Redistribution Of Alloying Elements Research Articles

Study of Welding Metallurgy, Alloying Element Redistribution, and Alloy Solidification in Hastelloy B-2 Superalloy Under Pulsating Current Waveform Gas Tungsten Arc Welding

Study of Welding Metallurgy, Alloying Element Redistribution, and Alloy Solidification in Hastelloy B-2 Superalloy Under Pulsating Current Waveform Gas Tungsten Arc Welding

Deposition Mechanism, Structure and Corrosion Protection of ZrO2-Based Conversion Coatings on AA6060 Primary and Recycled Aluminum Alloys

Chemical conversion coatings (CC) are amongst the most common surface pretreatments for aluminium alloys before further organic coating steps. The presence of a conversion layer can increase the resistance towards underpaint corrosion such as filiform corrosion (FFC). Here we explore the deposition mechanism, structure and protective properties against FFC of a ZrO2-based CC on AA6060 aluminium alloy by means of electrochemical techniques as open circuit potential measurements, potentiodynamic polarization and dynamic electrochemical impedance spectroscopy (dEIS). Surface composition and elemental distribution are characterized by X-ray photoelectron spectroscopy (XPS), Glow Discharge Optical Emission Spectroscopy (GD-OES) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The variation of important parameters such as charge transfer resistance and capacitance during the conversion layer deposition gives insight into the deposition mechanism. GD-OES and XPS shed light into the structure and composition of the formed CC layer constituted of a mixture of Zr (hydr)oxides based products, redistribution of alloy elements and the nature of deposited additives. Finally, FFC tests reveal a comparable performance between primary and recycled alloys.

Depth profiling analysis of the nitriding layer formed by gas nitriding of Ti13Nb13Zr alloy

The gas nitriding of Ti13Nb13Zr alloy was performed in a pure nitrogen atmosphere at 1200 °C with different holding times. The variations in the phase constituents, microstructure, and mechanical properties of the nitriding layer with depth were characterized by SEM, XRD, and nanoindentation. Depending on the phase distributions in depth, the nitriding layer could be sequentially divided into an external nitride layer mainly consisting of TiN, an internal nitride layer consisting of TiN, Ti2N and TiN0.3, and an N diffusion region consisting of TiN0.3. The mechanical properties were closely associated with the phase constituents, where the nanohardness of the internal nitride layer, internal nitride layer N diffusion region and substrate were about 17.126±0.399, 12.120±0.386, 5.627±1.080, and 3.632±0.116 GPa, respectively. In the nitriding process, the N diffusion region containing needle-like TiN0.3 precipitates formed in the initial nitriding stage, and then the precipitates were gradually converted to the external and internal nitride layers, whose thickness increased with the nitriding time. Furthermore, the influence of the alloying element redistribution on the N diffusion mechanism was discussed.

Nanoscale redistribution of alloying elements in high-burnup AXIOM-2 (X2®) and their effects on in-reactor corrosion

A comprehensive characterization study was carried out on 1 and 4 cycles X2®, aiming to reveal effect of irradiation-induced alloying element redistribution on the in-reactor corrosion kinetics. Using a combination of (scanning) transmission electron microscopy ((S)/TEM) and atom probe tomography (APT), the results strongly evidenced the existence of Nb-rich native precipitates and irradiation-induced platelets (IIPs)/nanoclusters in both metal and oxide. In addition, the suboxide may reject Nb content back into the metal matrix to achieve a thermodynamic equilibrium state. On the other hand, Fe-rich transition cracks were observed for the first time, along with Fe-rich oxide grain boundaries.

Effects of Cooling Rate on the Solidification and Microstructure of Nickel-Based Superalloy GTD222.

In this work, the microstructure and solidification behavior of nickel-based superalloy GTD222 at different cooling rates are studied. The solidification of the superalloy GTD222 proceeds as follows: L → L + γ, L → L + γ + MC, L → L + (γ/γ′)-Eutectic and L → η phase. Due to alloying element redistribution, the temperature of the solidus GTD222 superalloy, 1310 °C, is slightly lower than the temperature of the liquidus, which is 1360 °C. It was found that the dendrite arm spacing of the alloy decreased with the increase of the cooling rate from 200 μm at 2.5 K/min to 100 μm at 20 K/min.

Modeling of grain-boundary diffusion under nonstationary heating conditions

Grain boundary diffusion appreciably affects the physicomechanical and chemical properties of constructional materials. Experimental studies show that transfer processes run more intensively in materials with a larger area of internal boundaries. For these materials, the temperature required for diffusion activation is reduced. Much attention is paid to grain-boundary diffusion in such areas as materials science, physics and chemistry of metals, and metal science. However, there are practically no published works where the diffusion along the grain boundaries and phases under non-isothermal conditions has been studied. This work presents a two-dimensional model of alloying element redistribution from the amorphous coating into the substrate. The substrate is represented by alternating grains with a clear selection of the triple junction. The areas adjacent to the grain boundaries are clearly selected in the model and have a finite thickness. Different ratios between the grain sizes and the boundaries widths relate to diffusion in micro- and nanocrystalline materials. The redistribution of the alloying element is initiated by one or several thermal pulses associated with the action of the electron beam. It is taken into account when the heat part of the problem has been formulated that the typical scales of the heat and diffusion processes are essentially different. The diffusion problem takes into account the temperature dependence of the diffusion coefficients in the volume of grains and along the boundaries between them. The problem was solved numerically. Varying the parameters of the model, it was found by that the ratio of activation energies in the phases has the greatest influence on the diffusant distribution. Pulsed treatment compared to treatment with constant heating results in slower leads to a slow down of diffusion along the grain boundaries. The simulation results are qualitatively consistent with the data found in the literature.

Irradiation damage and hardening in pure Zr and Zr-Nb alloys at 573 K from self-ion irradiation

Zr-ion irradiation was performed on pure Zr, Zr-2.5Nb, and Zr-5Nb to a dose of 0.5 or 5 dpa at 573 K. The irradiation induced defects, alloying element redistribution, precipitation, and hardening were analyzed by Transmission Electron Microscopy and nanoindentation. In all the samples, irradiation induced the formation of ⟨a⟩ and ⟨c⟩ component loops in the α phase, and ½〈111〉 and 〈100〉 loops in the β phase. The presence of Nb in the α phase enhanced the formation of ⟨a⟩ component loops. As irradiation dose increased from 0.5 to 5 dpa, there is an apparent increase of the ⟨c⟩ component loop fraction with a corresponding decrease in ⟨a⟩ loops in the α phase. The Nb and Fe were observed to be redistributed from β to α phase. Irradiation significantly refined the size and increased the density of ω precipitates compared to thermal treatment alone. Irradiation caused noticeable hardening; compared with the defects in the α and β phase, the ω precipitates had a much more significant effect on hardening. This study suggests that decreasing the fraction of β phase in Zr-Nb alloys reduces the irradiation hardening.

Effect of Pulsed Helium Ion Fluxes and Helium Plasma on the Inconel 718 Alloy

The results of the irradiation of the Inconel 718 alloy with pulsed helium ion and helium plasma fluxes at a power density q = 107 W/cm2 and a pulse duration τ ≈ 100 ns in the Vikhr Plasma Focus setup are presented. The surface layer is not melted under the irradiation conditions. However, a slight increase in q causes melting of local regions in the surface and the formation of a wavy relief. Beam–plasma irradiation results in structural and phase changes in the irradiated surface layer, namely, the precipitation of microinclusions (complex niobium carbides), a redistribution of alloy elements, a slight decrease in the microhardness, and, accordingly, slight softening. These changes in the microstructure and the properties are determined by the melting of the irradiated surface in local regions, partial sputtering of solid-phase regions, and recrystallization in the near-surface layer during pulsed heating for each beam–plasma action.

Precipitate Stability in a Zr–2.5Nb–0.5Cu Alloy under Heavy Ion Irradiation

The stability of precipitates in Zr–2.5Nb–0.5Cu alloy under heavy ion irradiation from 100 °C to 500 °C was investigated by quantitative Chemi-STEM EDS analysis. Irradiation results in the crystalline to amorphous transformation of Zr2Cu between 200 °C and 300 °C, but the β–Nb remains crystalline at all temperatures. The precipitates are found to be more stable in starting structures with multiple boundaries than in coarse grain structures. There is an apparent increase of the precipitate size and a redistribution of the alloying element in certain starting microstructures, while a similar size change or alloying element redistribution is not detected or only detected at a much higher temperature in other starting microstructures after irradiation.

Impact of Mo on the ωo phase in β-solidifying TiAl alloys: An experimental and computational approach

Intermetallic structural materials based on TiAl have gained industrial importance in recent years. Especially β-solidifying alloys, such as the TNM alloy, have emerged, as they provide balanced mechanical properties attained via robust manufacturing routes. However, through the incorporation of the essential β-stabilizing elements additional phases, such as the ωo phase, are introduced, which predominantly precipitate within the βo phase.In this study the impact of the strong β-stabilizing element Mo on the formation mechanism of the ωo phase with particular emphasis on the prevailing elemental redistribution is investigated using differently heat-treated material conditions of a TNM alloy. To this end, scanning and transmission electron microscopy as well as atom probe tomography are employed. These methods provide evidence that ωo phase particles nucleate initially within the βo phase without significant diffusional alloying element redistribution. In contrast, the growth sequence is governed by diffusion, i.e. the alloying element Mo is rejected from the ωo particles and piles up in the βo phase as visualized and quantified by atom probe tomography. In order to understand the physical origin of the Mo redistribution phenomena, ab initio calculations are conducted. Calculations of the energies of formation reveal a thermodynamic destabilization of the ωo phase in the presence of dissolved Mo, while the βo phase is stabilized energetically. Supporting analysis of the valence charge density distribution shows an increase of TiMo bond directionality for the incorporation of Mo into the ωo structure in comparison to a Mo-free ωo phase.

Increasing of Durability of Surfaces by Plasma Nitriding Process

This article deals with properties of plasma nitrided layers in depth. The redistribution of alloy elements and C-N in depth of nitrided layers was analysed. The plasma nitriding process was used for creation of nitrided layers [1]. The nitrided layers were applied to 32CrMoV12-10 steel used for stressed machine parts. Samples were subsequently evaluated by metallographic, OES, hardness and microhardness methods. The results of measurement showed connection between chemical composition of alloying elements after chemical-heat treated process and universal hardness, microhardness.

Evolution of the Structural-Phase State of a Ti–Al– V–Mo Alloy During Severe Plastic Deformation and SubSequent Annealing

The effect of the initial phase composition of a Ti–Al–V–Mo alloy (VT16 according to Russian classification) on the evolution of its structural-phase state during the formation of ultrafine-grained structure and subsequent annealing is investigated by methods of optical and transmission electron microscopy and x-ray diffraction analysis. The structure is produced by cyclic pressing with a change of the deformation axis in each cycle combined with a gradual decrease of the pressing temperature from 1073 to 723 K. As this takes place, α″ → α + β and β → α phase transitions are found to develop in the test alloy. The phase state of the ultrafinegrained material thus produced depends for the most part on its elemental composition and severe plastic deformation regime. Annealing below the recrystallization temperature is shown to give rise to a β→α phase transition and alloying element redistribution. The foregoing processes allow for retaining a high level of the strength properties of the alloy.

Metastable phases in Zr-Excel alloy and their stability under heavy ion (Kr2+) irradiation

Zr-Excel alloy (Zr-3.5Sn-0.8Nb-0.8Mo, wt.%) has been proposed as a candidate material of pressure tubes in the CANDU-SCWR design. It is a dual-phase alloy containing primary hcp α-Zr and metastable bcc β-Zr. Metastable hexagonal ω-Zr phase could form in β-Zr as a result of aging during the processing of the tube. A synchrotron X-ray study was employed to study the lattice properties of the metastable phases in as-received Zr-Excel pressure tube material. In situ heavy ion (1 MeV Kr2+) irradiations were carried out at 200 °C and 450 °C to emulate the stability of the metastable phase under a reactor environment. Quantitative Chemi-STEM EDS analysis was conducted on both un-irradiated and irradiated samples to investigate alloying element redistribution induced by heavy ion irradiation. It was found that no decomposition of β-Zr was observed under irradiation at both 200 °C and 450 °C. However, ω-Zr particles experienced shape changes and shrinkage associated with enrichment of Fe at the β/ω interface during 200 °C irradiation but not at 450 °C. There is a noticeable increase in the level of Fe in the α matrix after irradiation at both 200 °C and 450 °C. The concentrations of Nb, Mo and Fe are increased in the ω phase but decreased in the β phase at 200 °C. The stability of metastable phases under heavy ion irradiation associated with elemental redistribution is discussed.

Behaviour of master alloy during sintering of PM steels: redistribution and dimensional variations

The addition of alloying elements in low alloyed PM steels in the form of a master alloy gives the advantage of introducing oxidation sensitive but less expensive elements and also allows manipulation in composition adjustment to achieve desired properties. In this work, interrupted sintering trials of the Fe–2MA–0.5C (%) (MA = Cu based master alloy) are performed. The behaviour of the liquid forming master alloy, for instance in terms of liquid phase formation, alloying element redistribution and effect on the dimensional changes, is investigated. The results show that master alloy particles melt over a range of temperature, which is also supported by the thermodynamic calculations. The low swelling in the master alloy system, compared to a reference system of Fe–2Cu–0.5C, is attributed to the progressive melting of the master alloy. The mean diffusion distance of Cu in Fe at the interparticle boundaries is 5.8 μm after 34 min of isothermal holding.

Atomic and nanoscale chemical and structural changes in quenched and tempered 4340 steel

Atom probe tomography and transmission electron microscopy (TEM) have been used to determine the location and distribution of carbon and alloying elements associated with the complex structural changes that occur at the atomic and nanoscale in 4340 steel after quenching to martensite and tempering at 325, 450 or 575°C. Tempering at 325°C resulted in carbide formation without partitioning of chromium, manganese, molybdenum, aluminum, nickel or phosphorus, but with early-stage silicon rejection from the carbide. TEM verified the presence of cementite and the Bagaryatsky orientation relationship with the tempered martensite matrix and detected complex precipitate structures. Tempering at 450 or 575°C developed concentrations of all alloying elements at ferrite–cementite interfaces: chromium, manganese and molybdenum partitioned into the cementite, and silicon, aluminum, nickel and phosphorus were clearly rejected from the cementite. These results provide direct evidence for staged cementite growth, where early-stage growth likely occurs under paraequilibrium conditions, followed by initial silicon redistribution and subsequent alloying element redistribution during late-stage growth. Tempering at 575°C induced spheroidization of the cementite, loss of the Bagaryatsky orientation relationship, and phosphorus concentrations at Cottrell atmospheres within the cementite and at ferrite–cementite interfaces, correlating with early observations of the retardation of spheroidization by phosphorus.

Progress in the understanding of Ni silicide formation for advancedMOSstructures

Metallic silicides have been used as contact materials on source/drain and gate in metal‐oxide semiconductor (MOS) structure for 40 years. Since the 65 nm technology node, NiSi is the preferred material for contact in microelectronic due to low resistivity, low thermal budget, and low Si consumption. Ni(Pt)Si with 10 at.% Pt is currently employed in recent technologies since Pt allows to stabilize NiSi at high temperature. The presence of Pt and the very low thickness (<10 nm) needed for the device contacts bring new concerns for actual devices. In this work,in situtechniques [X‐ray diffraction (XRD), X‐ray reflectivity (XRR), sheet resistance, differential scanning calorimetry (DSC)] were combined with atom probe tomography (APT) to study the formation mechanisms as well as the redistribution of dopants and alloy elements (Pt, Pd…) during the silicide formation. Phenomena like nucleation, lateral growth, interfacial reaction, diffusion, precipitation, and transient phase formation are investigated. The effect of alloy elements (Pt, Pd…) and dopants (As, B…) as well as stress and defects induced by the confinement in devices on the silicide formation mechanism and alloying element redistribution is examined. In particular APT has been performed for the three‐dimensional (3D) analysis of MOSFET at the atomic scale. The advances in the understanding of the mechanisms of formation and redistribution are discussed.

Control of Carbides and Graphite in Cast Irons Type Alloy’s Microstructures for Hot Strip Mills

The carbide and graphite formation and redistribution of alloy elements during solidification were investigated on high-speed steel (HS) and Ni-hard type cast irons with Nb and V. The crystallization of hypereutectic HSS proceeds in the order of primary MC, γ + MC, γ + M6C, γ + M7C3, and γ + graphite eutectic, in hypoeutectic alloys proceeds in the order of primary γ, γ + MC, γ + graphite, γ + M6C, and γ + M7C3 eutectic, and in Ni-hard proceeds in the order of primary γ, γ + MC, γ + M3C, and γ + graphite eutectic. The γ + graphite eutectic solidifies with the decrease of V, Nb, and Cr and the increase of Si and C contents in residual liquid during solidification. The behavior in graphite forming tendency in the residual liquid is estimated by the parameter ∑CLimi′. The eutectic graphite crystallizes at the solid fraction when ∑CLimi′ takes a minimum value. The amount of graphite increases with the decrease in ∑CLimi′ of initial alloy content in both specimens. Inoculation with ferrosilicon effectively increases the graphite content in both specimens.

Influence of the microstructure and alloying element on the polarization behaviour within the crevice of UNS S32304 duplex stainless steel

The influence of microstructure evolution and alloying element redistribution of UNS S32304 duplex stainless steel induced by annealing treatment on the crevice corrosion behaviour was studied. As the annealing temperature was increased from 1030 to 1150 °C, the crevice corrosion resistance was decreased and the active peaks were not only increased in magnitude but also shifted towards the more noble direction. Austenite and ferrite have greatly different polarization behaviour within the crevice. This study provides guidance to the material design and usage in industry field in consideration of different polarization behaviour induced by the evolution of microstructure and alloying elements.

Redistribution of Alloy Elements during Nickel Silicide Formation: Benefit of Atom Probe Tomography

The unique capabilities of atom probe tomography (APT) to characterize internal interfaces and layer chemistry with sub-nanometer scale resolution in three dimensions have been recently opened up to materials with poor electrical conductivity by the use of ultrafast laser pulses. The progress in sample preparation (focused ion beam) as well as in instrument performance enable now the analysis of relatively large volumes with typical diameters of 100 to 200 nm and depths of several hundred nm (this corresponds to an increase by several order of magnitude compared to the former instrument) of site specific samples. In this work, APT is used to study the effects of Pt on the formation and stability of Ni silicides. The precise location of this alloy element has been determined at the nanometer scale: In particular, APT allows us to quantify the amount of Pt in the grain boundaries (GB) of Ni2Si for about 100 different grain boundaries and thus to better characterize the GB diffusion and segregation.

Research on microstructure and alloy phases of AM50 magnesium alloy

Abstract Comprehensive researches on the microstructure and alloy phases of AM50 magnesium alloy of different status, varying from casting to solid-solution treated and aged ones, were carried out in the present paper. Special attention was drawn to Al–Mn phases and their effects on electrochemical properties of the alloy. The results indicated that Al–Mn phases were the major alloy phases in as-cast, solid-solution treated and aged AM50 alloys. They were quite heat-resistant and almost kept unchanged in terms of morphology, distribution and amount. The majority of Mg 17 Al 12 phase in as-cast alloy could be reduced when the temperature of solid-solution treatment was up to 410 °C and it precipitated as reinforcing phase during aging treatment. According to the microstructure and the corresponding microhardness analysis, it was believed that the strengthening mechanism of AM50 might not be limited to precipitation reinforcement; some other factors such as substructure and re-distribution of alloy elements might play critical roles too. Electrochemical experiments further showed that Al–Mn phases were detrimental to the corrosion resistance of the alloy in question, especially when Al-rich α-Mg and β-Mg 17 Al 12 phases were eliminated.