Microstructure and mechanical properties of a low activation cast WTaHfTiZr refractory high-entropy alloy

China Foundry - In the face of the requirement that nuclear fusion reactor materials exhibit more excellent thermal, mechanical and physical properties, a novel refractory high-entropy alloy,...     

Microstructure and electrical properties of Ti-Si-C-Ag nanocomposite thin films

Ti-Si-C-Ag nanocomposite coatings consisting of nanocrystalline TiC in an amorphous Si matrix with segregated Ag were deposited by dual magnetron sputtering from Ti₃SiC₂ and Ag targets. As evidenced by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, for Ag contents below 10 at.%, the Ag forms ∼ 10 nm large crystallites that are homogeneously distributed in the films. For higher Ag contents, coalescence during growth results in the formation of > ∼ 100 nm Ag islands on the film surface. The electrical resistivity of the coatings was measured in a four-point-probe setup, and ranged from 340 μΩcm (for Ti-Si-C coatings without Ag) to 40 μΩcm (for high Ag content).     

Hf含量对NbMo0.5HfxTiZrCrAl难熔高熵合金组织及力学性能的影响

采用真空非自耗电弧熔炼的方法制备了5种名义成分为NbMo0.5HfxTiZrCrAl(x=0, 0.25, 0.5, 0.75, 1, at.%)的难熔高熵合金。研究了Hf对合金组织及力学性能的影响规律。结果表明：添加Hf和不添加Hf的合金的微观组织均由两种BCC相（即BCC1和BCC2）和Laves相构成。在未添加Hf的合金中,BCC1相中主要富含Zr、Cr和Al元素,BCC2相中主要富含Nb、Ti和Mo元素。而在添加Hf的合金中,BCC1相中主要富含Hf、Zr、Cr、Mo和Al元素,BCC2相中主要富含Nb和Ti元素。随着Hf含量的增加,BCC1相的含量逐渐增加,且BCC1和BCC2相的晶格常数均有所增大。此外,随着Hf含量的增加,合金的硬度和脆性均逐渐增加,而合金在1200 ℃的抗压强度逐渐降低。     

第一性原理计算Mo含量及压力对CrZrNbTiMox难熔高熵合金性能的影响

利用基于密度泛函理论的第一性原理方法研究Mo含量和压力对CrZrNbTiMo_x难熔高熵合金相结构、弹性常数和弹性模量的影响。计算了CrZrNbTiMo_x难熔高熵合金的混合焓(ΔH_mix)、混合熵(ΔS_mix)、原子半径差(δ)、价电子浓度(VEC)以及参数Ω。结果表明,CrZrNbTiMo_x难熔高熵合金主要以单一的BCC无序固溶体结构为主。随着Mo含量的增加,合金的晶格畸变不断减小,但力学性能稳定且始终以金属键结合。Mo的添加对合金的抗体积变形能力提升较大、对抗切应变能力及刚度影响较小,合金表现为韧性材料。在0～25 GPa压力作用下,CrZrNbTiMo_0.6合金表现出优异的相稳定性和力学稳定性,体弹性模量(B)增幅较小,剪切弹性模量(G)、杨氏模量(E)变化较小;泊松比μ及G/B计算结果表明合金为韧性材料。     

Microstructural studies and electrical properties of Mg-doped SrTiO3 thin films

The structure–property relations of Mg-doped SrTiO₃ (ST) sol–gel thin films deposited on Pt/TiO₂/SiO₂/Si substrates have been investigated in order to determine the effect that Mg dopants have on the dielectric properties of SrTiO₃. It has been predicted that Mg-doped SrTiO₃ should exhibit a dielectric anomaly similar to that observed recently in Bi doped SrTiO₃ but, to date, no polar state has been reported. It has been suggested that this may relate to the low solubility of Mg on the A-site in bulk ceramics (<0.05 at.%). However, for Sr_1−xMg_xTiO₃ (SMT) (x ⩽ 0.30) films annealed at 750 °C, all Mg ions were accommodated in the perovskite lattice and for SMT films annealed at 900 °C, the solubility limit of Mg was x = 0.10, above which a Mg-rich ilmenite second phase was observed. Irrespective of the higher solid solubility limit of Mg in the ST lattice for sol–gel ST films compared to equivalent ceramics, no ferroelectric or relaxor phase transition was observed, refuting previous predictions for this dopant.     

Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys

Although refractory high-entropy alloys have exceptional strength at high temperatures, they are often brittle at room temperature. One exception is the HfNbTaTiZr alloy, which has a plasticity of over 50% at room temperature. However, the strength of HfNbTaTiZr at high temperature is insufficient. In this study, the composition of HfNbTaTiZr is modified with an aim to improve its strength at high temperature, while retaining reasonable toughness at room temperature. Two new alloys with simple BCC structure, HfMoTaTiZr and HfMoNbTaTiZr, were designed and synthesized. The results show that the yield strengths of the new alloys are apparently higher than that of HfNbTaTiZr. Moreover, a fracture strain of 12% is successfully retained in the HfMoNbTaTiZr alloy at room temperature.     

Microstructure and mechanical properties of the TiZrNbMoTa refractory high-entropy alloy produced by mechanical alloying and spark plasma sintering

The equiatomic refractory high entropy alloy (HEA) TiZrNbMoTa was investigated. The alloyed powders with face-centered cubic (FCC) structured solid solution phase were prepared by mechanical alloying (MA) and then sintered by spark plasma sintering (SPS) at 1300, 1400, 1500, and 1600 °C. The microstructure and mechanical properties of the bulk alloy were investigated. The body-centered cubic (BCC) structured solid solution phase and the ZrO₂ phase precipitated from the FCC structured solid solution phase during cool-down from sintering. The highest compression fracture strength (3759 MPa) and fracture strain (12.1%) were obtained in the refractory HEA sintered at 1400 °C. The grain boundary strengthening, precipitation strengthening, solid solution strengthening, transformation-induced plastic (TRIP) effect, and toughening effect of the ZrO₂ phase are the important factors for the high strength and ducitily of the refractory HEA prepared in this study.     

Correlation between electrical properties and point defects in NiO thin films

The wide band-gap semiconductor NiO has p-type characteristics and is an alternative to p-ZnO, due to conduction mechanisms coming from the Ni vacancies and O interstitials. The correlation between the electrical properties and point defects was studied with NiO thin films grown by sputtering at various temperatures and in pure Ar and O₂ atmospheres. The p-type characteristics of the NiO thin films were double-checked by Hall-effect and Seebeck coefficient measurements. Below 300 °C, the electrical resistivity of NiO films grown in an O₂ atmosphere was lower than those grown in an Ar atmosphere due to the suppressed point defects. On the other hand, the electrical resistivity over 300 °C became semi-insulating due to the relatively stoichiometric NiO. The optical transmittance of the NiO film deposited in an O₂ atmosphere and 600 °C was around 80% in the visible region. Finally, the p-NiO/n-ZnO heterojunction diodes showed a well-rectifying current-voltage curve.     

Microstructure characteristics and mechanical properties of NbMoTiVWSix refractory high-entropy alloys

Refractory high-entropy alloys are considered as potential structural materials for elevated temperature applications. To obtain refractory high-entropy alloys with high strength, different amounts of Si were added into the NbMoTiVW refractory high-entropy alloys. The effects of Si on the phase constitution, microstructure characteristics and mechanical properties of NbMoTiVWSi_x alloys were investigated. Results show that when the addition of Si is 0, 0.025 and 0.05 (molar ratio), the alloys are consisted of primary BCC and secondary BCC in the intergranular area. When the addition of Si is increased to 0.075 and 0.1, eutectic structure including silicide phase and secondary BCC phase is formed. The primary BCC phase shows dendritic morphology, and is refined by adding Si. The volume fraction of intergranular area is increased from 12.22% to 18.13% when the addition of Si increases from 0 to 0.1. The ultimate compressive strength of the NbMoTiVW alloy is improved from 2,242 MPa to 2,532 MPa. Its yield strength is also improved by the addition of Si, and the yield strength of NbMoTiVWSi_0.1 reaches maximum of 2,298 MPa. However, the fracture strain of the alloy is decreased from 15.31% to 12.02%. The fracture mechanism of the alloys is changed from mixed fracture of ductile and quasi-cleavage to cleavage fracture with increasing of Si. The strengthening of alloys is attributed to the refinement of primary BCC phase, volume fraction increment of secondary BCC phase, and formation of eutectic structure by addition of Si.

     

Post-deposition-annealed lanthanum-doped cerium oxide thin films:structural and electrical properties

The metal-organic-decomposed lanthanum cer-ium oxide(LaCeO2)solution was spin-coated on p-type Si substrate to form thin films.The method of microwave-as-sisted...     

Microstructure and mechanical properties of in-situ MC-carbide particulates-reinforced refractory high-entropy Mo0.5NbHf0.5ZrTi matrix alloy composite

Refractory high-entropy Mo_0.5NbHf_0.5ZrTi alloy matrix composite with MC-carbide particulates-reinforced was prepared by arc melting. It is found that Mo_0.5NbHf_0.5ZrTiC_0.1 and Mo_0.5NbHf_0.5ZrTiC_0.3 alloys consist of one disordered body-centered cubic (BCC) solid solution phase as the matrix phase and MC carbide phase. MC carbide is enriched with Zr and Hf due to the higher binding strength and without any Mo. Noticeable strengthening from the carbide is not observed for C_0.1 and C_0.3 alloys while the compressive plasticity is improved slightly due to the decrease of solution strengthening for the matrix BCC disordered solid solution phase.     

溶胶-凝胶法制备PST铁电薄膜

为了制备非致冷红外焦平面阵列,采用溶胶-凝胶法在Pt/Ti/SiO2/Si基片上制备出了PST铁电薄膜材料,研究了PST薄膜的结构及介电、铁电性能.XRD谱分析显示,PST膜呈现纯钙钛矿结构.SEM电镜照片显示,PST膜厚均匀一致,无裂纹、高致密.在室温且频率为1kHz时,PST膜介电常数为570,介电损耗为0.02,其热释电系数最大值为7.4×10-4C·m-2·K-1.     

Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy

A new refractory alloy, Ta₂₀Nb₂₀Hf₂₀Zr₂₀Ti₂₀, produced by vacuum arc-melting followed by hot isostatic pressing (HIPing) at T = 1473 K and P = 207 MPa for 3 h has predominantly a single-phase body-centered cubic (BCC) structure with the lattice parameter a = 340.4 pm. The alloy density and Vickers microhardness are ρ = 9.94 g/cm³ and H_v = 3826 MPa. The alloy has high compression yield strength (σ_0.2 = 929 MPa) and ductility (? > 50%). The alloy shows considerable strain hardening and homogeneous deformation. A simple model of solid-solution strengthening is proposed to explain the behavior.     

CrFeCoNiB0.05Tix高熵合金的显微组织和力学性能

采用机械合金化和放电等离子烧结工艺制备了CrFeCoNiB_0.05Ti_x(x=0.2、0.4、0.6、0.8、1.0)高熵合金材料,通过X射线衍射分析、扫描电镜观察和能谱分析以及维氏硬度测试和压缩强度测试等,研究了Ti含量对高熵合金微观组织和力学性能的影响。结果表明,CrFeCoNiB_0.05Ti_x(x=0.2、0.4、0.6、0.8、1.0)高熵合金由FCC、BCC和α相组成。当x=1.0时,合金由BCC结构转向HCP结构并析出新相Laves相,其具有最高硬度416.54 HV0.2。当x=0.8时,合金达到最大抗压强度586.3 MPa。     

Microstructures and mechanical properties of a welded CoCrFeMnNi high-entropy alloy

The response of the CoCrFeMnNi high-entropy alloy to weld thermal cycles was investigated to determine its applicability as an engineering structural material. Two processes were used: high-energy-density, low-heat-input electron beam (EB) welding and low-energy-density, high-heat-input gas tungsten arc (GTA) welding. Weldability was determined through comprehensive microstructural and mechanical property characterisation of the welds. The welds did not develop solidification cracking or heat-affected zone cracks. The microstructures in weld fusion zones are similar to that in the as-cast materials, consisting of large columnar grains with dendrite. The dendrite arm spacing and the extent of elemental segregation were less in the welds than in the cast ingot, and also were less pronounced in the EB weld than in the GTA weld. Compositional microsegregation between dendritic cores and interdendritic regions of the welds was insignificant. Both welds exhibited slightly higher yield strengths than the base metal. The EB weld possessed comparable tensile strength and ductility to that of the base metal. In comparison, the GTA weld maintained ～80% of the base metal’s tensile strength and 50% of the ductility.     

Microstructure and electrical properties of thin films of ReSi1.75 produced by co-sputtering

Microstructural evolution and changes in electrical resistivity of ReSi_1.75 thin films produced by co-sputtering has been investigated as a function of annealing temperature. Crystallization of amorphous ReSi_1.75 thin films occurs at 600 °C without forming any metastable phases. The crystallization temperature for ReSi_1.75 is considerably higher than those observed for other transition-metal disilicides. The crystal structure as well as the domain (twinned) structure observed for crystallites in ReSi_1.75 thin films annealed above 600 °C are essentially the same as those observed in bulk crystals of ReSi_1.75. Although the grain size of crystallites increases with the increase in annealing temperature, thin films annealed below 650 °C exhibit a nano-crystalline structure. Thin films of amorphous and crystalline ReSi_1.75 and bulk polycrystalline ReSi_1.75 all exhibit electrical resistivity values decreasing with the increase in temperature, indicating the semiconducting nature. Values of electrical resistivity for crystallized thin films are systematically higher than those for amorphous ReSi_1.75 thin films and bulk polycrystalline ReSi_1.75 and increase with the decrease in annealing temperature, exhibiting a peak at just above the crystallization temperature. The high values of electrical resistivity around the crystallization temperature are discussed in terms of the formation of the nano-crystalline structure in thin films, for which the effective medium approximation is not valid.     

铸态MnFeCoCuNix高熵合金的力学性能与断裂机理

采用真空感应熔炼法制备不同Ni含量的MnFeCoCuNix高熵合金.利用X射线衍射仪、扫描电镜以及能谱仪分析材料的相组成和结构,利用万能拉伸试验机测定试样的拉伸性能.结果表明,该高熵合金体系具有双相结构.其中,FCC1相富含Fe和Co,而FCC2相富含Cu和Mn.随着Ni含量的增加,Cu的偏析减少,伴随着FCC2相的减...     

Synthesis and thermal stability of a nanocrystalline MoNbTaTiV refractory high-entropy alloy via mechanical alloying

This work focused on the microstructure, morphology and thermal stability of a MoNbTaTiV refractory high-entropy alloy synthesized by mechanical alloying. The mechanically alloyed powder with a uniform chemical distribution and an average grain size of less than 15 nm was obtained after 40 h milling process. With a refinement of the grain size and an increase of the lattice strain, the milled powder transformed from large and inhomogeneous to refined and homogeneous. The mechanically alloyed powder exhibited excellent thermal stability at 1200 °C.     

Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy

Microstructure evolution in high-entropy alloy CoCrFeNiMn during plane-strain multipass rolling to a thickness strain of 80% at 293 and 77 K was studied. Deformation at both temperatures was found to be accompanied by twinning. At 77 K, twinning was more extensive in terms of the fraction of twinned grains and the length of the twinning stage thereby providing faster kinetics of the microstructure evolution. Micro-shear bands formed in the microstructure of the alloy at the late stages of rolling (at ε ≈ 80% at 293 K and ε ≈ 40% at 77 K). The ultimate tensile strength of specimens rolled at 77 K or 293 K was found to be 1500 or 1200 MPa, respectively while the strength in the initial homogenized condition was 440 MPa. The contribution of various mechanisms to the hardening of the alloy following rolling at 77 K and 293 K was analyzed quantitatively.