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封面论文

Zr-1.0Ti-0.35Nb合金热变形行为研究
乏燃料后处理强酸、强氧化性、强放射性的工作环境,对后处理溶解器选材、加工工艺提出了严苛要求。本论文研究了自主设计Zr-1.0Ti-0.35Nb合金在670 ~ 750 ℃温度范围、三种不同应变速率0.01、0.1和1 s-1条件下的热压缩变形行为,分析了热压缩过程中该合金的微观组织特征,并基于峰值应力构建了其热变形本构模型。结果表明,应变速率和变形温度对Zr-1.0Ti-0.35Nb合金热变形过程具有显著影响,流变应力随应变速率增加而增大,随变形温度的增加而减小,达到峰值应力后流变曲线呈现明显动态再结晶特征；提高变形温度有助于发生动态再结晶和晶粒长大；基于Arrhenius本构方程计算得到Zr-1.0Ti-0.35Nb合金的热变形激活能为225.8 kJ/mol,硬化指数为5.62，说明合金元素Ti使锆合金的热变形激活能升高；实验值与预测值之间的相关系数为0.97,平均相对误差为6.15%,证实此本构方程预测Zr-1.0Ti-0.35Nb合金流变应力的准确性,能够为新型锆合金热加工工艺优化提供理论指导。
Zr合金表面电沉积Ni/Cr涂层的高温氧化行为研究
日本福岛核事故后，耐事故燃料（Accident Tolerant Fuel, ATF）包壳技术引起业界广泛关注。在核反应堆堆芯核燃料包壳用锆（Zr）合金表面包覆Cr涂层被认为是短期内最有可能投入商业应用的技术。目前多数Cr涂层的制备方法存在设备昂贵负责、沉积速率偏低、形状适应性偏弱等缺点，而熔盐电沉积技术具有阴极电流效率高、电沉积速度快、基体形状适应性强等优点，有望解决包壳Zr合金表面高质量Cr涂层的高效低成本制备难题。为实现Zr合金表面Cr涂层的熔盐电沉积制备，本文采用水溶液电沉积和熔盐电沉积方法依次在Zr合金基体表面制备Ni过渡层和Cr涂层，对制备得到的Zr/Ni/Cr试样进行组织结构表征、结合力和纳米硬度测试及高温氧化行为研究。结果表明，Zr合金表面的Ni/Cr涂层均匀致密，与基体间的结合力约为151 N。Zr/Ni/Cr从内到外各层的硬度和弹性模量均逐渐升高，呈准梯度过渡。其中Cr涂层的表面粗糙度约为2 μm，硬度和弹性模量分别为2.86 GPa和172.86 GPa。Zr/Ni/Cr试样在1000℃和1200℃高温蒸汽氧化过程中分别表现出近抛物线和近线性规律，表明Ni/Cr涂层能够在1000℃下对Zr合金基体起到良好的保护效果。Zr合金表面Ni/Cr涂层的高温氧化失效机制与Ni过渡层的快速扩散、Cr层的氧化和扩散消耗以及Zr沿Cr晶界快速扩散导致的Cr层性能弱化密切相关。
Thermal Stability and Thermoplastic Formability of Pd20Pt20Cu20Ni20P20 High Entropy Metallic Glass
Thermal stability and thermo-mechanical properties of Pd20Pt20Cu20Ni20P20 high entropy metallic glass (HEMG) were investigated by differential scanning calorimetry, X-ray diffraction, and thermomechanical analysis. Results show that compared with other classical precious metal-based metallic glasses, Pd20Pt20Cu20Ni20P20 HEMG presents comparable performance with distinct characteristics.
Effect of Mo on Microstructure and Properties of AlCoCrFeNiMox High Entropy Alloy Coatings Prepared by Laser Cladding
AlCoCrFeNiMox (x=0, 0.5, 1.0, 1.5, 2.0) high entropy alloy (HEA) coatings were prepared by laser cladding method. The effect of Mo content on the microstructure, hardness, and corrosion resistance of the coatings was studied. Results show that with increasing the Mo content, the microstructure is changed from (Al, Ni)-rich body-centered cubic (bcc) phase (Mo-Cr-Fe)-rich σ phase into (Fe, Ni)-rich bcc phase (Mo-Cr-Fe)-rich σ phase (Al-Fe-Mo)-rich σ phase a little AlN (aluminum nitride). Additionally, the coating hardness (HV1) is increased from 6514.4 MPa to 10652.6 MPa. With increasing the Mo addition, the self-corrosion potential of the coating in 3.5wt% NaCl solution is also increased. The coating presents the optimal corrosion resistance at x=1.0.
Deposition Behavior of HVOF Sprayed WC-12Co Particles on AA7075
Abstract: WC-12Co particles were deposited on polished AA7075 (7075 aluminum alloy) substrate by HVOF (high velocity oxy-fuel) spraying. The microstructure, composition and hardness of the deposits were analyzed by SEM, EDS and nanoindentation hardness tester, respectively. The deposition behavior of six types of particles in three different molten states, including non-molten, semi-molten, and molten particles, was investigated. Results show that different types of particles have great impact on the substrate, which makes the AA7075 substrate deform or causes tears. The surface morphology and cross-sectional morphology of the deposits are different from those of the original powder. The surface of the deposits exhibits certain melting characteristics, and the cross-section is relatively dense. The semi-molten particles and molten particles generate some tearing to the substrate, and have a metallurgical bonding with the substrate to form a mutual meting zone. After the deposition of the particles, a hardened layer is formed on substrate surface with a thickness about 5 μm, and there is a certain gradient change in the hardness. The hardness near the surface is 3420 MPa, which is 1.56 times higher than that of the substrate (2200 MPa). The increase in hardness is originated from two factors: the peening effect of particles at high temperature and high speed, and the work hardening caused by particle extruding substrate. Key words:HVOF;AA 7075;deposition behavior;melt;metallurgical bond;harden layer
Effect of Heat Treatment on Microstructure and Properties of Titanium Alloy Welded Joint by Laser Welding with Flux-Cored Wire
Ti-6Al-4V titanium alloy plate was welded by a laser beam with self-developed titanium alloy flux-cored wire. The welded joint was solution treated at 920 °C for 1 h and aging treated at 650 °C for 2 h, and its microstructure and properties were compared with those of the as-welded joint. The results show that the heat-treated welded joint is composed of a typical tri-modal microstructure containing αp phase, αs phase colony, and αgb phase, as well as punctate distributed residue β phase. α' martensite microstructure in the as-welded joint is not found in the heat-treated joint, which makes the strength, plasticity, and toughness well balanced and maintained. The strength of the heat-treated welded joint is reduced, while elongation and impact toughness at room temperature are enhanced. The tensile fracture of the heat-treated welded joint is surrounded by massive shear lips. The dimples are deep and uniform, presenting as microvoid coalescence ductile fracture. In the as-welded joint, the proportion of large-angle grain boundaries with misorientation between grains in the weld zone greater than 15° accounts for 83.78%, and in the heat-treated welded joint, the proportion is about 90.21%. Through XRD test, it is discovered that the as-welded weld is mainly composed of α' martensite, with a small amount of extremely weak multi-angle α phase diffraction peak. In the heat-treated weld, the central angle position of α phase diffraction peak is consistent with that of α' martensite in the as-welded weld, with a sharp β phase (110) diffraction peak observed as well.
Uniform Elongation and Yield-Drop Phenomenon in Magnetically Annealed 1050 Aluminum Alloy Prepared by CryoECAP
The ultrafine grained (UFG) 1050 aluminum alloy was prepared by equal channel angular pressing at cryogenic temperature, namely cryoECAP process. The tensile behavior and microstructures of UFG 1050 aluminum alloy after annealing at 90–210 °C for 4 h without and with high magnetic field of 12 T were investigated by tensile tests, transmission electron microscope, and electron backscattered diffraction analyses. After cryoECAP and annealing treatments, the 1050 aluminum alloy has ultrafine grains with 0.7–1.28 μm in size, the ratio of ultimate tensile strength to yield strength is less than 1.24, and the uniform elongation is less than 2.3%. With increasing the annealing temperature from 90 °C to 210 °C, the yield-drop phenomenon becomes more obvious due to the decrease in mobile dislocations to maintain the applied strain rate during tensile deformation. The uniform elongation decreases from 1.55% to 0.55%, the dislocation density reduces from 5.6×1014 m-2 to 4.2×1013 m-2, and the fraction of high-angle grain boundaries (HABs) increases from 63.8% to 70.8%. These phenomena cause the higher annihilation rate of dislocations, thereby leading to the degradation of strain hardening effect. During annealing under high magnetic field at 90–210 °C, the low fraction of HABs (61.7%–66.2%) can provide a slower annihilation rate of dislocations, therefore resulting in the higher uniform elongation (0.64%–1.60%) and slower decrease in the flow stress after the yield peak.
Effect of Primary α-Phase on Micro-area Plastic Deformation of Ti6242s Alloy Under Dwell Fatigue
The relationship between microstructure characteristics and fatigue properties of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242s) alloy was investigated. According to the microstructure quantitative analysis results, the solution treatments at different temperatures have an obvious effect on the proportion and morphology of primary α-phase. The changes in microstructure characteristics slightly influence the tensile property and low-cycle fatigue property of Ti6242s alloy at room temperature, whereas the dwell fatigue life and the fatigue sensitivity index are sensitive to these changes. Additionally, it is verified that the relatively strong stress concentration and inhomogeneous micro-area plastic deformation occur in the Ti6242s alloy under dwell fatigue load. Moreover, the characteristics of small plane regions and the surrounding quasi-cleavage regions in the Ti6242s alloy under dwell fatigue load at room temperature are formed through the analysis of fatigue failure fracture morphologies. The related experiment results are in good agreement with the stress-strain distribution characterizations of microstructures of equiaxed primary α-phase and the surrounding soft phase/grain. Accordingly, the relatively low inhomogeneous micro-area plastic deformation in the alloy with equiaxed primary α-phase of low volume fraction is beneficial to reduce the probability of crack initiation and can delay crack propagation, thus improving the dwell fatigue property.