Needle-like Phase Research Articles

Thermal conductivity of binary Al alloys with different alloying elements

This work investigates the physical properties of several binary Al-Si/Ni/Fe/Cu/Mg alloys with varying alloying element contents and microstructures. The findings indicate that as the content of alloying elements increases, the number of secondary phases in binary Al alloys rises, and these phases become coarser. In hypereutectic compositions, primary phases in Al-Ni and Al-Fe alloys take on a needle-like morphology with high aspect ratios, forming a dendritic network. Regarding properties, in the hypoeutectic region, both thermal conductivity and electrical conductivity of binary alloys initially decrease sharply, followed by a more gradual decline with the addition of alloying elements. This trend occurs because thermal conductivity is mainly influenced by supersaturated solid solutions. In hypereutectic Al-Ni and Al-Fe alloys, the presence of large, needle-like primary phases significantly hinder the heat transfer, resulting in a more apparent reduction in both thermal conductivity and electrical conductivity. Among the five binary alloys, Al-Ni and Al-Fe alloys, which have lower solute concentrations of alloying elements in the Al matrix, exhibit the highest thermal and electrical conductivity. Furthermore, with increasing alloying element contents, the phonon thermal conductivity of Al-Si alloy increases more markedly compared to other alloys. The study will offer a fundamental understanding for the development of advanced alloys.

Effect mechanism of Nb addition on grain refinement and inhibition of discontinuous precipitation of Cu–15Ni–8Sn alloy

Cu–15Ni–8Sn-xNb alloys (x = 0, 0.2, 0.6 wt%) were prepared using a medium-frequency induction melting furnace. The effect of Nb addition on grain refinement and inhibition of discontinuous precipitation (DP) in Cu–15Ni–8Sn alloys was systematically studied to elucidate the mechanism by which microstructural characteristics contribute to strength improvement. The results indicate that the increase the Nb content from 0 to 0.6 wt% reduces the average grain size of the as-cast alloy from approximately 524.8 μm to approximately 81.3 μm, and significantly decreases the lamellar transition structure region (α+γ). During solution treatment, dispersed needle-like NbNi3 phases were observed in the Cu–15Ni–8Sn-0.2Nb alloy. After aging, the DP growth rate in the Cu–15Ni–8Sn-0.2Nb alloy was notably slower than those of the other alloys. This was attributed to NbNi3 phases at the grain boundaries hindering DP nucleation, with intragranular NbNi3 phases inhibiting DP growth. The phase transformation order of the solid solution Cu–15Ni–8Sn-0.2Nb at 673 K aging is: spinodal structure → D022 ordered phase → L12 ordered phase → DP. The hardness and tensile strength of the Cu–15Ni–8Sn-0.2Nb alloy peaked at 338.3 HV and 725.42 MPa, respectively, after aging for 120 min. Using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) and Arrhenius equations, the activation energies of DP in Cu–15Ni–8Sn-xNb (x = 0, 0.2, 0.6 wt%) alloys were calculated to be 93.19, 148.64, and 98.33 kJ/mol, respectively. These values suggest that the diffusion of DP atoms in the Nb-containing alloys is hindered, which effectively inhibits DP formation.

Effect of Al-5Ti-1B refiner content on the microstructure and mechanical properties of V-x(Al-5Ti-1B) alloys for hydrogen purification

Effect of Al-5Ti-1B refiner content on the microstructure and mechanical properties of V-x(Al-5Ti-1B) alloys manufactured by vacuum arc melting for hydrogen purification has been investigated in this work. Significant changes appeared in the grain morphology of V-xATB (x=2, 3, 4, 5) alloys with the Al-5Ti-1B (ATB) content increasing (2–5 wt%), while slender needle-like TiB phases were gradually precipitated inside the grains and increased in quantity, and grains were gradually refined. The precipitation of TiB phases in V-xATB alloys is advantageous for grain refining. Forming reasons of TiB phase and its morphology were discussed. The fracture mode for all five alloys is brittle fracture at room temperature. The combined strengthening effect of solid solution, fine-grained and precipitation of V-xATB alloys gradually improve with increasing ATB content, increasing the alloys' hardness and strength. The hardness and strength of the V-5ATB alloy are the highest, but the elongation is lower despite the fact that the grain size is the smallest among V-xATB alloys. This is because the extent of plasticity increase caused by grain refinement in the V-5ATB alloy is less than the extent of plasticity decrease caused by the strengthening of solid solution and hard-brittle TiB phase precipitation. This can be attributed to the significant reduction in plasticity caused by the solid solution and hard-brittle TiB phases precipitation of V-5ATB alloy.

Castable refractory materials from magnesium oxychloride cement-bonded cordierite-mullite

Cordierite-mullite ceramics are well-suited for use as kiln furniture due to their superior mechanical strength and thermal shock resistance. This study explores the development of cordierite-mullite refractory castables bonded with magnesium oxychloride (MOC) cement. These castables were created using calcined kaolin aggregates and a matrix comprised light-burned magnesia, calcined alumina, milled sand, and magnesium chloride hexahydrate. Interaction between light-burned magnesia and magnesium chloride hexahydrate produced a needle-like MOC cement phase, significantly enhancing the green strength of the castables. After firing at 1350 °C for 4 h, the MOC crystals decomposed, forming robust cordierite and mullite phases. The resulting sintered castables demonstrated a modulus of rupture of 14.55 MPa, a bulk density of 1.95 g/cm3, a porosity of 24.84 %, and a thermal expansion coefficient of 2.89x10−6°C−1. These findings indicate the effectiveness of MOC cement bonding for manufacturing high-strength cordierite-mullite refractory castables, making them ideal for kiln furniture applications.

Effect of Mg/Si ratio on synergistic improvement of formability and yield strength in Al-Mg-Si-Zn alloys

The design of Al-Mg-Si-Zn series alloys combines the advantages of 6xxx and 7xxx series alloys and breaks through the limitations of strength and formability matching of traditional alloys. In this study, we achieved an exceptional strength-formability combination in the Al-0.84Mg-0.79Si-3.38Zn (wt.%) alloy. Following a solution treatment at 550 °C for 3 min, the alloy exhibited an elongation of ∼36.8%, with a yield strength of ∼356 MPa after artificial aging at 175 °C for 8 h. The effects of different Mg/Si ratios (∼0.6, ∼1.2, and ∼2.5, at.%) on the strength-formability interplay in Al-Mg-Si-Zn alloys were investigated. The alloy with a low Mg/Si ratio (∼0.6) facilitated the precipitation of fine β'' phases characterized by a high number density (∼8.4 nm−3 × 10−5). A Mg/Si ratio of ∼1.2 proved advantageous for precipitating larger β'' phases with a higher volume fraction (∼0.69%), resulting in an elevated precipitation strengthening effect (∼290 MPa). However, with an increase in the Mg/Si ratio to ∼2.5, the reduced availability of Si atoms for forming Mg-Si clusters led to a decrease in the number density of precipitates. Additionally, the interaction among Fe, Mn, and Si elements resulted in the presence of blocky α-Al(FeMn)Si phases, as opposed to the needle-like β-AlFeSi phases, in the alloy with a Mg/Si ratio of ∼1.2. This effectively prevented the degradation of elongation. Our findings present a comprehensive strategy for the compositional design of high-performance aluminum alloys.

Ultrasonic peening-waterjet composite surface modification of 7075-T6 aluminum alloy for residual stress release and transformation mechanism

The release and transformation mechanism of residual stress contributes to the improvement of metal materials' processing and performance, enhancing their structural stability, surface integrity, and fracture resistance. This paper focuses on the Ultrasonic Impact Treatment-Solid Particle Entrainment by Waterjet (UIT-SPEWJ) composite surface modification of 7075-T6 aluminum alloy, investigating the effects of UIT-SPEWJ process parameters (jet pressure and target distance) on the alloy's surface quality, roughness, microhardness, residual stress, and the evolution mechanism of microstructure. The results indicate that the 7075-T6 aluminum alloy modified by UIT-SPEWJ mainly exhibits a "meteorite crater" effect, accompanied by significant smearing and ploughing phenomena. Surface roughness shows a trend of first decreasing and then increasing with the rise of jet pressure and target distance. The minimum surface roughness, 0.852 μm, is achieved at a jet pressure of 25 MPa and a target distance of 7.5 mm. The microhardness of the samples modified by UIT-SPEWJ gradually decreases from the surface to the substrate with increasing depth. The hardened layer depths of the samples UIT-SPEWJ-1–6 after modification are approximately 174 μm, 226 μm, 200 μm, 196 μm, 176 μm, and 172 μm, respectively. After UIT, SPEWJ, and UIT-SPEWJ surface modifications, the surface of 7075-T6 aluminum alloy mainly exhibits compressive residual stress (CRS). The surface residual stress σ_srs of samples modified by UIT and SPEWJ are approximately 195.161 and 215.752 MPa, respectively. The surface residual stresses of UIT-SPEWJ-1–6 samples are significantly improved to 277.089, 529.499, 393.615, 459.261, 368.084, and 220.635 MPa, respectively, with UIT-SPEWJ-2 sample having the highest surface residual stress, which is 2.7 and 2.4 times that of UIT and SPEWJ modified surfaces. Samples modified by UIT-SPEWJ present significant dislocation phenomena, and the precipitated phases mainly include the needle-like metastable phase η', as well as the equilibrium phase η (MgZn2) with a rod-like structure. At lower jet pressures and larger target distances, the 7075-T6 aluminum alloy exhibits a continuous PFZ with a size of approximately 12–23 nm. As jet pressure increases and target distance decreases, discontinuous grain boundary precipitate bands appear, sized around 8–17 nm. Moreover, when the jet pressure is 25 MPa and the target distance is 7.5 mm, the grain boundary precipitated phases are more dispersed.

Dynamic mechanical behavior of ultra-high specific strength lightweight Ti61Al16Cr10Nb8V5 multi-principal element alloy

This study investigated the dynamic mechanical property and microstructure evolution of the Ti61Al16Cr10Nb8V5 (Ti61) lightweight multi-principal element alloy(MPEA) with a density of 4.83 g/cm3. After melting and rolling, the Ti61 alloy was composed of a BCC matrix phase and dispersed B2 phase, and also contained a small amount of fine needle-like FCC phase and spherical HCP phase. Mechanical testing of Ti61 alloy revealed that the ultimate tensile strength is about 1223 MPa, and the tensile elongation is about 13.2 %, with a specific strength of around 253 MPa*cm3/g. Dynamic compression testing of Ti61 alloy showed a significant strain rate strengthening effect and excellent fracture strain greater than 50 %. Calculation and microstructural observations indicated that at low strain rates ranging from 10−3/s to 10/s, the strain rate sensitivity of Ti61 alloy was 0.0087, with the main plastic deformation mechanism of dislocation nucleation, dislocation slip, and stacking faults. At high strain rates ranging from 500/s to 5000/s, the strain rate sensitivity raises to 0.0963 and the main deformation mechanism of the Ti61 alloy involved higher-density dislocation nucleation, dislocation slip, stacking faults and twinning. At strain rates of 5000/s, HCP-FCC phase transition accompanied by FCC microtwins and BCC-α2HCP phase transition occurred. Furthermore, dynamic recrystallization also occurred in Ti61 alloy. The HCP-FCC has the orientation of (0001)HCP//(11−1)FCC, [1−210]HCP//[011]FCC. The BCC-α2HCP has the orientation of {101}BCC//{0001}HCP and [111]BCC//[2−1−10]HCP. Twining and phase transition strengthen the yield stress of Ti61 alloy, while dynamic recrystallization reduced the flow stress of Ti61 alloy. Ti61 alloy has high strength, high compression fracture strain, and ultra-high specific strength, which is a new LWMPEA with great application potential in military armor.

Evolution behavior of Fe and Fe-rich phases in high-Fe concentration Al-Si-Mg alloy with Mn, Cr, and Co alloyed

In this work, we report the evolution behavior of Fe and Fe-rich phases in the high-Fe concentration recycled Al-Si-Mg aluminum melt with and without alloying elements (Mn, Cr, Co) introduced. Firstly, the thermodynamic calculations reveal that the addition of Mn, Cr, and Co could improve the nucleation temperature of the Fe-rich phase. Moreover, four solidified ingots for Al-10Si-0.5Mg-1Fe alloy(base alloy) and the base alloy with 1.3 wt% Mn, 0.8 wt% Cr, and 0.8 wt% Co added, respectively, were prepared by holding the melt for 30 min at 610 ℃ after all the raw materials were fully melted. Microstructure examination revealed that the addition of Co resulted in the formation of the needle-like Fe-rich phase, which further leads to a poor settling effect and low iron removal efficiency. In contrast, the addition of Mn and Cr favors converting β-AlFeSi into α-AlFeMnSi, and forming the Chinese script and fish-bone Fe-rich phase. The size of the Fe-rich phase formed after adding Cr is smaller, while the residual Fe content after adding Mn is less. Also, the sludge phase at the bottom of the ingot presents a polygonal shape and long rod shape after adding Mn and Cr, respectively. Further, the Mn alloying increases the particle size of the sludge, while Cr refines the sludge phase and increases the aspect ratio (length/width ratio) of the sludge. Particularly, for Al-Si-Mg alloy with 1.0% Fe content, the superior mechanical properties are obtained by adding 0.8 wt% Cr, and the tensile strength reaches 136 MPa. After adding 1.3 wt% Mn, the Fe content of the upper section is reduced to 0.37 wt%, and the Fe removal rate achieves 63%. The obtained results provide some guidelines for achieving high iron separation efficiency in recycled aluminum-silicon alloy melts.

Improving the frost-resistance performance of supersulfated cement by reducing the crystalline-to-gel ratio through the addition of nano-SiO2

The poor frost resistance of supersulfated cement (SSC) is of great concerns for its application. This research assessed the frost resistance performance of SSC with 1 wt.% and 3 wt.% cement clinker activator (C1 and C3) and explored the effects of nano-SiO2 (NS) (1 wt.% and 3 wt.%, N1 and N3) on the frost resistance of SSC through macro- and micro-evaluation on loss of the compressive strength/the mass/the relative dynamic elastic modulus and XRD/MIP/SEM. Results showed that the frost resistance of C1 sample was superior than those of C3 sample. The addition of NS enhanced the frost resistance of SSC: the maximum mass loss was reduced by about 92.8% and the maximum relative dynamic elastic modulus loss was decreased by about 34.6%. This could be ascribed to the change of the porous microstructure: the portion of micro-pores smaller than 20 nm increased and the microstructure was densified. Moreover, Moreover, NS changes the long needle-like crystalline phase (AFt) shape to a short rod-like shape, the latter of which was more tightly wrapped by the gel phase, leading to a more compact microstructure, and the detachment of AFt from gel matrix (would be introduced by the difference of their contraction/expansion rate under F-T cycles) could result in the poor performance of SSC, while this could be relieved by the reduction of the AFt-to-gel ratio as seen from the NS-added SSC sample.

Recrystallization Behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si Alloy under Electroshocking Treatment

In this work, a novel electroshocking treatment (EST) method is employed to modify the microstructure of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy. The microstructural morphology and texture distribution after EST are characterized and analyzed to study the recrystallization behavior of the titanium alloy. After EST with 0.04 s, secondary αs phase transforms into β phase. After EST with 0.06 s, surrounding the αp phase, a significant amount of needlelike martensitic phase (αM) precipitates. Electron backscatter diffraction results reveal that EST reduces intragranular orientation gradients, resulting in a convergence of each point orientations within individual grains. Under longer EST time, grain boundaries display greater irregularity. After EST with 0.05 s, partial recrystallization takes place, and with an increase of EST time to 0.06 s, all deformed grains undergo complete recrystallization, forming defect‐free recrystallized grains. A substantial enhancement in texture intensity for both α and β phases, exhibiting prominent preferred orientations, and the increase in hardness values is contributed to the precipitation of αM phase. The stress analyses indicate that EST can optimize the distribution of residual stress and offer a potential solution for improving fatigue performance. In all results, it is shown that EST is an effective approach for manipulating the microstructure and optimizing the residual stress distribution of titanium alloys.

Effect of Ni addition on the microstructure, conductivities and mechanical properties of as-cast Al-Fe alloys

In this study, the microstructure, conductivity and mechanical properties of as-casted Al-1Fe-xNi (x= 0, 1, 4, 6 and 10, wt%) alloys were systematically investigated. The results exhibited the Al-1Fe-xNi alloys mainly consist of α-Al, Al3Fe, Al3Ni, and Al9FeNi phase. With the increase of Ni addition, the size of α-Al grain decreases and the dimension of Al9FeNi phase increased. The hardness of the as-cast alloys raises with increasing Ni content, whereas the elongation and electrical conductivity decreases and the ultimate tensile strength (UTS) first increases and then decreases dramatically. The Al-1Fe-6Ni alloy exhibited the highest Q-value, which was approximately 11.27% higher than that of the Ni-free alloys. This mechanism of mechanical properties change occurs mainly due to slightly Ni effectively refines the α-Al phase, and the excessive Ni content leads to formation of coarse needle-like or lath-like Al9FeNi phase. Additionally, the grain refinement and precipitation of the second phase upon Ni addition impede electron movement, leading to a decrease in the electrical conductivity of the alloy. The corresponding ductile fracture morphologies of Al-1Fe-xNi alloys are also analyzed. These high-strength Al-1Fe-xNi alloys could be helpful for developing novel Al-Fe alloys with a high strength and ductility.

Phase-field simulation for evolution of iron-rich phase during solidification of Al–Si–Fe alloys

Iron is a common impurity element in Al–Si alloys. The existence of Fe promotes the formation of needle-like iron-rich phase, which has an adverse effect on the macroscopic mechanical properties. In this study, the effects of casting temperature, Fe content and melt ultrasonic treatment on the morphology of β-Al9Fe2Si2 were investigated by phase-field simulation and experiment. The results show that when the casting temperature is close to the liquidus temperature, the grain size of the precipitated phase will be refined. The increase of iron content in the alloy will promote the increase of average grain size and volume fraction of iron-rich phase by inhibiting the growth of α-Al grains. In addition, the elevation of both casting temperature and Fe content promotes the transformation of the β-Al9Fe2Si2 from short rod-like to needle-like with large scale. The melt ultrasound inhibits the transformation to β-Al9Fe2Si2 by promoting the nucleation and growth of α-Al8Fe2Si. Melt ultrasound, casting temperature and iron content together cause the change of needle-like iron-rich phase morphology.

Microstructure Features and Mechanical Properties of Casted CoFeB Alloy Target

CoFeB alloy, as a promising magneto-resistive material, has attracted extensive attention concerning the magnetic properties of its thin film in the field of magneto-resistive random memory (MRAM). Although there are many studies on the magnetic properties of CoFeB thin films, there is relatively little research on the microstructure and mechanical properties of casted CoFeB alloy. In this work, Co20Fe60B20 (at%) alloy was fabricated through the vacuum induction melting method, and its microstructure features and mechanical performance were studied. Scanning electron microscopy (SEM), electron back scatter diffraction (EBSD), and transmission electron microscopy (TEM) were utilized to characterize the microstructure, which consists of the coarse, needle-like Fe2B phase that crystallizes first, the primary lamellar binary eutectic structure (Fe2B + bcc-Fe), and the ternary eutectic structure (Fe3B + Fe2B + bcc-Fe phase). It is found that Fe3B precipitates on the Fe2B with a core–shell structure. The orientation of bcc-Fe is randomly distributed, while there are two main kinds of textures in Fe2B: {100} <001> and Gaussian texture {110} <001>. In terms of mechanical properties, Co20Fe60B20 alloy’s tensile strength is 140MPa, and the yield strength is 87MPa. Because the cracks are easy to generate and expand along the needle-shaped pre-crystallized Fe2B, the plasticity of Co20Fe60B20 alloy is very poor, only 1%.

Effects of annealing on the mechanical and wear resistance properties of a designed novel eutectic high-entropy alloy

In this work, a eutectic high-entropy alloy, CoCrFeNi(TiNb)0.325, was successfully prepared via vacuum arc melting. In this investigation, the microstructures, mechanical properties, and wear behaviors of the samples in the as-cast state and after annealing at temperatures of 850 °C and 1160 °C were systematically examined. The results demonstrated that annealing significantly influenced the microstructures, mechanical properties and wear behaviors of CoCrFeNi(TiNb)0.325 EHEAs. After annealing at 850 °C, a substantial number of nanoscale L12 phases and needle-like phases precipitated within the alloy microstructure, remarkably increasing alloy hardness and yield strength from 635.34 HV and 1552.3 MPa in the as-cast state to 742.34 HV and 2537.7 MPa, respectively. Annealing at 1160 °C resulted in the disappearance of the eutectic structure within the alloy microstructure, causing a spherical transformation that enhanced the ductility of the alloy. The wear mechanisms for all three alloys were found to involve a combination of oxidative wear and delamination wear, although their severity varied. For the EHEA annealed at 850 °C, the combined effect of high hardness and the precipitation of needle-like phases with a noncoherent interface played a significant role in slowing delamination wear. As a result, this alloy exhibited the lowest wear rate at 3.41 × 10−5 (mm3/Nm).

Designing eutectic medium-entropy alloys CoFeNiTax with outstanding mechanical properties

The eutectic medium-entropy alloys CoFeNiTax (x = 0.4, 0.5, 0.6) were designed successfully based on binary eutectic systems and thermodynamics calculations in this study. With the increase of Ta content, the solidified microstructures evolved from hypoeutectic to full eutectic and finally to hypereutectic. The primary phases in the hypoeutectic and hypereutectic were FCC solid solution and C14-Laves phase, respectively. The solidified microstructures were coarsened and spheroidized by the heat treatment, and a high-density of needle-like Laves phase precipitated in the FCC phase of the CoFeNiTa0.4. The crystallographic orientation relationship between the precipitated phase and matrix phase was [2‾ 42‾ 3]Laves//[011]FCC, and the crystal plane (1 1‾ 1)FCC was almost parallel to (0 1‾ 12)Laves, just with a mismatch angle of about 6°. The compressive strength and fracture strain of the heat-treated alloys were within 2232∼2465 MPa and 22.5–32.8 %, respectively, being superior to most high-entropy alloys. The excellent combination of strength and toughness was attributed to the synergistic effects of various strengthening-toughening mechanisms, including the second-phase strengthening, the solid solution strengthening, the interface strengthening and toughening, and the twinning toughening.

Microstructure-Property Relationship and Embrittlement Mechanism of HR3C Steel with High-Temperature Service of 65000 h

HR3C steel, an austenitic helat-resistant steels, due to its good strength, high temperature behaviour and cost effectiveness, which lead to the extensive use in ultra-supercritical boilers. In order to study the microstructure characteristics and embrittlement mechanism of superheater tube of 660 MW thermal power boiler, the microstructure and chemical composition as well as the mechanical performance of HR3C steel pipe after service were tested. The results showed that HR3C steel, after approximately 65,000 hours of service, exhibited a continuous sheet-like distribution of M23C6 phasealong the grain boundaries, accompanied by needle-like or strip-like M23C6 phase growing into the grain boundaries, as well as the presence of Z phase (NbCrN) and σ-equivalent precipitation phase along the periphery of the grain boundaries. Following service, the hardness of HR3C steel experienced a slight increase, the tensile strength remained relatively unchanged, and the yield strength exhibited an increase of approximately 15%. However, the elongation after fracture significantly decreased, resulting in a decrease in plasticity decreased by 64% to 73% compared to its original state. HR3C steel displayed notable embrittlement after 65,000 hours of service at 650 °C, with a 96% reduction in impact toughness. The precipitation of M23C6 and σ phase were identified as the primary causes of embrittlement in HR3C steel.

Design and characterization of LaB6/NbNiTaTi refractory high-entropy alloy coatings: Effect of in-situ TaB phase on strengthening mechanism

The excellent strength and hardness obtained from the customized phase compositions provide vast spaces for surface protection application of refractory high entropy alloy (RHEA) coatings in titanium alloys. However, the introduction of high-strength brittle phase in RHEA coatings commonly increases the cracking tendency and affects their service life. In this work, we propose a design strategy to enhance the mechanical properties of RHEA coatings by introducing an extremely stable high strength boride combined with a ductile phase. Experimentally, LaB6/NbNiTaTi RHEA coatings mainly consist of BCC phase, in-situ TaB phase and NiTi/IM eutectic phase. TaB phase with high-density (110) nanotwins inside presents needle-like morphology with the long side axis parallel to the [001] direction. Excessive needle-like TaB phase is fully fused and fractured due to its preferential growth along a specific direction. The fracture surface analysis shows that BCC phase bears the main deformation, NiTi phase can relieve stress concentration at the boundary while bearing a small amount of deformation. By contrast, the low-density dislocation distribution and the low energy extended dislocation structure in TaB phase exhibit the phase stability. In addition, the amorphous islands are formed near the high-deformation regions when the defect concentration reaches the critical value.

Boron-induced microstructure evolution and mechanical properties of in situ Ti-based bulk metallic glass composites

The effect of boron(B) on the glass-forming ability (GFA), microstructure evolution and mechanical properties of (Ti48Zr20Nb12Cu5Be15)100-xBx bulk metallic glass composites (x = 0, 0.3, 0.5, 1.0 and 3.0 at. %) alloys was investigated. With increasing boron content to 0.3 at.%, the dendrite size is refiner than that of the other alloys, the B0.3 alloy presents good ductility and high strength at ambient temperature, which is mainly attributed to the strong interaction between dislocations, shear bands and the dendrite/glass interface. When the boron content is lower or higher than the critical value of 0.3 %, multiple slide steps decrease. Once the B content exceeds its maximum solid solubility, the phase instability of β-Ti dendrites and the glass matrix decreases, and more needle-like TiB2 phases and irregular block-like ZrB2 phases within and between the dendrites suppress the initiation of shear bands, resulting in brittle fracture.

Thermal post-treatment and material characterization of laser powder bed fusion additively manufactured Ti-6Al-4V

Laser powder bed fusion of Ti-6Al-4V (PBF-LB/Ti-6Al-4V) alloy results in the formation of non-equilibrium microstructures due to very high cooling rates, causing them to be unfit for direct applications; therefore, post-heat-treatment operations are required. This paper investigates the effects of different heat treatment operations on the microstructure, hardness and phase composition of as-built PBF-LB/Ti-6Al-4V samples. Six sets of heat-treatment operations have been designed in which samples have been first subjected to either above or below the beta (β) transus zone of 980 °C, then subjected to air cooling or water quenching followed by ageing at 510 °C. The novelty of this article is performing double quenching on as-built Ti-6Al-4V parts. Optical microscopy, Vickers microhardness testing, and XRD analysis have been performed on heat-treated samples for material characterization. Microstructural studies have revealed that depending upon the cooling medium and subjected temperature during heat treatment, the extent of dissolution of martensitic (α′) needle-like phases present in as-built samples into α-β phase varies, resulting in variation of hardness values. XRD analysis confirmed the presence of β-phase along with the α-phase in the matrix in air-cooled heat-treated samples. Maximum hardness was obtained in the case when the sample was solutionized at 1050 °C, followed by water quenching and ageing due to the formation of intermetallic precipitates.

SiCp/AlSi10Mg composites with simultaneously enhanced tensile strength and ductility fabricated by laser powder bed fusion additive manufacturing

Currently, SiCp/Al composites fabricated by laser powder bed fusion (LPBF) suffer from poor plasticity caused by their low density and a great deal of coarse needle-like brittle phases, which seriously limits their application in aerospace field. High scanning speed can shorten the reaction time between SiC and Al liquid due to the short existence time of molten pool, effectively suppressing the nucleation and growth of these needle-like brittle phases. Therefore, an LPBF with a high-speed scanning printing strategy was proposed to fabricate micro-SiCp/Al composites in this study. The effects of energy density (E) on the densification, microstructure and mechanical properties of the composites were studied systematically. The contradiction that the higher the density, the more and coarser needle-like phases produced was successfully resolved. The micro-SiCp/Al composites with simultaneously enhanced strength and ductility were achieved at a proper E of 47 J/mm3, whose tensile strength and elongation reached 356.7 ± 5.0 MPa and 11.6 ± 0.4%, respectively. Among them, the elongation is almost 4 times the maximum value reported in the literature, which is mainly attributed to the extremely high compactness without any visible defects, the moderate dissolution and uniform distribution of SiC particles, the refined grains, a few nano-sized Al4C3 and Al4SiC4, and the interfacial structure SiC–Al4SiC4–Al with high interfacial bonding strength. Because these factors greatly reduced the stress concentration inside the composites and made the stress evenly distributed during loading, thus improving the ductility. Especially these in-situ synthesized Al4C3 and Al4SiC4 can not only promote the transformation of α-Al columnar grains to cellular grains, but also hinder the bending and motion of dislocations, playing a role in grain refinement and dispersion strengthening. The fracture mode of the composites is ductile fracture. Our work provides theoretical guidance and technical support for fabricating SiCp/Al composites with simultaneously enhanced strength and ductility.