Needle-like Phase Research Articles

Microstructure Analysis of Progressive Secondary AlSi7Mg0.6 Alloy with Higher Fe Content Using Electron Metallography Techniques

This article investigates the effect of the higher Fe content on the formation of brittle Fe-rich needle-like phases in heat-treated secondary AlSi7Mg0.6 cast alloy. These secondary-recycled alloys contain an increased amount of impurities due to remelting of scrap. The common unwanted impurity found in these alloys is Fe. Its higher content negatively affects the structure and mechanical properties. Fe has low solubility in Al-alloys thus forming unwanted intermetallic phases such as β-Al5FeSi. Fe cannot be removed in the foundry conditions, so its content is reduced to an acceptable limit. One of the possibilities to eliminate the negative effect of high Fe content on the structure and mechanical properties is heat treatment. Heat treatment influences the size and morphology of structural components, which leads to a finer structure and thus better mechanical properties. Therefore, this study is focused on the changes in structure depending on the Fe content influenced by heat treatment applied to the secondary AlSi7Mg0.6 alloy with higher Fe content and evaluation using a scanning electron microscope, including methods of EDX analysis, and methods of deep etching.

Effect of heat treatment regime on microstructure and phase evolution of AlMo0.5NbTa0.5TiZr refractory high entropy alloy

This paper reports the effect of heat treatment process on the ensuing microstructure evolution, and hardness variation of a cast and homogenized AlMo0.5NbTa0.5TiZr refractory high-entropy alloy (RHEA). Heat treatments were carried out on the homogenized samples at 1000, 1100, and 1200 °C for 10, 24, and 48 h. It was revealed that the phase decomposition in the alloy took place in a much shorter time than that reported in the literature. The significant tendency of Zr and Ti in respect of separation from the solid solution was found as the main reason for the formation of multi-phase structure and causing thermal instability. The phase decomposition occurred though the formation of Zr-rich needle-like phase constituents and Ti-rich globular ones. The semi-stable Zr-rich phase constituents are believed to form on B2 lamellas, and during their growth, the crystal structure gradually transforms from body-centered cubic (BCC) to a hexagonal close-packed (HCP) configuration. The electron back-scattered diffraction (EBSD) analyses confirmed that the HCP phase fraction increased significantly with increase in annealing temperature and/or time. A remarkable increase of 78% in Vickers hardness was measured for the sample heat treated at 1100 °C for 24 h compared to that of the homogenized state. It turned out that the morphology, size, interface coherency, and volume fraction of the needle-like precipitates played the leading role in influencing the alloy’s hardness. A comprehensive mechanism has been proposed for the phase decomposition of the alloy, based on the microstructure and chemical composition variations.

Study of Coarse Particle Types, Structure and Crystallographic Orientation Relationships with Matrix in Cu-Cr-Zr-Ni-Si Alloy

Coarse particles in Cu-0.39Cr-0.24Zr-0.12Ni-0.027Si alloy were studied with scanning electron microscopy and transmission electron microscopy. Three types of coarse particles were determined: a needle-like Cu5Zr intermetallic phase, a nearly spherical Cr9.1Si0.9 intermetallic phase and (Cu, Cr, Zr, Ni, Si)-rich lath complex particles. The crystallographic orientation relationships of the needle-like and nearly spherical coarse particles were also determined. The reasons for formation and the role of the coarse phases in Cu-Cr-Zr alloys are discussed, and some suggestions are proposed to control the coarse phases in the alloys.

In-situ synthesized TiC/Ti-6Al-4V composites by elemental powder mixing and spark plasma sintering: Microstructural evolution and mechanical properties

Titanium matrix composites synthesized using powder metallurgy methods have attracted significant attention due to their extraordinary mechanical properties. In this study, cost-effective and high-performance TiC/Ti-6Al-4 V matrix composites were synthesized and characterized using a combined elemental powder mixing and spark plasma sintering method. Studies on the effect of graphene nanoplate (GNP) content on microstructures of Ti-6Al-4 V matrix composites showed typical Widmanstatten structures without apparent pores and microcracks. The grain size of composites was decreased significantly with the increase of GNP content, mainly due to the pinning effect of in-situ generated TiC particles at grain boundaries, which limited the rapid grain growth of the matrix. Mechanical test results showed that their yield strength and ultimate tensile strength were 889.0 MPa and 988.3 MPa, respectively, and their total fracture elongation was maintained at ∼14.2 %. GNPs/Ti-6Al-4 V exhibited superior strength (e.g., yield and ultimate tensile strength of 1028.4 and 1121.6 MPa, which are 14.4 % and 13.5 % higher than those of the matrix) and maintained a good ductility of ∼ 9.8 % with only 0.1 wt% GNP addition. Carbon nanomaterial (such as graphene nanoplates) induced the precipitation of needle-like nano-secondary phases in trigeminal grain boundary β phases, which strengthened the β-Ti soft phase. The reinforced strength of the composite is mainly attributed to the grain refinement, secondary α phases precipitation strength and dislocations strengthening. This work provides a new methodology for fabrication of high-performance titanium matrix composites (TMCs) combination blended elemental powder metallurgy (BEPM) and sintering technology.

Effect of Zn addition on microstructure and mechanical properties of Mg-3Y-2Nd-0.5Zr alloy

The effect of 0.5wt.% Zn addition on the microstructure and mechanical properties of Mg-3Y-2Nd-0.5Zr (WE32) alloy was investigated. The results indicate that WE32-0.5Zn alloy takes 48 h to reach peak hardness after solid solution treatment at 525 °C and aging at 200 °C, 10 h earlier than WE32 alloy, which implies an accelerated aging precipitation kinetics owing to the addition of 0.5wt.% Zn. A large quantity of fine-rod and rectangular block-like Zn-Zr precipitates in the α-Mg matrix are formed in the WE32-0.5Zn alloy, and numerous needle-like β1 phases are distributed at both ends of the Zn-Zr precipitates at peak-aged condition. In peak-aged condition, the ultimate tensile strength considerably increases from 263.2 MPa (WE32) to 309.6 MPa (WE32-0.5Zn), and the elongation dramatically increases from 4.3% (WE32) to 8.9% (WE32-0.5Zn). The β′ and β1 phases are the main precipitates of the WE32-0.5Zn alloy peak-aged at 200 °C. The β′ and β1 phases easily nucleate at the Zn-Zr precipitates, and the β1 phases are particularly likely to nucleate and grow at the interface between the two ends of the Zn-Zr precipitates, which accelerates aging precipitation kinetics and leads to a shorter time to achieve peak aging.

Characterization of Al Alloys Injected through Vacuum-Assisted HPDC and Influence of T6 Heat Treatment

AlSi12(Fe), AlSi10Mg(Fe), AlSi10MnMg, and AlMg4Fe2 die-casting alloys were produced by high-pressure die casting (HPDC) and vacuum-assisted high-pressure die casting (VADC) under a vacuum level of 200 mbar. The chemical composition, hardness, gas and shrinkage porosity, and mechanical properties were analyzed. The parts under study were subjected to a T6 heat treatment. The VADC led to a decrease in the percentage of defects in the as-cast state for all the alloys, due to a reduction in the amount of gas porosities. After heat treatment, the quantity of gas and shrinkage porosities increased. The efficiency and level of vacuum used were not sufficient to improve the mechanical properties in the as-cast state. The ductility of AlSi10Mg(Fe) and AlSi10MnMg alloys was improved after heat treatment; however, the YS and UTS of AlSi10Mg(Fe) did not increase. The primary aluminum alloys presented higher elongation values than the secondary aluminum alloys due to the reduced amount of the needle-like β-Al5FeSi phase.

Evolution of iron-rich intermetallics and its effect on the mechanical properties of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing

Si addition is commonly used to modify the iron-rich intermetallics in Al–Cu–Mn–Fe alloys, which is beneficial to increasing the use of recycled aluminum. Most of the available research has focused on the effect of Si content on the room-temperature mechanical properties of Al–Cu–Mn–Fe alloys. To expand the application of Al–Cu–Mn–Fe–Si alloys as light heat-resistant structural components in the automotive and aerospace industries, it is of great importance to investigate the evolution of iron-rich intermetallics and its effect on the fracture behavior of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing. In this work, the evolution of iron-rich intermetallics and the high-temperature mechanical properties of heat-treated Al-6.5Cu-0.6Mn-0.5Fe alloys with different Si contents after thermal exposure and high-temperature tensile testing were assessed by tensile tests, image analysis, scanning electron microscopy, X-Ray diffraction, transmission electron microscopy, and atomic probe tomography. The results indicate that the Al-6.5Cu-0.6Mn-0.5Fe alloys with 0.1Si and 0.5Si additions have excellent and stable high-temperature mechanical properties after long thermal exposure, which are better than those of most heat-resistant Al alloys. The high performance of the high-temperature mechanical properties is attributed to the high heat resistance of secondary intermetallics and precipitated particles. The addition of Si is detrimental to the strength of Al-6.5Cu-0.6Mn-0.5Fe alloys after long thermal exposure. This can be attributed to the solid-state phase transformation of iron-rich intermetallics from α-Fe to β-Fe, which results in the increase of needle-like Fe-rich phases and Si particles, the agglomeration of secondary intermetallics, and the consumption of Al2Cu phases.

Microstructural investigations on laser welded Al-30wt.% B4C composite

In order to tailor the microstructures and mechanical properties of laser welded Al-B4C composite, it is necessary to elucidate the chemical reactions between B4C reinforcement and Al matrix during laser welding process. In this study, 3 mm thick Al-30wt.% B4C plates were laser welded using different parameters. Then the microstructure, reaction products, tensile properties and fracture surface of welded joint were studied. The results show that the reaction products of the weld zone can be controlled by varying the welding energy input, but the full penetration welded joint requires high energy input (120 KJ/m in this paper). For the full penetration welded joint, all B4C particles and part Al matrix react to form irregularly particle-like AlB2C2 and coarse needle-like Al4C3 phases, the joint efficiency is 67%. The failure mechanism of AlB12C2 is cleavage fracture, and the failure mechanism of Al4C3 is interfacial decohesion and pull out. The interfacial decohesion between Al4C3 and Al matrix results in crack initiation due to their weak bonding interface. The cracks propagate, connect and penetrate the brittle AlB12C2 phase and eventually lead to brittle fracture of the joint.

The Oxidation Properties of a NiCrAlY Coating Fabricated by Arc Ion Plating

The microstructures, phase compositions, and high-temperature oxidation behavior of a NiCrAlY coating fabricated by arc ion plating were investigated by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), atomic force microscope (AFM), and X-ray diffractometer (XRD). The results indicate that the NiCrAlY coating was covered by a protective Al2O3 scale with excellent oxidation resistance after oxidation at 1050 °C for 100 h; the β phase rich in Al in the as-deposited coating was transformed into a γ/γ’ phase; the interdiffusion zone (IDZ), secondary reaction zone (SRZ), and needle-like TCP phases were detected in the superalloy substrate beneath the coating.

The microstructures and properties of diamond reinforced Cu–Ni–Si–Ti alloys

In this work, Cu–Ti–Ni–Si- diamond (Di) composites were successfully prepared by casting method based on Ti–Si–C system. The microstructures and properties of the prepared Cu-3.57Ni-1.8Ti-0.85Si-1Di composites were investigated. It is found that, besides core-shell Di@TiC particles formed by Ti-Di reaction, the Ni16Ti6Si7 phase will be formed prior to other Ni–Si phases during both casting and aging treatment processes. The morphologies of the aging precipitated Ni16Ti6Si7 depend on the concentration of Ni, Ti and Si solutes in the Cu matrix. If the concentration of the Ni, Ti and Si solutes is low, the needle-like Ni16Ti6Si7 with a large aspect ratio will be precipitated. On the contrary, when sufficient Ni, Ti and Si are dissolved in the Cu matrix, the Ni16Ti6Si7 will be changed from a needle-like phase to a spherical phase. The hot rolling can promote the solid solution of the dendritic Ni16Ti6Si7 formed during casting, and make the Ni16Ti6Si7 easier to precipitate in a spherical shape. As a result, the Cu–Ni–Ti–Si-Di composites will have excellent comprehensive properties after rolling, solution and aging treatment. The tensile strength of the Cu-3.57Ni-1.8Ti-0.85Si-1Di can get to 702.2 MPa with the elongation of 3.5% and the hardness of 291.1HV. At the same time, the conductivity is kept at 40%IACS.

The Influence of Fe Content on Corrosion Resistance of secondary AlSi7Mg0.3 Cast Alloy with Increased Fe-content

Production of primary aluminium is energetically enormously expensive. The use of secondary (recycled) aluminium, has therefore a high potential to save money and energy while reducing the negative environmental impact of aluminium production. Although the properties of secondary aluminium alloys are generally comparable to those of primary aluminium alloys, the increased Fe content can lead to a significant reduction in the corrosion resistance of these alloys. Secondary (recycled) AlSi7Mg0.3 cast alloy with different iron contents (0.123, 0.454, 0.679 and 1.209 wt. %) in the as-cast and after heat treatment (T6) condition was investigated. The quantitative analysis was focused on the evaluation of the Fe-phases, especially the needle-like Al5FeSi phase. The corrosion resistance was measured by a rapid corrosion test (AUDI test). The corrosion damage of the surface was observed macroscopically. The results show that Fe content higher than 0.454 % has no significant effect on the amount and size of needle-like phases of Al5FeSi. The corrosion resistance is mainly influenced by the size and length of the Al5FeSi phases. Increased Fe content decreases the corrosion resistance of AlSi7Mg0.3 alloy and accelerates the initiation of corrosion.

Sigma phase embrittlement-induced failures of heat-resistant stainless steel traveling grate links

Traveling grates (TG) are used as beds over which green pellets are subjected to a series of thermal cycles, namely preheating, induration, and firing to make the pellets suitable for charging in the blast furnaces of an integrated steel plant. In this work, chronic failures of TG links after a service life of 2.5–3 years are investigated. A comparative analysis of failed, used and new TG links was carried out. Fractography of the failed link revealed an intergranular brittle fracture near the surface followed by a transgranular fracture with a signature of decohesion in the bulk of the fracture surface. Microstructural analysis revealed the presence of pre-existing grain boundary chromium carbides (Cr23C6) in the new (unused) link, which can facilitate easy crack initiation and propagation. Furthermore, failed and used TG link revealed the presence of extensive precipitation of needle-like sigma phase confirmed by a combination of X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy techniques. Such precipitation of the sigma phase occurs during exposure to a susceptible high-temperature range. The presence of the sigma phase is known to embrittle the austenitic stainless steel and such embrittlement is confirmed by a significant increase in hardness and decrease in Charpy impact toughness of failed and used TG links compared to the new TG link. Thermodynamic and kinetic simulations confirmed a high susceptibility of the existing alloy composition to extensive sigma phase precipitation in a wide temperature range. A new alloy composition with higher nickel (∼30 wt%) and free from tungsten is proposed to reduce the susceptibility towards in-service embrittlement induced by sigma phase precipitation.

A new insight into LPSO phase transformation and mechanical properties uniformity of large-scale Mg-Gd-Y-Zn-Zr alloy prepared by multi-pass friction stir processing

A large-scale fine-grained Mg-Gd-Y-Zn-Zr alloy plate with high strength and ductility was successfully prepared by multi-pass friction stir processing (MFSP) technology in this work. The structure of grains and long period stacking ordered (LPSO) phase were characterized, and the mechanical properties uniformity was investigated. Moreover, a quantitative relationship between the microstructure and tensile yield strength was established. The results showed that the grains in the processed zone (PZ) and interfacial zone (IZ) were refined from 50 µm to 3 µm and 4 µm, respectively, and numerous original LPSO phases were broken. In IZ, some block-shaped 18R LPSO phases were transformed into needle-like 14H LPSO phases due to stacking faults and the short-range diffusion of solute atoms. The severe shear deformation in the form of kinetic energy caused profuse stacking fault to be generated and move rapidly, greatly increasing the transformation rate of LPSO phase. After MFSP, the ultimate tensile strength, yield strength and elongation to failure of the large-scale plate were 367 MPa, 305 MPa and 18.0% respectively. Grain refinement and LPSO phase strengthening were the major strengthening mechanisms for the MFSP sample. In particularly, the strength of IZ was comparable to that of PZ because the strength contribution of the 14H LPSO phase offsets the lack of grain refinement strengthening in IZ. This result opposes the widely accepted notion that IZ is a weak region in MFSP-prepared large-scale fine-grained plate.

Microstructure and enhanced tensile properties of AlCo CrFeNi high entropy alloys with high Co content fabricated by laser melting deposition

AlCoxCrFeNi (x = 2.2, 2.8) high entropy alloys (HEAs) were successfully prepared by multi-layer and multi-channel laser melting deposition (LMD). The tensile properties of the LMD-fabricated AlCoxCrFeNi HEAs were investigated. The phase evolution of these alloys was examined by X-ray diffraction and compared with existing models. The microstructure of the alloys was characterized using scanning electron microscopy and electron backscatter diffraction. It is found that Co element can promote the phase transformation from BCC phase to FCC phase in the as-deposited AlCoxCrFeNi HEAs, and the volume fraction of FCC phase increases from 51.4% to 74.6% as the Co content increases from 36.2 at% to 40.8 at%. With the increase of Co content, the grain size of BCC phase in the alloys decreases and a larger amount of fine needle-like BCC phase appears in the FCC matrix. Tensile testing shows that higher Co content in the deposited AlCoxCrFeNi alloy can enhance its plasticity without significantly compromising its ultimate strength. As the Co content increases, the fracture strain increases from 5.9% to 15.4%, while the yield strength reduces from 450 MPa to 360 MPa and the ultimate tensile strength increases from 734 MPa to 739 MPa. The variations in tensile properties of the AlCoxCrFeNi alloy result from phase structure changes and microstructure evolution. Through this research, it is demonstrated that enhancement of the tensile properties of the LMD-fabricated AlCoCrFeNi HEAs can be realized by increasing the content of Co element.

Effect of Sr on hot deformation behavior and microstructure of Al-4.6Mg alloy

The hot deformation behaviors of Al-4.6Mg and Al-4.6Mg-0.2 Sr alloys at temperatures of 300–450 ℃ and strain rates of 0.001–1 s−1 were comparatively studied. Optical microscopy (OM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to study the effect of Sr on the microstructures of the alloys. The constitutive equations of Al-4.6Mg and Al-4.6Mg-0.2 Sr alloys were established. The apparent activation energies of the two alloys were 165.632 and 157.456 kJ/mol, respectively. The microscopic analysis showed that Sr had no significant effect on the grains of the as-cast Al-4.6Mg alloy, the precipitation phase changed, and the needle-like Al4Sr phase was formed. During isothermal compression (IC), both alloy samples consisted of slender grains and new quasi-equiaxed grains. Compared with the Al-4.6Mg alloy, the Al-4.6Mg-0.2 Sr alloy had a lower LnZ value, a smaller grain size, and more sufficient dynamic recovery (DRV); in addition, the grain growth of the Al-4.6Mg-0.2 Sr alloy was suppressed. The main textures of the Al-4.6Mg alloy were Goss and cubic textures, which were replaced by cubic and brass textures after adding Sr. During the IC of the Al-4.6Mg-0.2 Sr alloy, a nanoscale precipitated τ phase, which was uniformly distributed inside spherical grains, was formed.

Effect of Ni on the microstructure and properties of Sn-6.5Zn-3.0Bi solders

ABSTRACT In this work, Sn-6.5Zn-3.0Bi-xNi solder were prepared using the covering agent smelting method. The effect of the addition of different contents of Ni on the microstructure, melting properties, corrosion resistance, and wettability of Sn-6.5Zn-3.0Bi lead-free solder was investigated. The results showed that the addition of Ni forms intermetallic compounds(IMCs) Ni5Zn21 and Ni3Sn4 with Zn and Sn, respectively, which gradually decreased the number of Sn-Zn eutectic regions and needle-like Zn phases in the microstructure until almost none, which can significantly refine the microstructure. However, with the increase of Ni content, the number of IMCs gradually increased and aggregated within the range of Ni addition, which made the coarsening of the solder microstructure, but the size of the IMCs only increased slightly. The addition of Ni caused two peaks in the DSC curve, the peak temperature of the second peak and melting range of solders both showed a sudden increase and a slow increase with the increase of Ni content. The corrosion resistance and wettability of the solder were the best when the Ni content was 0.1 wt.%.

Combination of severe plastic deformation and heat treatment for enhancing the corrosion resistance of a new Magnesium alloy

In the present study, various processes have been carried out on the extruded Mg-4Zn-4Al-0.6Ca-0.5Mn (wt%) alloy to achieve a low corrosion rate Magnesium (Mg) alloy. Microstructure studies revealed that the extruded alloy contained the needle-like Al-Mn second phases and the globular and elongated (Mg,Al)2Ca + ϕ(AlMgZn) phases. After solution treatment, the area fraction of the second phase decreased drastically from 5.1 % to 2.4 %. The later multi-directional forging (MDF) formed a modified ultrafine grain structure with an average grain size of 0.9 µm. The post-annealing process reduced the dislocation density of the heavily-deformed grains of the MDFed alloy, resulting in a grain size of 5.89 µm and an area fraction of 2.8 %. The annihilation of dislocations and homogeneous distribution of broken second phase particles after MDF and post-annealing processes improved the ultimate compressive strength and compressive strain from 352 MPa and 4.2 % for the extruded to 376 MPa and 5.1 % for the post-annealed alloy. Moreover, by applying solution treatment, MDF, and post-annealing, the corrosion rate of the extruded alloy drastically decreased from 2.13 mm/y to 0.76 mm/y due to grain refinement and reduced area fraction of phases and their random distribution. According to the corrosion results, the area fraction of second phase particles and the grain size obtained after different processes significantly influenced the corrosion properties, which was discussed in detail in the present study. On the one hand, the post-annealed alloy contained a significantly lower area fraction of particles than the extruded alloy, reducing micro-galvanic corrosion. On the other hand, significant grain refinement after the MDF process lowered the corrosion rate compared to the extruded alloy. However, the short-time annealing process annihilated the dislocations and high-energy sites while not significantly increasing the grain size and deteriorating the mechanical properties. Therefore, the combination of MDF and short-time annealing could be a remarkable route to obtaining the low corrosion rate and high strength Mg alloy.

Effect of Gd on the microstructure and mechanical properties of high-pressure die-cast Mg-La-Ce alloys at ambient and elevated temperatures

The microstructure and tensile properties of die-cast Mg-1.6 wt%La-1.0 wt%Ce alloys with Gd content ranging from 0 wt% to 3.0 wt% were investigated. The major intermetallic compound formed at the grain boundaries of the alloys was the skeleton-like Mg12RE phase, and the domains of {011} twins can be observed in Mg12RE. Three minor intermetallic compounds formed at the grain boundaries of the alloys with Gd addition, i.e., the irregular-shaped Mg2Gd phase, the needle-like Mg3Gd phase and the blocky Gd-Mn phase. The addition of Gd resulted in the increase of yield strength (YS) at both ambient and elevated temperatures. The YS of the Gd-free alloy was 135.3 MPa at ambient temperature and 57.3 MPa at 300 °C. The YS of the alloy with 3.0 wt% Gd was 162.1 MPa at ambient temperature and 97.3 MPa at 300 °C, which were increased by 19.8 % and 69.8 %, respectively, in contrast to the alloy without Gd addition. The addition of Gd increased the volume fraction of intermetallic compounds from 11.7 % to 18.5 %, while it had a very limited effect on the grain size of the Mg matrix phase. The increase in ambient YS can be mainly attributed to the increased secondary phase strengthening of intermetallic compounds at grain boundaries and the enhanced solid solution strengthening of Gd in the Mg matrix. The increased volume fraction of thermally stable intermetallic compounds Mg12RE, Mg2Gd and Mg3Gd contributed to the increase of YS at elevated temperatures. The fracture mechanism of the Gd-containing alloys at ambient temperature was a quasi-cleavage fracture.

Evolution of V-containing phases during preparation of Al-based Al−V master alloys

Al-based Al−V master alloys were prepared by both the stepwise heating melting experiment and stepwise melting cooling experiment with rapid solidification to investigate the transformation of V-containing phases which gave different effects on microstructures and properties of commercial Al alloys and Ti alloys, as both melting and solidification processes affect the evolution of V-containing phases largely. The results showed that the raw Al−50wt.%V alloy consisted of needle-like Al3V phase and Al8V5 phase (matrix), while petal-like Al3V, needle-like Al7V and plate-like Al10V phase were present in the Al−V master alloys. The metastable Al7V phase was evolved from Al3V phase and then evolved into Al10V phase during melting process. The number of Al10V phase increased with the decrease of temperature in the range of 800−1000 °C. Petal-like Al3V phases could be transformed from Al8V5 phase, pre-precipitated from Al−V molten liquid during melting process and precipitated directly during rapid solidification, respectively.

Second Phase Dissolution Evolution of an As-Cast Al-4.0Cu-1.0Li-0.4Mg-0.3Ag-0.1Zr Alloy during Multiple Homogenization Processes

Homogenization treatment is vital for eliminating eutectic structure and ensuring a preferable microstructure foundation for Aluminum Lithium (Al-Li) alloys. In this paper, solidification phases in an as-cast Al-Li alloy were revealed and their evolutions during multiple homogenization processes were analyzed by means of scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and differential scanning calorimetry (DSC) analysis. The results showed that the as-cast microstructure mainly contained needlelike Al2CuLi and Al2Cu phase, large size Cu-rich phases and netlike Ag-containing phases attached to them. As the alloy homogenized by 455°C/16h, except for needle-like phases inside grains, part of phases on grain boundaries had dissolved into the matrix and exhibited rounded shapes. As homogenized by 455°C/16h+495°C/16h, Ag-containing phase had completely dissolved into the matrix while the Cu-rich phases remained and showed two different morphologies depending on whether Mg element was contained. Prolonging the second homogenization time to 28h, no obvious change occurred for the Cu-rich phase. As homogenized by 455°C/16h+495°C/20h+512°C/20h, most Cu-rich phase had dissolved into the matrix while residual phase was mainly Fe-containing phase. This proposes an effective way to eliminate various solidification phases in Al-Li alloys and identify their contents.