Diffusion Of Alloying Elements Research Articles

Effect of grain size on high-temperature corrosion performance of laser cladding inconel 625 coating

As the calorific value of waste-to-energy incineration continues to rise, the corrosion damage to boilers becomes increasingly severe. The impact of the grain size on the high-temperature corrosion resistance of Inconel 625 coating was investigated. The results indicated that the corrosion performance of the coating in 1:1 NaCl–KCl salts at 700 °C for 120 h was highly dependent on the grain size of the coating. The coating fabricated with a laser power of 1800 W and an overlapping distance of 1.5 mm had an average grain size of 13.7 μm and highest hardness, exhibiting the best corrosion performance, with a corrosion weight loss of 1.4 mg/cm2. Coatings processed under different conditions all formed a three-layered structure, with a NiO outermost layer, a Cr2O3 intermediate layer, and an innermost Cr-depletion zone. Coatings with finer grains formed a continuous and dense protective oxide layer of NiO and Cr2O3 due to the rapid diffusion of alloy elements, effectively preventing the invasion of corrosive media and protecting the substrate from corrosion.

Pulsed Magnetic Field Treatment Effects on Undissolved Carbides in Continuous Casting Billets of GCr15 Bearing Steel

The study investigates the effect of pulsed magnetic fields on undissolved carbides in high-carbon chromium bearing steel GCr15 billets. The billets were subjected to heat treatment at 950 °C, with a pulsed magnetic field of varying durations applied during the process. The influence of the pulsed magnetic field on the distribution of undissolved carbides within the billets was investigated, and the thermodynamic and kinetic mechanisms of undissolved carbides dissolution were explored. The results indicate that the area percentage of undissolved carbides in the microstructure decreases from 1.68% to 0.06% after applying a pulsed magnetic field for 10 min, and the size of undissolved carbides decreases from 17.5 μm to 4.9 μm. When a pulsed magnetic field is applied for 30 min, all undissolved carbides dissolve. The statistics demonstrate that the average size of undissolved carbides is reduced from 14.19 μm to 0.63 μm, with a reduction percentage reaching 96%. Over the same duration, the number density of the undissolved carbides decreases from (0.19~0.55)/mm2 to (0.03~0.1)/mm2, and the percentage area of the undissolved carbides decreases from (1.26~1.68)% to (0~0.02)%. Thermodynamically, applying a pulsed magnetic field lowers the dissolution energy barrier of undissolved carbides and modifies their transformation temperature. Kinetically, the rate of alloy element diffusion is enhanced by increasing the frequency of atomic jumps. This research aims to provide new insights into enhancing the contact fatigue life of bearing steel, increasing the proportion of special steel, and optimizing the steel deep-processing process.

Martensite evolution in additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V-0.7Nb alloy under different annealing temperatures

Based on classical nucleation and growth theory, the phase evolution of martensite α′ phase of additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V-0.7Nb alloy under different annealing temperatures (600–960 °C) was systematically investigated. The results show that: Owing to insufficient diffusion of alloy elements during laser deposition, numerous β-stabilizing elements are retained in α′ phase and the microstructure is made up of 95.4% α′ phase and 4.6% β phase. According to TEM, EBSD, SEM/EDX results, the α′ phase can be divided into α′ phase containing more β-stabilizing elements (α′β), α′ phase containing more Al (α′Al). Under annealing condition, martensite decomposition tends to occur at the α′/α' interfaces and the matrix of α′β phase. In the matrix of α′β phase, fine adjacent α′ phase precipitates at β phase because a lot of β-stabilizing elements are consumed by precipitated β phase, thus improving the precipitation of adjacent α′ phase. The decomposition of α′ phase is controlled by diffusion of alloy elements. As the temperature increases, the diffusion of alloy elements and degree of α′ martensitic decomposition become more sufficient. When the Al content in the α′ phase reaches that in equilibrium α phase, the lattice parameters of α′ phase increase from a = 2.97 Å, c = 4.72 Å to a = 3.04 Å, c = 4.77 Å and meanwhile the α′ martensitic decomposition is finished.

Acceleration of Spheroidization in Low‐Density Ultrafine Pearlitic Steel by κ‐Carbide

The impact of κ‐carbide on the spheroidization process of low‐density ultrafine pearlitic steel (with 5 wt% Al and 5 wt% Mn additions) is investigated through isothermal annealing spheroidization. In the findings, it is demonstrated that κ‐carbides can significantly refine the lamellar spacing of low‐density ultrafine pearlite during the isothermal transformation. Additionally, under identical conditions, κ‐pearlite exhibits a higher number of ferrite/κ‐carbide interfaces. In the first stage of pearlite spheroidization, an abundance of dislocations and sub‐grain boundaries facilitate rapid fusion of the lamellar pearlite, while in the second stage, they provide a high‐speed diffusion pathway for carbon diffusion. Moreover, the reduced lamellar spacing shortens the distance for carbon and alloying elements diffusion, thereby promoting granular κ‐carbide formation and accelerating spheroidization in low‐density ultra‐fine pearlite.

Industrially produced 2.4 GPa ultra-strong steel via nanoscale dual-precipitates co-configuration

An ultra-high strength martensitic steel, strengthened by the dual-precipitation of NiAl and M2C nano-particles, has been successfully developed and produced on an industrial scale. It has achieved a remarkable tensile strength of 2467 MPa and exhibited ductility approaching 10%. The precipitation configurations of NiAl and M2C nano-particles during ageing at 510 °C for durations ranging from 0.5 to 10 h were characterized using atom probe tomography. The results indicate that the preferential precipitation of NiAl, facilitated by minimal lattice misfit and low interfacial energy, promotes the uniform nucleation of M2C particles within the matrix while restricting their growth. As a result, both types of precipitates maintain a high number density. With increasing ageing time, the distribution density of the precipitates decreases, their size increases and the increase in yield strength gradually decreases. Furthermore, the addition of element cobalt effectively reduces the diffusivity of other major alloying elements, synergistically inhibiting the growth of NiAl and M2C particles. The dual precipitates co-configuration provides a substantial hardening effect (1046 MPa at 510 °C for 220 min) compared to the individual hardening effect of M2C observed in AerMet100 steel.

Development of Multicomponent Nanostructured Nitride Coatings to Protect against Corrosion Products from Titanium Alloy

Phase-structural characteristics and the corrosion resistance of coatings ZrN, (Zr,Ti)N, (Zr,Hf)N, (Zr,Nb)N, (Ti,Zr,Hf)N and (Ti,Zr,Nb)N, which were deposited on a Ti6Al-4V titanium alloy substrate, were investigated. It was found that the titanium substrate has a crystalline structure, including grains with high (up to 24 at.%) and low (less than 2 at.%) vanadium content. Thus, during the deposition process, the coating can form adhesive bonds with local areas of the substrate that have quite different compositions. The diffusion of the coating elements into the substrate takes place up to a depth of 200 nm. The diffusion of titanium alloy elements (primarily titanium and vanadium) into the adhesive sublayer of the coating to a depth of 100 nm is also observed. Corrosion studies were carried out in 1M solutions with acidic (H2SO4), alkaline (NaOH) and neutral (NaCl) media at a constant temperature of 50 °C. The actual change in the mass of the samples during corrosion tests is extremely small. The protective coatings under study have very high anti-corrosion characteristics and practically do not react with solutions that imitate the liquid environments of the human body.

Study on the process and microstructure of Fe34Mn13.5Al3Cr7.5Ni alloy prepared by spark plasma sintering based on elemental powder

FeMnAlCrNi shape memory alloy exhibits constant superelasticity over a broad temperature range, which has application prospect in aerospace field. In this paper, Fe34Mn13.5Al3Cr7.5Ni superelastic alloy was prepared by spark plasma sintering, and the effects of pre-sintering and sintering process parameters, element diffusion process, and superelastic properties of the alloy were investigated. The orthogonal experiment was conducted to determine the optimal parameters and laws revealed that the compactness degree and hardness of this sintered alloy reached 99.7 % and 291HV, respectively, which were treated by a process of 620 ℃ and 15 min pre-sintering temperature and time, and final sintering temperature of 1000 ℃ under the sintering pressure 45 MPa. In FeMnAlCrNi alloy with multiple elements and significant differences in their melting points, Al element with low melting point will affect the diffusion and microstructure of alloy elements during sintering. The results show that Al element was in a liquid state during the sintering process, which helps to improve the density of the alloy. The remaining Al element in the alloy generated a small amount of pores due to the Kirkendall effect during heat treatment. After heat treatment, the superelastic recovery of the alloy was 0.46 % under 3 % compression.

Characterization of microstructural evolution with pre-strain in BG801 bearing steel: Grain, carbides, retained austenite and martensite

The method of multiple cryogenic and tempering treatments significantly limits the microstructure regulation of BG801 bearing steel. In this work, the pre-strain was introduced in the vacuum-melted BG801 steel combing with heat treatment (HT) to adjust the microstructures. The samples were subjected to uniaxial tensile at room temperature with different strains (engineering strain of 0, 0.15, 0.175, 0.2), then heating to 1323 K and water-cooling. The results showed that higher pre-strain was beneficial to obtain finer prior austenite grain (PAG) and martensite grains, and a low fraction of retained austenite (RA) in BG801 steel. With increasing pre-strain, the PAG was elongated along the RD direction (rolling direction) without a change in size. The recrystallized grains with a size of about 5–6 μm nucleated inside the PAG during HT. Meanwhile, the high-density dislocations were introduced in the tensile samples by pre-strain, promoting the growth of micron-M6C by accelerating the diffusion of alloy elements during HT. And the evolution of RA and martensite indicated that the pre-strain accelerates transformation from austenite to martensite by transferring elements from the matrix to M6C and introducing shear bands into the matrix to promote the nucleation of martensite.

Electronic and strain-elimination effects of solute–vacancy interaction in molybdenum

The interaction between a solute and vacancy is crucial for the diffusion and precipitation of alloy elements and ultimately affects the creep resistance and safety of refractory molybdenum (Mo) alloys. In this work, the first-principles calculations of the interaction between solutes (5 light impurity atoms and 23 transition metal elements) and vacancies show that Mo is sensitive to all the light impurity elements, which indicates that these solutes are easy to combine with an Mo alloy and have relatively strong interaction. In most cases, the interaction between the 23 transition metal solutes and vacancy is attractive, but the interaction between solutes V, Ti, and the first-nearest vacancy is repulsive, and the interaction between the early elements in the 3d, 4d, and 5d series and the second-nearest vacancy is also repulsive. Then, we discuss the main factors that affect the solute–vacancy interaction, that is, the effects of electron and strain-elimination. Finally, a solute–vacancy interaction database based on the above effects is established, which would contribute to the research on heat- and radiation-resistant Mo alloys.

Investigation of Hafnium Oxide Containing Zirconium in the Scaled Region on the Surface of As-Cast Nickel-Based Single Crystal Superalloy Turbine Blades

Surface scale is usually formed in the aerofoil part of as-cast nickel-based single crystal turbine blades by the strong interaction between the mould wall and the melt, and the subsequent oxidation of the fresh metallic surface of the casting. For better understanding of the scaling, the scaled region was investigated, and an interesting region containing hafnium oxides and a rhenium-rich particle was found. Generally, a continuous aluminium oxide layer was detected on the outer surface of the base material and covered the surface of an unscaled region. In contrast, there was no oxide on the surface of a scaled region, but it was replaced by several tiny particles remaining locally on the outer surface of the base material. SEM-EDX and TEM-EDX point analysis of these particles indicated not only the existence of high amounts of hafnium, but also several particles such as hafnium oxide, aluminium oxide, and even tiny metallic particles. Most of all, STEM-EDX point analysis clearly detected zirconium in the hafnium oxide. Furthermore, a rhenium-rich particle was also detected towards the outer surface of the base material, which suggested that the surface of the scaled region might be exposed to high enough temperatures to allow the diffusion of heavy alloying elements. Based on the observation, the formation mechanism of hafnium oxide containing zirconium and its meaning was discussed.

Segregation kinetics of immiscible alloying elements for understanding phase separation in multicomponent alloys

The diffusion and segregation rates of immiscible alloying elements in Cu-based alloys with one (V) or five refractory metals (V, Nb, Mo, Ta and W) at 300 to 500 °C were investigated for understanding the kinetics of phase separation in multicomponent alloys. At a comparable grain size and a similar influence of grain boundary diffusion (more dominant at 300 °C), the summed rate constant of segregation of the five alloying elements was higher than that of the one element, attributable to the larger entropy change for vacancy formation and atom exchange. The rate constant and time exponent of segregation depended on the melting temperature of the alloying element (W > Ta > Mo > Nb > V), while the activation energy of diffusion, 5.1 to 12.3 kJ/mole (W < Mo < V < Nb < Ta), was inversely related to the delta mixing enthalpy (to Cu) or electronegativity.

The role of copper in microstructure and toughness of intercritically reheated coarse grained heat affected zone in a high strength low alloy steel

This paper presents an investigation on the role of Cu addition in microstructure and toughness of intercritically reheated coarse grained heat affected zone (ICCGHAZ), aiming at elucidating the influencing mechanism of Cu addition on toughness in terms of reverted transformation, alloying element diffusion and variant selection. The element of Cu has a significant effect on reverted structure and Cu-rich precipitates by influencing the diffusion of alloying elements and variant selection. On the one hand, the addition of Cu changed the diffusion kinetics of Ni and Mn, decreasing the segregation of elements at grain boundaries. Accordingly, the reverted structure at grain boundary with an adverse effect on toughness was reduced. Moreover, Cu addition changed the feature of CP group during variant selection, and thus increased the high angle grain boundary density. On the other hand, Cu-rich precipitates were increased after Cu addition, which was easier to originate crack. But, the ductile character of the reverted structure would overwhelm the harmful effect of Cu-rich precipitates on the toughness. As a result, a higher low temperature toughness with impact energy of 72.8 J at −40 °C was achieved in 1.5Cu steel than that in 0Cu steel (36.3 J).

Uneven hardness mechanism and related fracture characteristics in laser-MIG hybrid welded Al–Mg–Si alloy butt

In this paper, a 6082-T6 high-strength aluminum alloy with 5 mm thickness was taken as the research object. The TRUMP Trudisk 10002 disc laser and the Transpils synergic 4000 welder were used to weld it by laser-Metal-Inert-Gas composite welding. The property transition and uneven hardness of the base metal (BM), the heat affected zone (HAZ), and the partially melting zone (PMZ) were analyzed by a transmission electron microscope, a scanning electron microscope, and an energy dispersive spectrometer. The results showed that: grain coarsening was not the main reason for softening in the HAZ but the evolution of precipitates is the main reason. Precipitates annexation in HAZ was the main reason for hardness decrease in the HAZ. Solid diffusion occurred in PMZ, and the enrichment of main strengthening elements at grain boundary played a positive role in hardness recovery. A large number of spherical GP zones and the β″ phase had been found in the PMZ, which played a major role in strengthening and increased the hardness of the PMZ. The tensile results showed that the BM had the highest elongation and tensile strength. The strength and plasticity of the HAZ were lower than those of the BM. A lamellar second phase with lamellar tearing was found at the bottom of the fracture in HAZ. The softening fracture morphology was different from that of the BM. This paper mainly focuses on the softening of HAZ of welded joints and the hardness bump in PMZ. The distribution of precipitates, grain size, alloy element diffusion distribution, and mechanical properties were analyzed. The uneven hardness behavior of the joint and its influence on mechanical properties were further revealed.

Multilayer Maraging/CoCrNi Composites With Synergistic Strengthening-Toughening Behavior

A novel multilayer maraging/CoCrNi composite with good mechanical properties was successfully fabricated by a vacuum hot-rolling and aging treatment. The yield strength, tensile strength, uniform elongation, and fracture elongation reached 1,151, 1,380 MPa, 15.7, and 24% respectively, realizing the aim of synergistic strengthening–toughening by effectively improving the yield strength of the CoCrNi alloy and strain-hardening capacity of the maraging steel. The vacuum state, high rolling reduction ratio, and alloy element diffusion are beneficial in strengthening the clad interface. The good work-hardening capacity of the CoCrNi alloy compensates for the poor strain-softening behavior of the maraging steel, effectively delaying the premature localized necking of the multilayer composites. The strengthening–toughening mechanism of the multilayer maraging/CoCrNi composites is mainly attributed to the strong interface, nanoscale precipitation, and strain-induced twinning.

Eliminating segregation defects during additive manufacturing of high strength β-titanium alloys

Segregation defects, known as β-flecks, have long detracted from the widespread use of metastable β-Titanium alloys produced via solidification routes. These defects form when segregated β-stabilising solute such as chromium create localised phase instabilities which is detrimental to the alloy’s aging response and mechanical properties. This work explores whether Additive Manufacturing is a viable process for preventing the formation of these defects in the Beta-C alloy (Ti-3Al-8V-6Cr-4Mo-4Zr). Although it is observed that β-flecks initially form during deposition of the top layer, each new subsequent layer and thermal cycle promotes solid-state diffusion of alloy elements that effectively heals the segregation defect. Additive Manufacturing is thus a promising pathway for producing high quality metastable β-titanium alloys free of this defect.

Effect of External Factors on Diffusion Acceleration Mechanism During Steel Microarc Surface Impregnation

The considerable duration of steel chemical heat treatment (CHT) processes makes it necessary to intensify them. For this purpose, it is advisable to use microarc surface impregnation that provides acceleration of CHT in all stages. During microarc alloying steel products are immersed in a metal container filled with coal powder, followed by passage of an electric current in the circuit. Microarc discharges occur within the powder and are concentrated around the product with formation of a microarc halo and rapid heating to the diffusion saturation temperature. When the coal powder is heated a saturated atmosphere based on carbon monoxide is formed within the container that leads to steel product carburization. Preliminary application of a coating containing a diffusant to the surface provides preparation of multi-component coatings. This technology provides acceleration of CHT in all stages. The surface being treated is affected by microarc discharges that lead to accelerated formation of active diffusant atoms and ions and their movement towards a treated surface. On transfer into the metal there is an effect of electrotransport whose operating factors are a force for action of an electric field on the diffusant ions, and the strength of the interaction of atoms and diffusant ions with directional flow of conduction electrons that drives them in the direction of their own movement (“electron wind”) as well as heat transfer, whose impact is described by a vacancy mechanism (Wirts model), the mechanism of entrainment of ions by electrons under the action of a temperature gradient, as well as the mechanism of diffusing atom attraction by phonons of the crystal lattice (“phonon wind”). Experimental studies are conducted for the intensity of diffusion flow depending on the direction of the electric current for various alloying elements, as well as a calculated estimate of the values of these forces. It is found that the main factor determining intensification of diffusion saturation during microarc alloying is the “electron wind” strength. Calculated values of alloying element diffusion coefficients for various versions of microarc heating correspond to the scheme proposed for acting forces.

The Effect of Ni Interlayer on the Hot-Rolled and Quenched Stainless Steel Clad Plate.

The vacuum hot-rolled SUS314/Q235 stainless steel clad plate has many drawbacks including serious interface alloy element diffusion, stainless steel cladding’s sensitization, and carbon steel substrate’s low strength. In this study, the comprehensive properties were systematically adjusted by changing the thickness of the Ni interlayer (0, 100, 200 μm) and the quenching temperature (1000~1150 °C). The results showed that the Ni interlayer can obviously hinder the diffusion of carbon element, so as to achieve the purpose of eliminating the decarburized layer and reducing the carbon content of the carburized layer. Meanwhile, the perfect metallurgical bonding between the substrate and cladding can be obtained, effectively improving the stainless steel clad plate’s tensile shear strength and comprehensive mechanical properties, and significantly reduce the brittleness of the carburized layer. As the quenching temperature increases, the grains coarsening of carbon steel and stainless steel became more and more serious, and the sensitization phenomenon and the thickness of the carburized layer are gradually decreased. The stainless steel clad plate (Ni layer thickness of 100 μm) quenched at 1050 °C had the best comprehensive mechanical properties. Herein, the interface shear strength, tensile strength and the fracture elongation reached 360.5 MPa, 867 MPa and 16.10%, respectively, achieving strengthening and toughening aim. This is attributed to the disappearance of the sensitization phenomenon, the grain refinement and the lower interface residual stress.

Microstructure evolution and improved properties of laminated titanium matrix composites with gradient equiaxed grains

With the purpose of improving both the strength and ductility, gradient equiaxed grains were successfully achieved in the matrix of the laminated TiB/Ti-TiB/Ti-6.58Al-1.76Zr-1.04V-0.89Mo composite via water quenching (WQ) and thermal compressing deformation. Gradient equiaxed grains varied from approximately 1.0 µm in TiB/Ti-6.58Al-1.76Zr-1.04V-0.89Mo layer to 5.5 µm in TiB/Ti layer. The formation of the gradient structure was related to the alloying elements diffusion during the initial sintering process, and the equiaxed shape was constructed by dynamic recrystallization during thermal compressing. WQ treatment before thermal compressing was adopted to obtain fine lamellar structure, which promoted the segmentation of a lamellae, and accelerated the dynamic recrystallization process. Raising the quenching temperature can increase the proportion of equiaxed grains in the composite, which improved both the bending strength and ductility. Compared with the as-sintered specimen, the specimen with gradient equiaxed grains exhibited nearly 30% enhancement in flexural strength (from 1719 to 2218 MPa), and the ultimate bending fracture strain was increased from 7.0% to 17.2%. This significant improvement should be attributed to the coordination deformation by interface gradient grains, the grain refinement strengthening and the good balance between strength and ductility of the recrystallized equiaxed grains.

Evaluation of microstructure and mechanical properties of Fe–1.2Mn–0.3Cr–1.4Ni–0.4Mo–C steel welded joints

The microstructural variations in different sub-regions of the Fe–1.2Mn–0.3Cr–1.4Ni–0.4Mo-C steel welded joint were investigated by means of optical microscope and scanning electron microscope equipped with electron backscattered diffraction. This study focused on the correlation between microstructural characteristics and mechanical properties of the welded joint which was carried out by gas metal arc welding. The microhardness of the fusion line (FL) and fine-grained heat affected zone was inferior to other locations of welded joint due to the diffusion of alloy elements and granular bainite phase transformation, respectively. The tensile strength of the welded joint was about 980 MPa, which reached the required strength of the 1000 MPa grade steel plate for hydropower station. The final fracture was located at the weld metal (WM) because that the crack easily nucleated and propagated in proeutectoid ferrite. The impact toughness at −60 °C of various sampling locations for welded joint was higher than 47 J. The low temperature impact toughness improved gradually as the distance between the V-notch and weld center line increased. The inclusions in the WM and non-uniformity of microstructure and composition near the FL could increase the deterioration on impact toughness at −60 °C.

Microstructure evolution and stress rupture properties of K4750 alloys with various B contents during long-term aging

The relationship among B content, microstructure evolution and stress rupture properties of K4750 alloy during long-term aging were investigated. After aging at 800 ℃ for 1000 h, the decomposition degree of MC carbides of K4750 alloys with 0B, 0.007 wt. % B and 0.010 wt. % B were basically identical, which indicated that B has no inhibition on MC carbide decomposition during long-term aging. The MC carbide decomposition was accompanied by the formation of M23C6 carbides and a small number of η phases, which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from the γ matrix. In addition, M23C6 carbides in boron-free alloy were in continuous chain and needle-like η phases were precipitated near them, while M23C6 carbides in boron-containing alloys remained in granular distribution and no η phases precipitation around them. Adding B could delay the agglomeration and coarsening of M23C6 carbides during long-term aging, which was because the segregation of B at grain boundary retarded the diffusion of alloy elements, thus weakened the local fluctuation of chemical composition near grain boundary. The stress rupture samples of K4750 alloys with various B contents after aging at 800 ℃ for 1000 h were tested at 750 ℃/380 MPa. The results indicated that the stress rupture properties of boron-containing alloys were significantly better than that of boron-free alloy, which could be attributed to the increase of grain boundary cohesion strength and the optimization of M23C6 carbide distribution due to the addition of B.