Selective Laser Melting of Al-7Si-0.5 Mg-0.5Cu: Effect of Heat Treatment on Microstructure Evolution,Mechanical Properties and Wear Resistance

Al-7Si-0.5 Mg-0.5Cu alloy specimens have been fabricated by selective laser melting (SLM). In this study, the effects of solution treatment, quenching, and artificial aging on the microstructural evolution, as well as mechanical and wear properties, have been investigated. The as-prepared samples show a heterogeneous cellular microstructure with two different cell sizes composed of α-Al and Si phases. After solution-treated and quenched (SQ) heat treatment, the cellular microstructure disappears, and coarse and lumpy Si phase precipitates and a rectangular Cu-rich phase were observed. Subsequent aging after solution-treated and quenched (SQA) heat treatment causes the formation of nanosized Cu-rich precipitates. The as-prepared SLMs sample has good mechanical properties and wear resistance (compressive yield strength: 215 ± 6 MPa and wear rate 2 × 10^-13 m³/m). The SQ samples with lumpy Si particles have the lowest strength of 167 ± 13 MPa and the highest wear rate of 6.18 × 10^-13 m³/m. The formation of nanosized Cu-rich precipitates in the SQA samples leads to the highest compressive yield strength of 233 ± 6 MPa and a good wear rate of 5.06 × 10^-13 m³/m.     

Sub-solvus Cellular Recrystallization and P Phase Formation in a Single-Crystal Superalloy Containing Re

Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of c0 phase in the recrystallized aggregates which lower the activation energy for atomic migration.     

Preparation of Flaky Hexagonal Nanoporous Au-Cu and Au Particles via Dealloying

Nanoporous metals have attracted significant attention owing to their excellent physical, chemical, and biological properties. However, preparing ultrafine nanoporous metal particles (1-5 μm) with specific geometries remains challenging. Herein, we report a simple strategy to prepare ultrafine flaky hexagonal nanoporous Au-Cu and Au particles via dealloying. Mg-based alloy ribbons with ultrafine flaky hexagonal Mg-Au(Cu)-Gd particles dispersed in a Mg-Cu(Au)-Gd metallic glassy matrix were prepared. The size and morphology of the precipitated flaky hexagonal Mg-Au(Cu)-Gd particles were controlled by the solidification process of a Mg₆₁Cu₂₁Au₇Gd₁₁ alloy melt. Ultrafine flaky hexagonal nanoporous Au-Cu particles (diagonal diameter 2.58 ± 0.44 μm, ligament size ~ 28 nm), Au_-1 particles (diagonal diameter 2.38 ± 0.35 μm, ligament size ~ 83 nm) and Au_-2 particles (diagonal diameter 2.39 ± 0.44 μm, ligament size ~ 66 nm) were prepared via ultrasonic-assisted dealloying of Mg₆₁Cu₂₁Au₇Gd₁₁ alloy ribbons in 0.25 M HCl/ethanol, 1 M HCl/ethanol and 0.25 M HNO₃/ethanol solutions, respectively. The ultrafine flaky hexagonal nanoporous Au-Cu and Au particles with a large specific surface area have a uniform particle size and shape, implying that they possess adequate powder fluidity and excellent catalytic properties. Moreover, the formation mechanism of the MgAu(Cu)Gd phase in solidified Mg-Cu-Au-Gd alloys was discussed. This study provides a novel approach for synthesizing nanoporous metal particles with a specific geometry.     

固溶时效处理对Ti-5322钛合金组织与性能的影响

通过对Ti-5322合金板材进行固溶时效处理,发现在相变点以下的α+β两相区温度固溶时,初生α相随固溶温度的升高而减少,β相所占体积分数升高;在相变点以上的β单相区温度固溶时,初生α相全部溶入β基体之中,温度越高,晶粒越粗大。在510~590 ℃时效处理过程中,细小弥散的板条状次生α相在β基体内析出,起到了析出强化作用,且随着时效温度的升高,次生α相长大,所占体积分数减少,导致合金的强度下降,塑性升高。对Ti-5322合金热处理后的试样进行室温拉伸试验,发现与β单相区温度固溶时效相比,合金在α+β两相区温度固溶及不同温度时效处理后强度与塑性的匹配更加优良,因此选择在两相区进行固溶处理是最合适的,最佳工艺为870 ℃×1 h固溶+550 ℃×6 h时效。     

Effects of Ribbon Thickness on Structure and Soft Magnetic Properties of a High-Cu-Content FeBCuNb Nanocrystalline Alloy

The effects of ribbon thickness (t) on the structure and magnetic properties of a Fe_82.3B₁₃Cu_1.7Nb₃ alloy in melt-spun and annealed states have been investigated. Increasing the t from 15 to 23 μm changes the structure of the melt-spun ribbons from a single amorphous phase to a composite with dense α-Fe nanograins embedded in the amorphous matrix. The grain size (D_α-Fe) of the α-Fe near the free surface of the ribbon is about 6.7 nm, and it gradually decreases along the cross section toward the wheel-contacted surface. Further increasing the t to 32 μm coarsens the D_α-Fe near the free surface to 15.2 nm and aggravates the D_α-Fe ramp along the cross section. After annealing, the ribbon with t = 15 μm has relatively large α-Fe grains with D_α-Fe > 30 nm, while the thicker ribbons possessing the pre-existing nanograins form a finer nanostructure with D_α-Fe < 16 nm. The structural uniformity of the ribbon with t = 23 μm is better than that of the ribbon with t = 32 μm. The annealed ribbons with t = 23 and 32 μm possess superior soft magnetic properties to the ribbon with t = 15 μm. The ribbon with t = 23 μm exhibits a high saturation magnetic flux density of 1.68 T, low coercivity of 9.6 A/m, and high effective permeability at 1 kHz of 15,000. The ribbon with t = 32 μm has a slightly larger coercivity due to the lower structural uniformity. The formation mechanism of the fine nanostructure for the ribbons with suitable t has been discussed in terms of the competitive growth effect among the pre-existing α-Fe nanograins.     

Temperature Effect on Early Oxidation Behavior of NiCoCrAlY Coatings: Microstructure and Phase Transformation

Initial oxidation behavior of NiCoCrAlY coating prepared by arc-ion plating has been studied in air at 900, 1000 and 1100 °C. The results showed that phase transformation from transient θ-Al₂O₃ to α-Al₂O₃ was highly related to the temperature and oxidation time. The oxide scale in the initial stage was mainly composed of θ-Al₂O₃ at 900 °C. Instead, more amount of α-Al₂O₃ emerged out with increasing oxidation temperature. The elemental distribution after oxidation confirmed that faster chromium diffusion to the oxide scale played an important role in the speedy transformation from θ-Al₂O₃ to α-Al₂O₃. Y segregation at scale/coating interface resulted in less cavity formation and hence improved the oxide scale adherence.     

化学成分对GH4169合金组织与力学性能的影响

针对不同Nb、Al、Ti含量的GH4169合金,对其进行固溶时效热处理,研究微观组织与拉伸性能的变化规律。研究表明,一定的Al/Ti和(Al+Ti)/Nb值下,随Nb含量增加,δ相含量增加,并且具有较好的高温稳定性,高温固溶可有效阻止晶粒的长大,主要通过稳定的δ相来保证合金的细晶强化。当Al/Ti与(Al+Ti)/Nb比值较低时,时效后γ″、γ'含量有所降低,γ″相形貌由唇状变为圆盘或芝麻粒状,其中Nb元素主要形成δ相。力学性能表明,Al/Ti和(Al+Ti)/Nb值一定时,拉伸强度主要受晶粒尺寸影响,而δ相含量影响较小。但当Al/Ti与(Al+Ti)/Nb比值较低时,由于强化相γ″和γ'数量与形貌的变化,拉伸力学性能显著降低,其微观硬度也随拉伸强度的降低而降低。     

Correlation Between Magnetic Properties and Allotropic Phase Transition of Fe–Co–V Alloy

In this study, the allotropic phase transition and its effect on the magnetic behavior of Fe Co–7 wt%V alloy were investigated. It was found that c phase is observed in the microstructure in the as-cast condition, and it diminishes after severe cold rolling(90% reduction). After annealing at temperatures higher than 500 up to 750 ℃, the c phase is observed in the structure, again. But, this phase is disappeared by annealing at temperatures above 750 ℃ due to the formation of vanadium-rich precipitates. Thermocalc software was used in order to elucidate the influence of vanadium percent on the stability of c phase in Fe–Co alloys. Also, magnetic studies showed that the saturation induction is reduced by annealing at temperatures from 500 up to 750 ℃, which is related to the formation of residual non-magnetic γ phase.     

脉冲电流冲击处理对TC11钛合金组织与性能的影响

利用光学显微镜、扫描电镜、X射线衍射仪、万能材料试验机和显微硬度计等研究了脉冲电流冲击处理(EST)对TC11钛合金微观组织和力学性能的影响。结果表明:不同脉冲数对TC11钛合金微观组织中α相的比例、β转变组织的板条尺寸和残余压应力大小影响明显,随EST脉冲数的提升,α相含量及板条状β转变组织中次生α相含量呈现先增加后减少的趋势。经最佳工艺参数(900 A、50 Hz、25个脉冲)脉冲电流冲击处理后,TC11钛合金中α相细小均匀,β转变组织板条长度、厚度和间距较处理前试样分别减小了51.9%、58.0%和36.8%,此时合金力学性能提升最为明显:伸长率提高了12.7%,显微硬度增加了4.7%,残余压应力增加48.4%。     

Effect of heat treatment on microstructures and mechanical properties of A356 alloy cast through rapid slurry formation (RSF) process

The effect of T5 heat treatment on microstructure and mechanical properties of A356 alloy was observed. The as-cast A356 alloy exhibited coarse dendrites and long Si needles. RSF process changed the dendritic α-Al phase to globular morphology which helps in improving the mechanical properties of the alloy. The addition of 0.6wt-% Al–5Ti–1B grain refiner refined the average grain size of primary α-Al phase. T5 heat treatment at 170 °C for 20 h in different processing conditions was given to A356 alloy. T5 heat treatment led to further refinement of α-Al phase and Si needles, precipitation hardening due to Mg₂Si phase and reduction in the porosity level (%). The Quality Index for A356 alloy in different processing conditions was also measured. Results showed that RSF process with the use of baffles, addition of grain refiner and T5 heat treatment had improved the mechanical properties over other processing conditions.     

Internal cooling to produce aluminium alloy slurries for rheocasting

Rheocasting is becoming the choice of the casting industry which relies on the semi-solid processing for high integrity structural parts. It is thus of great technological interest to identify simple methods to prepare slurries at reduced cost. The potential of internal cooling to produce slurries for rheocasting was investigated in the present work for several aluminium casting alloys which are technologically important for aluminium foundries. Alloys quenched directly from the liquid state were predominantly dendritic while the slurry samples quenched after an initial fraction of solid phase was first formed, were dominated by α-Al rosettes and globules. The solidification of the remaining liquid phase occurred through the growth of the α-Al rosettes and globules which have formed during internal cooling before quenching.     

Effect of Heat Treatment on the Microstructure,Mechanical Properties and Corrosion Resistance of Selective Laser Melted 304L Stainless Steel

The influence of heat treatment holding temperatures from 600 to 1300 °C on the microstructure, mechanical properties and corrosion resistance in selective laser melted(SLMed) 304L stainless steel is investigated in this work. The results reveal that there is no remarkable microstructure change after holding at 600 °C for 2 h, while recrystallization leads to a slight decrease in grain size in the temperature range of 700–900 °C. The heat treatment at temperatures from 1000 to 1300 °C for 2 h ob...     

Structural, thermodynamic, and electrochemical properties of TixZr1−x(VNiCrMnCoAl)2 C14 Laves phase alloys

The crystal structure of the monoclinic phase η-Al₁₁Cr₂ of the space group C2/c, a ≈ 1.76 nm, b ≈ 3.05 nm, c ≈ 1.76 nm, β ≈ 90° [L.A. Bendersky, R.S. Roth, J.T. Ramon, D. Shechtman, Metall. Trans. A 22A (1991) 5] has been determined by single-crystal X-ray diffraction. The structure model, refined to a final R value of 0.0441, has the composition of Al_83.8Cr_16.2. a = 1.77348(10) nm, b = 3.04555(17) nm, c = 1.77344(10) nm, monoclinic angle β = 91.0520(12)°. There are 80 (66Al + 14Cr) independent atomic positions in a unit cell, of which all Cr atom sites and 8 Al atom sites have icosahedral coordination. These icosahedra are interconnected forming icosahedral chains along , (1 0 1) icosahedral layer blocks as well as a three-dimensional icosahedral structure.     

The Minor Precipitation at the Final Stage of U720Li Solidification

The minor precipitations caused by B and Zr which are the normal constituents of U720 Li alloy have been studied by analyzing the solidification process and the composition evolution. The present study aims to supply the elementary information about the existing form of B and Zr in the as-cast microstructure, which is helpful for the subsequent processing, such as homogenization treatment. The M_3B_2 and Ni_5Zr phases were observed in the U720 Li alloy in as-cast state, which were usually accompanying with each other together with g-Ni_3 Ti phase at the edge of eutectic(γ+γ'). Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following: c matrix, eutectic(γ+γ'), g-Ni_3Ti, M_3B_2 and Ni_5Zr. The in situ composition analysis by EDS and EPMA revealed that the precipitation and microstructure were governed by the composition evolution in the liquids. The solidification of c matrix increased the Ti concentration in the residual liquids and resulted in the eutectic(γ+γ') formation; the(γ+γ') formation increased the Ti/Al radio in the liquids and the g-Ni_3Ti was formed in front of the eutectic(γ+γ'); the g-Ni_3Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M_3B_2 boride is formed;the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni_5Zr is formed finally. The melting points are in the ranges of 1130–1140 °C for Ni_5Zr phase, 1180–1190 °C for M_3B_2 boride and1190–1200 °C for g-Ni_3Ti phase.     

Influence of Rust Layers on the Corrosion Behavior of Ultra-High Strength Steel 300M Subjected to Wet–Dry cyclic Environment with Chloride and Low Humidity

The influence of rust layers on the corrosion behavior of ultra-high strength steel 300 M subjected to a simulated coastal atmosphere was investigated by corrosion weight loss, surface analysis techniques, and electrochemical methods.The results exhibit the presence of a large proportion of c-Fe OOH and a-Fe OOH and a small amount of Fe3O4 in the outer rust layer. During the wet–dry cyclic process, the bonding performance and the density of outer rust layer deteriorate with the thickness of outer rust. The inner rust layer plays a main role on protectiveness, which can be attributed to the formation of an ultra-dense and adherent rust film with major constituent of a-Fe OOH and a-Fe2O3 on the steel.     

Revealing the Diversity of Dendritic Morphology Evolution During Solidification of Magnesium Alloys using Synchrotron X-ray Imaging: A Review

In this paper, the diversity of complicated dendrite microstructure and its evolution behavior during solidification in different magnesium alloys under various processing conditions were illustrated using synchrotron X-ray imaging technique. A variety of dendritic morphologies and branching structures were revealed, i.e., sixfold plate-like symmetric structure in Mg-Al-based structure, 12-branch structure in Mg-Zn-based alloys and 18-branch structure in Mg-Sn- and Mg-Ca-based alloys as well as seaweed like hyper-branched structure in Mg-38wt%Zn alloy. In addition, a dendrite morphology and orientation transition with increasing addition of Zn content were also observed in Mg-Zn alloy, with dendrite growth pattern transform from anisotropy (low Zn addition) with sixfold symmetric snow-flake structure to relative isotropy (intermediate Zn addition) where seaweed morphology presented and then back to anisotropy (high Zn addition) when only 12 branches with preferred < $11\overline{2}1$ > orientations were observed. The phase-field model representing the typical dendritic morphologies and branching structures under various conditions was also depicted and discussed. Further, the two-dimensional (2D) real-time dendrite growth dynamics in different Mg-based alloys captured using synchrotron X-ray radiography for unveiling the originate of the α-Mg dendrite was reviewed. Following this, the four-dimensional (3D + time) synchrotron X-ray tomographic in situ observation of dendritic morphology evolution indicating the formation mechanism of the diverse dendritic morphology during Mg-Sn- and Mg-Zn-based alloys was also summarized. Finally, the future study on exploring the complicated dendritic morphologies and their origination during solidification of Mg-based alloys is prospected.     

Directional Growth of Tin Crystals Controlled by Combined Solute Concentration Gradient Field and Static Magnetic Field

A new method was introduced to achieve directional growth of Sn crystals. Microstructures in liquid(Pb)/liquid(Sn) diffusion couples were investigated under various static magnetic fields. Results show that the β-Sn crystals mainly reveal an irregular dendritic morphology without or with a relatively low static magnetic field(B0.3 T). When the magnetic field is increased to 0.5 T, the β-Sn dendrites close to the final stage of growth begin to show some directional character. With a further increase in the magnetic field to a higher level(0.8–5 T), the β-Sn dendrites have an enhanced directional growth character, but the dendrites show a certain deflection. As the magnetic field is increased to 12 T, the directional growth of the β-Sn dendrites in the center of the couple is severely destroyed. The mechanism of the directional growth of the β-Sn crystals and the deflection of the β-Sn crystals with the application of static magnetic field was tentatively discussed.     

Microstructure Evolution and Phase Transformation of Ti–6.5Al–2Zr–Mo–V Alloy During Thermohydrogen Treatment

Thermohydrogen treatment(THT) is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys.In the current work,as-cast Ti–6.5Al–2Zr–Mo–V alloy was hydrogenated with different hydrogen contents and processed solution aging.Accordingly,the microstructure evolution and phase transformation were analyzed.Results show that during solution aging,eutectoid decomposition occurs and the product is a mixture of coarse primary a,fine eutectoid product and undecomposed bH.The size of primary a is closely dependent on the hydrogen content,and large primary a can be obtained at medium hydrogen content.Further,the influence of hydrogen content on the growth of primary a phase was revealed.The primary a is much fine,and the eutectoid product is relatively homogeneous with 0.984 wt% H.After THT,the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.     

Texture of TiNi shape memory alloy sheets produced by roll-bonding and solid phase reaction from elementary metals

We have developed a new process to produce Ti–Ni shape memory alloys from titanium and nickel sheets. This process is composed of repetitive roll-bonding for producing a Ti/Ni laminated sheet from them, and of subsequent heat treatment for forming a TiNi intermetallic phase by reactive diffusion. The Ti/Ni laminated sheet changed to a nearly single phase TiNi material through the formation of Ti₂Ni and Ni₃Ti intermediate products. TiNi sheets obtained by this process consisted of a near <111>//ND texture such as {223}<110> in the B2 austenite phase. Recoverable strain associated with martensitic transformation of TiNi was fairly isotropic in the sheet plane due to the relatively isotropic character of {111}<uvw> texture. The formation of such texture through phase transformations by reactive diffusion has been discussed in view of texture inheritance.     

3D Fe-Rich Phases Evolution and Its Effects on the Fracture Behavior of Al-7.0Si-1.2Fe Alloys by Mn Neutralization

The evolution of the 3D Fe-rich phases of Al-7.0Si-1.2Fe alloys with different Mn contents was visualized and characterized using synchrotron X-ray computed tomography, and the effect of Fe-rich phases with typical morphologies on the fracture behavior during tensile testing was analyzed. The results showed that the Fe-rich phase changed from platelet-like β-Al₅FeSi into α-Al₁₅(FeMn)₃Si₂ with various morphologies after the addition of Mn. The Mn addition not only significantly reduced the volume fraction, equivalent diameter and interconnectivity of the Fe-rich phase but also greatly increased the sphericity, surface thickness, and distribution of the mean curvature and surface thickness. Furthermore, the equivalent diameter of α-Al₁₅(FeMn)₃Si₂ had an inverse exponential function relationship with its sphericity. The 3D morphology of α-Al₁₅(FeMn)₃Si₂ can be summarized as massive and regular polyhedrons, hollow and regular polyhedrons, and multibranched polyhedrons. The fraction of the different 3D morphologies in each alloy is related to the Mn content, where excess Mn increased the number and volume fraction of the large Fe-rich particles with a low sphericity. The ductility of each alloy was significantly improved by the addition of Mn but gradually decreased when the Mn/Fe ratio exceeded 1.2. The increase in large α-Al₁₅(MnFe)₃Si₂ with a low sphericity was the main reason for the decreased ductility of alloys with a high Mn content.