Grain Boundary Particles Research Articles

Study on Microstructure and Mechanical Properties Modification of Cu-Ti3AlC2 Composites by Ni Element

Ti3AlC2 three-layered ceramics-reinforced copper matrix composites show not only the strength of the copper matrix but also better wear resistance, all while preserving its conductive property and, ultimately, reducing the cost of preparation. However, decomposition in Cu-Ti3AlC2 composites mainly occurs due to the severe interdiffusion of Al and Cu elements from Ti3AlC2 particle-grain boundaries, leading to the formation of TiCx. This work explored a practical method to produce Cu reinforced with a Ti3AlC2 particle which keeps an effective ternary layered structure by adding a nickel element. Results show that the addition of Ni elements results in a microstructure composed of Ti3AlC2, TiCx, NixAl, NixTi, and a Cu(Ni) matrix in the Cu-Ni-Ti3AlC2 composites. As the volume fraction of Ti3AlC2 particles increases, the morphologies change from a finely dispersed reinforcement phase to a continuous network, leading to a reduction in hole number and volume compared to Cu-Ti3AlC2 composites. This reduction is especially significant when the volume fraction of Ti3AlC2 exceeds 50%. The formation of NixAl and NixTi compounds at the grain boundary of the reinforcement phase after Ni element alloying restricts the diffusion of Al elements. The addition of Ni elements improves the mechanical properties of the composites.

Coexistence of Intermetallic Complexions and Bulk Particles in Grain Boundaries in the ZEK100 Alloy

Magnesium-based alloys are highly sought after in the industry due to their lightweight and reliable strength. However, the hexagonal crystal structure of magnesium results in the mechanical properties’ anisotropy. This anisotropy is effectively addressed by alloying magnesium with elements like zirconium, zinc, and rare earth metals (REM). The addition of these elements promotes rapid seed formation, yielding small grains with a uniform orientation distribution, thereby reducing anisotropy. Despite these benefits, the formation of intermetallic phases (IP) containing Zn, Zr, and REM within the microstructure can be a concern. Some of these IP phases can be exceedingly hard and brittle, thus weakening the material by providing easy pathways for crack propagation along grain boundaries (GBs). This issue becomes particularly significant if intermetallic phases form continuous layers along the entire GB between two neighboring GB triple junctions, a phenomenon known as complete GB wetting. To mitigate the risks associated with complete GB wetting and prevent the weakening of the alloy’s structure, understanding the potential occurrence of a GB wetting phase transition and how to control continuous GB layers of IP phases becomes crucial. In the investigation of a commercial magnesium alloy, ZEK100, the GB wetting phase transition (i.e., between complete and partial GB wetting) was successfully studied and confirmed. Notably, complete GB wetting was observed at temperatures near the liquidus point of the alloy. However, at lower temperatures, a coexistence of a nano-scaled precipitate film and bulk particles with nonzero contact angles within the same GB was observed. This insight into the wetting transition characteristics holds potential to expand the range of applications for the present alloy in the industry. By understanding and controlling GB wetting phenomena, the alloy’s mechanical properties and structural integrity can be enhanced, paving the way for its wider utilization in various industrial applications.

Study on the liquid phase-derived activation mechanism in Al-rich alloy hydrolysis reaction for hydrogen production

Bulk Al-Ga-In-Sn alloys are potential on-demand hydrogen generation materials, and Ga, In, and Sn with fascinating liquid characteristics manifest extraordinary activation capabilities in an Al–H2O reaction for hydrogen production. In the present work, the hydrogen production performances of Al-rich alloys were explored and the corresponding activation mechanism was investigated. The actual composition of the grain boundary (GB) phase in each alloy was determined by the Ga-In-Sn ternary phase diagram. A significant morphological transformation in the grain boundary particles (GBPs) was observed. Based on the surface tension of GBPs, the “Marangoni effect” was used to explain the hydrogen release rates of different alloys during hydrolysis. In addition, a liquid phase-derived activation mechanism during Al hydrolysis was proposed. The as-cast Al-Ga-In-Sn alloys were also used as energy carriers, and the power-consuming load was successfully supplied through the on-demand hydrogen supply mode, verifying the feasibility of liquid phase-activated Al-rich alloys in hydrogen energy applications.

On the complex intermetallics in an Al-Mn-Sc based alloy produced by laser powder bed fusion

The recently-developed Al-Mn-Sc based alloys specific for laser powder bed fusion (LPBF) have shown excellent mechanical performance. However, the complicated intermetallic particles in the microstructure remain to be identified, hindering the deep understanding of their effects on mechanical properties and further property improvement. In this work, a range of phases in a LPBF-built Al-Mn-Sc based alloy have been systematically studied by atom probe tomography. The results clarify characteristic intermetallic phases in two different grain-size regions in the microstructure. In the fine grain (FG) region, three distinct phases have been identified. A Sc-rich phase having an average composition of Al3.3(Sc0.8Zr0.2) is observed at both the grain boundary (GB) and grain interior, but they have different morphologies. The Mn-rich phase with an average composition of Al4.3(Mn0.9Fe0.1) is only observed along GBs. In addition, Mg-rich oxide has been observed either with a separate distribution or attached to Sc-rich particles. In the coarse grain (CG) region, the GB particles exhibit different size and composition from FG region. The Sc-rich GB particles contain Mg enrichment rather than Zr and the composition is determined to be Al3.4(Sc0.75Mg0.25). The Mn-rich GB particles in the CG region, with higher Fe contents, are Al4.3(Mn0.8Fe0.2) along the GBs and Al4.5(Mn0.85Fe0.15) inside the grains. Many smaller Mg-rich oxides and Sc-rich particles are also observed in the internal grains of the CG region.

Microstructure, mechanical properties and oxidation behavior of NbTaTi and NbTaZr refractory alloys

This study reports the microstructure, phase composition, mechanical properties and oxidation behavior of NbTaTi and NbTaZr ternary equiatomic alloys. These ternary components, together with earlier reported NbTiZr and MoNbTi, are commonly observed in refractory high entropy alloys (RHEAs) with a single-phase BCC or multi-phase crystal structures. Therefore, the reported information about these ternary alloys is thought to be useful for the development of advanced RHEAs. While NbTiZr and MoNbTi are single-phase BCC structures, NbTaTi and NbTaZr are multi-phase alloys. NbTaTi consists of a BCC matrix and discontinuous, Ti-rich precipitates with BCC and FCC crystal structures. NbTaZr consists of a co-continuous spinodal mixture of two BCC phases, one of which is rich in Ta and Nb and another is rich in Zr, and it also contains a Zr-rich HCP phase in the form of coarse grain-boundary particles. NbTaTi has compression yield stress of 724 MPa at 25 °C, 268 MPa at 800 °C and 129 MPa at 1200 °C. NbTaZr is stronger, with the compression yield stress of 1027 MPa at 25 °C, 530 MPa at 800 °C and 183 MPa at 1200 °C. Both alloys show rapid oxidation with formation of complex oxides during holding at 1200 °C.

Multiscale structural characterization of yttria dispersed copper alloys fabricated by hot isostatic processing of mechanically alloyed powders

Multi-scale characterization techniques, including computed tomography (CT) based on serial sectioning by focused ion beam (FIB) and scanning transmission electron microscopy (STEM), have been employed to elucidate the microstructural hierarchy inside yttria (Y2O3) dispersed copper alloys designed for fusion reactors. The alloys were fabricated by the combination of mechanical alloying of metal and oxide powders and subsequent hot isostatic pressing. Optical microscopy, together with electron micro-probe analysis revealed bimodal size distribution of powders, where coarse Cu powders of several hundred micrometer are composed of metallic core surrounded by a crust of Cu oxides, while fine powders of several ten micrometer are mostly of oxides. Scanning electron microscopy (SEM) on the fabricated bulk alloys revealed heterogeneous inner structure, which is best described by an assembly of large Cu grain regions bordered by small Cu grain regions. Distribution of Y2O3 particles are visualized three dimensionally by serial sectioned FIB-SEM images and by CT-STEM, both of which revealed the particles are enriched in small Cu grain regions. These multi-scale characterization, together with mechanical bend testing, demonstrated that excess Y2O3 particles in grain boundaries leads to brittle fracture, while that large Cu grains are responsible for the ductility of the alloy.

Current Trend of Second Phase Particle-grain Boundary Interaction Research using Computer Simulations

Current Trend of Second Phase Particle-grain Boundary Interaction Research using Computer Simulations

Cooling Rate and Microstructural Investigation of Rapidly Solidified Spherical Mono-Sized Copper Particles

Spherical copper particles with diameter ranging from 120.6 to 437.0 μm were prepared by the pulsated orifice ejection method (termed “POEM”). These spherical copper particles exhibit a good spherical shape and a narrow size distribution, suggesting that the liquid copper can completely break the balance between the surface tension and the liquid static pressure in the crucible micropores and accurately control the volume of the droplets. Furthermore, the relationship between cooling rate and microstructures of spherical copper particles was carried out with a specific focus on different cooling atmosphere and particle diameter during the rapid solidification. The cooling rate of spherical copper particles is evaluated by a Newton’s cooling model. It is revealed that the cooling rate was depended on cooling medium and particle diameter. The cooling rate decreases and the grain size increases with the increase of particle diameter during the rapid solidification, while the grain boundary of same particle diameter with larger cooling rate in argon gas is smaller, while the grain boundary of particles with smaller cooling rate in helium gas is larger. When the particle diameter is larger than 100 μm, the cooling rate of the cooper droplet in argon gas achieves 1.0×104 K/s. Meanwhile, the cooling rate decreases rapidly when the particle diameter increased between 70.6 and 149.6 μm. It is an effective route for fabrication of high-quality spherical copper particles.

Direct measurement of the maximum pinning force during particle-grain boundary interaction via molecular dynamics simulations

Interaction between coherent Ag particles and Cu grain boundaries (GBs) has been investigated by molecular dynamics simulations. Through measuring GB energy evolution as a function of GB position during the interaction, the maximum pinning force was directly determined and found to agree well with theoretical predictions. The relationship between the maximum pinning force and the pinning efficiency for different GBs has been discussed. It is concluded that the two parameters are irrelative to each other, as also observed in experiments.

Compositional variation effects on the microstructure and properties of a refractory high-entropy superalloy AlMo0.5NbTa0.5TiZr

An AlMo0.5NbTa0.5TiZr baseline alloy was shown earlier to have good high temperature strength but poor ductility below 600°C due to coarse intermetallic grain boundary particles and a continuous ordered B2 matrix phase. Systematic composition changes intended to remove the deleterious microstructural features and to improve mechanical properties were explored in the present work. The baseline alloy and the new alloys studied here, AlMo0.5NbTa0.5TiZr0.5, AlNbTa0.5TiZr0.5, Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr, all had an ordered B2 matrix crystal structure. Additionally, coherent BCC nanoscale precipitates were present at a high volume fraction inside the B2 matrix grains in AlMo0.5NbTa0.5TiZr, Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr, and/or coarse, grain-boundary particles existed in AlMo0.5NbTa0.5TiZr and AlMo0.5NbTa0.5TiZr0.5. The mechanical properties were assessed with microhardness and compression testing at 25°C and 1000°C. Al0.5Mo0.5NbTa0.5TiZr showed the highest hardness (Hv=6.4GPa) and strength (σ0.2=2350MPa) at 25°C and modest strength (σ0.2=579MPa) at 1000°C. AlMo0.5NbTa0.5TiZr0.5 had the highest strength (σ0.2=935MPa) at 1000°C, but was brittle at 25°C. High-temperature deformation produced a desirable microstructure in Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr alloys consisting of a continuous BCC phase and discontinuous B2 nano-precipitates. The relationships between the composition, microstructure, and properties were identified and discussed.

Effect of Aging Treatment before Extrusion on Microstructure and Mechanical Properties of AZ80 Magnesium Alloy

This paper investigated the effect of aging treatment before extrusion on microstructure and mechanical properties of AZ80 magnesium alloy. Fractural microstructures of the test specimens were also analyzed by scanning electron microscopy. Results show that aging treatment before extrusion can remarkably promote grain refinement; the dispersed Mg17Al12 particles precipitated from the matrix during aging treatment before extrusion distribute in the grain boundaries; during the subsequent extrusion process these particles can pin grain boundaries migration and lead to fine microstructure; however, with prolonging the aging time, the grain refinement effect is weakened. After aging treatment and subsequent extrusion, the yield strength, tensile strength and elongation are increased. Based on fracture surface analysis, the premature cracks occur around big particles in grain boundaries lead to loss of elongation. The strengthening effect of aging before extrusion in AZ80 magnesium alloy proven in this paper provides a new method for the design of the relatively high-performance magnesium alloys.

Particle-grain boundary interactions: A phase field study

A detailed phase field model is used to investigate particle-grain boundary interactions. The model takes into consideration both the curvature-driven grain boundary motion and particle migration by surface diffusion. The phase field model relaxes all the restrictive assumptions usually employed in the classical and sharp-interface models of the problem. Our 2D and 3D simulations demonstrate that the model captures all possible particle-grain boundary interactions proposed in theoretical models and observed in experiments. For high enough surface mobility, the particles move along with the migrating boundary as a quasi-rigid-body (i.e., without change in shape or size), albeit hindering its migration rate as compared to the particle-free case. For less mobile particles, the migrating boundary can separate from the particles. For the case of steady-state motion of the particles with the migrating boundary, evolution equations for the grain size were derived that predict a strong dependence of the grain growth rate on the number of particles, particle size, and surface diffusivity. For the case of boundary breakaway, the separation condition was found to agree well with predictions from theoretical calculations. However, our results show that non-uniform particle distribution promotes boundary detachment. The results also demonstrate that it is easier for a migrating boundary to separate from a spherical particle than from a cylindrical particle, and hence 2D simulations underestimate boundary breakaway.

Enhancing corrosion resistance of 7150 Al alloy using novel three-step aging process

The effects of a novel three-step aging process (T76+T6) on the electrochemical corrosion behavior of 7150 extruded aluminum alloy were evaluated and compared with those of the conventional retrogression and re-aging process (T77). The open circuit potential (OCP), cyclic polarization and electrochemical impedance spectra (EIS) of the Al alloys were measured after treatment in three solutions (3.5% NaCl (mass fraction); 10 mmol/L NaCl + 0.1 mol/L Na2SO4; 4 mol/L NaCl + 0.5 mol/L KNO3 + 0.1 mol/L HNO3). The parameters including the corrosion potential, pitting potential, pit transition potential and steepness, and potential differences were extensively discussed to evaluate the corrosion behavior of the Al alloys. The electrochemical and scanning electron microscopy (SEM) data show that compared with the 7150-T77 Al alloy, the T76 + T6 aged 7150 Al alloy exhibits better resistance to pitting corrosion, inter-granular corrosion (IGC) and exfoliation corrosion, which is attributed to further coarsening and inter-spacing of the grain boundary particles (GBPs) as revealed by transmission electron microscopy. Furthermore, the hardness tests indicate that an attractive combination of strength and corrosion resistance was obtained for the 7150 Al alloy with T76 + T6 treatment.

Study on preparation and properties of molybdenum alloys reinforced by nano-sized ZrO2 particles

The nano-sized ZrO2-reinforced Mo alloy was prepared by a hydrothermal method and a subsequent powder metallurgy process. During the hydrothermal process, the nano-sized ZrO2 particles were added into the Mo powder via the hydrothermal synthesis. The grain size of Mo powder decreases obviously with the addition of ZrO2 particles, and the fine-grain sintered structure is obtained correspondingly due to hereditation. In addition to a few of nano-sized ZrO2 particles in grain boundaries or sub-boundaries, most are dispersed in grains. The tensile strength and yield strength have been increased by 32.33 and 53.76 %.

Critical quenching rate for high hardness and good exfoliation corrosion resistance of Al-Zn-Mg-Cu alloy plate

By means of the end-quenching technique, we investigated the relationship between quenching rate and hardness as well as exfoliation corrosion rating for Al-2.21 Zn-3.59 Mg-0.45 Cu-0.038 Zr (at%) alloy plate. In order to achieve an exfoliation corrosion rating of P or EA, the quenching rate must be greater than approximately 460 °C/min and 300 °C/min, respectively, and the drop degree in hardness should simultaneously be lower than approximately 2.0% and 3.5%, respectively. The results of microstructural and microchemical examination using a scanning transmission electron microscope indicate that a lower quenching rate leads to a higher content of Zn, Mg, and Cu in the grain-boundary particles and a greater width of precipitate-free zones near grain boundaries; therefore, grain-boundary particles with Zn and Mg contents less than approximately 13.39% and 10.23% (at%), respectively, and precipitate-free zones near grain boundaries with widths less than about 107 nm can contribute to an exfoliation corrosion rating better than EA. The amount of quench-induced η-phase particles, which lead to lower hardness, increases with decreasing quenching rate, and the area fraction of these particles is approximately 2.9% at a quenching rate of 300 °C/min.

On the notch ductility of a magnesium-rare earth alloy

The room-temperature notch ductility of magnesium-rare earth alloy WE43 is investigated for two loading orientations. This material is endowed with quasi-isotropic plastic flow properties, higher strength and similar uniaxial ductility in comparison with other commercially available Mg alloys. The authors have recently shown that the notch ductility of a Mg–Al–Zn alloy is greater than its uniaxial ductility over a wide range of notch geometries. This paper investigates whether the same trends hold for WE43, discusses the orientation dependence of ductility and the propensity for intergranular fracture at high levels of hydrostatic tension. The latter mode of fracture is analyzed by means of detailed fractography in order to elucidate the role of grain-boundary particles and precipitates in the fracture process.

Study on the evolution mechanism of oxidation and copper diffusion and precipitation phenomena and their effect on the surface quality of steel plates

Purpose – Identification of the critical process conditions that enhance Cu diffusion in ferrite grain boundaries and promote precipitation of Cu-rich particles in the proximity of steel semi-finished products surface is crucial for every steel maker as it leads to the creation of hot shortness cracks in final products deteriorating surface condition. The purpose of this paper is to reveal the possible effect of Cu segregation in the metal/oxide interface, its role in surface crack initiation and, finally, to propose actions to prevent from hot shortness issues throughout the production chain of steel products. Design/methodology/approach – The here presented study was based on S355 steel plate production starting from re-melting of scrap in an EAF, followed by metallurgical treatment in a Ladle Furnace, continuous casting, re-heating (RH) and thermo-mechanical rolling in a reversing mill. For the purposes of this study, more than ten heats, 100 t of steel each, were analyzed. Here presented are depicted steels in the high and low end of the permitted Cu-wt-% spectrum, 0.4 wt-% Cu (0.15 wt-% C, 1.1 wt-% Mn, VTi micro-alloyed steel) and 0.25 wt-% Cu (0.09 wt-% C, 1.2 wt-% Mn, NbTi micro alloyed steel), respectively. Findings – Although Cu levels of 0.25-0.40 wt-% are well below the Cu solubility in austenite and ferrite (8 percent wt-% and 3 wt-% Cu, respectively) and within specifications, precipitation of Cu-rich particles is observed in industrial semi-finished and/or final products. Cu-rich precipitates and Cu segregation along grain boundaries near the steel surface lead to hot shortness cracks in industrial products. Research limitations/implications – Hot shortness surface defects related to Cu presence in steel having significantly lower Cu amounts than its maximum solubility in austenite and ferrite does not make sense in first place. Correctly, Cu is expected to remain in solid solution. Identification of Cu-rich particles is explained on the basis of the development of double diffusion actions: interstitial diffusion of carbon (decarburization) and substitution diffusion of copper. Root cause analysis and reliable countermeasures will save financial and material resources during steel production. Originality/value – Automobile scrap re-melting results in noticeable Cu amounts in EAF produced steel. Presence of Cu-rich particles in grain boundaries near the surface of intermediate or final products deteriorates surface quality through relevant surface defects. Identification of Cu-rich particles is explained on the basis of the development of double diffusion actions: interstitial diffusion of carbon and substitution diffusion of copper. Pre condition for metallic Cu precipitation in ferrite is the Cu amount to be above 3 wt-%, which is ten times higher than the usual permitted Cu amount in such steel grades. This pre-condition is met through austenite oxidation during RH.

The effect of oxide particles on the strength and ductility of bulk iron with a bimodal grain size distribution

The strength and ductility of bulk nanostructured and ultrafine-grained iron containing 0.39% oxygen by weight was determined by tensile tests. Samples were obtained by consolidation of milled iron powder at 500°C. Heat treatments were designed to cover a wide range of grain sizes spanning from 100 to 2000nm with different percentages of coarse and nanostructured grain areas, which was defined as a bimodal grain size distribution. Transmission electron microscopy was used to determine the diameter, volume fraction and location of oxides in the microstructure. The strength was analysed following two approaches. The first one was based on the strong effect of oxides and involved the use of a mixed particle-grain boundary strengthening model, and the second one was based on simple grain boundary strengthening. The mixed model underestimated the strength of nanostructured samples, whereas the simple grain boundary model worked better. However, for specimens with a bimodal grain size, the fitting of the mixed model was better. In this case, the more effective particle strengthening was related to the dispersion of oxides inside the large ferrite grains. In addition, the bimodal samples showed an acceptable combination of strength and ductility. Again, the ferrite grains containing oxides promoted strain hardening due to the increase in dislocation activity.

Pinning of grain boundary migration by a coherent particle

We studied single-particle pinning of grain boundary (GB) migration during grain growth. A phase-field model was formulated to simulate the pinning by a coherent particle and validated quantitatively by comparison with analytical prediction. A study of GB migration velocity using this model revealed that second-phase coherent particles have a previously unknown restraining effect over the whole of the GB-particle interaction range, which is qualitatively different from the interaction between GB and incoherent particles.

Tensile and fracture properties of an Mg-RE-Zn alloy at elevated temperatures

Magnesium alloy EZ10 (Mg-RE-Zn) was deformed in tension at temperatures from 20 up to 520 °C. A rapid decrease of the yield and tensile strength with temperature was observed at temperatures higher than 300 °C. On the other hand, ductility of samples rapidly increased in the same temperature range. Light microscopy and scanning electron microscopy was used to reveal the reason for these behaviours. Intermetallic particles in grain boundaries are responsible for excellent mechanical properties at lower temperatures. Diffusional processes occurring at temperatures higher than 300 °C significantly influenced the deformation mechanism as well as the fracture character.