostructure with an average grain size of 7 nm by the utilization of the in situ high-pressure diamond anvil cell (DAC) technique. To verify the DAC findings, compact nanostructured bulk InAs with grain sizes ranging from 2-20 nm (average = 8 nm) and large dimensions (3.2 mm × 3.2 mm × 0.5 mm) was successfully synthesized from single-crystal InAs using a large volume press (LVP). The smaller work function (3.86 eV) and larger bandgap energy (2.64 eV) of the compact nanostructured bulk InAs phase compared to those of single-crystal InAs demonstrated that the nanostructure affected the macroscopic properties of InAs. The findings confirm the feasibility of synthesizing nanostructured bulk materials via RPPT.

As hard and refractory materials with high chemical resistance and mechanical strength, lanthanide hexaborides (LnB6) have attracted much attention. Among the family of LnB6, gadolinium hexaboride (GdB6) occupies a special position due to the half-filled 4f shell of Gd. Here, using in situ synchrotron radiation angle-dispersive X-ray diffraction in a diamond anvil cell at room temperature, the structural stability and compression behavior of GdB6 are investigated extensively, GdB6 is observed to be structurally stable up to 73 GPa, and the bulk modulus of 177 GPa is obtained under hydrostatic compression. In this paper, an interesting observation of pressure-induced spotty diffraction rings of GdB6 is reported, its formation mechanism can be well described by the reversible regional texture effect, which highlights the rearrangement of crystal grains under high pressure. The rearrangement mechanism has been well explained by investigating the pressure dependence of full width at half maximum, macro-differential stress and grain size. Collective grain rotation behavior motivated by stress difference is critical for the rearrangement process, the strong isotropy and strong stability of GdB6 structure also provide necessary conditions for high-pressure grain rotation behavior. These results will help to promote the understanding of high-pressure structural properties of GdB6, and provide novel insights on the high-pressure grain behavior in hard materials with strong isotropy and three-dimensional skeleton constituted of strong covalent bonds.