查看更多>>摘要：In additive manufacturing especially in directed energy deposition, it is hard to control crystal growth pattern due to the high temperature gradient in AM. Essentially, the crystal grows epitaxially from the substrate and columnar crystals with strong texture are exhibited in AM components. Here, small spot diameter was used for DED-L of Ni-based alloys and various crystal growth patterns were obtained by changing power density. Samples processed under low power density were shown to exhibit wide and shallow spindle-like melt pool, along with apparently hierarchical planar-columnar-equiaxed micro-structure. While samples prepared under relatively high power density were shown to exhibit narrower and deeper melt pool with two sharp turning points, exhibiting inclined columnar grains and several discontinuous central axial columnar crystals. When highest power density and small spot diameter were applied, crystal growth with weak texture was achieved. Under this deep and narrow melt pool, the crystal growth can be separated into four regions: nearly-equiaxed grains with random grain orientations; horizontally symmetrically grown crystal grains; axial columnar in the center; columnar grains grew approximately vertical to the boundary of melt pool. Correlations were investigated between melt pool shape, solidification parameters and microstructure. The transformation of melt pool morphology was mainly attributed to the change of power density. Solidification parameters were shown to be different under different melt pool morphology. Diverse crystal growth patterns were achieved under different melt pool morphology controlled by power density, showing the feasibility of site-specific of microstructure control in DED-L according to the required mechanical properties.
查看更多>>摘要：The evolution of the crystallographic orientation and the microstructures in triangular adapters of grain continuators (GCs) with different flare angles and thicknesses prepared by a 3rd-generation single crystal (SX) superalloy were investigated under different withdrawal rates. The microstructures were observed by an optical microscope and the crystallographic orientation of the GCs was measured by electron backscatter diffraction (EBSD). The directional solidification process was simulated by a commercial simulation software ProCAST The experimental results showed that the [001] misorientation between both sides of the GCs at top cross section increased with the increasing of the flare angle, the thickness of specimens, and the withdrawal rate. These phenomena were attributed to the changing of the isotherm morphology in the mushy zones and the velocity of the solidification front. As for a low flare angle, the freckle defect formed at one side of specimens under the withdrawal rate of 1 mm/min. With the increasing of flare angle, the freckle defect cannot be found at the same position of the specimens. The formation mechanisms of freckle defect in the triangular adapters of GCs were further discussed.
查看更多>>摘要：Recent studies report that eutectic high entropy alloys (EHEA) which possess both high strength and high ductility have potential industrial applications. In the present work, the solidification behaviors and mechanical properties of directionally solidified AlCoCrFeNi_(2.1) EHEA obtained at different growth velocities are investigated. The microstructure of the as-cast AlCoCrFeNi_(2.1) EHEA is composed of bulky dendrites (NiAl phase) and lamellar eutectic structures which consists of the CoCrFeNi (FCC) phase and the NiAl (BCC) phase. Although the actual composition of the alloy is shown to slightly deviate from the eutectic point, it is interesting to observe that the full lamellar structure of this alloy is obtained through directional solidification. In order to explain this contradiction, the maximum interface temperature criterion and the interface response function (IRF) theory are applied to calculate the velocity range of the transition from the primary phase to the eutectic, which is 1.2 um/s-2 ×10~4 um/s. Furthermore, the tensile test indicates that the directionally solidified AlCoCrFeNi_(2.1) EHEA possesses a good combination of strength (1340 Mpa) and ductility (30.5%) at 100 um/s, which can be attributed to the full lamellar eutectic structure after directional solidification. In addition, the compression tests demonstrated that the compressive strength of the axial specimen is greater than the radial specimen at the same growth velocity. Thus the directional solidification can effectively adjust the alignment of the eutectic structure and achieve better mechanical properties along the growth direction.
查看更多>>摘要：In this work, a series of (l-x)MgAl_2O_4-xMg_2TiO_4 (expressed as (l-x)MA- xMT) (0.0 < x < 0.9) ceramics were successfully prepared via a solid-state reaction, and the effect of Mg_2TiO_4 content on the phase compositions, microstructures and microwave dielectric properties of MgAl_2O_4 ceramics have been investigated. The MgAl_2O_4 could form a solid solution with Mg_2TiO_4, and the grain size of the (l-x)MA-xMT ceramics gradually increased with the increase of sintering temperature. The dielectric constants (ε_r) of (l-x)MA-xMT ceramics steadily increased with the increase of Mg_2TiO_4 content. The ε_r values were mainly affected by secondary phase and dielectric polarizability. The quality factor (Qf) of (l-x)MA-xMT ceramics was closely related to the relative density, secondary phase, cation vacancies and Ti valence; whereas the temperature coefficient of resonance frequency (τ_f) highly depended on the Al-site bond strength. For the sample at x = 0.9, the 0.1MA-0.9MT ceramics sintered at 1350 °C for 4 h exhibited the optimized microwave dielectric properties: ε_r = 12.36, Qf = 236,600 GHz and τ_f = -61 ppm/°C.
查看更多>>摘要：The authors regret that the layer thickness and hatch spacing in Table 2 should be 0.05 and 0.12 mm respectively in the original paper. Meanwhile, an incorrect version of Fig. 7 was used. The correct version is exhibited below.
查看更多>>摘要：In the present study, correlative corrosion testing and electron microscopy, including quasi-in-situ electron microscopy, are employed to investigate the corrosion behaviour of 2A97-T8 Al-Cu-Li alloy extrusion in 3.5 wt% NaCl solution. It is found that the corrosion behaviour of the alloy is influenced by synergetic effect of the plastic deformation introduced by the thermomechanical processing during alloy fabrication and the precipitation of T_1 phase during T8 treatment. Localized corrosion initiates and propagates preferentially within non-recrystallized regions of the partially recrystallized alloy. Corrosion initiated at coarse constituent intermetallic particles is confined to its periphery, which doesn't develop further when the particles are disconnected from alloy matrix.
查看更多>>摘要：Carbonate materials have been increasingly favoured in terms of the development of flame retardants because of the eternal subject of inorganic carbon sequestration. In this regard, anhydrous zinc carbonate (ZnCO_3) has been deemed as suitable candidate thanks to its excellent flame retardancy compared to traditional hydroxid flame retardant. However, the single crystals growth, accurate characterizations of structural and thermal properties are still not entirely clear from previous studies. With this in mind, ZnCO_3 single crystals were synthesized under high-pressure-temperature conditions (high P-T; 3 GPa and 973 K). The crystal structure of impurity-free ZnCO_3 was determined by means of single crystal X-ray diffraction (XRD). The symmetry was identified as R3c, while the unit cell parameters were a= 4.6463(3) A and c= 15.0015(11) A with a final R value of 0.0229. The quantitative analyses of Raman spectrum and infrared absorption indicate that the as-synthesized ZnCO_3 is anhydrous phase. Using Thermogravimetric (TG) / Differential Scanning Calorimeter (DSC) measurements, ZnCO_3 was decomposed in the temperature range of 593-773 K, whereas the heat capacity and the endothermic peak were determined. According to the single crystal XRD from 150 K to 383 K, the thermal expansion coefficients were quantified as α_a= 7.90 × 10~(-6) K~(-1) and α_c=22.8×10~6 K~(-1) as well as aVunit ceii=38.8× 10~(-6) K~(-1). These findings provide a precise characterisation and obtain important thermal parameters for the evaluation of its flame retardant properties.
查看更多>>摘要：The development of novel lead-free materials operating at high temperature and able to replace lead-based compounds and their derivatives in different devices is currently a remarkable challenge. In spite of having a high Curie temperature, the ferroelectric Bi_4Ti_3O_(12) is rarely used as a piezoelectric due to its high conductivity. In this work, Bi ions in Bi_4Ti_3O_(12) were partially replaced with Yb and Er aiming to compensate oxygen vacancies and improve others physical properties. In particular, Bi_(3.8-x)Er_(0.2)Yb_xTi_3O_(12) compounds with x = 0.05, 0.1, and 0.2 were prepared by a low-cost solid-solid state reaction process, and their structural, optical and electrical properties were investigated. It was observed that, at low temperatures, the conductivity, impedance and permittivity are mainly governed by grain effects, and by grain boundaries and grains effects at high temperatures. The electrical conductivity at 700 K decreases with Yb content and shows a minimum for x = 0.2. The relative permittivity at high temperatures is colossal while the tangent loss is negligible. High permittivities accompanied with low loss factor were obtained for Bi_(3.6) Er_(0.2)Yb_(0.2)Ti_3O_(12). These results open a wide application domain for this particular compound.
查看更多>>摘要：The analysis of the phase nucleation requires not only the in-situ images but also the accurate nucleation information. Thus, the nucleation of both the primary Ni_3Sn_2 and peritectic Ni_3Sn_4 phases were studied during peritectic solidification of Sn-Ni alloy using the high temperature confocal laser scanning microscope (HT-CLSM) and differential scanning calorimetry (DSC). The results by both methods confirm that the nucleation undercooling AT of the solid phases decrease with increasing cooling rates, which is demonstrated to be caused by higher concentration difference at larger cooling rates. Besides, the freezing range of the alloy is also found to decrease with the increase in cooling rate. However, the nucleation undercooling of the peritectic Ni_3Sn_4 phase determined by HT-CLSM is larger than that by DSC at each cooling rate. This is attributed to the slower capture of peritectic reaction when using HT-CLSM. Furthermore, the nucleation of peritectic Ni_3Sn_4 phase is confirmed to be controlled by the heterogeneous nucleation on primary Ni_3Sn_2 phase by comparison between the HT-CLSM observation and DSC result. In addition, the wetting angles for heterogeneous nucleation of both phases which are important parameters in analyzing the nucleation of solid phases were determined through the entropy of solidification data from the DSC results. The smaller wetting angle of peritectic Ni_3Sn_4 phase as compared with that of the primary Ni_3Sn_2 phase further proving the smaller nucleation undercooling of Ni_3Sn_4 phase during solidification.
查看更多>>摘要：Positron beam-based Doppler broadening results on He ion-implanted FeCrCoNi show distinct changes in defect-sensitive S-parameter corresponding to the formation of helium-vacancy complexes, nucleation/ growth of helium bubbles, and finally the helium bubble rupture, with isochronal annealing. Data analyses of the positron beam-based ratio curves provide clear information on the kinetics of bubble growth and rupture. The difference data in S-parameter (AS), which corresponds to the bubble growth, displays a maximum at 1123 K, with annealing temperature. Comparing the AS value to that reported for nickel, it is concluded that FeCrCoNi has higher bubble growth resistance than nickel.
NSTL
Elsevier