Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B...Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B and C in vacuum.By adjusting the B4 C/MOxratios,diboride mixtures without detectable MOxwere obtained at 1600℃,while high-entropy diboride(HEB)powders with particle size of<1μm was obtained at 1800℃.The phase,morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated.Although X-ray diffraction pattern indicated the powders synthesized at 1800℃ were in a single-phase Al B2 structure,elemental mappings showed that(Ta,Ti)-rich and(Zr,Nb)-rich solid solution coexisted in the HEB powders.The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C.(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2 ceramics with a relative density of 97.9%were obtained after spark plasma sintering the HEB powders at 2050℃ under 50 MPa.Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050℃,and the averaged grain size increased from 6.67 to 41.2μm.HEB ceramics sintered at 2000℃ had a Vickers hardness of 22.44±0.56 GPa(under a load of 1 kg),a Young’s modulus of^500 GPa and a fracture toughness of 2.83±0.15 MPa m1/2.This is the first report for obtaining high density HEB ceramics without residual oxide phase,benefiting from the high quality HEB powders obtained.展开更多
In this communication, the crystal structure of Cr_4AlB_4, a new MAB phase compound(where M is a transition metal, A is Al or Si, B is boron) discovered in Cr-Al-B system is reported. This new MAB phase was synthesize...In this communication, the crystal structure of Cr_4AlB_4, a new MAB phase compound(where M is a transition metal, A is Al or Si, B is boron) discovered in Cr-Al-B system is reported. This new MAB phase was synthesized from a mixture of CrB and Al powders at 1000?C and its crystal structure was determined by a combination of X-ray diffraction, first-principles calculations and energy dispersive X-ray spectroscopy(EDS). Cr_4AlB_4 crystallizes in an orthorhombic structure with Immm space group. The lattice constants are a = 2.9343(6) ?, b = 18.8911(0) ?, c = 2.9733(7) ?, and the atomic positions are Cr1 at 4 g(0, 0.2936(5),0), Cr2 at 4 h(0.5, 0.5859(7), 0), Al at 2 b(0, 0.5, 0.5), B1 at 4 h(0, 0.3839(8), 0.5) and B2 at 4 g(0.5, 0.6646(2),0.5).展开更多
基金financially supported by the National Natural Science Foundation of China (51521001 and 51832003)the Fundamental Research Funds for the Central Universities
文摘Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B and C in vacuum.By adjusting the B4 C/MOxratios,diboride mixtures without detectable MOxwere obtained at 1600℃,while high-entropy diboride(HEB)powders with particle size of<1μm was obtained at 1800℃.The phase,morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated.Although X-ray diffraction pattern indicated the powders synthesized at 1800℃ were in a single-phase Al B2 structure,elemental mappings showed that(Ta,Ti)-rich and(Zr,Nb)-rich solid solution coexisted in the HEB powders.The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C.(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2 ceramics with a relative density of 97.9%were obtained after spark plasma sintering the HEB powders at 2050℃ under 50 MPa.Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050℃,and the averaged grain size increased from 6.67 to 41.2μm.HEB ceramics sintered at 2000℃ had a Vickers hardness of 22.44±0.56 GPa(under a load of 1 kg),a Young’s modulus of^500 GPa and a fracture toughness of 2.83±0.15 MPa m1/2.This is the first report for obtaining high density HEB ceramics without residual oxide phase,benefiting from the high quality HEB powders obtained.
基金financial supports from the National Natural Science Foundation of China under grant No.51672064 and No.U1435206
文摘In this communication, the crystal structure of Cr_4AlB_4, a new MAB phase compound(where M is a transition metal, A is Al or Si, B is boron) discovered in Cr-Al-B system is reported. This new MAB phase was synthesized from a mixture of CrB and Al powders at 1000?C and its crystal structure was determined by a combination of X-ray diffraction, first-principles calculations and energy dispersive X-ray spectroscopy(EDS). Cr_4AlB_4 crystallizes in an orthorhombic structure with Immm space group. The lattice constants are a = 2.9343(6) ?, b = 18.8911(0) ?, c = 2.9733(7) ?, and the atomic positions are Cr1 at 4 g(0, 0.2936(5),0), Cr2 at 4 h(0.5, 0.5859(7), 0), Al at 2 b(0, 0.5, 0.5), B1 at 4 h(0, 0.3839(8), 0.5) and B2 at 4 g(0.5, 0.6646(2),0.5).
