High-temperature elemental segregation induced structure degradation in high-entropy fluorite oxide

Fluorite-structured oxides constitute an important category of oxides with a wide range of high-temperature applications.Following the concept of high entropy,high-entropy fluorite oxides(HEFOs)have showcased intriguing high-temperature application potential.However,unlocking this potential necessitates an assessment of their long-term stability under high-temperature conditions.In this study,we conducted a prolonged heat treatment at 1000℃on typical HEFO,specifically(CeHfZrGdLa)O_(x).After 100 h,high-intensity X-ray diffraction(XRD)revealed a transition from a single-phase fluorite to a multi-phase configuration.Further investigation by analytical electron microscoy(AEM)demonstrated that this degradation resulted from facilitated element diffusion and consequent escalating chemical fluctuation at high temperatures,leading to spontaneous segregation and separation of Ce and La elements,forming Ce-rich,La-poor,and La-rich phases.Notably,the La-rich phase spontaneously transformed from a fluorite structure(space group Fm3m)to a bixbyite structure(space group Ia3)at elevated temperatures,resulting in the appearance of superstructure reflection in XRD profiles and electron diffraction patterns.Despite the intricate phase decomposition,the energy band gap showed minimal variation,suggesting potential property stability of(CeHfZrGdLa)O_(x)across a broad range of compositions.These findings offer valuable insights into the future applications of HEFOs.

Microstructure evolution and mechanical property of Cu-15Ni-8Sn-0.2Nb alloy during aging treatment

Aging treatment of Cu-based alloys is essential to enhance their strength that is desirable for their extensive engineering applications in electrical industry,whereas the underlying mechanism of strengthening is essential for massive manufacturing of these alloys.Here,the microstructure evolution of a supersaturated solid solution Cu-15Ni-8Sn-0.2Nb alloy aged at 400℃for different time was characterized at atomic scale using state-of-the-art transmission electron microscopy(TEM)and the corresponding mechanical property was also measured.The results reveal that the modulated structure,DO_(22)/L1_(2)ordering,and discontinuous precipitation(DP)appeared in the advances of aging time.At the early stage of aging treatment,component modulation waves and satellite spots appeared from spinodal decomposition and the modulation wavelength was identified in the range of 1-7 nm.Subsequently the modulated structures formed-poor-rich solute regions,of which DO22ordering was present in the Ni-poor region while L1_(2)ordering appeared in the Ni-rich region.The sequence of ordering precipitates was further verified by density functional theory(DFT)simulations.Furthermore,orientation relationships and interfacial structures between DO_(22),L1_(2)phases and the parent matrix were determined.The measured hardness of alloy reached a maximum value of 335 HV after aging for 120 min due to the coherence between the two ordering phases and matrix.These results illustrated the importance of aging on structural evolution and mechanical property of Cu-15Ni-8Sn alloy at various heat treatment stages,which could potentially help in manufacturing promising alloys for their extensive engineering applications.

Enhanced densification and mechanical properties of β-boron by in-situ formed boron-rich oxide

We report nearly full densification of polycrystalline rhombohedral beta(β)-boron without the addition of sintering aids via spark plasma sintering(SPS).The analytical aberration corrected transmission electron microscope observations have revealed in-situ growth of nanocrystalline boron-rich oxide precipitates that contain approximately 4 at.%of oxygen and beget the densification of β-boron.Further electron energy loss spectroscopy and diffraction analysis confirmed that the newly formed boron-rich oxide(nominally B_(96)O_(4))structure with B-O σ-bonding belongs to space group R■m.Depth sensitive nanoindentation showed boron-rich oxide phase has a hardness of about 41±2 GPa,which is 10%higher than that of β-boron matrix.The estimated hardness and fracture toughness of β-boron were approximately 31 GPa and 2.2 MPa m^(1/2),respectively,using Vickers microindentation,which falls in the range of those commercially used boron carbides.These results suggest that the enhanced densification and mechanical properties arise from the newly formed boron-rich oxide inβ-boron during SPS experiments.