Yunbo Wang

Solid solution alloys of AlCoCrFeNiTix with excellent room-temperature mechanical properties

Alloys with composition of AlCoCrFeNiTix (x: molar ratio; x=0,0.5,1,1.5) were designed by using the strategy of equiatomic ratio and high entropy of mixing. The alloy system is composed mainly of body centered cubic solid solution and possesses excellent room-temperature compressive mechanical properties. Particularly for AlCoCrFeNiTi0.5 alloy, the yield stress, fracture strength, and plastic strain are as high as 2.26GPa, 3.14GPa, and 23.3%, respectively, which are superior to most of the high-strength alloys such as bulk metallic glasses.

Deformation behavior of thermal aged duplex stainless steels studied by nanoindentation, EBSD and TEM

The cast duplex stainless steels were investigated in order to understand the plastic deformation behaviors and fracture mechanisms after thermal aging at 400°C for up to 10,000 h. Tensile strength has an obvious improvement, but the yield strength has a slightly increase after long-term thermal aging. Microcracks initiate in the ferrite and extend to phase boundaries, leading to the fracture of ferrite before the tensile failure of specimen. Nanoindentation tests indicate that hardness in ferrite continuously increases with aging time, but not strongly affected by deformation degree. Electron backscattered diffraction observations reveales that high-stress concentration on the phase boundaries causes the phase boundary separating. The dislocations pile-up at spinodal decomposition precipitates in ferrite phases and along G-phase boundaries, leading to the reduction of mechanical property of the ferrite.

POSITRON-ANNIHILATION STUDY ON THE TL-1223 SUPERCONDUCTOR

We have measured the Doppler-broadened radiation spectra at the various temperatures from 77K to room temperature for the sample of T1-system 1223 phase. The temperature dependence of the line-shape parameters (S, W, S/W) show the anomalous changes in the temperature range 10-20K above Tc, which indicates that electron momentum distribution alters anomalously due to the localization of the electrons. the result is discussed.

Atomic position and electronic field gradient of Fe dopant in Y‐124 superconductor

We attempt to provide a model calculation of the electric field gradient. With reference to the observed 57Fe Mossbauer hyperfine interaction parameters, the effective valence and electric field gradient on different Fe lattice sites have been computed for the Fe‐doped Y‐124 superconductor by means of the point‐charge‐effective‐valence model. According to the present calculations, the sign of electric‐field gradient for the pyramid Fe(B’) site and the quasioctahedral Fe(B) is estimated to be positive and negative, respectively, and furthermore Fe is found to be displaced relative to the CuO2 plane in Y‐124 superconductors.

PAT study on stress-field pinning in high- T c superconductors

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra were performed by positron annihilation analysis on Eu-doped Y-123 superconductors. When the doping concentration reaches 30% and 70%, the density of defeat and its stress-field reach the maximum for (Y1-xEux)Ba2CU3O7-y The pinning mechanism of such superconductors is discussed, and the results support the stress pinning mechanism.

The excess oxygen model for Fe-doped superconductors

Abstract A excess oxygen model that explains the Fe-doping mechanism in 1223 Tl Ba Ca Cu O superconducting phase is presented. So far the details of the coordination of Fe dopant is not understood completely yet, for Fe dopant substitution in Tl-system cuprate oxide. The similarity in the ionic radii and electronegativity of Cu and Fe lead to the Fe occupying Cu lattice site. However, the difference between the oxidation states of Fe and Cu means that Fe will bring the excess oxygen into lattice to keep the charge balance. We show that, for the evidence of the excess oxygen., the bonding energy of Fe-cluster in Cu O layer reach its minimum when Fe make a shift from the center of Cu O square network. Moreover, under the scheme of effective valence, the calculated value of Fe effective valence is relatively close to that of Mossbauer measurement.

Anomalous behaviors in high temperature superconductors: Lattice effect and d-electron delocalization effect of the normal state on the Bi, Tl, and Y-system superconductors

Abstract The dynamic studies of the hyperfine interactions and the lattice vibrational behavior are performed by means of temperature-dependent Mossbauer spectroscopy on the Bi-, Tl-, and Y-based superconductors, respectively. The temperature-dependent Mossbauer recoil-free factor f show that, above, but relatively close to, the transition temperature, there is an anomalous lattice effect which is related to the softening of phonon vibrational modes for Bi-2223, Tl-1223, and Y-123 superconductors. Moreover, for such three systems, the anomalous decrease of the Mossbauer isomer shift above Tc reveal the delocalization effect of d-electrons. Since the lattice effect is accompanied by d-electron delocalization effect at the almost same temperature region, it is believed that the structural instability involved in phonon softening is strongly interrelated to the change of the electronic states, and the phase separation behavior.

Positron annihilation study on stress-field pinning in (Eu,Y)-123 superconductors

Abstract In order to investigate the reason of Jc increment in (RE,Y)-123 superconductors, we prepare Eu-doped (Eu x Y 1−x )-123 ceramic samples with different Eu concentration. Positron lifetime and Doppler-broadening spectrum are simultaneously used to investigated the defects (stress field) verse different Eu-substitution. The experimental results support the stress-field pinning mechanism.