Intragranular Precipitates Research Articles

Effect of thermal strain on the microstructure evolution and post-aging mechanical properties of Al-Zn-Mg-Cu alloy in simulating hot stamping process

To identify the effect of thermal strain on the microstructure evolution and mechanical properties of Al-Zn-Mg-Cu alloys in simulating hot stamping process, the alloys were strained at elevated temperature and then artificially aged at 120 °C for different time. The post-aging mechanical properties of the alloys were examined, and the microstructures after aging were systematically characterized via using electron backscattered diffraction (EBSD), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Especially, the effect of dislocation on precipitation behavior and its mechanism on properties were analyzed in detail. The results show that an 8.96% rise of ultimate tensile stress (UTS) can be found in solid solution specimens and the peak-aging time shortens by 50% with the increase of thermal strain from 0% to 30%. The deformed grains are slightly refined from 12.62 μm to 10.99 μm when the strain increases to 30%, while aging time has no obvious effect on the grain size and orientation. Experiments demonstrate that dislocations definitely exist after thermal strain, and will not be completely devoid because of recovery. To a certain extent, dislocations can be used as the favorable nucleation core for the precipitates, promote the rapid formation of strengthening phases, and finally result in a shorter peak-aging time with mixed intragranular precipitates of a large amount of η' and a small amount of GPII zones.

Reducing stress corrosion cracking susceptibility of high-strength aluminum alloy and its fastener by a novel electromagnetic shocking treatment

A novel electromagnetic shocking treatment (EST) is put forward to reduce the stress corrosion cracking (SCC) susceptibility of high-strength aluminum alloys and its fasteners by selectively modifying grain boundary (GB) microstructure. The effect of EST on the microstructure, mechanical properties and SCC susceptibility of T73-Al7075 alloy and its high-lock nuts has been investigated. Mechanical properties, electrochemical performance and constant strain SCC test have been carried out for T73-Al7075 alloy, microhardness test and SCC test under wet-dry cyclic condition have been carried out for its high-lock nuts, as well as microstructure characterizations via SEM, EBSD and TEM have been carried out for T73-Al7075 alloy. Comparing to the received sample, the strength of EST sample basically keep consistent while the elongation increases slightly, besides, the electrical conductivity and electrochemical corrosion performance improves. The SCC susceptibility of T73-Al7075 alloy and its high-lock nuts have been reduced after EST. Microstructure characterization shows that the grain morphology and matrix intragranular precipitate distribution of the EST sample keep almost the same as those of the received sample, which is consistent with the almost unchanged mechanical properties of alloy sample. However, EST induces the transformation of grain boundary precipitates (GBPs) and the formation of stripy/wavy GB as observed experimentally, which indicates that EST promotes nonlinear GB wetting or pre-melting and even grain bridging. The phase transformation and GB complexion transition induced by EST are main factors attributing to the obvious improvement of SCC resistance of T73-Al7075 alloy and its high-lock nuts. It can be deduced that anodic dissolution cracking will be relieved because of the dissolution and coarsening of GBPs. Hydrogen induced cracking and crack propagation can be relieved with the occurrence of grain bridging. They are both beneficial to improve SCC resistance of Al7075 alloys. This work provides a novel EST method to targetedly embellish GB microstructure, optimize GB complexion and connectivity, and then enhance the service performance of solid alloys and even finished parts.

Extra-ductile and strong tin bronze alloy via high-density intragranular ultra-nano precipitation with minimal lattice misfit

Here we report an “intragranular ultra-nano precipitation” strategy to strongly plasticizing and simultaneously strengthen polycrystalline Cu alloys. Our strategy relies on a high density (more than 1023 m−3) and uniform dispersion of extremely fine Fe nanoprecipitates (5.0 ± 2.7 nm) with minimal lattice misfit (theoretically 0.69%) inside Cu grains. The intragranular dispersion of ultra-nano particles not only considerably enhance tensile ductility of a plasticity-poor tin bronze alloy more than 2 times, but also elevate tensile strength above 20% in the meantime, arising from more stable and greater work hardening. The superb plasticizing and hardening of this class of Cu alloy are based on minimal lattice misfit to achieve maximal precipitate dispersion and durable intragranular nanoprecipitation-dislocation interactions (i.e., dislocations cut through fully-coherent Fe nanoprecipitates and hence produce plastic deformation), and we envisage that this intragranular ultra-nano precipitation strategy may be applied to many other metallic alloys.

Precipitation behavior of γ′ precipitates in 21Cr-32Fe-41Ni heat-resistant alloy at different aging temperatures

The precipitation behavior of γ′ precipitates and mechanical properties of 21Cr-32Fe-41Ni alloy at different aging temperatures were investigated by scanning electron microscopy, transmission electron microscopy, chemical phase analysis, mechanical property tests. The results showed that the precipitates in 21Cr-32Fe-41Ni alloy included γ′ precipitates, MC phases, σ phases after aging heat treatment. There were two precipitation modes of γ′ precipitates: intragranular dispersed precipitation and grain boundary discontinuous precipitation. The number and size of intragranular γ′ precipitates increased with the increase in aging temperature. During the coarsening process, the γ′ precipitates were kept spherical. When the aging temperature was above 740 ℃, the discontinuous precipitation reaction occurred under the combined effect of grain boundary migration and solute diffusion-controlled equilibrium phase growth. According to SEM images, there were four different kinds of morphologies of discontinuous precipitated cellular γ′ at the grain boundaries. They were lamellae precipitates, short rod precipitates, granular precipitates and bidirectional parallel precipitates. When the aging temperature was between 650 ℃ and 740 ℃, the strength of the alloy increased rapidly due to the precipitation of a large number of intragranular spherical γ′, and the precipitation of grain boundary cellular γ′ was one of the main reasons for the toughness of alloy decreased rapidly above 740 ℃.

Quantified effect of quench rate on the microstructures and mechanical properties of an Al–Mg–Si alloy

It is well known that lower quench rate leads to the formation of precipitate free zones (PFZs) adjacent to grain boundaries in aging hardening materials such as Al–Mg–Si alloys. However, the combined effect of PFZs and intragranular precipitates on the yield strength of Al–Mg–Si alloys is not systematically understood. To clarify this, an Al–Mg–Si alloy was water/oil/air quenched after solid solution treatment, then aged at 180 °C for 6 h. It was found that a lower quench rate led to coarser precipitates and wider PFZs. The effects of PFZs and precipitates on the yield strength of the Al–Mg–Si alloy were separated by quantitative characterization of multi-scale microstructures and mechanical simulations. Wide PFZs were found to have a notable effect on yield strength, amounting over half the strengthening effect of intragranular precipitates. This effect increases as the width of PFZ increases, due to more serious strain localization caused by PFZs. A corresponding equation has thus been proposed to describe the deleterious effects of PFZs on yield strength of Al–Mg–Si alloys.

Microstructures and properties of wire-arc additively manufactured ultra-high strength aluminum alloy under different heat treatments

The microstructures, mechanical properties and corrosion behaviors of the ultra-high strength Al–Zn–Mg–Cu-Sc aluminum alloy fabricated by wire-arc additive manufacturing process using a self-prepared 7B55-Sc filler wire were systematically investigated under different heat treatments. The results showed that the microstructures of the as-deposited, T6, T73, and retrogression and re-aging (RRA) heat treatments were all composed of fine equiaxed grains with a size of about 6.0 μm. The grain boundary precipitates (GBPs) of the as-deposited sample were very coarse and continuously distributed along the grain boundaries, and the intragranular precipitates (IGPs) mainly consisted of a small amount of ηMg(Zn,Cu,Al)2 and η′ phases. After T6 heat treatment, the GBPs became much finer, but still continuously distributed along the grain boundaries. The IGPs mainly consisted of fine GP zones and η′ phases. After RRA heat treatment, the GBPs became coarser and discontinuously distributed along the grain boundaries. The IGPs were composed of η′ phases and ηMg(Zn,Cu,Al)2 phases. After T73 heat treatment, the GBPs became much coarser and sparsely distributed along the grain boundaries. The IGPs mainly consisted of coarser ηMg(Zn,Cu,Al)2 phases. The T6 heat-treated sample achieved the highest tensile strength of 618 MPa. While the strength of T73 heat-treated sample was sacrificed by up to 17% compared with the T6 heat-treated sample, but the corrosion resistance was the best of all heat-treated conditions. After RRA heat treatment, the strength was about 10% lower than the strength of the T6 condition, but the corrosion resistance was better than that of the T6 heat-treated sample.

Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression

Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression at 450 °C and 0.001 s−1 was investigated in detail. Based on microstructure analysis, Mg-Gd-Y-Zn-Zr alloys with bimodal grain size distribution and bimodal texture distribution were obtained by uniaxial isothermal compression. There were more heavily bimodal levels in grain size and texture at true strain greater than 0.8. There was a significant inhomogeneous plastic deformation between and within the grains in Mg-Gd-Y-Zn-Zr alloy during the deformation, which is attributed to the difference the grain orientation and a large number of intergranular block-shaped LPSO phases. Further studies revealed that particle-stimulated nucleation (PSN) caused by a large intergranular LPSO phases and continuous dynamic recrystallization (CDRX) developed layer by layer toward the grain interior were the main grain refinement mechanisms. This PSN and CDRX mechanism contributed to the fine grains in bimodal grain size distribution and the random texture in bimodal texture distribution. However, the original coarse grains with basal orientation were preserved during deformation due to smaller plastic deformation and energy storage. Meanwhile, grain boundary segregation and intragranular lamellar LPSO precipitate promoted CDRX by retarding DDRX. In addition, the grain boundary pinning effect dominated by grain boundary segregation and intragranular lamellar LPSO precipitate hindered the coarsening of fine DRXed grains, which was the ultimate guarantee for the formation of bimodal microstructure. Ultimately, inhomogeneous plastic deformation, DRX, and grain boundary pinning effect were demonstrated as the necessary conditions for the formation of heterogeneous bimodal microstructure during thermal deformation of Mg-Gd-Y-Zn-Zr alloy.

Significant strain hardening ability of AZ91 magnesium alloy fabricated by spark plasma sintering

The strain hardening behavior of AZ91 alloys fabricated by spark plasma sintering (SPS) under different sintering temperatures was investigated by compression at room temperature. The sintered AZ91 alloys with random texture exhibit high yield strength and significant strain hardening ability. The hardening abilities (including strain hardening rate, strain hardening exponent and hardening capacity) increase with increasing sintering temperature. The strain hardening of sintered AZ91 alloys is dominated by dislocation slip and accompanied with extension twinning. The increment of hardening abilities is mainly due to the increasing of dislocation induced strain hardening because the rate of dislocation propagation is enhanced by coarse-grain. Additionally, the strain hardening induced by twinning is suppressed by fine-grain and intragranular lamellar Mg17Al12 precipitate.

Influence of microstructure evolution on hot ductility behavior of austenitic Fe–Mn–Al–C lightweight steels during hot tensile deformation

Four alloys based on the Fe–30Mn–(8.5–12)Al–(1.0–1.3)C (wt%) system were prepared to investigate the effects of microstructure evolution and κ-carbide precipitation behavior on hot ductility behavior of austenitic lightweight steels. Hot tension tests were carried out at temperatures of 500–1230 °C using a Gleeble simulator. At high temperatures above 1000 °C, dynamic recrystallization occurred in all alloys, leading to high tensile ductility. At temperatures of 700–900 °C, the ductility decreased in all alloys due to the intragranular precipitation of κ-carbide, with increases in the amounts of Al and C contents then leading to a greater loss of ductility due to the formation of coarse intergranular κ-carbides. The addition of Cr and Mo suppressed the precipitation of κ-carbide, reducing the extent of ductility loss. At 500 °C, the ductility was recovered due to a reduction of inter-/intragranular κ-carbide precipitation and the development of slip bands caused by planar gliding of dislocations through κ-carbide shearing. The spacing among slip bands then became coarse with an increase in the Al and C contents, resulting from the coarsening of κ-carbide. Meanwhile, dynamic strain aging (DSA) behavior was observed in all alloys deformed at 500 °C. This occurred because the hot tensile tests were carried out under a high strain rate condition; therefore, the mobility of the dislocations was fast and thus solute atoms pinned the dislocations despite deformation at a high temperature. With a coarsening of κ-carbide, the extent of serration was reduced, resulted from the fact that the content of solute C decreased due to the greater precipitation of κ-carbide; i.e., the amounts of solute C atoms to cause the DSA behavior were reduced.

Microstructure and mechanical properties of the new cladding austenitic stainless steel of 15–15Ti–Y after aging at 500–700 °C for up to 1000 h

As a promising material for fuel cladding, 15Cr–15Ni–Ti (15-15Ti) austenitic stainless steel is widely used in core materials of nuclear reactors due to its excellent creep and oxidation resistance. To further enhance the stability of 15–15Ti steel in ultra-long service time at high temperature, the yttrium-modified 15Cr–15Ni–Ti (15-15Ti–Y) was aged at 500 °C, 600 °C and 700 °C for up to 1000 h. The grain size and precipitation behavior including precipitates species, morphology, quantity, formation sequence and location were systematically investigated. Through the detection and analysis of electron backscatter diffraction, scanning electron microscope, X-ray diffraction and transmission electron microscope, the results indicates that Y is a strong solute segregation element, which hinders the formation and coarsening of M23C6 phase along the grain boundary, thus facilitating the formation of fine intragranular precipitation. At the same time, the addition of Y inhibits recrystallization, grain growth and intergranular precipitation, and promotes intragranular precipitation. Therefore, after aging at 500 °C for 1000 h, the Chib particles only precipitated in 15–15Ti–Y steel, while the size of precipitates in grains interiors and area fraction of M23C6 phase along grain boundaries in 15–15Ti–Y steel were always smaller than that of 15–15Ti steel. Besides, the effect mechanism of Y addition on precipitation behavior and mechanical properties of 15–15Ti–Y after aging is also discussed. The comparison shows that the hardness of the two steels has little change after aging, but the Charpy impact toughness and tensile properties have decreased to varying degrees. As the main reason affecting the mechanical properties, the intergranular precipitation in the chain after low temperature aging weakens the impact toughness of 15–15Ti, while the fine intragranular precipitation after high temperature aging can improve the strength of 15–15Ti–Y. Therefore, compared with 15–15Ti steel, 15-15Ti–Y steel shows higher hardness, impact toughness and uniform elongation after aging at 500–700 °C for 1000 h.

Development of the high-strength ductile ferritic alloys via regulating the intragranular and grain boundary precipitation of G-phase

• Although the nano-dispersed precipitation of G-phase within the grain hardens the alloy remarkably, its large amount of precipitation at the grain boundary leads to serious alloy embrittlement. • For the first time, the crystal structure transition of nano-dispersed precipitates from the Fe 2 TiSi-L2 1 to the Ni 16 Ti 6 Si 7 -G phase is observed within the ferrite at the early stage of aging. • Guided by current TEM and DFT results, the interfacial energy is predicted to play a leading role in forming the cube-on-cube orientation relationship between α-Fe and G-phase. • Adopting the “cold-rolling and aging” process to effectively avoid its precipitation at grain boundaries, a class of high-performance ferroalloys with both high strength and good ductility is realized. A typical G-phase strengthened ferritic model alloy (1Ti: Fe-20Cr-3Ni-1Ti-3Si, wt.%) has been carefully studied using both advanced experimental (EBSD, TEM and APT) and theoretical (DFT) techniques. During the classic “solid solution and aging” process, the superfine (Fe, Ni) 2 TiSi-L2 1 particles densely precipitate within the ferritic grain and subsequently transform into the (Ni, Fe) 16 Ti 6 Si 7 -G phase. In the meanwhile, the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the (Ni, Fe) 16 Ti 6 Si 7 -G phase are also observed. These nanoscale microstructural evolutions result in a remarkable increase in hardness (100 - 300 HV) and severe embrittlement. When the “cold rolling and aging” process is used, the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening effect. Superhigh yield strength of 1700 MPa with 4% elongation at break is achieved. This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary. These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.

High strength Al−Mg−Sc−Zr alloy with heterogeneous grain structure and intragranular precipitation produced by laser powder bed fusion

In this study, the intrinsic heterogeneous grain structure derived from the special temperature gradient in additive manufacturing and the intragranular precipitation were utilized for constructing a high-strength Al−Mg−Sc−Zr alloy. The alternative distribution of the coarse-grained (CG) layer in the middle of the molten pool and ultrafine-grained (UFG) layer at the boundary between the adjacent two molten pools were produced by using laser powder bed fusion (L-PBF) technology. High contents of Sc and Zr elements were introduced into the Al−Mg alloy, which contributed to the stabilization of UFG boundaries in the form of sub-micro primary Al3(Sc, Zr) particles and resulted in the formation of supersaturated Sc and Zr solid solutions in CG. The afterward aging treatment or well-controlled hot isostatic pressing (HIP) induced uniform precipitation of the high-density nano-scale coherent Al3(Sc, Zr) phase in CG. The HIP also promoted a significant decrease in porosity of the alloy and resulted in a combination of high ultimate tensile strength of 560 MPa and elongation of 12 % of the L-PBF Al−Mg−Sc−Zr alloy. The high strength of the alloy was attributed to the hierarchical formation of the Sc-rich phase, exerting a “dragging effect” on intergranular boundary migration as well as a “pinning effect” on intragranular dislocation sliding.

Effects of electropulsing frequency on mechanical properties, corrosion behavior and microstructure of a creep-aged 7050 aluminum alloy

In the experiment, the effects of electropulsing frequency (150 Hz, 300 Hz, 450 Hz, 600 Hz) on the creep age formability, mechanical properties and corrosion behavior of 7050 aluminum alloy were studied. The results show that the creep age formability of the alloy is improved as the electropulsing frequency increases from 150 Hz to 600 Hz. The tensile strength and yield strength of the 600 Hz sample are 627 MPa and 590 MPa, which correspondingly increased by 5.2% and 6.1% compared with the 150 Hz sample. In addition, the maximum corrosion depth of the 600 Hz sample is 25.9 μm while that of the 150 Hz sample is 61.9 μm. The application of higher frequency electropulsing promotes the dislocation movement, so the dislocation density decreases and the creep strain is larger. The electropulsing accelerates the precipitation by enhancing the movement of vacancy and solute atoms and the depletion of vacancy inhibits the growth of precipitates. Hence, the higher frequency electropulsing leads to finer and denser intragranular precipitates, which contributes to the optimized mechanical properties. The electropulsing frequency of 600 Hz results in the discontinuous distribution of grain boundary precipitates and the reduction in the width of PFZ, which are beneficial to improve the corrosion resistance of the alloy.

Extrinsic and intrinsic contributions to the electrostrain in precipitation-hardened barium calcium titanate

Deconvoluting the extrinsic and intrinsic contributions to electrostrain is of great importance to understand the hardening mechanism of piezoceramics. Here, in situ electric-field high-energy x-ray diffraction measurements are performed to investigate the polycrystalline barium calcium titanate hardened by precipitation, a recently developed hardening technique that pins domain walls with fine intragranular precipitates. The effect of precipitates on extrinsic and intrinsic mechanisms is examined. Under a low-frequency and large-signal field, the precipitates suppress non-180° wall motion, which is the major source of loss, by 40%. Anisotropy is observed in the field-induced lattice strain, which is dominantly contributed by an intergranular effect instead of pure piezoelectricity. At small fields, the lattice strain is barely affected by precipitates, while both lattice strain and strain from non-180° domain wall motion are suppressed and are coupled with each other at large fields, leading to an unchanged relative percentage of the extrinsic contribution.

Effect of Carbon on Stress-Relief Cracking Susceptibility of T23 Steel

The coarse-grained heat-affected zone (CGHAZ) samples were prepared from T23 steel with three different carbon contents on a thermal simulator, then isothermal slow strain rate tensile tests were carried out at 500° ~ 750°C, and the effect of carbon content on the stress-relief cracking (SRC) sensitivity of T23 steel was evaluated according to fracture ductility. The microstructure evolution of the CGHAZs in the process of SRC was observed by optical microscopy, structural equation modeling energy-dispersive spectroscopy, and transmission electron microscopy energy-dispersive spectroscopy/selected area electron diffraction to reveal the mechanism of SRC. Finally, the feasibility of adjusting carbon content to prevent SRC was discussed. The results showed the classical theory of intragranular precipitation hardening cannot explain the cause of SRC, and a new viewpoint was put forward that the precipitation of M23C6 at the grain boundary was the main reason for SRC. The precipitation of M23C6 carbide cannot only provide a preferred site for void nucleation but also leads to the depletion of alloy elements in the matrix near the grain boundary. The interaction of these two aspects leads to the direct weakening of grain boundaries. Moreover, the SRC of T23 steel could be inhibited when the carbon content was determined to be close to the ASME standard lower limit of 0.04 wt-%. In summary, reducing the carbon content can inhibit the SRC sensitivity of T23 steel, which is not due to reducing the relative weakening of the grain boundary by reducing the precipitation hardening in the grain interior but to inhibiting the direct weakening of the grain boundary by reducing the precipitation of M23C6 at the grain boundary.

Mechanical properties, corrosion behavior and microstructure evolution of zinc and scandium co-strengthened 5xxx alloy

Nowadays, it is a challenge to improve the strength of 5xxx aluminium alloy while maintaining its distinctive properties. To address this, the Zn and Sc modified high strength 5xxx aluminium alloys with good intergranular corrosion resistance were prepared through chill casting, and the effects of minor Sc on mechanical properties, corrosion behavior and microstructure evolution of Al-5.0Mg-3.0Zn alloys were systematically investigated. The results show that the Sc containing alloy has high number density and small size of intragranular precipitates, combined with the solid solution strengthening of Sc and the Al3(Sc1–x,Zrx) second phase strengthening, the strength of Al-5.0Mg-3.0Zn-0.1Sc alloy is obviously enhanced. Meanwhile, the intergranular corrosion resistance of the Sc containing alloy is also improved due to the formation of a large number of low angle grain boundaries and the weakening of the continuity of precipitated phases along the grain boundaries.

Revealing a correlation between microstructure, stoichiometry, coherency strains and mechanical behavior of bulk polycrystalline oxide-based age hardened/toughened ceramic alloys

The processing challenges and microstructural inhomogeneity associated with ceramic (nano)composites render it important to develop hardened/toughened oxide-oxide ceramic alloys, possessing good mechanical properties, via a facile solid-state precipitation-based approach. However, detailed understanding of processing-microstructure-property correlations in age-hardened/toughened polycrystalline ceramics is still lacking. The present research, therefore, focuses on understanding the possible effects of variation of dimension/stoichiometry of coherent second-phase precipitate particles in such ceramic alloys on the coherency strains, associated residual stresses and mechanical properties. Polycrystalline MgO-based ceramic alloys have been developed via controlled aging treatments of partially supersaturated Fe(oxide)-containing MgO solid solution. Air-quenching post sintering-cum-solution treatment results in the formation of ultra-fine (∼5–20 nm) coherent intragranular Mg-ferrite precipitates; with subsequent aging treatments causing increases in their size and amount. The intragranular precipitate particles remain coherent with the MgO matrix even upon aging at 1000 °C for 20 h, despite coarsening to ∼130 nm. Detailed compositional analysis using EDS (with HAADF-STEM) and 3D APT reveal progressive increment in Fe-content of the precipitates with aging duration, such that the ultra-fine particles obtained right after solution treatment correspond to non-stoichiometric Fe-deficient Mg-ferrite (with ∼6 at.% Fe), whereas the relatively coarser particles obtained after 20 h of aging correspond to near-stoichiometric MgFe2O4 (with ∼27 at.% Fe). Such increment in Fe-content results in increase in lattice misfit/coherency strains, which dominates the hardening response and improves the hardness despite loss of solution hardening. Furthermore, increase in compressive residual stress in the matrix due to increment in misfit/coherency strains results in improved resistance to crack propagation.

Effect of cobalt on the mechanical properties of 718 base alloy with 1.2 wt% Al

In this paper, the effect of Co (0–5 wt%) on the mechanical properties of 718 alloy and its strengthening mechanism were studied to clarify the function of Co on the mechanical properties of 718 base alloy. The experimental results showed that the addition of Co refined the grain. After the heat treatment, the addition of Co promoted the precipitation of Laves phase and γ′ phase, but inhibited the precipitation of γ″ phase. With the increasing of the Co content, the tensile strength was increased first and then decreased. It was found that the increase in the tensile strength of the alloy was caused by refining the grains and promoting the intergranular precipitation of the Laves phase. Nevertheless, Co increased the volume fraction of intragranular γ′ phase, while suppressed the precipitation of intragranular precipitation of γ″, thus the intragranular strength of the alloy was reduced. Accordingly, the alloy with 3 wt% Co shows the highest yield strength among the experimental alloys. Besides, the Co content extent 3 wt% will show a yield plateau under room temperature tension, but not in the case of 650 °C tension. It was revealed by analyzing the TEM morphology of the Laves phase in the room-temperature tensile samples and 650 °C tensile samples that the dislocation could shear through the intergranular Laves phase under the 650 °C tensile test, but it was not the case under room temperature tensile test. Therefore, the yield plateau in the stress-strain curves is due to the intergranular Laves phase impeding the dislocation motion.

Enhancing the intergranular corrosion resistance and mechanical properties of Al–Mg–xSi–Cu–Zn alloys by synergistic intergranular and intragranular precipitation behaviors

Enhancing the intergranular corrosion resistance and mechanical properties of Al–Mg–xSi–Cu–Zn alloys by synergistic intergranular and intragranular precipitation behaviors

Powder densification behavior and microstructure formation mechanism of W-Ni alloy processed by selective laser melting

W-Ni alloy has been an important engineering material in military, nuclear industry and many other fields because of its excellent properties such as high strength and superior heat/corrosion/oxidation resistances. As a mainstream additive manufacturing technology based on powder-bed-fusion principle, selective laser melting (SLM) is suitable for the rapid fabrication of very complex W-Ni alloy components. However, researches on the powder densification behavior and microstructure formation mechanism of SLM W-Ni alloy are not thorough enough. In this work, a series of W-xNi (x = 16, 23, 30 wt.%) binary alloy samples were fabricated by SLM, using W/Ni mixed powders as the raw materials. The densification behaviors of the W/Ni mixed powders and the microstructure formation mechanisms of the different SLM W-Ni samples were studied. The results showed that when the W/Ni mixed powder bed was scanned by the laser, Ni powders with a relatively lower melting point would melt completely whereas W powders with a relatively higher melting point would rearrange themselves in the liquid Ni and only part of them could be dissolved into the liquid Ni. As the initial content of Ni powders increased, the rearrangement and dissolution of W powders proceed more sufficiently, thus leading to the improvement in densification degree. The relative densities of the W-16Ni, W-23Ni and W-30Ni SLM samples are 92.87 ± 0.67%, 98.66 ± 0.26% and 99.67 ± 0.18%, respectively. During the solidification processes of W-Ni molten pools, the precipitation of W grains from liquid Ni, the transformation of liquid Ni to γ-Ni solid solution and the precipitation of Ni2W from γ-Ni solid solution would occur in sequence. For the W-16Ni and W-23Ni samples, the grain morphology of the W precipitations is dendritic. For the W-30Ni samples, the W precipitations are globular. As a result of the unique melting- solidification behaviors, all the SLM samples possessed a tri-modal microstructure composed of unmelted W particles, dendritic/globular W grains, and γ-Ni solid solution with Ni2W intragranular precipitation. The average microhardness of the SLM W-Ni alloy samples decreased with the increase of Ni content, which could be explained by the variation in defect and microstructure features.