Long-and short-range orders in 10-component compositionally complex ceramics

Neutron diffraction and total scattering are combined to investigate a series of single-phase 10-component compositionally complexfluorite-based oxides,[(Pr_(0.375)Nd_(0.375)Yb_(0.25))2(Ti_(0.5)Hf_(0.25)Zr_(0.25))_(2)O_(7)]_(1-x)[(DyHoErNb)O_(7)]_(x),denoted as 10CCFBOxNb.A long-range order-disorder transition(ODT)occurs at x=0.81±0.01 from the ordered pyrochlore to disordered defectfluorite.In contrast to ternary oxides,this ODT occurs abruptly without an observable two-phase region;moreover,the phase stability in 10CCFBOs deviates from the well-established criteria for simpler oxides.Rietveld refinements of neutron diffraction patterns suggest that this ODT occurs via the migration of oxygen anions from the position 48f to 8a,with a smallfinal jump at the ODT;however,the 8a oxygen occupancy changes gradually(without an observable discontinuous jump).We further discover diffuse scattering in Nb-rich compositions,which suggests the presence of short-range order.Using small-box modelling,four compositions near ODT(x=0.75,0.8,0.85,and 1)can be betterfitted by C2221 weberite ordering for the local polyhedral structure at nanoscale.Interestingly,10CCFBO_(0.75)Nb and 10CCFBO_(0.8)Nb possess both long-range pyrochlore order and short-range weberite-type order,which can be understood from severe local distortion of the pyrochlore polyhedral structure.Thus,weberite-type short-range order emerges before the ODT,coexisting and interacting with long-range pyrochlore order.After the ODT,the long-range pyrochlore order vanishes but the short-range weberite-type order persists in the long-range disordered defectfluorite structure.Notably,a drop in the thermal conductivity coincides with emergence of the short-range order,instead of the long-range ODT.

Fatigue behaviors of Z2CND18.12N stainless steel under thermal-mechanical cycling

Tests under mechanical strain control were performed to investigate the TMF behavior of Z2CND18.12N within the temperature range between 150–550 ？ C. Different strain amplitudes and phase-angles were applied. Total strain controlled low cycle fatigue test was also performed at the peak temperature of TMF cycling. The results show that the cyclic stress response of the material displayed an initial hardening regime followed by a saturation period and then cyclic softening till failure. The TMF cycling leads to the development of significant amounts of mean stress. Some life prediction models were employed to predict the TMF life of Z2CND18.12N, and the results indicate that the energy-based models provide good prediction on the thermal-mechanical fatigue behaviors of this material. An optical microscopic observation shows that the surface crack initiations and crack propagations are typically transgranular mode.