查看更多>>摘要：A FeCoNiCu multi-principal element alloy (MPEA) was undercooled and its microstructures, mechanical and tribological performance were investigated for different solidification conditions. All the microstructures were consisted of two phases, i.e., the primary Cu-depleted phase and the secondary Cu-rich phase. With the increase of undercooling, a transition of the primary phase from coarse dendrites to fine equiaxed grains happened and the volume fraction of the secondary phase decreased considerably. The primary phase was supersaturated by Cu at high undercooling and segregation of Cu was inhibited considerably by rapid solidification. Consequently, the hardness and yield strength were improved remarkably due to grain refinement strengthening and solution strengthening, and the wear volume was decreased significantly due to both improved mechanical properties and oxidation induced by friction. For the largest undercooling obtained in this study, (i.e., ΔT = 226 K), the yield strength was increased by about 50% and the wear volume was decreased by about 45% as compared with the as-cast case. The current work shows rapid solidification could be an effective way to modulate both mechanical and tribological properties of MPEAs and hence is helpful for their potential applications as engineering materials.
查看更多>>摘要：This work focused on the texture evolution related to the interactions between α phases and β phases for a Ti80 Pilger cold rolling pipe during dual-phase region annealing of 1223 K, 1243 K and 1263 K. The key observation was that the annealing temperature significantly affected the components and intensities of the texture. At 1223 K, the main factor affecting the texture evolution was the globularization of the primary α phases (αP), and the globularization of αP was greatly influenced by the crystallographic orientation in reference to the loading direction during cold rolling. Those globularized αP maintaining the Burgers orientation relationship (BOR) with the residual β phases were more prone to growing up. At 1243 K, more αP were dissolved and residual β phases grew up accompanying by the precipitation of more secondary α phases (αS). The texture was closely related to the preferential growth of the β phase due to the same crystallographic orientation of αS with adjacent αP grains. Those β phases suffering little deformation during the cold rolling were more likely to grow up since the initial volume advantage and the maintaining of BOR with αP. The β phases undergoing larger deformation would be pinned by αP during the growing up, which was attributed to the recrystallization of β phases and the broken of BOR with αP. At the annealing temperature of 1263 K, the degree of variants selection (DVS) of αS was significantly reduced due to the dissolution of abundant αP during β→αS transformation, thus the texture intensities were reduced.
查看更多>>摘要：In this work, to develop new quantum cutting (QC) luminescent materials and clarify the QC process, a series of Tm3+-activated Ca9Gd(PO4)7 (CGP) phosphors were prepared by conventional high-temperature solid state reaction method. The phase purity of the samples was checked by XRD patterns, and the structural features were analyzed on the basis of Rietveld refinement. The emission spectra of Tm3+ were measured by exciting at different wavelengths, in which the QC was identified. The main QC model was proposed when the Tm3+ is excited into 1D2 level. The occurrence of QC was further verified by comparing the decay lifetimes on 3H4 level under 357, 470 and 685 nm excitation. With increasing Tm3+ concentration, it was found that the 3H4→3H6 transition intensity around 806 nm exhibits faster increase rate than other ones by exciting at either 357 or 470 nm. This increase rate is more effective under 357 nm excitation thanks to the existence of QC process. The above investigation could show instructional significance for the development of other rare earth ions doped luminescent materials.
查看更多>>摘要：Novel Mg alloy anodes with high efficiency have great potential in seawater batteries for marine explorations. This work investigated the effect of indium addition and equal-channel angular pressing on discharge performance and electrochemical properties of commercial AZ61 alloy via hydrogen evolution, weight-loss test, electrochemical and discharge measurements in 3.5 wt% NaCl. The results indicated that indium addition promoted the formation of second phases due to the segregation effect. The pressing process effectively refined the grains and contributed to the uniform grain distribution, which was beneficial to the discharge performance and corrosion resistance of the Mg anodes. The pressed AZ61-In exhibited improved discharge activity and anode utilization compared to as-cast AZ61 and AZ61-In alloys.
查看更多>>摘要：This paper illustrated the effects of aging treatment on microstructural evolution and mechanical properties of the SLM printed Al-Zn-Mg-Cu-Si-Zr alloy systematically. Various aging time and aging temperature were adopted in the experiment to obtain the optimal aging parameters, and maximum tensile strength of 508 MPa, yield strength of 451 MPa and elongation to failure of 7.6% were achieved. The XRD spectrum shows the appearance of Mg2Si phases after aging treatment which was precipitated in the matrix. In addition, Al3Zr phases in the 150 °C/4 h sample were observed by EBSD and confirmed by TEM, which was responsible for the high mechanical properties of alloys. As the aging temperature increased from 150 °C to 230 °C, the average grain size increased from 3.08 to 4.18 μm and most of precipitated phases were transformed from small subsphaeroidal particles into large band-like phases.
查看更多>>摘要：FexCu100?x alloys where x (wt%) = 25, 35, 50, 65, and 75 were prepared via mechanical alloying. Microstructural characterization revealed a single-phase face-centered cubic structure for a wide range of compositions. Atomic volumes were estimated from measured lattice parameters suggesting a high volume state consistent with Weiss model predictions of the ferromagnetic ground state. Saturation magnetic moments at 2 K monotonically decreased from 2.28 μB/Fe for starting Fe powder to 0.65 μB/Fe in Fe25Cu75 alloy. Samples with Fe content higher than 35% exhibited magnetic transition temperatures (Tc) higher than 350 K. On the other hand, the Fe25Cu75 sample exhibit a Tc around 250 K as determined from the gradient method. According to Banerjee's criterion, the magnetic transition is determined to be second order for the Fe25Cu75 alloy. Moreover, modified Arrott plots were used to reveal the critical behavior of the Fe25Cu75. Temperature dependencies of the resistivity were measured for all samples. At low temperatures, the Fe25Cu75 alloy exhibited T3/2 dependence of the resistivity. At higher temperatures (100–300 K) all mechanically alloyed FeCu exhibited unusual T linear dependence resistivity. Such behavior is often called “strange metallicity” or non-Fermi Liquid behavior. The possible origin of such behavior was also discussed.
查看更多>>摘要：This work investigates the fabrication of Ti6Al4V/xAg composites processed by powder metallurgy. An atomized Ti6Al4V powder was blended with 0, 5, 10, 15 and 20 vol% of Ag particles, compacted in a die, and sintered above Ag melting point. The compressibility of the mixtures was analyzed, and the sintering kinetics was evaluated by dilatometry tests. The characterization of sintered samples by microtomography, SEM and diffraction analysis allowed determining the effect of Ag amount on the sintering process and on microstructural changes arising during sintering. The mechanical properties of sintered materials were investigated by microhardness. The compressibility of Ti6Al4V powder was found to be improved by adding soft Ag particles, which resulted in denser packing and larger plastic deformation. The densification during compaction can be predicted by Heckel model and the rule of mixture. It was next found that sintering was driven by volume diffusion in the solid state for Ag content lower than 10 vol% and involved a liquid phase for Ag content higher than 15 vol%. In the latter case, the liquid fills the interconnected pores under the action of capillary forces as the quantity of liquid increased. The microstructure was composed mainly of α-Ti, Ti2Ag, and Ag, which filled the pores between Ti6Al4V particles. Microhardness data showed a maximum value with 20 vol% of Ag. This value is similar to those reported for highly dense Ti6Al4V. It has thus proved that close to full-dense Ti6Al4V/20Ag composites can be processed by powder metallurgy.
查看更多>>摘要：Switching instability significantly affects the development of resistive random access memories. An enhanced resistive switching uniformity was developed by embedding well-ordered Pt/Au nanostructures (NSs) in Pt/NiFe2O4/Pt devices. The well-ordered Pt/Au NSs arrays were developed on Pt/Ti/SiO2/Si substrates using 2D colloidal crystals as the deposition templates. The NiFe2O4 (NFO) thin films were then deposited on the Pt/Au NSs array via a spin coating technique, followed by annealing treatment. The memory devices were fabricated via Pt top electrode sputtering on the NFO film surface. The embedded device showed more uniform SET and RESET voltages and lower forming voltage than the non-embedded device. Distortion of electric field and temperature gradient around the Pt/Au NSs suppresses the random development of conductive filaments (CFs). Furthermore, the temperature gradient near the Au NSs could be relatively high during the forming process than in Pt NSs since Au has high thermal conductivity, promoting oxygen vacancy migration. Therefore, CFs easily nucleate in HRS and develop near Au NSs under the same bias voltage, leading to better resistive switching properties in the Au NSs array device than in the Pt NSs array device. This study provides an effective path for enhancing the resistive switching performances of devices.
查看更多>>摘要：The CoCrFeMnNi high entropy alloys (HEAs) performs large work strengthening with excellent deformability. The work strengthening and deformation mechanism are facilitated by dislocation activities and dislocation accumulation. In this study, the dislocation density and microstructure of the CoCrFeMnNi HEA were characterized by the neutron line profile analysis using convolution multiple whole profile (CMWP) method and EBSD in a comparison with a binary FeNi alloy. The CoCrFeMnNi HEA and FeNi alloy were plastically deformed by rotary swaging until 85% area reduction. The characteristics (e.g., low stacking fault energy, local variation with different atom species) rising from the high compositional complexity of the CoCrFeMnNi HEA cause different dislocation activities and levels of dislocation accumulation from the binary FeNi alloy. The dislocation density of the CoCrFeMnNi HEA continuously increased during the cold swaging and was significantly larger than the FeNi alloy. The larger dislocation accumulation of the CoCrFeMnNi HEA is facilitated by higher compositional complexity, extensive dislocation arrangement, and strong grain fragmentation. The increasing heterogeneity of dislocation distribution in the CoCrFeMnNi HEA was contributed by dislocation cell formation and increasing geometrically necessary dislocations (GNDs). The larger work strengthening in CoCrFeMnNi HEA is correlated with large total dislocation density during cold swaging.
查看更多>>摘要：Chromium tellurides have triggered tremendous investigations in physics and materials science, where their unique properties offer excellent opportunities for spintronic applications. Here, we report anisotropic magnetoresistance (AMR) of ferromagnetic CrTe single crystal flake grown by chemical vapor deposition. An individual hexagonal flake exhibits an easy magnetization axis along its normal direction, and cooling from room temperature to low temperature, its electrical resistance measured with the magnetization parallel to current geometry is larger than that for the perpendicular to the current geometry, indicating the presence of an AMR behavior. The AMR ratio has been found to increase with decreasing temperature and is 5.2% at 5 K, comparable with general magnetic transition metal and alloy. This suggests CrTe single crystal flake to be a promising magnetic material for spintronics applications.
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Elsevier