Segregation Of Alloying Elements Research Articles

Effect of aging temperature on mechanical properties of TC21 alloy with multi-level lamellar microstructure

This paper aims to reveal the effect of aging temperature on strength, impact toughness and fracture toughness of TC21 alloy with multi-level lamellar microstructure. The multi-level lamellar microstructures of TC21 alloy were prepared by solid solution at 1000 °C, annealing at 800 °C and respectively aging at 550 °C, 600 °C and 650 °C. The multi-level structures include β grains, α colony (αc), primary α phase (αp) and aged α plates (αs). The microstructures were performed on using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), laser scanning confocal microscope (LSCM) and image analysis software. Results show that as aging temperature goes up, the size of αs increases and the segregation of alloy elements increases, while the β grain, grain boundary α phase (Gα) and αc almost remain unchanged. Hall-Petch formula is used to reveal that αs phase is the minimum control unit of strength. Fine grain strengthening, volume fraction of α phase (Vα) and element segregation have great influence on mechanical properties. In the impact experiment, microstructure deformation and secondary cracks consume more energy, while cleavage fracture consumes less energy. The TEM analysis found that the coordination of microstructure deformation became worse after aging. Fracture analysis shows that the size of αc plays an important role in crack propagation, and the crack propagation path always tends to cut αc vertically and pass through it. Whether the size of αc is large or small, it is not conducive to the increase of crack propagation path. Crack propagation path length and the interface energy consumed by different fracture mechanisms should be taken into consideration when there are more crack propagation paths and the fracture toughness decreases in the multi-level lamellar microstructure of TC21 alloy.

Effect of surface segregation on the oxidation resistance of Cu3Pt(100)

Alloying element segregation often occurs under a reactive environment but its interplay with the subsequent surface oxidation of the alloy remains unclear. Using synchrotron-based ambient-pressure x-ray photoelectron spectroscopy, we dynamically monitor the surface segregation in ${\mathrm{Cu}}_{3}\mathrm{Pt}(100)$ in response to temperature and oxygen gas. Vacuum annealing leads to surface segregation of Cu along with the enrichment of Pt in the subsurface region. Upon switching to the ${\mathrm{O}}_{2}$ atmosphere, dissociative chemisorption of oxygen does not change the surface segregation profile from that under the vacuum annealing condition. A stepwise increase in the oxygen pressure results in the transformation pathway of $\mathrm{Cu}\ensuremath{\rightarrow}{\mathrm{Cu}}_{2}\mathrm{O}\ensuremath{\rightarrow}\mathrm{CuO}$, in which the selective oxidation of Cu gives rise to further accumulation of Pt underneath the oxide/alloy interface that hinders the supply of Cu from the bulk to the oxide/alloy interface, thereby leading to the termination of the surface oxidation after the ${\mathrm{Cu}}_{2}\mathrm{O}\ensuremath{\rightarrow}\mathrm{CuO}$ conversion is completed. This differs from the transformation pathway of $\mathrm{Cu}\ensuremath{\rightarrow}{\mathrm{Cu}}_{2}\mathrm{O}\ensuremath{\rightarrow}{\mathrm{Cu}}_{2}\mathrm{O}$/CuO for the oxidation of pure Cu and Cu-Au alloys, in which the oxidation of Cu continues and the ${\mathrm{Cu}}_{2}\mathrm{O}$/CuO bilayer growth is constantly maintained. These key differences provide useful insight into alloy design for controlling the surface properties such as corrosion resistance and catalytic performance of Cu base alloys.

Electrochemical co-reduction of Y(III) and Al(III) in a fluoride molten salt system and electrolytic preparation of Y–Al intermediate alloys

In this study, a molten salt co-reduction method was proposed for preparing Y–Al intermediate alloys and the electrochemical co-reduction behaviors of Y(III) and Al(III) and the reaction mechanism of intermetallic compound formation were investigated by transient electrochemical techniques. The results show that the reduction of Y(III) at the Mo electrode is a reversible electrochemical process with a single-step transfer of three electrons, which is controlled by the mass transfer rate. The diffusion coefficient of Y(III) in the fluoride salt at a temperature of 1323 K is 5.0238 × 10−3 cm2/s. Moreover, the thermodynamic properties associated with the formation of Y–Al intermetallic compounds were estimated using a steady-state electrochemical method. Y–Al intermediate alloy containing 92 wt% yttrium was prepared by constant current electrolysis at 1323 K in the LiF–YF3–AlF3–Y2O3 (6 wt%)–Al2O3 (1 wt%) system at a cathodic current density of 8 A/cm2 for 2 h. The Y–Al intermediate alloy is mainly composed of α-Y2Al and Y phases. The development and application of this innovative technology have solved major technical problems, such as a long production process, high energy consumption, and serious segregation of alloy elements at this stage.

Effect of electromagnetic stirring on metallurgical quality of GCr15 electroslag ingot

To further improve the metallurgical quality of electroslag remelting, remove the large inclusions in electroslag ingot and refine the solidification structure of electroslag ingot, an electroslag remelting furnace with electromagnetic stirring was designed, and the influence of different magnetic induction intensities on metallurgical quality of GCr15 electroslag ingot was studied. Inclusions with different sizes and types were analyzed by an ASPEX scanning electron microscope, and the morphology and composition of inclusions were further observed by a JSM-6510LV scanning electron microscope. The distribution of alloying elements on the cross section of electroslag ingot was analyzed by original position analysis (OPA). The results show that whether or not electromagnetic stirring is used, the inclusions in electroslag ingot are mainly composed of Al2O3, MnS, MnS-oxide and other oxides, among which Al2O3 is the most. Compared with an electroslag ingot without electromagnetic stirring, the number of inclusions decreases considerably, and the proportion of small inclusions increases while the proportion of large inclusions decreases when the electromagnetic stirring with remelting current of 1.1 kA and magnetic induction intensity of 62 Gs is applied. However, excessive electromagnetic force will cause the number and diameter of inclusions to increase again. Electromagnetic stirring has different effects on the segregation of different elements. Under the experimental conditions, the weak electromagnetic force with 1.1 kA and magnetic induction intensity of 108 Gs has little effect on the segregation of C, but decreases the segregation of P, and the excessive electromagnetic force aggravates the segregation of alloy elements.

Primary microstructure characterization of Co-20Ni-9Al-7W-3Re-2ti superalloy

The characterization of the primary microstructure of the new Co-based superalloy of Co-20Ni-9Al-7W-3Re-2Ti type was shown in this article. The investigated alloy was manufactured by induction melting process from pure feedstock materials. The fundamental technological problem related to Co-Al-W-X multicomponent alloys casting process is a strong susceptibility to interdendritic segregation of alloying elements, especially tungsten and rhenium. The performed analysis revealed that the observed effect of alloying elements segregation was detectable and much stronger than for Co-9Al-9W and Co-20Ni-7Al-7W alloys, related to titanium, nickel, and aluminium migration to inter-dendritic spaces. Consequently, the tungsten concentration gradient between dendritic and interdendritic zones was higher than for Co-9Al-9W and Co- 20Ni-7Al-7W alloys. The same situation was in the case of rhenium and cobalt, but Co concentration in the interdendritic zone was only slightly lower.

In-situ TEM investigations of dislocation loop annealing kinetics in neutron-irradiated 9%Cr RAFM steel

Post-irradiation annealing (PIA) is beneficial in recovering degraded mechanical properties of irradiated materials due to the evolution of irradiation-induced defects. To understand the underlying responsible mechanisms, isothermal PIA experiments were carried out in this work on a neutron-irradiated 9%Cr reduced-activation ferritic-martensitic steel (EUROFER97) using in-situ transmission electron microscopy. In general, PIA at 600 °C led to an appreciable reduction of the dislocation loop density and increment in the loops mean size. This is found mainly associated with the vacancy-mediated loops shrinkage phenomenon. The driving force for this process is loop's curvature force. Nevertheless, the spontaneous evolution of the loops neighboring microstructure substantially alters their annealing kinetics. Therefore, in this study, loops’ annealing kinetics in EUROFER97 is discussed in terms of various scenarios originating in terms of the presence or absence of the local external sources and sinks. In addition, influence of loops type, alloying elements segregation and free surface are also discussed.

The Influence of the Cooling Bores on Crystal Orientation and Lattice Parameter in Single-Crystalline Cored Turbine Blades.

The areas located near the cooling bores of single-crystalline cored turbine blades made of nickel-based CMSX-4 superalloy were studied. The blades were solidified by the vertical Bridgman technique in the industrial ALD furnace. Longitudinal sections of the blades were studied by Scanning Electron Microscopy, X-ray diffraction topography, X-ray diffraction measurements of the γ′-phase lattice parameter a, and the α angle of the primary crystal orientation. The local changes in α were analyzed in relation to the changes of the dendrite’s growth direction near the cooling bores. It was found that in the area approximately 3 ÷ 4 mm wide around the cooling bores, changes of α and a, both in the blade root and in the airfoil occurred. The local temperature distribution near the cooling bores formed a curved macroscopic solidification front, which caused changes in the chemical composition and, consequently, changes in the a value in a range of 0.002 Å to 0.014 Å. The mechanism of alloying elements segregation by tips of the dendrites on the bent solidification front was proposed. The multi-scale analysis that allows determining a relation between processes proceed both on a millimeter-scale and a micrometric and nanometric scale, was applied in the studies.

Microstructure and corrosion behaviour of wire arc additive manufactured AA2024 alloy thin wall structure

In the present work, the microstructure and corrosion behaviour of wire arc additive manufactured (WAAM) AA2024 alloy thin wall structure are investigated. Three distinctive zones are present in the WAAM structure, including melt pool zone (MPZ), melt pool border (MPB) and heat affected zone (HAZ). Localized corrosion initiates at the multi-phase lamellar eutectic structure via de-alloying of S-phase (Al2CuMg) intermetallics. MPB is most susceptible to localized corrosion due to its relatively high volume fraction of eutectic S-phase resulted from alloying elements segregation during solidification.

Evolution of Soft Zone in a Simulated 1.25Cr-0.5Mo Steel-Welded Joint During Post-weld Heat Treatment

In the present study, the effect of post-weld heat treatment on the soft zone of 1.25Cr-0.5Mo steel welded joint has been systematically studied. Outstanding features of the soft zone, including grain size, number density of carbides, volume fraction of carbides, and hardness distribution, are quantitatively analyzed by means of scanning electron microscopy and a microhardness testing. Results show that ferrite grain growth, carbide coarsening, and alloying element segregation are main factors accounting for hardness variations.

GTAW Welded Inconel 625 Alloy Fuel Cladding for the Canadian SCWR: Microstructure and Mechanical Property Characterization

Abstract Inconel 625 is considered one of the candidate materials for reactor fuel cladding in the Canadian supercritical water reactor (SCWR) design. Gas tungsten arc welding (GTAW) is being evaluated as a joining technique for SCWR fuel cladding since this method is widely used to join components in the power and nuclear industry. During the GTAW process, the welding thermal cycle produces different types of microstructures in both the heat-affected zone (HAZ) and fusion zone (FZ) that affect the material's mechanical properties. A series of welding experiments at various weld conditions were performed using an automatic GTAW orbital process on Inconel 625 alloy tubing. Simple analytical heat conduction and grain growth models were developed to predict weld temperature profiles and metallurgical transformations. Weld characterization included mechanical tests, optical microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) elemental analysis, and microhardness measurements. Weld microstructural characterization revealed that a characteristic dendritic structure was formed in the FZ, while the HAZ exhibited larger equiaxed grains than those found in the base material (BM). SEM-EDS analysis showed no distinct alloying element segregation in both the HAZ and FZ. Welds produced with heat inputs of about 3.00 J/mm3 presented similar mechanical properties as those observed in the BM. In these welds, grain growth was homogenously minimized in the FZ. It is concluded that the effective welding heat input control can optimize the weld microstructure and the weld mechanical properties in Inconel 625 tubing used as Canadian SCWR reactor fuel cladding.

Effect of Cr/Mn segregation on pearlite—martensite banded structure of high carbon bearing steel

The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling. With the use of an optical microscope, scanning electron microscope, transmission electron microscope, and electron probe microanalyzer, the segregation characteristics of alloying elements in cast billet and their relationship with hot-rolled plate banded structure were revealed. The formation causes of an abnormal banded structure and the elimination methods were analyzed. Results indicate the serious positive segregation of C, Cr, and Mn alloy elements in the billet. Even distribution of Cr/Mn elements could not be achieved after 10 h of heat preservation at 1200°C, and the spacing of the element aggregation area increased, but the segregation index of alloy elements decreased. Obvious alloying element segregation characteristics are present in the banded structure of the hot-rolled plate. This distinct white band is composed of martensitic phases. The formation of this abnormal pearlite—martensite banded structure is due to the interaction between the undercooled austenite transformation behavior of hot-rolled metal and the segregation of its alloying elements. Under the air cooling after rolling, controlling the segregation index of alloy elements can reduce or eliminate the abnormal banded structure.

Effects of cooling method and elements segregation on the martensite-pearlite banded structure of high carbon bearing steel

In this study, the effects of the cooling method and alloying element segregation on the abnormal zonal structure of high-carbon bearing steel were investigated by reheating, controlled rolling, and controlled cooling of the sample. The microstructure and alloy segregation characteristics of the abnormal strip structure in the hot-rolled plate of the steel were revealed by optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron probe x-ray microanalysis. The JMatPro thermodynamic software was used to examine the relationship between the alloy elements and banded structure defects of the hot-rolled sheet and analyse the causes of the defects and their elimination methods. The results show distinct positive segregation of Cr/Mn and negative segregation of C in martensite–pearlite banded structure defects in hot-rolled plates. The martensite–pearlite banded structure is formed by the interaction between the cooling rate and the segregation of alloying elements after hot rolling. Studies have found that segmented cooling (slow cooling–air cooling) can eliminate the martensite–pearlite banded structure.

Influence of aging heat treatment on microstructure and tensile properties of laser oscillating welded TB8 titanium alloy joints

Investigations were conducted on TB8 titanium alloy to assess the influence of aging heat treatment on microstructure and phase transformations induced by laser oscillating welding, and establish a correlation between tensile properties and microstructure. Low-temperature aging joint consists of ω phase and β phase, while high-temperature aging joint is comprised of α phase and β phase. The formation of poor and rich precipitation zone in weld area was due to segregation of major alloying elements like Al, Mo and Nb. Low-temperature aging deteriorates plasticity of joints, but high-temperature aging improved ambient-/elevated-temperature tensile properties. The effect of aging temperature and aging time were discussed in depth. After aging treatment at 450° Celsius for 1 h, intermediate ω phase in gathering place transformed to α phase, but dispersed ω phase still retained. With the increase of aging temperature and aging time, size of α precipitate increased. Note that there is a significant increase (85%) in tensile strength after high-temperature aging treatment and Portevin-Le Chatelier (PLC) effect can be avoided under thermo-mechanical loading conditions. It was found that after 1 h of aging treatment at 550° Celsius followed by air cooling, the welded joints owned the best tensile properties at both ambient and elevated temperature.

Adding chemical composition variable to traditional time–temperature-transformation (TTT) diagrams with an experimental cost-effective method

Time-temperature-transformation diagrams are the most important tools for studying the phase transformations kinetics in materials science/engineering. Despite the many benefits and applications, their use has some limitations. One of the major drawbacks is that, despite spending a lot of time/cost in drawing a TTT diagram, the information can only be used for a specific chemical composition. Therefore, the purpose of this work was to take into account simultaneously the effect of composition on these diagrams using the potential of alloying elements segregation during solidification. As such, 3-dimensional time–temperature-composition-transformation (TTCT) diagrams can be used to replace the simple TTT diagrams. Using such comprehensive diagrams, it is possible to study the influence of three parameters of temperature, time and composition, simultaneously, on phase transformation kinetics and to find the most appropriate conditions for manufacturing/heat treatment of different materials.

OPTIMISATION OF BALLISTIC PROPERTIES OF NANOSTRUCTURED BAINITIC STEEL PLATES FOR CONTAINER ARMOUR SYSTEM

Abstract: An innovative armour system containing plates made of nanostructured bainite steel is under development to improve operating properties of a light armoured observation-protective container (LAOC) providing the 2nd protection level according to STANAG 4569. Armour system solution of the container has been modified because the new plates have different mechanical and technological properties than currently used armour plates. The paper presents results of investigation of material characteristics and firing tests of plates made of nanostructured steel in the semi-industrial scale. To optimise parameters of heat treatment of the plates the kinetics of phase transformation was examined, and mechanical properties were measured, and changes in microstructure in the area of projectile interaction at firing tests were analysed for different variants of treatment. The reported investigations precede the industrial scale production process for plates made of the new grade steel designed for the armour system of LAOC. Optimisation of mechanical properties of the plates has been carried out by selection of such production parameters as chemical composition of the steel, hot working, interprocess heat treatment and temperature and time of final heat treatment (isothermal annealing). Effect of primary segregation of alloying and residual elements on protective properties of the plates is indicated. As a result of applying the annealing temperature in the range of 210-225°C (Ms temperature is ca. 200°C at cooling rate 1-2°C/s) and time in the range of 120-70 hours respectively, the following properties have been achieved: YS0.2 (yield strength) 1400-1500 MPa; UTS (ultimate tensile strength) 2000-2150 MPa; TE (total elongation) 10-13%, impact toughness KV at room temperature 10-16 J and hardness 590-610 HV (53-54 HRC). Microstructure of the plates consists of carbide free lathy nanobainite and 10-21% (volume fraction) of retained austenite. Plates of thickness 6.3 and 7.5 mm and ammunition type of 5.56x45 mm M193 and 7.62x51 mm API BZ have been used in the firing tests. Based on results of firing tests and microstructure examination in the area of projectile interaction the phenomena have been identified indicating high efficiency of ballistic protection of investigated plates, manifested by high ability to absorption and dissipation of projectile energy without susceptibility to cracking. Based on the results of mechanical properties measurements and firing tests a chemical composition of the nanostructured bainitic steel adjusted for industrial production of plates of thickness in the range of 6-8 mm has been developed. An analysis of container armour system mass reduction possibility, basing on results of investigation of the nano-structured bainite steel plates made in the semi-industrial scale, has been carried out. Basing on the analysis of the container construction and operational conditions some sensitive areas of ballistic protection have been specified to be subjected to firing tests. Ballistic examination will be carried out using the container segments representing the selected sensitive areas.

Effect of prior austenite grain boundaries on corrosion fatigue behaviors of E690 high strength low alloy steel in simulated marine atmosphere

A correlative method based on TEM, EBSD, and SKPFM was used to investigate the corrosion fatigue behaviors of E690 steel in simulated marine atmosphere. Results revealed E690 steel is susceptible to corrosion fatigue cracking in this environment and PAGBs play a vital role in crack initiation and propagation. Intergranular microcracks were prone to initiate along PAGBs probably due to high-angle grain boundaries and micro-galvanic effect between martensite-austenite (M-A) constitutes and adjacent ferrite. Segregation of alloy elements within M-A constitutes introduced Volta potential variation between M-A constitutes and adjacent ferrite, resulting in micro-galvanic corrosion between them and eventually intergranular crack initiation along PAGBs.

Alloying Element Segregation and Grain Boundary Reconstruction, Atomistic Modeling

Grain boundary (GB) segregation is an important phenomenon that affects many physical properties, as well as microstructure of polycrystals. The segregation of solute atoms on GBs and its effect on GB structure in Al were investigated using two approaches: First principles total energy calculations and the finite temperature large-scale atomistic modeling within hybrid MD/MC approach comprising molecular dynamics and Monte Carlo simulations. We show that the character of chemical bonding is essential in the solute–GB interaction, and that formation of directed quasi-covalent bonds between Si and Zn solutes and neighboring Al atoms causes a significant reconstruction of the GB structure involving a GB shear-migration coupling. For the solutes that are acceptors of electrons in the Al matrix and have a bigger atomic size (such as Mg), the preferred position is determined by the presence of extra volume at the GB and/or reduced number of the nearest neighbors; in this case, the symmetric GB keeps its structure. By using MD/MC approach, we found that GBs undergo significant structural reconstruction during segregation, which can involve the formation of single- or double-layer segregations, GB splitting, and coupled shear-migration, depending on the details of interatomic interactions.

First-principles study on the equilibrium shape of nanometer-sized body-centered cubic Cu precipitates in ferritic steels

The knowledge of thermodynamic parameters like interfacial energies is of great importance to understand microstructure evolution and the strengthening effect of nanometer-sized precipitates. While the strengthening of Cu particles has been widely applied in ferritic steels, the energy of Cu precipitate/Fe matrix interface and its orientation dependence have not been duly addressed. In the present study first-principles calculations were performed to obtain the interfacial energies of 13 different bcc-Cu/bcc-Fe interface orientations, based on which the equilibrium shape of Cu precipitates was explored by Wulff construction. The result seems to agree with essentially spherical morphology of Cu particles in the ferrite matrix, and could be used as reference in the study of alloying element segregation at coherent Cu/Fe interface and its influence on interfacial energy.

Remarkable improvement in resistance spot weldability of medium-Mn TRIP steel by paint-baking heat treatment

In this study, a remarkable improvement in resistance spot weldability of medium-Mn transformation-induced plasticity (MT) steel through a paint-baking heat treatment was addressed using dissimilar resistance spot weldments between MT and dual phase (DP) steels with various soaking temperatures. Experimental results showed that the cross-tension strength in the dissimilar MT/DP weldment increased up to 128% when the soaking temperature was more than 110 °C, and was at its highest at 210 °C. The improved cross-tension strength is attributed to the failure mode change from a pull-out to a partial pull-out failure. The brittle-to-ductile transition of martensitic matrix in coarse-grained heat-affected zone in MT steel was also considered to be a factor. Austenite reversion and carbide precipitation occurred in the martensitic matrix during the paint-baking heat treatment, whereas there was no significant change in alloying element segregation at the prior austenitic grain boundary. The driving force for the tempering was discussed based on the calculated carbon diffusion distance.

Segregation of the major alloying elements to Al3(Sc,Zr) precipitates in an Al–Zn–Mg–Cu–Sc–Zr alloy

Solute segregation of Zn, Mg and Cu in and around Al3(Sc,Zr) precipitates in an Al-Zn-Mg-Cu-Sc-Zr alloy was systematically studied using atom probe microscopy, aberration-corrected scanning transmission electron microscopy and first-principles simulations. Results show that the Al3(Sc,Zr) precipitates occur as a Sc-rich ‘core’ and Zr-rich ‘shell’ structure. Zn segregates to the Zr-rich shell of Al3(Sc,Zr) precipitates and substitute mainly for the Al atoms, and the Zn segregation intensifies with increasing ageing time. Mg and Cu atoms segregate to the Zr-rich shell during the early stages of ageing, while the Mg atoms prefer the Al-matrix with longer ageing time. Density functional theory calculations demonstrate that such segregation is energetically favored and highlight the diverse role of Al3(Sc,Zr) in influencing the distribution of the major alloying elements.