Microstructure and mechanical properties of a novel refractory AlNbTiZr high-entropy alloy

The microstructure and mechanical properties of a novel refractory AlNbTiZr high-entropy alloy (HEA) with a low density of ～5.85?g?cm^?3 were investigated after arc melting and homogenisation at 1473?K for 5?h. The as-cast HEA exhibits a single-phase ordered body-centred cubic (B2) structure. A hexagonal Zr₅Al₃-type second phase is introduced into the HEA through homogenisation treatment, resulting in increase of the yield strength, ultimate compressive strength and fracture strain by 70?MPa, 308?MPa and 9.2%, respectively. These results indicate that the introduction of the hexagonal Zr₅Al₃-type second phase into the B2 matrix can simultaneously improve the HEA strength and ductility, showing a strength–ductility combination superior to those of most reported refractory HEAs.     

新型高密度合金的组织与性能

基于面心立方固溶体结构和时效强化机理,设计出一种新型高密度合金NiW750。利用SEM,TEM对合金微观组织进行观察,采用分离式Hopkinson压杆实验研究合金在动态压缩条件下的特点,并将此合金与其领域常用材料超高强度钢G50及钨合金93WNiFe进行对比。结果表明:NiW750合金在3种材料中综合性能最好。在750℃/5h时效后,合金抗拉强度可达1746MPa,冲击韧度( a kU )可达113J/cm^2。在动态加载条件下,材料存在应变率硬化效应,其动态流变应力可达到2250MPa左右。试样在与中心轴线成45°方向形成绝热剪切带,在应变率约5500s -1 条件下,带宽80~150μm,过渡区较宽,避免材料剪切断裂过早出现。     

The role of microstructural morphology on creep properties of a [0 0 1] oriented nickel‐base single crystal superalloy

The experimentally observed microstructure of nickel‐base single crystal alloy consists of a large volume of cuboidal γ′ precipitates coherently embedded in the γ matrix. In calculation, a representative volume element is usually used to represent the whole structures due to the regular γ/γ′ topological structures. Here, three experimentally found microstructures have been extracted to generate the representative volume elements. One is constituted by one cuboidal γ′ phase surrounded by γ phase. The other two consisted of two cuboidal γ′ phases and one rectangle γ′ phase with different arrangement of the two γ′ phases. The misfit stress is taken into consideration by different thermal expansion coefficients of the two phases. The influences of different microstructures on the macro‐creep strain evolution, rafting and stress distributions are discussed.     

Serrated flow behavior in a near alpha titanium alloy IMI 834

Serrated behavior of near alpha titanium alloy IMI 834 has been studied at elevated temperature from 400 °C to 475 °C. Serrations morphology was found as A type of locking serrations followed by B type serrations at 400 °C. E type of serrations has been observed at higher strains at 425 °C. B type and unlocking serrations of C_A type at 450 °C and again A and C_B type serrations were found at 475 °C. In strength parameters, anomalous tensile behavior was found in the variation of tensile strength and yield strength with test temperature in the temperature range between 400 °C and 475 °C. However, the variation of normalized flow stress showed regions I–III with test temperature. Regions I and III correspond to normal tensile behavior and region II corresponds to anomalous tensile behavior. Blue brittle temperature of IMI 834 was attributed at 450 °C by confirming minimum ductility of 8.2%. In present study, a different approach has been adopted to show the change in deformation behavior during serrated region called as abrupt change in strain path. Maximum irregularity in flow behavior has been observed at 450 °C and 475 °C. Room temperature fractographic features showed transgranular features whereas mixed ductile and cleavage fracture has been observed in the temperature range between 400 °C and 475 °C. However, reverse slope behavior has been observed in the plot of critical strain versus test temperature at 450 °C, which could be due to silicide precipitation. In the present study, interaction of dislocations with interstitial/substitutional solutes is responsible for dynamic strain aging in IMI 834.     

Microstructure characterizations and strengthening mechanism of multi-principal component AlCoCrFeNiTi0.5 solid solution alloy with excellent mechanical properties

It was reported that AlCoCrFeNiTi_0.5 alloy exhibits excellent comprehensive mechanical properties. In this letter, this alloy was further studied on its microstructures and strengthening mechanism. The super-high strength and good plasticity of AlCoCrFeNiTi_0.5 alloy should be attributed to its microstructure of intrinsic strong body-centered cubic solid solution, and effective multiple strengthening mechanisms like solid solution strengthening, precipitation strengthening, and nano-composite strengthening effects, etc.     

Effects of thermal martensite content and deformation on damping capacity of a Co–32 wt.% Ni alloy

The damping capacity of Co–32 wt.% Ni alloy was investigated as a function of the amount of thermal and strain-induced martensite under non-magnetic and 900 Oe magnetic fields, respectively. The damping capacity of the Co–32 wt.% Ni alloy containing martensite without magnetic field consists of the magneto-mechanical damping capacity of mainly α phase, damping capacities of α and phases without magneto-mechanical damping effect. Under a magnetic field of 900 Oe, the more the thermal martensite mass fraction the higher the damping capacity. However, the damping capacity of the deformed Co–32 wt.% Ni alloy with the strain-induced martensite decreases with increasing deformation degree despite the increase in total martensite fraction, because the lattice defects like dislocations introduced during deformation act as barriers to movement of damping sources such as magnetic domain walls, stacking faults boundaries in both α and phases, and α/ interfaces.     

Effect of Mn on damping capacities, mechanical properties, and corrosion behaviour of high damping Mg–3 wt.%Ni based alloy

The effect of Mn on the damping capacities, mechanical properties, and corrosion behaviour of high damping Mg–3 wt.%Ni based alloys has been studied. The damping vs. strain amplitude spectrum of the studied alloys could be divided into three parts. The strain amplitude weakly dependent part appears again when the microplastic strain occurs at high strain amplitude. The mechanical properties of as-cast Mg–3 wt.%Ni alloy could be improved by the addition of Mn, which is due to the refinement of α-Mg dendrites and solid solution strengthening by Mn. In addition, the corrosion resistance of the alloys could also be improved remarkably by the addition of Mn.     

电冲击处理(EST)对TC11钛合金微结构和力学性能的影响研究

为寻求优化钛合金组织和力学性能的新思路,本文采用电冲击处理方法对TC11钛合金进行组织结构调控,并利用SEM和EBSD对处理前后材料的微结构、相含量和织构分布进行表征分析,同时利用处理前后的硬度分布和压缩性能分析来体现力学性能变化。微结构研究表明,当电冲击处理时间增加至0.04 s,材料发生针状二次α向β相转变,β相含量从10.1%增加到14.4%,且初生α/β相界面平滑度增加,主要归因于电冲击处理过程的热效应和非热效应。织构研究表明,电冲击处理0.04 s后,α相织构强度从6.77增大到10.53,β相织构强度增加不明显,α相和β相织构变化与电冲击处理能量集中引起的相变有关。力学性能研究结果显示,电冲击处理0.06 s后,显微硬度和屈服强度明显提高,主要原因是样品内部析出大量细小的针状马氏体α相,起到弥散强化作用。综上,利用电冲击处理来调控钛合金微结构将是一种新的探索,可为钛合金力学性能改善提供一条新途径。     

Investigation of α platelet boundaries in a near-α titanium alloy

Transmission electron microscopy (TEM) of a bimodal near-α titanium alloy revealed the existence of retained β phase layers and silicide precipitates at the α platelet boundaries inside transformed β grains. The β to α phase transformation accompanied by the precipitation of silicide resulted in the formation of a large number of dislocations at α platelet boundaries. Orientation relationships between silicide, β phase and α phase were also identified. However high-resolution TEM (HRTEM) revealed crystal mismatches between these phases generating high strains at α platelet boundaries. The strengthening effects of the platelet boundaries are discussed in terms of dislocations slip across the boundaries. The mechanism that governs the β to α phase transformation is also discussed.     

Correlation between precipitate microstructure and mechanical properties in AA7075‐T6 aluminum alloy friction stir welded joints

Microstructural evolution and mechanical properties of friction stir welded AA7075‐T6 aluminum alloy were examined. Grain structure and precipitate evolution in the stir zone and heat‐affected zone were evaluated using optical microscope and differential scanning calorimetry. A significant grain refinement and dissolution of η′ precipitates in the stir zone were found, but chromium‐bearing dispersoids remained nearly unchanged. The main particles in the stir zone and heat‐affected zone were η precipitates as well as Guinier‐Preston zones formed during post‐weld natural aging. The small recrystallized grains were observed in the thermo‐mechanically affected zone next to the stir zone. A W‐shaped hardness distribution where soft region was produced in the heat‐affected zone at a short distance from the stir zone were obtained. Hardness profiles of the welds were explained by precipitate distributions. Friction stir welding resulted in the reversion and coarsening of η′ precipitates. The formation of Guinier‐Preston zones in the stir zone and some parts of the heat‐affected zone during post‐weld natural aging increased the hardness. In transverse tensile specimens, fracture occurred in a location with the minimum hardness at either advancing or retreating side randomly. Further, influences of welding parameters on mechanical properties were investigated.     

烧结方法对LiTaO3/Al2 O3复相陶瓷微观组织及力学性能的影响

为了研究铁电／压电陶瓷颗粒对结构陶瓷力学行为的影响规律及机理,分别采用真空热压烧结法和冷等静压成型无压烧结法制备了稳定共存的LiTaO3/Al2O3(LTA)复相陶瓷,对比研究了其微观组织与力学性能,结果表明,1500℃真空热压烧结制备的LTA复相陶瓷中,LiTaO3相熔化,冷却时分布在Al2O3基体晶界,基体晶粒粗化,力学性能较低,200MPa冷等静压成型,1300℃无压烧结制备的LTA复相陶瓷中,LiTaO3颗粒弥散分布,基体晶粒细小,ψ（LiTaO3)为5％的LTA复相陶瓷的力学性能显著改善,强韧化机理为细晶与压电效应和／或电畴运动耗散能量强韧化。     

Effect of precipitates on damping capacity and mechanical properties of Ti–6Al–4V alloy

The present study was concerned with the effects of over-aging on damping property and fracture toughness in Ti–6Al–4V alloy. Damping property and toughness become important factors for titanium implants, which have big modulus difference between bone and implant, and need high damping capacity for bone-implant compatability. Widmanstätten, equiaxed, and bimodal microstructures containing fine α₂ (Ti₃Al) particles were obtained by over-aging a Ti–6Al–4V alloy. Over-aging heat treatment was conducted for 200 h at 545 °C. Fracture toughness, Charpy impact, and bending vibration tests were conducted on the unaged and the over-aged six microstructures, respectively. Charpy absorption energy and apparent fracture toughness decreased as over-aging was done, even if the materials were strengthened by precipitation of very fine and strong α₂-Ti₃Al particles. On the other hand, damping properties were enhanced by over-aging in Widmanstätten and equiaxed microstructures, but was weakened in bimodal microstructure due to the softening of tempered martensite and the decreasing of elastic difference between tempered martensite and α phase contained α₂ particles, etc. These data can provide effective information to future work about internal damping and fracture properties of Ti–6Al–4V alloy.     

Effect of pimelic acid on the crystallization, morphology and mechanical properties of polypropylene/wollastonite composites

The pimelic acid (PA) was used as a new surface modifier for wollastonite. The effects of PA treatment on the crystallization, morphology and mechanical properties of polypropylene/wollastonite composites were investigated. The Fourier transform infrared spectroscopy analysis revealed that the PA bonded to the wollastonite's surface and formed the calcium pimelate after reacting with the wollastonite. The results of wide angle X-ray diffraction, differential scanning calorimetry and polarized light microscopy proved that the PA treated wollastonite induced more β-crystalline form and decreased the spherulites sizes of polypropylene. The results of scanning electron microscopy showed that the PA treatment enhanced the interfacial adhesion between the filler and the matrix, indicating the improvement of the compatibility between polypropylene and wollastonite. The toughness of the composites was improved by the more ductile β-form spherulites. When 2.5 wt% of PA treated wollastonite was added, the Izod notched impact strength reached its maximum, a value of 17.33 kJ/m², which was 3.19 times greater than that of the blank polypropylene.     

Microstructural characterization of precipitates in Cu–10 wt.%Al–0.8 wt.%Be shape-memory alloy

Microstructural characterization of α₁-plate and γ₂ phase precipitated in hypoeutectoid Cu–10 wt.%Al–0.8 wt.%Be shape-memory alloy (SMA) aged at 200 °C for different periods of time (20–160 h) is researched in this study. High-resolution transmission electron microscope (HRTEM) was employed to investigate the α₁-plate with 18R long period stacking order structure (LPSO) in the SMA aged for 20 h. According to the atomic shuffling revealed in HRTEM-micrograph, the atomic model of the 18R LPSO is proposed. The quantitative mapping of electron energy loss spectrometry shows that the α₁-plates in the SMA aged for 160 h contain lower aluminum concentration than the parent phase matrix. The lattice image of the nanometer-sized γ₂ phase precipitated homogeneously in the SMA aged for 160 h is also revealed by using HRTEM. Precipitation of the nanometer-sized γ₂ phase cannot be impeded by means of the addition of beryllium and quenching, and such precipitate does not grow up in the SMA aged for periods of time less than 160 h.     

Dynamic strength and failure behavior of titanium alloy Ti-6Al-4V for a variation of heat treatments

In our study, samples of Ti-6Al-4V were subjected to modifications of an aging treatment where temperatures for solution annealing and final aging as well as the cooling rate were varied. The titanium alloy was annealed above and below the β-transus temperature followed by cooling in a vacuum furnace or by water quenching. Additionally, the final annealing temperature was varied. Compression tests under quasistatic and dynamic loading rates were performed to determine the flow stress and strain hardening behavior. Furthermore, instrumented Charpy impact tests on U-notch specimen were performed at room temperature to monitor the load-time response of deformation and fracturing. The obtained high rate mechanical properties are discussed and correlated with the present microstructure. Our results reveal a very strong effect of the microstructure on the material behavior and will assist to choose the appropriate heat treatment technology, especially if impact loaded or safety structures have to be considered.     

Interaction of dissolved atoms and relaxation due to interstitial atoms in hcp metals

The literature data on the mechanism of internal friction maxima induced by O, N, and C in α-Ti, α-Zr, and α-Hf, are contradictory. They do not answer the question which kind of complexes induces relaxation: interstitial atoms or interstitial atoms with substitutional atoms. To clarify this question, modeling of the short-range order and atomic displacement fields around the solute atom clusters was carried out by the Monte-Carlo technique for typical Ti–O–Zr alloys. The energies of strain-induced (elastic) O–O and O–Zr interactions and displacement fields of host atoms around the solute atoms were calculated and used in modelling. The concentration dependence of relaxation strength due to diffusion under stress of oxygen atoms was evaluated using the values of local displacement around the solute atom complexes. It is shown that the developing short-range order cannot be described by the single O–O or O–Zr pair and the associated relaxation, as simple reorientation of any specific atomic pairs. It seems likely that in many cases the internal friction is caused by more complicated clusters constituted by interstitial and substitutional atoms.     

Study of autoabsorption for total α and β counting

The RaMsEs Group (Radioprotection et Mesures Environnementales) of the IPHC performs research and offers services mainly in the field of radioactivity measurements and sample analysis. This report will describe some of our recent experience using a semiautomatic evaporation system to prepare large area thin deposits for total α and β counting and gives experimental and simulated results for the autoabsorption coefficients.     

Effect of heat treatment on structure evolution and properties of nano zinc titanate ceramics

Nano zinc titanate ceramics are prepared using a conventional solid state method. The obtained compacts are sintered at 800, 900, 1000 and 1100 °C for 3 h. The prepared compacts are analyzed using X‐ray diffraction (XRD) and scanning electron microscopy (SEM) for structural and microstructural studies. Based on the X‐ray diffraction (XRD) data, it is observed the coexistence of ZnTiO₃ and α‐Zn₂TiO₄ phases together at low temperature (800 °C) without the presence of TiO₂ (rutile) contradicting the general mechanism stating the transformation of ZnTiO₃ to α‐Zn₂TiO₄ and TiO₂ at higher temperatures. A new mechanism is proposed to explain the formation of nano ZnTiO₃ and α‐Zn₂TiO₄ structures depending on the role of TiO₂ in achieving this mission. According to this mechanism, we propose a partial diffusion of TiO₂ in the ZnO lattice forming the ZnTiO₃ phase. The second part of TiO₂ acts as a catalyst that facilitates the transformation of nano ZnTiO₃ to nano α‐Zn₂TiO₄. The catalytic power of rutile is achieved from its enhanced tensile stress that induces the phase transition from nano ZnTiO₃ to nano α‐Zn₂TiO₄.