Dissolution Of Secondary Phases Research Articles

Effect of electric current on secondary phase dissolution and elements migration behavior of a Ni-based single crystal superalloy

Effect of electric current on secondary phase dissolution and elements migration behavior of a Ni-based single crystal superalloy

Influence of rotational speed on the microstructural, mechanical and wear behaviour of FSPed cast AZ91 magnesium alloy

ABSTRACT In this study, the Magnesium AZ91 alloy underwent a process known as Friction Stir Processing (FSP) at different tool rotational speeds: 1160 r.p.m., 850 r.p.m., and 580 r.p.m. subjected to a constant transverse speed/feed rate (50 mm/min). The average grain size of the FSP-processed samples fell dramatically due to dynamic recrystallisation (DRX), from 43.65 μm in the as-received Mg AZ91 to less than 4.21 μm in the alloy processed at 850 rotational speed. The UTS was enhanced from 145.49 MPa to 184.51 MPa for as-received material to alloy processed at 850 rotational speeds due to the dissolution of secondary phases and the grain refinement. Also, this increase in rotational speed resulted in an increase in total elongation (TE), which increased from 5.465% to 9.92% due to the removal of defects during the processing. The wear resistance of FSPed materials was better than that of the as-received material at all tool rotational speeds. Better wear resistance was observed in the case of samples processed at 850 r.p.m. All these alterations in properties were found due to the existence of refined grains and the removal of defects due to stirring action.

Re-dissolution behavior of secondary phases and mechanical response of CNT/2024Al composites

Heat treatment is one of the most effective strategies for regulating microstructure and enhancing the mechanical properties of aluminum matrix composites. In this work, the effects of solution treatment on the static recrystallization behavior, characteristics of secondary phases dissolving, and mechanical response of carbon nanotubes reinforced aluminum matrix composites (CNT/2024Al) were systematically investigated using transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). Since plenty of deformation stored energy in as-extruded composites is consumed, the residual energy is insufficient to induce static recrystallization during solution treatment. Besides, various interfacial phases aggregated around the Al2Cu phases, which reduced the effective contact area between the Al2Cu phases and matrix, and severely hindered the dissolution process of the Al2Cu phases. The yield strength, tensile strength, and elongation of as-solution treated composites were 23.2%, 44.9%, and 23.0% higher than those of as-extruded composites, respectively. The enhanced yield strength is attributed to solid solution strengthening induced by the dissolution of secondary phases, thermal mismatch strengthening, and the strengthening originating from the formation of Al4C3 phases.

Effects of vapor hydration and radiation on the leaching behavior of nuclear glass

During the long-term disposal of high-level nuclear waste glass in the geological disposal facility, the nuclear glass could be exposed to an unsaturated vapor environment prior to the direct contact with groundwater. Additionally, glass will experience damages due to self-irradiation. In this study, experiments were performed to investigate the chemical durability of glass after being hydrated and irradiated. This study proposes an attempt to investigate the impact of ionizing effect by applying alpha or gamma radiations to the pre-hydrated glass. The impact on chemical durability is evaluated by leaching the hydrated and irradiated glass samples. With the studied doses, 8 Giga Gy of alpha radiation or 100 kGy of gamma radiation, no significant modification of chemical durability is observed upon irradiation. On the other hand, leaching of pre-hydrated glass shows the increase of the initial release of Si, which is considered to be related to the dissolution of secondary phases. Furthermore, the total dissolution of hydrated layer is observed, which would result in the release of radionuclides at the same rate as Si does and thus needs further investigation.

Second Phase Dissolution Influenced by Simultaneously Enhanced Mg, Cu Contents during Homogenization of As-cast Al-Zn-Mg-Cu Alloys

The dissolution of second phase with relatively high melting point in as-cast Al-Zn-Mg-Cu alloys was closely related to Mg and Cu contents. In present work, second phases in three Al-Zn-Mg-Cu alloys with simultaneously enhanced Mg and Cu contents (named by LMC alloy, MMC alloy and HMC alloy as Mg and Cu contents progressively enhanced) were analyzed and the correlated dissolution during homogenization was investigated. The results showed that both Mg(Zn,Cu,Al)2 phase and Cu-rich phase existed in as-cast alloys while HMC alloy possessed more eutectic phases. As homogenized by 470°C/24h, Mg(Zn,Cu,Al)2 phase had dissolved completely, LMC alloy contained little Al2CuMg phase and the amount of it for the three alloys was arranged as LMC alloy < MMC alloy < HMC alloy. As furtherly homogenized by a second stage at 480°C for 12h, no endothermic peak for Al2CuMg phase was observed for LMC alloy and only Fe-rich phase existed. Meanwhile, Al2CuMg phase still remained in MMC and HMC alloy. As the homogenization time prolonging to 36h, Al2CuMg phase in MMC alloy dissolved completely while that still existed in HMC alloy. Adding a third stage at 490°C for HMC alloy, no Al2CuMg phase could be observed for 24h. This gave rise to a method by incrementally grading homogenization temperature combined with prolonging soaking time to fulfill the dissolution of second phase for Al-Zn-Mg-Cu alloys with enhanced Mg and Cu contents

Correlation of microstructure with corrosion performance in high zinc 7068 aluminum alloy aged using different T6 conditions

High-strength 7068 aluminum alloy containing >7 wt% zinc deserves evaluation of its SCC performance in different ageing conditions, because of reported less effectiveness of over-ageing in newly developed high‑zinc 7xxx series alloys. Microstructural features resulting from different ageing treatments are correlated with mechanical properties, localised corrosion behaviour, and SCC susceptibility. Localised attack manifested in dissolution of second phase precipitates which occurs from selective leaching of magnesium and aluminum. The micro-galvanic effect of iron-containing precipitates on the matrix dissolution is less as compared to Al 2 MgCu phase. Peak and overaged alloy exhibited higher resistance to intergranular corrosion and SCC because of discontinuous grain boundary precipitates and fewer Al 2 MgCu/α-Al micro-galvanic couples. • Differently aged high-Zn 7068 alloy is characterized using DSC, XPS, SEM and TEM. • Peak aged condition showed intermittent distribution of GBPs. • Corrosion of 7068 alloy in different T6 temper conditions is compared. • Over-ageing is not necessary for satisfactory SCC performance in this alloy.

Flow stress stabilization of Zn-Cu-Mn-Mg alloys using thermomechanical processing

Zinc-based alloys are potential candidates for bioabsorbable metallic devices due to their application-appropriate corrosion rates and biocompatibility. However, strain softening and rate sensitivity in tensile testing remain as challenges for their use in load bearing applications. In this study, three different Zn-xCu-yMn-0.05Mg (x = 0.5, 1.0 wt%, y = 0.4, 0.6 wt%) alloys were formulated and their microstructure and tensile properties in the room-temperature rolled condition were characterized. Additionally, the effect of short-time annealing at 320 °C on the strain softening and strain rate sensitivity of alloys was studied. The results indicate that dissolution of secondary phases and grain coarsening lead to the suppression of strain softening and strain rate sensitivity. The evolution of microstructure during the room-temperature tensile testing indicates that dynamic recrystallization is responsible for strain softening and can be eliminated by tuning the fraction of secondary phases and underlying grain size. The formulated alloys are not susceptible to natural aging and show good thermal stability during aging up to 200 °C for 60 h due to the pinning effect of MnZn13 precipitates on the grain boundaries.

Nano-treating promoted solute dissolution for novel high strength Al-Cu-Mg alloys

High-strength Al-Cu-Mg alloys are normally designed with a low Mg content because of their tendency towards solidification cracking and a limited solubility of Cu when the Mg content exceeds 2 wt.%. Nano-treating is an emerging metallurgical method to improve the manufacturability and properties of alloys by adding a small volume fraction of ceramic nanoparticles. In this study, approximately 1.0 vol.% of TiC nanoparticles was added to cast Al-4.6Cu-xMg-0.3Mn alloys with a high Mg content ranging from 1.5 wt.% to 4.0 wt.%. Nanoparticles not only eliminated shrinkage porosities during casting but also significantly improved the dissolution of secondary phases during solution treatment of the alloys. Moreover, it was discovered that TiC nanoparticles promoted an unusual solute dissolution behavior to break the thermodynamic solubility limit of Mg in the Al-4.6Cu-xMg-0.3Mn system to achieve high mechanical performance. Theoretical analysis and TEM suggest that the promoted solute dissolution was driven by an overall reduction in interfacial energy between nanoparticles and the S phase during solutionization. The density of primary S’ precipitates was significantly increased by nano-treating, as an unusually high supersaturation was achieved in the nano-treated samples.

Effect of preaging temperature on microstructure evolution, mechanical and corrosion behavior of RRA‐treated high‐zinc 7068 alloy

AbstractMechanical and corrosion behavior of high‐strength, high‐zinc (>7 wt%) containing 7068 aluminum alloy is investigated after employing different retrogression and reaging (RRA) treatments. The effect of preaging conditions on the distribution of copper, zinc, and magnesium, the volume fraction of η′ phase, and the width of precipitate free zones (PFZ) have been investigated. Microstructural and compositional features characterized by scanning electron microscope‐energy dispersive spectroscopy, scanning transmission electron microscope, and differential scanning calorimetry are correlated with hardness and corrosion performance. The localized attack is manifested in the dissolution of second phase precipitates which occurs from selective leaching of magnesium and aluminum. A combination of two opposite effects, that is, the presence of nobler, high‐copper grain boundary precipitates and microgalvanic effect of PFZ along with the distribution of alloying elements, that is, Cu, Zn, and Mg govern the electrochemical behavior of RRA‐treated 7068 alloy. Optimum preaging and RRA conditions are identified for this high‐zinc 7xxx series alloy.

Affecting mechanism of deep cryogenic treatment on nugget zone softening in dissimilar aluminum alloys friction stir welded joint

As far as the 2219 Al-ceramic Al dissimilar friction stir welded joint is concerned, the weld nugget zone surprisingly shows a notable softening phenomenon after the deep cryogenic treatment (DCT). The effect of DCT on microstructure evolution of nugget zone in 2219 Al was investigated in this paper in order to explore the reasons for the unique softening phenomenon. The results indicate that the DCT further promotes recrystallization and causes the grain growth slightly. The diffusion of Ti element from ceramic Al to 2219 Al occurs during the dissimilar friction stir welding, leading to the formation of Ti-rich secondary phases in the nugget zone of 2219 Al. The DCT can facilitate the precipitate dissolution to a certain extent, and thus the size and distribution density of the secondary phase particles are both reduced. Essentially, DCT produces internal stress in the dissimilar friction stir weld, and the rotation of grain boundaries from dislocation movement and the dissolution of secondary phases are able to release the internal stress, which are fundamental reasons for the softening of nugget zone after DCT. • DCT by liquid nitrogen results in NZ softening in 2219 Al-ceramic Al FSW joint. • DCT leads to grain growth, stronger shear texture and dislocation reduction in NZ. • Dissolution of secondary phases occurs in the NZ after DCT. • Fundamental reasons for NZ softening after DCT are discussed.

Dependence of the microstructure and properties of joints on acoustic intensity during the ultrasonic soldering of 7075 Al alloys

• Acoustic intensity of solder remarkably affects the microstructure and mechanical properties of joints. • High acoustic intensity was produced by applying a small lap width without increasing input ultrasonic power. • The melting point of Zn increased by 73.8 °C under the high pressure caused by bubble collapse. • Zn grains were significantly refined due to undercooling–induced direct solidification caused by cavitation. • Shear strength and hardness respectively increased by 24% and 17% when lap clearance decreased by 50%. In this work, 7075 Al alloys were ultrasonically soldered by pure Zn solder in air under different acoustic intensities. The dependence of the microstructure and mechanical properties of joints on acoustic intensity was revealed. Acoustic intensity is innovatively controlled by applying different lap widths without increasing the input ultrasonic power. The acoustic pressure inside the solder was simulated by finite element simulation, and results showed that the acoustic intensity increased by more than six times when the lap width decreased by 50 %. High acoustic intensity led to intense cavitation, thereby causing a pronounced erosion of substrate. The Al content inside the joint increased from 17.3%–24.5% when the clearance width decreased from 0.8 mm to 0.4 mm. The Clausius–Clapeyron equation helped explain that the melting point of Zn increased by approximately 73.8 °C under the high acoustic pressure caused by cavitation bubble collapse, which could cause the direct solidification of Zn and served as the main reason of grain refinement. Zn grains were refined from 6.9 μm to 4.2 μm when soldered at 0.8 mm and further to 3.5 μm when soldered at 0.4 mm because of the strong cavitation caused by high acoustic intensity. Acoustic intensity had a remarkable effect on the mechanical properties of joints. Joint shear strength and hardness increased from 117.3 MPa and 93.3 HV to 145.1 MPa and 109.4 HV, respectively, when the lap clearance width decreased from 0.8 to 0.4 mm. Particularly, the joint soldered at 0.4 mm failed through the substrate because of the high strength of the joint seam and the weakness of the substrate, which were explained through the Hall–Patch relationship, the Schmid factor, the dissolution of secondary phases, and the increase in high angle boundaries. This work provides a novel method by producing strong joints with fine grains without increasing input power.

Defects, microstructure and mechanical behaviour upon multi-pass friction stir processing of magnesium alloy with spiral tool path

Friction stir processing is an advance technique to modify the microstructure of metals for enhanced mechanical properties. Previous works have demonstrated that the friction stir processing of Mg alloys is feasible. These works are restricted to small areas or sample sizes. The objective of this work is to adopt spiral tool path strategy to process a complete blank of Mg alloy and study the effect of process parameters on the microstructure and hardness. The friction stir processed samples are prepared with different combinations of tool shoulder overlaps and tool rotation directions. Experimental results show fully consolidated processed Mg blanks are achieved with the tool rotation direction, which forces the advancing side towards the ‘yet to be processed’ side of the tool. Further, tool rotation direction plays a crucial role towards defect formation and retention upon friction stir processing of Mg alloy blanks with spiral tool path. The microstructural studies and hardness test are performed for the fully consolidated processed blanks. The microstructure of the processed region consists of refined grains, with banded regions comprising even finer grains. The friction stir processing of the as-cast Mg alloy also lead to the dissolution of second phase and more uniform distribution of the alloying elements in the Mg matrix. The grain refinement and second phase dissolution lead to enhanced hardness upon friction stir processing. Higher tool shoulder overlap promotes higher temperatures and plastic deformation, which leads to more grain refinement and second phase dissolution with corresponding increase in hardness levels. The local variation in average hardness within a processed sample corresponds well to the local variation of the average grain size. This work shows that large samples of Mg alloy can be friction stir processed towards microstructure modification and mechanical property enhancement, with appropriate combination of tool rotation direction and tool shoulder overlap.

Controlling the Microstructure and Texture Using Multidirectional Forging (MDF) to Develop a Low Corrosion Rate Mg Alloy

A new Mg alloy was cast, then extruded, and finally multidirectionally forged (MDF) at 180°C (MDF180) and 300°C (MDF300). The corrosion behavior was evaluated using electrochemical and immersion techniques. The mechanical property was assessed using tension and compression tests. The microstructures were analyzed using optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction (XRD), and thermodynamic calculations, while texture was studied by XRD and electron backscatter diffraction. Results indicated that due to grain refinement and good distribution of second phase after MDF, yield strength as well as elongation were improved. The corrosion rates were reduced for MDF180 due to the uniform distribution of the second phase along with uniform distribution of grains. Corrosion rate was more reduced for MDF300 due to dissolution of second phase and elimination of worked grains.

Rapid Dissolution of Secondary Phase in Cold-Rolling Al-Cu-Mg Alloy by Electropulsing Treatment

The Al-Cu-Mg alloy is a precipitation-strengthening alloy, which is traditionally dissolving lots of secondary phase and precipitating in subsequent aging process, thus having been applied to aerospace and automobile industry due to its low density and high strength. However, the high temperature and long operating time needed for the dissolution of secondary phase consume numerous energy. In this work, rapid dissolution of secondary phase in cold-rolling Al-Cu-Mg alloy was achieved by coupling treatment of thermal field and pulsed electric current. The energy consumption for dissolving secondary phasewas reduced from 495°C operating 1h to 450°C operating several minutes. Therefore, the coupling treatment of thermal field and pulsed electric current significantly improves the dissolving rate and decreases the energy consumption.

Microstructure Evolution and Mechanical Properties Improvement of AlZnMgCu Alloys Induced by a Novel Double Electroshocking Treatment

Herein, a novel solution method of AlZnMgCu alloy using double electroshocking treatment (D‐EST) is proposed. Compared with the conventional solid solution treatment (SST) at 475 °C for 30 min, this method can provide more desirable results, although it only takes 10.2 s. Single tensile test results show that the ultimate strength and elongation after D‐EST followed by T6 aging are, respectively, increased by 2.6% and 6.8% relative to that of SST. In addition, the microstructure of the alloy after D‐EST and SST is characterized by the scanning electron microscopy, transmission electron microscopy, and electron backscattering diffraction (EBSD). The experimental results reveal that D‐EST can give rise to more adequate dissolution of secondary phases especially intermetallic compounds and long rod‐shaped dispersion phase, finer grains, and fewer grain boundaries in the range of 5°–8°. The kinetic mechanism of D‐EST is interpreted from the point of view of electromigration and the nonuniform distribution of temperature on the microscale.

Metallurgical and corrosion characterization of electron beam welded duplex stainless steel joints

Electron beam welding (EBW) process was employed autogenously to fabricate 12 mm and 18 mm thick full-penetration butt welds of 2205 duplex stainless steel (DSS). The welds were evaluated for their metallurgical characteristics (microstructure, ferrite content and microhardness) and corrosion (intergranular and pitting) behavior in the as-welded, aged (850 °C/30 min) and aging followed by solution treated (1050 °C/30 min) conditions. The EB welds showed a relatively higher propensity to pitting corrosion than the base metal while they remained immune to the intergranular corrosion based on the degree of sensitization. Thermal aging-induced Cr and Mo-rich intermetallic σ-phase and Cr-based carbides further led to degradation of pitting resistance while their sensitization behavior remained largely unaltered. The solution treatment promoted the formation of austenite and partial dissolution of secondary phases in the welds which favored the pitting resistance restoration tendencies. This study highlights that two factors viz. microstructural phase imbalance and weld zone size would play a significant role in influencing the corrosion properties of DSS EB welds. Thus, better pitting corrosion performance could be expected from the welds fabricated using welding procedures that would result in small sized fusion zones and a lesser degree of phase imbalance.

A systematic investigation of secondary phase dissolution in Mg–Sn alloys

Abstract As-cast Mg–Sn alloys (3, 6, and 9 wt% Sn) were solution treated at 653, 703 and 753 K (380, 430 and 480 °C) for 1, 4, 8, 12 and 24 h to determine the variation of secondary phase with respect to Sn content, temperature and time. Mg-3 wt% Sn exhibits Mg2Sn dissolution at all solution treatment temperatures whereas Mg-6 and 9 wt% Sn alloy displays Mg2Sn reprecipitation and dissolution depending on the heat treatment temperature. In addition, a combined mathematical model that predicts the secondary phase dissolution and solute redistribution as a function of temperature and time is presented in this work. This model is a significant improvement compared to the previous studies where the dissolution and homogenization processes are considered independently. The effect of grain size and solute mobility upon the dissolution and homogenization kinetics is discussed as well.

Microstructure of Mg96Zn2Y2 alloy joint formed by resistance spot welding

Mg96Zn2Y2 alloy sheets were welded by employing a resistance spot welding technique. The microstructures of the joint were observed and analyzed using scanning electron microscopy and transmission electron microscopy to investigate the difference between it and those of the base material. The joint comprises a base metal zone, heat-affected zone, fusion zone, and nugget. In the heat-affected zone, the dissolution of secondary phases occurred locally. In the fusion zone, the α-Mg grains were coarsened; and the secondary phase was transformed into a network distribution. In the nugget, the α-Mg grains were approximately 30 μm in diameter; and most of the secondary phase possessed a long-period stacking-ordered structure of 18 periods rhombohedral lattice. The results reveal that the number of secondary phases and Mg3Zn3Y2 compounds formed in the nugget were smaller than those observed in the base metal.

The effects of alloying with Cu and Mn and thermal treatments on the mechanical instability of Zn-0.05Mg alloy

The detrimental effect of natural aging on mechanical properties of zinc alloys restricts their application as bioresorbable medical implants. In this study, aging of Zn-0.05Mg alloy and the effect of 0.5 Cu and 0.1 Mn (in weight percent) addition on the microstructure and tensile properties were studied. The alloys were cold rolled, aged and annealed; aiming to investigate the effects of precipitates and grain size on the mechanical properties and their stability. TEM analysis revealed that in ultrafine-grained binary Zn-0.05Mg alloy, the natural aging occurred due to the formation of nano-sized Mg2Zn11 precipitates. After 90 days of natural aging, the yield strength and ultimate tensile strength of Zn-0.05Mg alloy increased from 197±4 MPa and 227±5 MPa to 233±8 MPa and 305±7 MPa, respectively, while the elongation was drastically reduced from 34±3% to 3±1%. This natural aging was retarded by adding the third element at either 0.1Mn or 0.5Cu quantities, which interacted with Mg in Zn solid solution and impeded the formation of Mg2Zn11 precipitates. The addition of Cu and Mn elements increased alloy's strength, ductility, and its mechanical stability at a room temperature. The measured tensile strength and elongation were 274±5 MPa and 41±1% for Zn-0.1Mn-0.05Mg and 312±2 MPa and 44±2% for Zn-0.5Cu-0.05Mg, respectively. Annealing the alloys at elevated temperatures caused increase in both grain size and dissolution of secondary phases, and both affected alloy deformation mechanisms.

High temperature strength of refractory complex concentrated alloys

Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, Tm) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000 °C and 1200 °C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 Tm, so that the alloys with higher Tm retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000 °C is probably due to dissolution of secondary phases.