Main Precipitated Phase Research Articles

Effect of swinging laser on porosity, microstructure, and mechanical properties of laser welded aluminum alloy joints with different lap forms

Oscillating and conventional linear laser welding were conducted to enhance the bonding strength of dissimilar aluminum alloys. The effect of lap joint configuration on porosity, microstructure, and mechanical properties of laser-welded dissimilar aluminum alloy joints was investigated. It was found that the solidification rate of the melt pool was the dominant factor in the difference in porosity for oscillating laser-welded joints with different lap joint configurations. The main precipitated phase in welds with different lap joint configurations was eutectic Si, and the proportion of eutectic Si was higher in oscillating laser welds than in linear laser welds. The elemental distribution in oscillating laser welds was more uniform. The strengthening mechanism for tensile shear strength of oscillating laser welded lap joints was analyzed. The improved tensile shear strength and elongation were attributed to the reduced porosity and solid solution strengthening for the oscillating laser welding. The tensile strengths of the two lap types of oscillating laser welded joints increased by 25.1 MPa and 34.5 MPa, and the elongation increased by 0.7% and 4.5%.

Microstructure and mechanical properties of Al-Zn-Mg-Cu alloy fabricated by multi-wire arc-based directed energy deposition

ER5356, ER2319 and Zn wires were synchronously fed using a multi-wire gas metal arc directed energy deposition (DED) system. This allowed the successfully fabrication of Al-6.0Zn-2.5Mg-1.5Cu alloy. The microstructure and mechanical properties of the fabricated parts were studied. It is shown that the main precipitated phases are η and nanoscale η'. The upper and lower parts of the deposited parts are composed by coarse equiaxed grains and columnar dendrite structures, respectively. The average microhardness of the fabricated alloy was 118.5 HV. The ultimate tensile strength and elongations in the perpendicular to the building direction (BD) and parallel to the BD were 267 MPa and 2.7 %, 238 MPa and 2.3 %, respectively. The average tensile strength is higher than that of 7075-O alloy. The fracture mode of the samples composed brittle and ductile features, although the former dominated. This multi-wire arc DED approach provides a new choice for the in-situ synthesis of Al-Zn-Mg-Cu alloys.

The investigation on microstructure evolution and mechanical behavior of extruded Mg-1.5Al-xLa-Mn (x=0.5, 1, 2wt%) alloys

In this study, the microstructure evolution and mechanical behaviour of Mg-1.5Al-xwt%La-Mn (ALaM-x) (x = 0.5, 1, and 2) alloys with low-cost were investigated. Microstructure analysis of the cast alloys revealed that the Al11La3 was preferentially precipitated, followed by Al2La, and finally Mg12La, as the Al/La ratio decreased. Nonetheless, the main precipitated phase in the alloy was Al11La3 phase, and it gradually coarsened and the content increased, with the increase of La content. The fine rod-like Al11La3 was broken and refined after extrusion, while the coarse Al11La3 was not completely broken, and aggregated unevenly within Mg matrix. As a result, the grain size of the alloys gradually increased, and the strength gradually decreased. When adding 0.5 wt%La, compared with pure Mg, the ALaM-0.5 alloy exhibited finer grain microstructure with an average size of 1.83 μm and a grain refinement rate of 90%; the corresponding tensile yield strength (TYS) and ultimate tensile strength (UTS) are 262.5±3.0 MPa and 275.0±2.4 MPa, with an increase rate of 124.7% and 52.1%, respectively. The calculation and analysis of strengthening mechanism showed that the high strength of the alloy was mainly attributed to fine-grained strengthening with an accounting for up to 42%. Besides, the inverse relationship between the square root of the reciprocal of grain size (d−1/2) and strain hardening exponents (n) was found in this study, and the precipitates were found to delay the weakening trend of strain hardening capacity with decreasing grain size.

Evolution of the second phases in the weld seams of 2.25Cr-Mo-0.25 V steel in different heat treatment states

Abstract The evolution characteristics of the second phases in the weld seams of 2.25Cr-Mo-0.25 V steel in three states – welded, post-weld heat-treated and creep states – during manufacturing and service were evaluated by calculations and observations using optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction spectrometry. The results showed that the second phase was M 3C in the as-welded state. The types and quantities of the precipitated phases increased greatly after post-weld heat treatment (PWHT), and M 3C, M 2C, M 7C3, M 23C6 and MX were the main precipitated phases. During PWHT, the amount of M 7C3 decreased, the amounts of M 23C6 and MX increased, and the sizes of the second phases increased. The precipitates coarsened after long-term creep, and M 7C3, M 6C, M 23C6, M 2C, and MX were the main precipitate phases in this state. The size of the MX phase remained relatively stable during long-term creep, which contributed greatly to the creep resistance.

Study on multi-field composite strengthening, surface integrity, and microstructural evolution mechanism of 7075-T6 aluminum alloy

Multi-energy field composite reinforcement technology has become a critical technique for improving the service life of key components in aviation engines. This paper aims to investigate the strengthening mechanism of high-speed cutting and solid particle-entrained waterjet peening (HSC-WJP) composite reinforcement on 7075-T6 aluminum alloy. Samples subjected to single solid particle-entrained waterjet peening (WJP) reinforcement were selected as the control group. The surface quality and microstructure evolution of 7075-T6 aluminum alloy after composite reinforcement were examined using SEM, EDS, XRD, TEM, and HRTEM techniques. The research results indicate that, under the same jet parameters, HSC-WJP composite reinforcement is superior to single WJP reinforcement in terms of surface roughness, except when the jet pressure is below 25 MPa and the track spacing is 0.53. The surfaces of the 7075-T6 aluminum alloy workpieces after reinforcement are mainly characterized by the presence of pits and micro-pores. Surface roughness shows a positive correlation with jet pressure and nozzle traverse speed. After HSC-WJP composite reinforcement, the surface roughness decreases by 0.132 μm compared to single WJP reinforcement, and the size of surface pits is reduced by 4–20 μm. Surface roughness decreases first, then increases, and then decreases again with increasing target distance, while it increases first and then decreases with increasing path spacing. A significant precipitation-free zone (PFZ) is present in the 7075-T6 aluminum alloy after reinforcement, and its width is positively correlated with the nozzle traverse speed. When the nozzle traverse speed is 120 mm/min, the average size of the PFZ at grain boundaries after single WJP reinforcement is 12–20 nm, while it decreases by approximately 3 nm after HSC-WJP composite reinforcement. The main precipitate phase in the reinforced aluminum matrix is η′ phase, and under the same jet parameters, the HSC-WJP grain size is smaller, approximately 25–34 nm, and exhibits significant dislocation walls and dislocation tangles, with a higher dislocation density compared to single WJP reinforcement.

Analysis of Precipitation Phase of Alloy1.4957 Heat-Resistant Steel for Gasoline Turbocharger

Recently, strengthened environmental regulations have required the downsizing of gasoline engines, and as a result, demand for gasoline turbochargers has rapidly increased. The vane of a turbocharger controls the flow of gases toward the turbine, and it is manufactured by powder metallurgy due to its complex shape. Gasoline engines have a high exhaust gas temperature (~1000<sup>o</sup>C), and thus, Alloy1.4957 (GX15CrNiCo21-20-20) containing large amounts of Cr, Ni, and Co is used. In this study, Alloy1.4957 powders were sintered by hot isostatic pressing (HIP), and then homogenized and thermally exposed to exhaust gas temperatures. Then, a microstructural analysis was conducted to observe the changes that occurred for each process. M<sub>6</sub>X carbonitride containing Si, called Cr<sub>3</sub>Ni<sub>2</sub>SiX, was observed to be the main precipitate phase in this alloy. In general, it is known that Cr<sub>3</sub>Ni<sub>2</sub>SiX is only rarely observed in heat-resistant steel. However, in Alloy1.4957, a large amount of Cr<sub>3</sub>Ni<sub>2</sub>SiX was precipitated or dissolved depending on the process, and this is probably due to the high Si and N content of Alloy1.4957. In addition to Cr<sub>3</sub>Ni<sub>2</sub>SiX, Cr<sub>23</sub>C<sub>6</sub> and NbX were observed. Cr<sub>23</sub>C<sub>6</sub> was dissolved during the homogenization process, but NbX, which has high thermal stability, retained a fine size during the homogenization process and provided a nucleation site for Cr<sub>3</sub>Ni<sub>2</sub>SiX during thermal exposure.

The influence of porosity and precipitates on the corrosion behavior of A356 aluminum alloy

In this article, the pore and precipitation phases of A356 aluminum alloy were regulated by adding element Cu, and the corrosion kinetic behavior was investigated by electrochemical experiments and salt spray test (SST), while the corrosion mechanism of A356 aluminum alloy was studied by scanning electron microscopy (SEM), 3D laser confocal microscopy (LSCM) and atomic force microscopy (SKPFM). The results of both electrochemical and salt spray experiments showed that the corrosion rate increased with increasing Cu content, and the average corrosion rate increased from 0.040 mm y-1 to 0.329 mm y-1 when the Cu content increased from 0 wt% to 7 wt%, and the Cu free alloy exhibited a greater charge transfer resistance. This is mainly due to the addition of Cu on the one hand to increase the number of precipitation phase, Al2Cu become the main precipitation phase, SKPFM test results show that the precipitation phase relative to the Al matrix potential difference in order: Al2Cu (350 ∼ 380 mV), Si (250 ∼ 275 mV), Mg2Si (80 ∼ 110 mV), the increase in potential difference between the two phases to promote the corrosion of occurrence. On the other hand, CT results showed that the average diameter of pores increased from 89.95 µm to 281.00 µm, and the volume fraction increased from 0.05 % to 0.18 %, and the larger and more porous pores help promote corrosion. Therefore, how to control the porosity and precipitation phase is an important factor in regulating the corrosion behavior of A356 aluminum alloy.

Insights into the effect of γ′- or γ′′-precipitated phase types on resistance against harmful atoms of heterogeneous interface in nickel-based superalloys

To improve the service performances of nickel-based superalloys by controlling the type and proportion of precipitate phases, the resistances against harmful atoms (weakening effect caused by harmful atoms and capacity for hindering intrusion of harmful atoms) of the representative γ-matrix/γ′- or γ′′-precipitated phase heterogeneous interfaces (γ-Ni/γ′-Ni3Al, γ-Ni/γ′-Ni3Ti, and γ-Ni/γ′′-Ni3Nb interfaces) was investigated. The results show that, the harmful atoms (H, O, and S atoms) can be stable adsorbed on heterogeneous interfaces and weaken the interfacial bonding strength, resulting in the fracture position change from γ′- or γ′′ slab to interface during tensile deformation. Because of the minimum changes on interfacial bonding characteristics after the intrusion of harmful atoms, γ-Ni/γ′′-Ni3Nb heterogeneous interface (HIγ″−Ni3Nb) demonstrates the strongest resistance against weakening effect, presented as the smallest deterioration of interfacial ideal work of adhesion and tensile strength. Meanwhile, the energy barrier of harmful atoms diffusing along HIγ″−Ni3Nb is much larger than those along HIγ″−Ni3Al and HIγ″−Ni3Ti, leading to a smallest diffusion coefficient for harmful atoms diffusing along HIγ″−Ni3Nb. The result indicates HIγ″−Ni3Nb also shows the strongest capacity for hindering the intrusion of harmful atoms. This phenomenon is attributed to the strongest interaction between harmful atoms on saddle point and the neighboring atoms on HIγ″−Ni3Nb during the diffusion process. To summary, the Ni-based superalloys with γ′′-Ni3Nb as the main precipitated phase demonstrate the most promising properties in resistance against harmful atoms. This work not only contributes to the further improvement of Ni-based superalloys, but also provides the creative ideas for the research of other superalloys.

Effect of rejuvenation heat treatment on the microstructure and stress relaxation behavior of nickel-based superalloy with excess hardness

The repair of R26 high-temperature alloy bolts with excessive hardness after ∼60,000 h of service using rejuvenation heat treatment was evaluated. Stress relaxation tests with different initial stresses were performed on R26 superalloy before and after rejuvenation heat treatment. The effect of rejuvenation heat treatment on the microstructure of the R26 superalloy was evaluated through a combination of scanning electron microscopy, transmission electron microscopy, and evaluation of the Brinell hardness and stress relaxation behavior. The results showed that the main precipitated phases in the R26 superalloy are the γ’ phase, Ni(Cr32Mo9Fe3), TiC, and Mo23C6. Rejuvenation heat treatment can effectively reduce the hardness of the alloy by dissolving intergranular carbides. In the stress relaxation test, the γ’ phase effectively impeded dislocation movement, whereas Ni(Cr32Mo9Fe3), TiC, and Mo23C6 impeded grain boundary slippage. The specimens without rejuvenation heat treatment showed normal stress relaxation, whereas the specimens subjected to rejuvenation heat treatment showed abnormal stress relaxation. This is attributed to the precipitation of solid-solution Mo from the γ matrix to form a precipitated phase, which induces lattice shrinkage of the γ matrix and size reduction (shrinkage) of the R26 superalloy. The γ’ phase spacing was smaller and dislocation movement was more difficult in the sample subjected to rejuvenation heat treatment than in the congener without rejuvenation heat treatment. The superalloy exhibited better resistance to stress relaxation, resulting in plastic strain during stress relaxation, which was less than the lattice shrinkage. In order to maintain a constant total strain, the external force needs to be increased; thus, the superalloy showed an abnormal stress relaxation phenomenon with increasing stress.

Multi-stage precipitation for the eco-friendly treatment of phosphogypsum leachates using hybrid alkaline reagents

The leachate of waste phosphogypsum (PG) contains substantial amounts of phosphate, sulfate, and fluoride, which poses an immense threat to the safety of the aquatic ecosystem. In this study, a multi-stage precipitation approach is proposed to treat PG leachates using hybrid alkaline reagents including barium hydroxide (Ba(OH)2) and quicklime (CaO). To avoid potential environmental impacts, hazardous environmental contaminants are removed from the PG leachates by adding alkaline reagents in a specific order. The multi-stage precipitation approach achieves nearly 100% removal efficiencies for phosphate, fluoride, and metal ions as well as satisfactory removal efficiencies for sulfate and ammonia nitrogen (93.6% and 94.8% respectively). The compositions of generated mineral precipitates are comprehensively investigated by analytical techniques including X-ray diffraction (XRD), transmission electron microscope (TEM), etc. The main precipitated mineral phases are barite (BaSO4), brushite (CaHPO4·2H2O), and hydroxyapatite (Ca5(PO4)3OH). This is further proven by the geochemical modeling using Visual MINTEQ, in which the precipitation of mineral phases is calculated. Furthermore, a straightforward economic analysis indicates the proposed method has a lower cost of the use of alkaline reagents. Overall, this study establishes an effective and eco-friendly approach for the treatment of waste PG leachates to remove hazardous environmental contaminants and recover valuable precipitates.

Microstructure and properties of Cu-Zn-Cr-Zr alloy treated by multistage thermo-mechanical treatment

A high-strength and high-conductivity Cu-0.4Zn-0.35Cr-0.2Zr alloy was designed and prepared by adding galvanized scrap copper into Cu-Cr-Zr alloy. The effects of multistage thermo-mechanical treatment involving the microstructure and properties of the alloy were investigated by some means, e.g. mechanical property, conductivity testing and transmission electron microscopy observation. After multistage thermo-mechanical treatment, the Cu-Zn-Cr-Zr alloy had tensile strength of 672 MPa, yield strength of 670 MPa, hardness of 201 HV, and electrical conductivity of 72.1 %IACS. Zn was uniformly dissolved in the Cu matrix, and the nano-Cr phase was the main precipitation phase. The strengthening mechanism of the alloy was combination of strain strengthening, fine grain strengthening, precipitation strengthening and solid solution strengthening. Adding an appropriate amount of Zn can improve the strength of the alloy without significantly reducing the electrical conductivity in combination with the appropriate thermo-mechanical treatment. The novel formulation could present an idea for high-quality reuse of waste copper containing Zn to fabricate high-performance copper alloys.

Variations in microstructure and mechanical properties of selective laser melted AlSi10Mg alloy with different heat treatments

In the present study, the variations of precipitated phases, microstructure and mechanical properties of different heat treatments were investigated. The main precipitated phases were Al and Si in different heat-treated specimens. The horizontal microstructures of as-deposited and below 280°C annealing appeared the interwoven structures, and the vertical microstructures were fish-scale structures. As the annealing temperatures were higher than 280°C, the interwoven and fish-scale structures disappeared because the Si phase precipitated and coarsen along the boundaries. With regard to the specimen was heat-treated by solution + artificial aging (T6), the strengths increased and ductility decreased, respectively compared with those of specimens at 420°C for 2 h. Moreover, The AlSi10Mg specimen annealing at 280°C for 2 h has excellent strength and ductility. This investigation indicates that the microstructure and mechanical properties of SLM AlSi10Mg alloy be tailored by different heat treatment processes.

The effects of continuous and discontinuous β-Mg17Al12 on compression creep properties of Mg-8.0Al-1.0Nd-1.0Gd alloys

The microstructure evolution of β-Mg17Al12 in as-cast Mg-8.0Al-1.0Nd-1.0Gd alloy during 420 °C solution and then 210 °C aging treatment was investigated. The results indicate that the discontinuous precipitation (DP) distributes along grain boundaries, and the needle-shaped continuous precipitation (CP) with crystallographic orientation relationship (OR) of [1‾101]Mg//[334‾]β, (01‾11)Mg//(313)β, (1‾011‾)Mg//(33‾0)β are the main intragranular precipitated phases. Compression creep tests of the specimens were carried out at a stress of 60 and 90 MPa at 150 and 180 °C. The microstructure observation reveals that the considerable 86° {101‾2} tension twins assist in the creep deformation for all crept specimens. The solid solution treated alloys achieve excellent creep resistance compared with the aged alloys owing to the strong co-segregation between Nd and Gd in α-Mg and β-Mg17Al12 at 180 °C/90 MPa. However, the creep properties of the aged alloys are superior to that of the solid solution treated alloys at 180 °C/60 MPa and 150 °C/90 MPa, which can be attributed to the inhibited dislocation slipping via needle-shaped and spherical CP. In addition, increasing the CP region and reducing recrystallized grains could improve the creep resistance. Combining the micro-morphology of creep structure and creep parameters (n, Q), the dominated creep mechanism for solid solution treated alloys is grain boundary sliding (n = 1.5–1.8, Q = 32–36 kJ/mol), while the dislocation climb combined with grain boundary diffusion (n = 4–6, Q = 60–80 kJ/mol) is regarded as the controlling creep mechanism for the aged Mg-8.0Al-1.0Nd-1.0Gd alloys.

Achieving crack-free CuCrZr/AlSi7Mg interface by infrared-blue hybrid laser cladding with low power infrared laser

A coaxial infrared-blue hybrid laser with low power infrared laser was used to clad high-reflectivity CuCrZr alloy on the AlSi7Mg substrate, and the formability and solidification behaviour in cladding with the high crack sensitivity materials under the infrared, blue and hybrid laser were compared. We demonstrated that the combination of a small power (100–200 W) infrared laser with 960 W blue laser can effectively eliminate the cracking and balling phenomenon existed in the cladded samples under the action of both infrared and blue laser. Compared with the coarser Al-dendrite (1.37–2.50 µm) and Al2Cu phase in the cladded tracks formed under the infrared laser (2600 W) and the hybrid laser with high infrared power (1000–2600 W), finer Cu-dendrite (0.45–0.89 µm) is the main precipitation phase in the cladded tracks under the combination of 960 W blue laser and low power infrared laser (100–200 W). The improvement of the formability and solidification behaviour should be related to the stable and high absorption rate (∼65%) of blue laser, as well as the concentrated power density of low power infrared laser. This work validated the potential of the infrared-blue hybrid laser with low power infrared laser in cladding with high-reflectivity and high crack-sensitivity alloys.

Influence behavior and mechanism of γ′- or γ′′-precipitated phase types on damage resistance of heterogeneous interface in nickel-based superalloys

The microstructure adjustment of Ni-based superalloys focuses heavily on the types of precipitated phases because the failures of Ni-based superalloys often occur at γ-matrix/γ′- or γ′′-precipitated phase heterogeneous interfaces. As different types of precipitate phases result in different interface damage resistances, the influences of the types of precipitated phases on the damage resistances of heterogeneous interfaces were investigated in this work. Based on the results of this work, the interfacial works of adhesion for the three interfaces composed of the γ-matrix and typical precipitated phases (γ′-Ni3Al, γ′-Ni3Ti, and γ′′-Ni3Nb) are 4.81, 3.88, and 4.26 J/m2, respectively, indicating that the γ-Ni/γ′-Ni3Al interface shows the highest cleavage fracture resistance. This can be attributed to the highest interfacial average charge density among all three interfaces. During interface tensile processing, all fractures occur in the precipitated phase, indicating that the ductility of heterogeneous interfaces is determined by that of the precipitated phase. Due to the highest ductility of γ′-Ni3Ti, the γ-Ni/γ′-Ni3Ti interface demonstrates the best plastic fracture resistance with the smallest Rice ratio of 4.89 and the largest maximum strain of 10 %, followed by the γ-Ni/γ-Ni3Al and γ-Ni/γ′′-Ni3Nb interfaces. The γ-Ni/γ′-Ni3Ti interface also exhibits the highest fatigue fracture resistance attributed to its strongest ability in hindering dislocation slipping with the lowest unstable stacking fault energy, followed by the γ-Ni/γ′′-Ni3Nb and γ-Ni/γ′-Ni3Al interfaces. These results conclude that Ni-based superalloys with γ′-Ni3Al as the main precipitated phase demonstrate high strength, while those with γ′-Ni3Ti as the main precipitated phase demonstrate high plastic and fatigue fracture resistances.

Influence of Y2O3 addition on the microstructure of TiC reinforced Ti-based composite coating prepared by laser cladding

TiC reinforced Ti-based composite coatings were prepared on the Ti6Al4V surface via laser cladding with Ti6Al4V/NiCr-Cr 3 C 2 /Y 2 O 3 mixed powders. Influence of Y 2 O 3 on forming quality and microstructure of composite coatings was investigated systematically. The cracks were eliminated completely and the number of pores was drastically minimised when 2 wt% Y 2 O 3 was added. In addition, the reinforcement TiC and the matrix sloid solution β-Ti were the main precipitated phases within the coating. The addition of Y 2 O 3 inhibited the generation of Ti 2 Ni at the grain boundary, however, promoted the generation of Cr 1.93 Ti 1.07 in the coating. With the addition of Y 2 O 3 , the dendrites became largely developed and a large number of tertiary dendrites were found. Besides, a more obvious regional segregation appeared in the distribution of Ni and Cr elements in the coating with 2 wt% Y 2 O 3 . Furthermore, some nano-sized β-Ti were distributed on the surface of small TiC particles in both coatings. Combined with the theoretical analysis of the two-dimensional mismatch degree, the β-Ti and TiC interface exhibited a semi-coherent relationship. Therefore, β-Ti can nucleate with TiC as the heterogeneous nucleation matrix. In the meanwhile, there was a certain dependent growth relationship between the β-Ti and TiC. • Y 2 O 3 addition eliminated cracks completely and reduced porosity of the coating. • Y 2 O 3 addition led to sufficient growth of the developed dendrite TiC. • Irregular Ti 2 Ni disappeared and Cr 1.93 Ti 1.07 precipitated in coating with Y 2 O 3 . • Y 2 O 3 addition resulted in severe regional segregation of Cr and Ni. • β-Ti can nucleate with TiC as the heterogeneous nucleation center.

Effect of boron on aging strengthened phase and properties of Cu–Cr–Zr alloy

The effect of trace B on the properties and microstructure of Cu–Cr–Zr alloy was investigated in this work. Aging precipitation phases in Cu matrix were analyzed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It was found that Cr is the main precipitation phase and its precipitation process is affected by B. Boron can increase the precipitation rate of the precipitation phase and promote the precipitation of the precipitation phase. After Cu–Cr–Zr alloy aged at 480 °C for 2 h, nanosized Cr particles were coherent with the Cu matrix and formed coherent G.P zones. In Cu–Cr–Zr alloy with 0.03wt% B addition, the aging precipitation process was accelerated, resulting in most of the aging precipitate being bcc Cr. Effect of trace B on hardness and electrical conductivity of Cu–Cr–Zr alloy was tested. It is indicated that B addition improves the hardness and electrical conductivity of the alloy and 0.03wt% B corresponds to the peak value of conductivity and hardness of Cu–Cr–Zr–B alloy.

Effect of rare earth elements on the segregation behavior and microstructure of super austenitic stainless steel

Super austenitic stainless steel (SASS) was prepared by non-consumable vacuum arc melting for the first time. The effects of rare earth (RE) elements on its solidification microstructure and segregation behavior were investigated. The σ phase is the main precipitation phase in SASS. The addition of RE elements can refine the solidification microstructure and the dendrite spacing. The addition of RE elements reduced the secondary dendrite spacing at top (S1) by 5.22 μm. The elements as Cr, Mn, and Mo have positive segregation, and elements as Ni and Fe have negative segregation. The addition of RE elements has the trend of reducing element segregation, thereby inhibiting the σ phase precipitation. And the addition of RE reduces the hardness of the secondary phase.

Segregation behavior and precipitated phases of super-austenitic stainless steel influenced by electromagnetic stirring

Super austenitic stainless steel was prepared by vacuum non-consumable arc melting furnace. The influence of electromagnetic stirring on segregation behavior and the precipitates of super-austenitic stainless steel was investigated via a combined thermodynamic calculation and an experimental characterization approach. The σ phase is the main precipitation phase in super austenitic stainless steel, which has been proved by Thermo-Calc software and experimental characterization. Electromagnetic stirring significantly reduces the dendrite spacing and refines the solidification microstructure. In super-austenitic stainless steel, Cr, Mn, and Mo are positively segregated, while Ni and Fe are negatively segregated. Si and Cu have no obvious segregation. The segregation coefficient (K) shows that the segregation of Mo element is the most serious. Compared with no electromagnetic stirring, the segregation of elements in the electromagnetic stirring sample is significantly weakened.

Effect of MgO on solidification and crystallization properties of ultrahigh-basicity mold flux

The effect of MgO on the solidification and crystallization properties of ultrahigh-basicity mold flux was analyzed (i.e., comprehensive basicity R = 1.75). Results showed that with the increase of MgO content (0–6 wt%), the viscosity, melting temperature and break temperature decreases firstly, and then increases. The change of slag viscosity caused by the change of composition is indistinct, but the variation trend of melting temperature and break temperature is obvious. With the increase of MgO content, the crystallization temperature of ultrahigh-basicity slag decreases gradually, while the crystallization rate decreases firstly, and then increases. When the MgO content reaches 3%, the crystallization rate is the minimum. Research found that Ca4Si2F2O7 is the main precipitated phase in the slag, and as the MgO content continues to increase, the new phases of CaF2 and Ca3MgSi2O8 appear gradually.