Revealing the phase-transformation path in a FeCoNiSnx eutectic high entropy alloy system by crystallographic orientation relationships

High entropy alloys are the focus of current research.An accurate description of their phase-transformation path,however,is a challenge when their phase constituent and transformation process are complex.In this study,a FeCoNiSn x eutectic high entropy alloy(EHEA)system was investigated and a novel FeCoNiSn EHEA composed of BCC+HCP phases was reported.The transition from the hypoeutectic to the fully eutectic and then to the hypereutectic microstructure with the Sn addition was characterized by the electron backscatter diffraction(EBSD)technology,and the phase-transformation path was clari-fied by crystallographic orientation relationships.The studies reveal that the primary phase of FeCoNiSn x(x=0.2,0.4)is FCC structure,and a further Sn addition induces an obvious phase-transformation from FCC to BCC in both the primary phase and eutectic lamellar,which satisfies the Kurdjumov-Sachs(K-S)or Nishiyama-Wasserman(N-W)variant orientation relationship.The mechanical results confirm that the phase structure and microstructure transition caused by Sn addition do significantly improve the strength and hardness of FeCoNiSn x EHEAs,but have serious adverse effects on plasticity.This study would be of significance to understanding the phase-transformation process in HEAs and preparing the HEAs with aimed mechanical properties.

Synthesis process and growth mechanism of γ′-Fe4N nanoparticles by phase-transformation

Multiphase Fe/N nanoparticles were synthesized by means of chemical vapor reaction, the influence of the preparing parameters on the properties of particles was studied carefully dur-ing the first nitriding process. The optimum process was determined. Single phaseγ’-Fe4N was prepared by twice-nitriding. Multiphase iron-nitride really transformsγ’-Fe4N nanoparticle of sin-gle-phase and uniform. Moreover, the mechanism of nanoparticle nucleation and growth, including phase-transformation, was revealed. In addition, the mircograph, particle size, physical phases, schemical constituents and magnetic properties before and after phase-transformation were char-acterized initially.

Electro-chemo-mechanical analysis of the effect of bending deformation on the interface of flexible solid-state battery

Flexible solid-state battery has several unique characteristics including high flexibility,easy portability,and high safety,which may have broad application prospects in new technology products such as rollup displays,power implantable medical devices,and wearable equipments.The interfacial mechanical and electrochemical problems caused by bending deformation,resulting in the battery damage and failure,are particularly interesting.Herein,a fully coupled electro-chemo-mechanical model is developed based on the actual solid-state battery structure.Concentration-dependent material parameters,stress-dependent diffusion,and potential shift are considered.According to four bending forms(k=8/mm,0/mm,-8/mm,and free),the results show that the negative curvature bending is beneficial to reducing the plastic strain during charging/discharging,while the positive curvature is detrimental.However,with respect to the electrochemical performance,the negative curvature bending creates a negative potential shift,which causes the battery to reach the cut-off voltage earlier and results in capacity loss.These results enlighten us that suitable electrode materials and charging strategy can be tailored to reduce plastic deformation and improve battery capacity for different forms of battery bending.

Microstructural Evolution of Localized Shear Bands Induced during Explosion in Ti-6Al-4V Alloy

Microstructures of the localized shear bands generated during explosion with a thick-walled cylinder specimen in Ti-6AI-4V alloy, were characterized by TEM and SEM. The results show that the twinning is a major mode of deformation, and the distortion-free grains in the bands with the size of 10μm in diameter were proposed to be the re-crystallization during dynamic explosion. The further observations show that the α→α2 phase transformation may occur in the bands, and this kind of transformation could be confirmed by its dark field image and electron diffraction analysis. Analysis shows that there is specified orientation between the α and α2 Phases.

Stress-dilatancy of Zipingpu gravel in triaxial compression tests

Experimental investigations of the dilatancy and particle breakage of gravelly material from the Zipingpu concrete-faced rock- fill dam, which was subjected to high-intensity seismic load during the 2008 Wenchuan earthquake, were conducted through a series of large-scale drained triaxial compression tests. A hyperbolic relation between the input plastic work and the degree of particle breakage was found for Zipingpu gravel, independent of the initial void ratio and confining pressures. The stress-dilatancy for Zipingpu gravel was analyzed and compared with data from two rounded alluvial and three angular quar- ried gravelly and rockfill materials in the literature. A nearly linear relationship between the dilatancy Dp and the stress ratio η was found at medium-to-large stress ratios before the peak stress ratio. The slope of the stress-dilatancy line before peak had slight dependence on the void ratio and confining pressure of the gravel. After peak, the stress-dilatancy relation shifts down compared with that before peak for the gravel specimen. The phase-transformation stress ratio decreased with increased con- fining pressure, with the exception of sub-rounded gravel with little particle breakage. A nearly linear relationship was found between the phase-transformation stress ratio Mf and the state parameter ψ for the Zipingpu gravel, regardless of the void ratio and confining pressure of the specimens.

Reaction mechanism and luminescence properties of orange-red SrAlSi4N7：Eu2＋ phosphor for white-LEDs

The well-crystalline SrAlSi_4N_7：Eu^（2＋） phosphor could be synthesized by the solid-state reaction method using different types of Si_3N_4 raw material.The corresponding reaction mechanism was studied by means of X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy-dispersive spectrometry（EDS） and fluorescence spectra.It was found that the structural difference between α-Si_3N_4 and β-Si_3N_4,and the rearrangement of Si_3N_4 microstructures at high temperature had an important influence on the formation of SrAlSi_4N_7 phase.Results showed that the α-β transformation of Si_3N_4 was included in the synthesis reaction of SrAlSi_4N_7.So the reaction mechanism of SrAlSi_4N_7：Eu^（2＋） phosphor should be：α-Si_3N_4→β-Si_3N_4;β-Si_3N_4＋AlN＋Sr_2N→SrAlSi_4N_7.The β-Si_3N_4 seeds addition were actually helpful to the grain growth of α-β transformation of Si_3N_4 and then increased the synthesis reaction rate of SrAlSi_4N_7.Pure SrAlSi_4N_7：Eu^（2＋） phosphor with higher luminous efficacy and better crystal morphology was finally obtained,indicating that the as-synthesized phosphor was an attractive candidate material as an orange-red component for warm white light LEDs.

FORMATION MECHANISM ON PHASE 5 AND PHASE 3

From the viewpoint of crystallography and thermodynamics the formation and phase-transformation reaction of phase 5[Mg,(OH)5Cl·4H2O] and phase 3[Mg2(OH)3Cl·4H2O] as a major resultant for the hardening reaction of magnesium oxychloride cement have been systematically investigated. It is recognized that the formation of phase 5 and phase 3 is mainly controlled by the molar ratio of MgO, MgCl2, and H2O in the initial reaction mixture. Phase 5 and phase 3 are formed by the reaction between MgO or Mg2+ dissolved out from MgO and the aqueous solution of MgCl2. In the system of MgO-MgCl2-H2O both phase 5 and phase 3 are metastable phases, and under given conditions phase 5 may transform into phase 3 and the latter also into Mg(OH)2.