The effect of hydrogen charging on the mechanical behaviour of 5083 wrought aluminum alloy

The effect of hydrogen cathodic charging on the mechanical behaviour of 5083 wrought aluminum alloy has been studied. Hardening of the surface layers of the examined alloy, due to hydrogen absorption, was observed. The tensile tests revealed that the ductility of 5083 wrought aluminum alloy decreased with increasing hydrogen charging time, for a constant value of charging current density, and with increasing charging current density, for a constant value of charging time. However, the ultimate tensile strength of the examined alloy was slightly affected by the hydrogen charging procedure. The cathodically charged 5083 wrought aluminum alloy exhibited brittle transgranular fracture at the surface layers and ductile intergranular fracture at the deeper layers of the alloy.     

Hydrogen embrittlement of the 7B05-T5 aluminum alloy for high-speed trains

The effect of hydrogen cathodic charging on the mechanical behavior of the 7B05-T5 aluminum alloy has been studied. The hydrogen susceptibility of the 7B05-T5 aluminum alloy was evaluated by slow strain rate tensile testing. The tensile tests revealed that the ductility of the 7B05-T5 aluminum alloy decreased when charged cathodically for 96 hr with a constant current density of 2 mA/cm ². The charged 7B05-T5 aluminum alloy exhibited pitting corrosion at the surface layers and brittle intergranular fracture at the deeper layers of the alloy. The decrease in the ductility of the aluminum alloy was attributed to hydrogen-reducing atomic binding force.     

Effect of atomic ordering on the hydrogen-induced environmental embrittlement of Ni4Mo intermetallics

Hydrogen induced environmental embrittlement of a Ni₄Mo alloy in different degree of ordered conditions was investigated by tensile tests in various atmosphere. The results show that the disordered Ni₄Mo alloy is not susceptible to embrittlement in hydrogen gas, but very susceptible to embrittlement in hydrogen charging. However, for the ordered Ni₄Mo alloy, there is similar deterioration in ductility when the environment changes from oxygen to hydrogen gas and simultaneous hydrogen charging. It indicates that the atomic ordering does not influence the dynamic hydrogen charging-induced environmental embrittlement, but has a considerable effect on the gaseous hydrogen-induced environmental embrittlement. In addition, hydrogen absorption and desorption of the Ni₄Mo alloy with disordered and ordered structures were also investigated using gas chromatographic analysis. The results show that the atomic ordering can promote gaseous hydrogen absorption at room temperature. This suggests that the atomic ordering accelerates the kinetics of the catalytic reaction for the dissociation of molecular hydrogen into atomic hydrogen due to the change of the outer layer electron structure and therefore exacerbates the hydrogen gas-induced environmental embrittlement.     

Nickel–boron plating on magnesium and AZ91D alloy by a chromium-free electroless process

Ni–B coatings have been deposited directly on commercial purity magnesium and AZ91D magnesium alloy by a chromium-free electroless process and characterized using scanning electron microscopy, glow discharge optical emission spectroscopy, gravimetric measurements and, for corrosion evaluation, hydrogen evolution. The surfaces of the coatings reveal a typical cauliflower-type appearance and form at a rate of ~ 8 to 13 μm/h. The nickel and boron contents of the coatings are relatively uniform across the whole thickness. The addition of NH₄HF₂ to the electroless bath improves the corrosion resistance provided by the coatings during immersion of coated substrates in sodium chloride solution.     

X70管线钢电化学充氢后的力学行为研究

应用拉伸试验和慢拉伸试验，研究X70管线钢电化学充氢后材料拉伸性能的变化．结果表明：电化学充氢对X70管线钢的强度没有显著的影响，主要降低了材料的塑性，从而降低了材料的断裂延性和断裂强度．在静态电化学充氢条件下，材料的塑性随充氢时间的增加，依次降低．在慢拉伸条件下，动态电化学充氢显著降低材料的塑性．断口分析表明：静态电化学充氢后的断口以韧窝为主要特征，但韧窝直径变小；慢拉伸的动态电化学充氢断口出现准解理断裂．     

Phase precipitation behavior and tensile properties of as-cast Ni-based superalloy during heat treatment

The phase precipitation behavior and tensile properties of an as-cast Ni-based alloy, IN617B alloy, after solution heat treatment and long-term aging treatment were investigated. Ti(C,N), M₆C and M₂₃C₆ are the primary precipitates in as-cast microstructure. After solution heat treatment, most of carbides dissolve into the matrix except a few fine Ti(C,N) within grains. During long-term aging at 700 °C, the phase precipitation behaviors of the alloy are characterized as follows: (1) M₂₃C₆ carbides at grain boundaries (GBs) transform from film-like shape to cellular shape and gradually coarsen due to the decrease of the surface energy and element aggregation to GBs; (2) M₂₃C₆ carbides within grains have a bar-like morphology with a preferential growth direction [110] and have a cube-on-cube coherent orientation relationship with the matrix γ; (3) γ′ particles inhibit the coarsening of M₂₃C₆ within grains by constraining the diffusion of formation elements. Furthermore, the tensile strength of the alloy obviously increases, but the ductility significantly decreases after the aging for 5000 h. The alloy has a relatively stable microstructure which guarantees the excellent tensile properties during long-term aging.     

STUDY ON AGING PRECIPITATION BEHAVIOR OF Al–5.2Cu–0.4Mg–1.02Ag ALLOY

研究了Al--5.2Cu--0.4Mg--1.02Ag合金在不同时效制度下的力学性能和显微组织, 并详细分析了合金的主要析出相Ω的形核与粗化, 同时提出了浓度台阶粗化机制. 结果表明: 合金的主要强化相是Ω相和θ'相. 欠时效时出现了大量细小的Ω相和少量的θ'相; 峰时效时Ω相和θ'相的体积分数大大增加, 且Ω相与基体呈半共格关系; 过时效时出现了球状的平衡θ'相, Ω相略为长大, 而θ'相的长度和厚度明显增大. Mg/Ag原子簇是时效初期Ω相的形核核心; Mg, Ag和Cu的浓度差异引起的台阶迁移是Ω相粗化的驱动力. 由于Mg和Ag原子在Ω相与基体界面存在时降低了晶格的畸变能, 使得Cu原子向Ω相迁移的速率受到限制, 因此Ω相能够在长时间下保持片状而不发生共格失稳.     

Effect of Second-Phase Precipitates on Local Elongation in Extruded Magnesium Alloys

The effect of precipitates on room-temperature tensile ductility, specifically post-uniform elongation (local elongation), was investigated in three magnesium alloys. The alloys were prepared by extrusion and had different types of precipitate morphology. Tensile tests revealed an almost total absence of local elongation in an alloy with intergranular precipitate particles (a few hundred nanometers in diameter), in spite of a relatively high total elongation to fracture of 17%. On the other hand, an alloy with densely dispersed fine intragranular precipitates (up to a few tens of nanometers in diameter) showed the highest local elongation at 17% and an alloy with a sparse dispersion showed moderate local elongation. The fracture surfaces of the latter two alloys had a primarily ductile appearance with dimples. For the specimen that showed the highest local elongation, twinning was observed in the necked portion, whereas very few twins were observed in the uniformly elongated portion. It was suggested that fine precipitates in the grain interior inhibit the formation of twins, a feature that can be exploited to enhance local elongation in magnesium alloys.     

钛合金电子束焊接接头的氢行为

采用恒载电解充氢的方法和X射线衍射分析方法，研究了Ti1023电子束焊接接头引起氢致开裂氢含量门槛值以及Ti1023合金基体与焊缝中由于相成分的不同造成对氢致开裂的敏感性的差异；同时通过扫描电镜分析了不同充氢状态下试件的断口特征。采用等效人工时效的方法，借助离子探针分析技术，对Ti1023合金电子束焊接接头中氢的动态行为进行了研究。     

Hydrogen embrittlement and fracture toughness of a titanium alloy with surface modification by hard coatings

The effect of hydrogen embrittlement on the fracture toughness of a titanium alloy with different surface modifications was investigated. Disk- shaped compact- tension specimens were first coated with different .hard films and then hydrogen charged by an electrochemical method. Glow discharge optical spectrometry (GDOS), scanning electron microscopy (SEM), and x- ray diffractometry (XRD) were applied to analyze the surface characteristics. The results revealed that fracture toughness of the as- received titanium alloy decreased with the increase of hydrogen charging time. Fracture toughness of the alloy after plasma nitriding or ion implantation, which produced a TiN _x layer, decreased as well, but to a lesser extent after cathodic charging. The best result obtained was for the alloy coated with a CrN film where fracture toughness was sustained even after hydrogen charging for 144 h. Obviously, the CrN film acted as a better barrier to retard hydrogen permeation, but it was at the sacrifice of the CrN film itself.     

Effect of interfacial solute segregation on ductile fracture of Al–Cu–Sc alloys

Three-dimensional atom probe analysis is employed to characterize the Sc segregation at θ′/α-Al interfaces in Al–2.5 wt.% Cu–0.3 wt.% Sc alloys aged at 473, 523 and 573 K, respectively. The interfacial Sc concentration is quantitatively evaluated and the change in interfacial energy caused by Sc segregation is assessed, which is in turn correlated to yield strength and ductility of the alloys. The strongest interfacial Sc segregation is generated in the 523 K-aged alloy, resulting in an interfacial Sc concentration about 10 times greater than that in the matrix and a reduction of ～25% in interfacial energy. Experimental results show that the interfacial Sc segregation promotes θ′ precipitation and enhances the strengthening response. A scaling relationship between the interfacial energy and precipitation strengthening increment is proposed to account for the most notable strengthening effect observed in the 523 K-aged alloy, which is ～2.5 times that in its Sc-free counterpart and ～1.5 times that in the 473 and 573 K-aged Al–Cu–Sc alloys. The interfacial Sc segregation, however, causes a sharp drop in the ductility when the precipitate radius is larger than ～200 nm in the 523 K-aged alloy, indicative of a transition in fracture mechanisms. The underlying fracture mechanism for the low ductility regime, revealed by in situ transmission electron microscopy tensile testing, is that interfacial decohesion occurs at the θ′ precipitates ahead of crack tip and favorably aids the crack propagation. A micromechanical model is developed to rationalize the precipitate size-dependent transition in fracture mechanisms by taking into account the competition between interfacial voiding and matrix Al rupture that is tailored by interfacial Sc segregation.     

N18锆合金氢致裂纹延迟开裂临界温度研究

研究了N18锆合金(Zr-1Sn-0.3Nb-0.3Fe-0.1Cr)发生氢致延迟开裂(DHC)临界最大开裂温度(Tc)和临界最小止裂温度(Th)随氢含量的变化规律;同时对裂纹尖端偏聚氢含量及静水应力和发生DHC的临界氢含量进行了理论分析,建立理论模型对临界温度进行理论计算.结果表明:N18合金发生氢致延迟开裂的临界温度介于相同氢含量下溶解固溶温度与析出固溶温度之间,且最大开裂温度小于最小止裂温度,计算的临界温度值与实验值相当吻合,因此该理论模型能够真实反映N18锆合金的氢致延迟开裂的物理过程.     

时效对TiNiCr形状记忆合金马氏体相变与超弹性的影响(英文)

通过X射线衍射分析(XRD)、透射电子显微观察(TEM)、差式扫描量热分析(DSC)与拉伸实验研究时效处理对Ti48.4Ni51.1Cr0.5合金显微组织、马氏体相变与超弹性的影响规律与机制。经400°C时效处理30 min后,合金中形成Ti3Ni4析出相。当时效温度介于400°C和500°C之间时,合金表现出两步马氏体相变。经时效处理的Ti48.4Ni51.1Cr0.5合金在室温下表现出优异的超弹性。随时效温度自300°C升高到450°C,超弹性恢复率增加。继续升高时效温度,恢复率下降。超弹性应力滞后表现出相反的变化趋势。通过分析Ti3Ni4析出相随时效处理的演化规律解释了时效处理与马氏体相变和超弹性之间的关系。     

In situ thermomechanical processing to avoid grain boundary precipitation and strength-ductility loss of age hardening alloys

To avoid grain boundary (GB) precipitation during aging, a new strategy of in situ thermomechanical processing for age hardening alloys was proposed. Specifically, high-density nanoscale precipitates were introduced into ultrafine grain (UFG) interiors of 7075Al alloy by equal-channel-angular (ECAP) processing at 250 °C for 8 passes, thus avoiding GB precipitation. Tensile test results indicated that the UFG 7075Al alloy exhibits superior mechanical properties (yield strength of 350 MPa, ultimate tensile strength of 500 MPa, uniform elongation of 18% and tensile ductility of 19%) compared with the UFG 1050Al counterpart (yield strength of 170 MPa, ultimate tensile strength of 180 MPa, uniform elongation of 2.5% and tensile ductility of 7%). Fracture surface morphology studies revealed numerous homogeneous micro shear bands in necking shrinkage areas of both UFG 7075Al and 1050Al alloys, which are controlled by cooperative GB sliding. Moreover, the introduction of nanoscale precipitates in UFG 7075Al matrix weakened the tendency of shear fracture, resulting in a higher tensile ductility and more homogeneous deformation. Different from the GB precipitation during postmortem aging, in situ thermomechanical treatment dynamically formed GBs after precipitation, thus avoiding precipitation on GBs.     

Precipitation behavior and its effects on tensile properties of FeCoNiCr high-entropy alloys

In this work, we present a systematic study on the precipitation behavior and mechanical properties of a FeCoNiCr-based high-entropy alloy alloyed with dilute amounts of Ti and Al, (FeCoNiCr)_100-x-yTi_xAl_y (where x = 1–3, y = 4–9 at.%). It was found that, upon aging, nano-sized L1₂-Ni₃(Ti, Al) particles are formed within grains, whilst L2₁-(Ni, Co)₂TiAl Heusler particles are formed mainly along grain boundaries. The relative thermal stability of the two phases were studied at different aging temperatures (700–900 °C) with various durations of time (up to 48 h) and the results were directly compared with Thermo-calc calculations. Tensile tests were also conducted on alloys aged under different conditions. The measured properties, including strength and ductility, were correlated with the microstructure of aged (FeCoNiCr)_100-x-yTi_xAl_y alloys, with particular attention on the distribution and morphology of the two kinds of precipitate. Whereas both phases could contribute to the strengthening of the alloys via either Orowan bowing or particle shearing mechanism, the brittle (Ni, Co)₂TiAl Heusler phase was found to mainly affect the tensile plasticity. A simple composite model was proposed to describe the plastic strain of alloys. Based on observed microstructure and its corresponding mechanical performance, the alloy with the composition of (FeCoNiCr)₉₄Ti₂Al₄, when aged between 700 and 800 °C, gives the best balanced strength/ductility properties.     

Hydrogen uptake in austenitic stainless steels by exposure to gaseous hydrogen and its effect on tensile deformation

Hydrogen solubility and diffusivity of Fe–Cr–Ni austenitic stainless steels were measured through exposure to gaseous hydrogen at a pressure of 10 MPa over the temperature range 110–235 °C. The hydrogen solubility depended on the alloy compositions, whereas the diffusion coefficients were nearly identical at a given temperature. Hydrogen uptake in the stable austenitic steels by exposure to high-pressure gaseous hydrogen led to some loss of ductility, while their fracture surfaces showed evidence of plastic deformation. This was attributed to the enhanced inhomogeneity of plastic deformation in the presence of hydrogen and the increased stress for plastic instability with increasing hydrogen concentration.     

Microstructure, Mechanical Properties, and Age-Hardening Behavior of an Al-Si-Fe-Mn-Cu-Mg Alloy Produced by Spray Deposition

It has been recognized generally that the spray-deposited process is an innovative technique of rapid solidification. In this paper, Al-20Si-5Fe-3Mn-3Cu-1Mg alloy was synthesized by the spray atomization and deposition technique. The microstructure and mechanical properties of the spray-deposited alloy were studied using x-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and tensile tests. It is observed that the microstructure of spray-deposited Al-20Si-5Fe-3Mn-3Cu-1Mg alloy is composed of the α-Al,Si and the particle-like Al₁₅(FeMn)₃Si₂ compounds. The aging process of the alloy was investigated by microhardness measurement, differential scanning calorimetry analysis, and TEM observations. The results indicate that the two types of precipitates, S-Al₂CuMg and σ-Al₅Cu₆Mg₂ precipitate from matrix and improve the tensile strength of the alloy efficiently at both the ambient and elevated temperatures (300 °C).     

Hydrogen embrittlement of high strength pipeline steels

A comparison was made between three API grade pipeline steels (X60, X80 and the X100 grade) from the point of view of their susceptibility to hydrogen embrittlement. The main aim was to determine whether the development of higher strength materials led to greater susceptibility to hydrogen embrittlement. This was achieved by straining at 2.8 × 10⁻⁵ s⁻¹ after cathodic charging. The results showed that there is a distinct susceptibility to loss of ductility after charging and this tends to increase with the strength level of the steel at a charging current density above 0.44 mA mm⁻². All three steels exhibited fine cracks parallel to the major rolling direction after charging and an increasing amount of brittleness on the fracture surface.     

Bismuth nanoprecipitation at grain boundaries during microstructural evolution in (Sr, Ba)TiO3 ceramics

Metallic Bi nanoprecipitates (NPs) were observed at grain boundaries (GBs) in a series of Bi-doped Sr_1−xBa_xTiO₃ ceramics sintered at low temperature, using analytical electron microscopy. Most GBs fell into two groups, according to their local structure and chemistry, one with the NPs and the other with titania-based amorphous film. The solution of Ba into the lattice was only about one-third of the nominal value, and the remaining Ba dopants were driven into triple pockets or wetted the GBs during the sintering. Lower Ba doping initiated less liquid, resulting in a core–shell structure. Bi dopants were co-soluted with Ba, thereby suppressing the Bi segregation at GBs. The extra Bi could not diffuse out through the dewetted GB as fast as via the titania-based film, compelling them to precipitate locally. These metallic NPs may initiate several modes of polarization that increase the dielectric constants detected at 1 kHz.     

Influence of Li addition on mechanical property and aging precipitation behavior of Al-3.5Cu-1.5Mg alloy

The influence of Li addition on mechanical property and aging precipitation behavior of Al-3.5Cu-1.5Mg alloy was investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The results show that the tensile strength can be significantly improved with the slightly decreased ductility and the form of fracture morphology is converted from ductile fracture into ductile/brittle mixed fracture by adding 1.0% Li. Besides, the peak aging time at 185 °C is delayed from 12 to 24 h and the main precipitation phase S'(Al₂CuMg) is converted into S' (Al₂CuMg)+δ'(Al₃Li), while the formation of S'(Al₂CuMg) is delayed.