Role of second phase particles in pitting corrosion of 3003 Al alloy in NaCl solution

The second phase particles in 3003 aluminum (Al) alloy were characterized by scanning electron microscope, energy‐dispersive X‐ray analysis and X‐ray diffraction techniques. The role of second phase particles in Al alloy pitting corrosion was investigated by cyclic polarization measurement, and scanning vibrating electrode technique. Results demonstrated that the second phase particles in 3003 Al alloy are mainly Al_x(Fe,Mn) intermetallics, with an average diameter of about 5 µm. The enrichment of Mn in second phase particles forms a galvanic cell effect relative to the adjacent Al alloy substrate. The initiation of pitting corrosion of 3003 Al alloy is the local dissolution of Al substrate around the second phase particles. When a sufficient amount of Al is dissolved away, the second phase particles drop off from the Al substrate, forming large pitting cavities that are usually linked each other.     

Analysis of phase in Cu–15%Cr–0.24%Zr alloy

The vacuum medium-frequency induction melting technology was employed to prepare the Cu–15%Cr–0.24%Zr alloy. The scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to analyze the phase composition, morphology and structure of the alloy. The results reveal that the as-cast structure of the alloy consists of Cu matrix, Cr dendrite, eutectic Cr and Zr-rich phase. A large number of Cr-precipitated phases occur in the Cu matrix, and Cu₅Zr particles can be found in the grain boundary of Cu matrix. The HRTEM images prove that there is a semi-coherent relationship between Cu₅Zr and Cu matrix.     

Surface damage of a TiAl-based alloy during high temperature annealing

The near surface stability of a Ti–45Al–2Mn–2Nb alloy has been investigated. Samples of the alloy were annealed at two different temperatures, 1200 and 1000°C for various periods of time. It was found by detailed SEM and TEM investigation that a significant loss of aluminum occurred from the surface of the specimens during annealing, resulting in the formation of Al-depleted damaged layer near the surface of the specimens. The microstructure of the damaged layer was found to be considerably different from that of the bulk. The damaged layer was found to consist of α₂ phase near the interface between the damaged layer and the undamaged bulk, and α₂ phase and B2 phase with ω-type phase precipitates near the outer edge of the specimen. It was also observed that the thickness of the damaged layer increased with the annealing time and also with the annealing temperature. The thickness of the damaged layer exhibited a parabolic relationship with annealing time for both the temperatures.     

Effect of grain size on hot cracking resistance of the HAZ metal of welded joints in creep-resisting alloys

Summary

Al₂O₃/6061 aluminium alloy composite was welded to aluminium alloys (6061‐T6, 5052‐H14, 2017‐T4) using an inertia type friction welding machine to investigate the microstructures and hardness distributions of friction welded joints. Regardless of the aluminium alloys used, finer alumina particles and finer grain structures were observed in the area near the weld interface. And the occurrence of mechanical mixing and moving of alumina particles into a region other than the composite were observed clearly on the weld interface. The distance moved by the alumina particles became greater in the order of the alloys, 6061, 5052 and 2017. A softened area was observed in the heat affected zone of all the aluminium alloys. It was found 30 days after welding that the hardness of the welded joint between the composite alloy and 6061 alloy had not changed significantly, whilst that of the welded joint between the composite alloy and 2017 was observed to have recovered to the previous standard 14 days after welding.     

FIB-SEM investigation on corrosion propagation of aluminium–lithium alloy in sodium chloride solution

Abstract

Microstructural characterisation of 2A97-T4 aluminium–lithium alloy was carried out using electron probe microanalysis and transmission electron microscopy (TEM). Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy facilities has been employed to examine localised corrosion sites after immersion in sodium chloride solution. A dual beam microscope, which integrates a focused ion beam and an electron beam in one powerful instrument, has also been employed to investigate the development of intergranular corrosion from both surface and cross-section. It was found that localised corrosion is generally initiated at θ phase particles, which represents only 8.4% of the intermetallic (IM) particles in 2A97-T4 aluminium–lithium alloy. θ phase particles exhibit preferential dissolution of aluminium during corrosion testing, with trench formed at their periphery as well. Initiation of intergranular corrosion is relatively late with respect to the attack of IM particles. Owing to the presence of θ phase particles at intergranular corrosion sites and non-uniform distribution of T1 (Al₂CuLi) grain boundary precipitates, it is supposed that dealloyed θ phase particles and grain boundary precipitates cooperate to provide the driving force for grain boundary attack.     

Microstructure and properties of Cr3C2-modified nickel-based alloy coating deposited by plasma transferred arc process

The nickel-based alloy with 30 wt.% chromic carbide (Cr₃C₂) particles has been deposited on Q235-carbon steel (including 0.12 wt.% C) using plasma transferred arc (PTA) welding machine. The microstructure and properties of the deposited coatings were investigated using optical microscope, scanning electron microscope (SEM) equiped with X-ray energy spectrometer (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), microhardness testes, and sliding wear test. It was found that the γ(Ni, Fe), M₇(C,B)₃, Ni₄B₃, and (Cr,Fe)₂B phases existed in the Cr₃C₂-free nickel-based alloy coating obtained by PTA process. The typical hypoeutectic structure and composition segregation in the solid solution could be found clearly. The addition of 30 wt.% Cr₃C₂ particles led to the existing of Cr₃C₂ phase and the microstructure changing from hypoeutectic structure into hypereutectic structure. The composition segregation in the solid solution could not be found clearly. The average microhardness of the Cr₃C₂-free nickel-based alloy coating increased by 450-500 HV after the addition of 30 wt.% Cr₃C₂ particles. The partial dissolution of Cr₃C₂ particles led to the enrichment of carbon and chromium in the melten pool, and hence caused the formation of more chromium-rich carbides after the solidification process. The undissolved Cr₃C₂ particles and the increasing of chromium-rich carbides was beneficial to enhance the hardness and wear resistance of the Cr₃C₂-modified nickel-based alloy coating deposited by PTA process.     

Alloying with copper to reduce metal dusting of nickel

Copper is thought to be noncatalytic to carbon deposition from gas atmospheres, and owing to its extremely low solubility for carbon, inert to the metal dusting reaction. Thus, the addition of copper to nickel, which forms a near perfect solid solution, may be able to suppress or greatly retard the metal dusting of the alloy, without the need for a protective oxide scale on the surface. The dusting behaviour of Ni‐Cu alloys containing up to 50 wt% Cu, along with pure Cu, was investigated in a 68%CO‐31%H₂‐1%H₂O gas mixture (a_C: 19) at 680°C for up to 150 h. Surface analysis showed that two types of carbon deposits, graphite particle clusters and filaments, were observed on pure Ni and Ni‐Cu alloys with Cu contents of up to 5 wt%. Alloys with more than 10 wt% Cu showed very little coking, forming filaments only. SEM and TEM analyses revealed metal particles encapsulated by graphite shells within the graphite particle clusters, and metal particles at filament tips or embedded along their lengths. A kinetic investigation showed that alloy dusting rates decreased significantly with increasing copper levels up to 10 wt%. At copper concentrations of more than 20 wt%, the rate of metal dusting was negligible. Although pure copper is not catalytic to carbon formation, scattered carbon nanotubes were observed on its surface. The effect of copper on alloy dusting rates is attributed to a dilution effect.     

Quantitative analyses of dissolution and precipitation kinetics of θ-Al2Cu phase in an Al-6.2Si-2.9Cu alloy using electrical resistivity

The dissolution kinetics of the θ phase during solution treatment and isothermal precipitation kinetics during aging in an Al-6.2Si-2.9Cu alloy were quantitatively examined through electrical resistivity, hardness, and transmission electron microscopy (TEM). The electrical resistivity successfully predicted that pre-existing θ particles in the as-cast Al-Si-Cu alloy were fully dissolved after 4.5 h at 500 °C, which was in a good agreement with that directly observed by TEM. The electrical resistivity immediately decreased during aging at 250 °C due to the precipitation of θ′ phase and was saturated within 2 h. A maximum hardness peak appeared in 1 h at 250 °C due to the formation of a metastable θ′ phase, and then gradually decreased due to the coarsening of the θ′ phase as well as the annihilation of the θ″ phase. The θ′ phase was finally transformed to θ phase after 3.5 h at 250 °C.     

Precipitation location of secondary phase and microstructural evolution during static recrystallization of as-cast Ti-25V-15Cr-0.3Si titanium alloy

The microstructural evolution and precipitation location of the secondary phase of an as-cast Ti-25V-15Cr-0.3Si titanium alloy were investigated via isothermal compression experiments and heat treatment. The average aspect (length-to-width) ratio, average area and size of the grains at different heat treatment temperatures and holding time were analyzed and the effects of deformation and annealing time on the grain area and size were considered. It was found that the grain size was strongly influenced by the height reduction and holding time. Grain growth was significant when annealing time increased from 10 min to 2 h at 950 °C and height reduction of 30%; however, grain growth was minimal at annealing time between 2 and 4 h. Many dispersion particles were observed to form in continuous chains; the precipitation location was confirmed to be along initial grain boundaries, and the dispersion particles were identified to be Ti₅Si₃ phase by TEM.     

Investigation of an as‐sprayed NiCoCrAlY overlay coating – microstructure and evolution of the coating

A low pressure plasma sprayed (LPPS) NiCoCrAlY aircraft turbine blade overlay coating was investigated in the as‐sprayed condition by X‐ray diffraction (XRD) and analytical transmission electron microscopic (TEM) techniques. γ‐Ni, β‐NiAl and γ′‐Ni₃Al phases were identified by XRD showing predominant γ phase at the expense of β phase if compared to the fully processed state with γ and β at about equal portions. Besides grains of γ‐Ni and β‐NiAl, amorphous metallic grains and Cr‐rich oxides were found by TEM. Close to the surface to the atmosphere γ′‐Ni₃Al phase was localized in composite grains with an off‐plane oriented γ‐γ′ fibrous eutectic structure. They were neighbored by amorphous metallic grains. The evolution of these structures is discussed within the scope of crystal growth behavior. The dominant occurrence of face‐centered cubic at the expense of body‐centered phases observed in as‐sprayed coatings is attributed to their higher growth rates on quenching. Also benefiting from rapid growth the γ‐γ′ fibrous eutectic grains will form, hereby relying on essentially binary phase compositions of highly extended solubility ranges. Accumulation of alloy constituents like Cr and Ti by segregation to the growth front is considered to pave the way for the evolution of amorphous grains at the coating surface on top of γ‐γ′ grains. The potentials of the microstructures in service are addressed.     

Structural stability and the alloying effect of TiB polymorphs in TiAl alloys

Borides have been widely used in cast TiAl alloy for grain refinement and a variety of stoichiometry and crystal structure of borides were reported. Here the effects of alloying elements Nb, Ta, and Mn on the structural stability of fine boride precipitates in TiAl alloys have been studied combining transmission electron microscopy (TEM) and first-principles calculations. The results show that most boride particles have the TiB stoichiometry. In the alloy containing Nb and Mn, all the TiB particles have the B27 structure and are highly enriched with Nb but depleted with Mn. In the alloy containing Nb and Ta, however, the intergrowth of B_f and B27 structure has been observed, and the TiB particles are enriched with both Nb and Ta. First-principles calculations reveal different effects of Nb, Ta, and Mn on the structural stability of TiB polymorphs. Nb stabilizes B27 but destabilizes B_f. Ta strongly stabilizes both B27 and B_f structures. Mn strongly destabilizes both B27 and B_f structures.     

Al9Fe0.7Ni1.3相对喷射沉积Al-Zn-Mg-Cu-Ni合金组织和性能的影响

本文利用喷射沉积技术合成含Ni的Al-Zn-Mg-Cu合金,合金中的Ni元素以亚微米球状Al₉Fe_0.7Ni_1.3化合物的形式存在。利用扫描电镜和电子背散射衍射、透射电镜以及拉伸测试研究了Al₉Fe_0.7Ni_1.3颗粒对合金固溶处理后组织和性能的影响。结果发现：Al₉Fe_0.7Ni_1.3颗粒主要在晶界附近分布,说明该颗粒在固溶过程中具有有力的抑制再结晶作用。固溶处理后,合金的拉伸强度为603 MPa,断裂延伸率为11.79%,主要断裂方式为穿晶延性断裂。实验结果表明亚微米球状Al₉Fe_0.7Ni_1.3化合物对合金性能有重要影响,可以产生细晶强化和Orowan强化,是合金发生穿晶延性断裂的主要原因。     

Localized corrosion of 2024 T351 aluminium alloy in chloride media

The corrosion behaviour of 2024 T351 alloy was studied in chloride solutions. The polarization curve of this alloy showed two breakdown potentials. The first was related to the dissolution of the coarse intermetallic Al₂CuMg particles. A severe Mg dealloying of the particles resulted in Cu-rich remnants. The matrix surrounding the particles was systematically found to be dissolved. Indeed, the particles are surrounded by a dispersoid-free zone which is anodic in comparison to both the rest of the matrix and the S phase particles. The second breakdown potential corresponded to the matrix breakdown potential. Pits but also intergranular corrosion developed. Grain boundaries were preferentially attacked because they are anodic in comparison to the grain body and the previous dissolution of the intergranular precipitates made them fragile.     

Solidification structures and phase selection of iron-bearing eutectic particles in a DC-cast AA5182 alloy

The solidification structures of a commercial DC-cast AA5182 alloy have been studied through the thickness of a rolling ingot. The dendrite arm spacing, grain size, size of primary eutectic particles and macro segregation in the ingot have been measured. The types of the iron-bearing primary particles in the as-cast alloy have been identified by using electron microprobe and selected area electron diffraction pattern in TEM. It has been found, for the first time, that the dominant iron-bearing primary particles are Al_m(Fe,Mn) and Al₃(Fe,Mn) instead of Al₆(Fe,Mn) and Al₃(Fe,Mn). The evolution of the relative fraction of the Al_m(Fe,Mn) particles in the alloy through the thickness of the ingot has been measured. The phase selection principle for the iron-bearing primary particles in the alloy is discussed with reference to the phase diagram and growth kinetics of the different intermetallic phases.     

NiTi形状记忆合金的相变温度滞后

用透射电镜、正电子湮没和电阻测量,研究了NiTi形状记忆合金的组织结构与相变滞后的关系结果表明,经不同制度时效处理的组织,其相变温度滞后大小的顺序是:片状马氏体>R相>束状马氏体。Ti_(11)Ni_(14)相质点周围的共格应力场对这些相的可逆转变起障碍作用。正电子湮没Doppler展宽能谱S参数值与试样的温度滞后值之间存在线性关系,从而确认Ti_(11)Ni_(14)相析出的错配位错密度及由此而建立的晶体中弹性应力场分布是决定NiTi合金相变温度滞后的主要因素。     

Microstructural study on effects of C-alloying on Cu-Fe-P cast alloy

The effects of C-alloying on the microstructure and secondary phase particles of Cu-2.5Fe-0.1P cast alloy were investigated by transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), and image simulation. The mechanical and electrical properties were slightly improved, and grain refinement and fine γ-Fe particles were evident. Additionally, the Fe₃P particles were characterized, and abnormally grown Fe₃P particles were observed at the grain boundary in the non C-alloyed and C-alloyed specimens.     

HYSTERESIS OF PHASE TRANSFORMATION TEMPERATURE IN NiTi SHAPE MEMORY ALLOY

The relationship between structure and hysteresis of phase transformation temperature inNiTi shape memory alloy has been investigated by means of TEM observation,positron an-nihilation and electrical resistivity measurement.The sequence of hysteresis for the alloy agedunder different regimes was found to be:plate martensite>R-phase>tie-like martensite.The reversible displaeement of phase boundaries of these transformations is blocked by the co-herent stress field around Ti_(11)Ni_(14)phase particles.A linear relationship between S paramet-er of positron annihilation and maximum values of temperature hysteresis showed that themismatch dislocation and elastic stress field established by Ti_(11)Ni_(14)phase precipitation arethe main factor to determine the temperature hysteresis of phase transformation in NiTi shapememory alloy.     

Study on localized corrosion mechanism of 2195 Al‐Li alloy in 4.0% NaCl solution (pH 6.5) using a three‐electrode coupling system

Localized corrosion morphologies of 2195 Al‐Li alloy with various heat treatment in 4.0% NaCl solution (pH 6.5) were investigated, and its corrosion mechanism was studied using a three‐electrode coupling system of α (Al) substituting for the precipitate‐free zone (PFZ), simulated bulk θ′ (Al₂Cu) and T1 (Al₂CuLi). θ′ acts as cathodic zone in the alloy. At the initial stage, T1 phase is active with respect to θ′ and α (Al), and endures the main anodic current, indicating that anodic dissolution occurs on T1. However, its potential moves to positive direction with immersion time, due to dealloying of Li from T1. As a result, the main anodic dissolution occurs on α (Al) at a later stage. At this stage, as only T1 and α (Al) are coupled, T1 is cathodic to α (Al). In real 2195 alloy, T1 phase is very tiny, and anodic dissolution of T1 and PFZ occurs alternately. These results show that its intergranular corrosion or intersubgranular corrosion is caused by alternate anodic dissolution of T1 phase and PFZ along grain and subgrain boundaries.     

TiC strengthened CoCrNi medium entropy alloy: Dissolution and precipitation of TiC and its effect on microstructure and performance

In order to improve the strength and corrosion resistance of CoCrNi medium entropy alloy, TiC strengthened CoCrNi medium entropy alloy (CoCrNi/(TiC)_x (x=0.1, 0.2, 0.4)) was designed by addition of different amounts of TiC. The effects of TiC content on the microstructure, mechanical properties, and corrosion resistance of the alloy were investigated. It was found that the precipitation morphologies of TiC changed from lamellar eutectic to needle structure with the increase of TiC content, and finally formed mixed needled and bulk TiC particles. TiC appears as a dissolution?precipitation phenomenon in the CoCrNi alloy, which is important for the mechanical properties and corrosion resistance of the CoCrNi/(TiC)_x alloy. The strength of alloy was enhanced obviously after the addition of TiC. The compressive yield strength of CoCrNi/(TiC)_0.4 alloy reached 746 MPa, much larger than that of the CoCrNi medium entropy alloy, 108 MPa. Additionally, the addition of TiC was found to improve the corrosion resistance of CoCrNi medium entropy alloy in the salt solution.     

Solidification of Mg-28%Zn-2%Y alloy involving icosahedral quasicrystal phase

Applying XRD, DTA, SEM and TEM techniques, an investigation on the solidification microstructure and solidification sequence of Mg-rich Mg-28%Zn-2%Y （mole fraction） alloy was carried out. It is found that, a-Mg dendrites, Mg7Zn3 phase and icosahedral quasicrystal phase coexist in the as-solidified alloy, where the icosahedral quasicrystal, whose structure is indentified to be a face-centered type, originates from a peritectic reaction occurring at 416 ℃. The primary phase of this peritectic reaction has the composition of Mg20Zn66Y14, which is coincident with the H phase reported by TSAI as （Zn, Mg）5Y. Furthermore, the single I-phase grain morphology was observed and its growth evolution was also discussed.