Diffusion Distance Of Atoms Research Articles

Effect of sintering activation energy on Si3N4 composite ceramics

This study investigated the activation energy and kinetic characteristics of silicon nitride (Si3N4) composite ceramics produced using different preparation methods. The effects of the linear shrinkage, expansion ratio, and heating rate on the sintering process were analysed in detail. The obtained results reveal that the finer particle size produced using the urea homogeneous precipitation method obviously enhanced the densification rate and reduced the atomic diffusion distance. Moreover, the densification sintering process was carried out efficiently, and the largest densification rate was achieved in advance. According to the Arrhenius curve, the activation energy of the Si3N4 composite ceramics calculated using the urea homogeneous precipitation method (310.94 kJ/mol) was lower than that achieved using the mechanical ball-milling method (365.11 kJ/mol). Additionally, the flexural strength and hardness of the Si3N4 composite ceramic prepared using the urea homogeneous precipitation method were 740 ± 42 MPa and 16.20 ± 30 GPa, respectively, which is attributed to this composite ceramic's higher diffusion rate and small grain size.

Effect of phosphorus on interface characterization of steel–copper bimetals in solid–liquid composite casting

Carbon steel and Cu-Pb-Sn bimetallic samples were prepared by the solid–liquid composite method. The preheating temperature of the steel matrix was 1150 ± 30 °C and the Cu-Pb-Sn alloy smelting temperature was 1200 ± 20 °C. The Cu-Pb-Sn alloy was subsequently poured into the preheated steel matrix to form a bimetallic composite interface. The influence of phosphorus (P) on the microstructure and mechanical properties of the interface and the behavior mechanism of P at the bimetallic interface were examined. The results showed that when the P content was 0.1% (all contents reported herein are in wt%), the carbon steel and Cu-Pb-Sn alloy formed an ideal metallurgical bonding interface. It was found that a nanoscale Fe3P phase was precipitated at 750 °C at the grain boundary of the ɑ-Cu interface. The Fe3P interplanar spacing of the phase was 2.1 Å, and the interplanar spacing of ɑ-Cu was 2.07 Å. When the P content reached 0.2%, the Fe3P phase precipitated at 1020 °C and aggregated into micrometer size at the interface, which formed a transition zone with ɑ-Cu. The transition zone widened with an increase in the P content in the alloy liquid. The interface atomic diffusion distance between Fe and Cu was 11.55 µm when the P content was 0.1%, and the sample shear strength reached a maximum value of 157.89 MPa. With increasing P content, the interface became hard and brittle because it emerged in the precipitated transition zone, which decreased the shear strength.

New criteria for the applicability of combustion synthesis: The investigation of thermodynamic and kinetic processes for binary Chemical Reactions

Combustion synthesis is a novel technique that utilizes the exothermic heat of a chemical reaction to maintain the reaction and to rapidly prepare materials. But, hitherto, none of unified criterion for the validation of combustion synthesis has been proposed. Herein, we proposed the conditions need to be met. In terms of kinetics, at the adiabatic temperature (Tad), the diffusion distance of atoms (lTad) within 0.1 s should be larger than the particle size of the reactants(d), that is, lTad≥d. For systems that satisfy Tad/Tm,L≥1(where Tm,L is the melting point of the low-melting point component of the reactants), the presence of a liquid phase significantly increases the atomic diffusion distance from nanometers to tens of microns, making the criterion lTad≥d simplified to Tad/Tm,L≥1 in most situations. In terms of thermodynamics, the system needs to ensure that the reaction components are in an activated state, that is, Tad/Tm,H≥0.7, where Tm,H is the melting point of the high-melting point component. The criteria for the SHS reactions proposed in this study further improve the theoretical understanding of SHS reactions, and provide guidance for exploring the ultra-fast synthesis of binary and multicomponent compounds.

Internal friction of Ni-Al intermetallic compound formation in sintering process

The Ni-Al intermetallic compounds, as important high-temperature structural materials, have clear target requirements in a number of fields. Powder metallurgy is an important candidate for preparing the Ni-Al intermetallic compounds. Clarifying the formation and transformation process of Ni-Al intermetallic compounds in sintering process and determining the solid diffusion reaction temperature and types of intermetallic compounds are greatly important for tailoring sintering process and optimizing product quality. In this paper, the internal friction behaviors of Ni-Al powder mixture compacts in the sintering process are systematically investigated by the internal friction technique. A typical internal friction peak is observed in the internal friction-temperature spectrum. The peak height decreases with the measuring frequency increasing, but the peak temperature is independent of frequency. Moreover, the internal friction peak shifts toward higher temperature and the peak height increases as the heating rate increases. It is reasonable that the internal friction peak belongs to the typical phase transformation internal friction peak which is associated with the formation of intermetallic compounds NiAl<sub>3</sub> and Ni<sub>2</sub>Al<sub>3</sub> in the heating process. Furthermore, the microstructure of the Ni-Al powder mixture can be tailored by mechanical ball-milling. The internal friction peak shifts toward lower temperature and the peak height decreases with the ball-milling time increasing, which indicates that the solid diffusion reaction can be activated at lower temperature with a slower reaction rate. This decrease is related to the refinement of powder particles, the lamellar formation of powder mixture, the enhancement of solid solution degree and surface energy, and the shortened atomic diffusion distance due to the mechanical ball-milling. It is also indicated that the mechanical ball-milling can effectively reduce the initial temperature of solid diffusion reaction, thus lowering sintering temperature.

Design of a new Al-Cu alloy manipulated by in-situ nanocrystals with superior high temperature tensile properties and its constitutive equation

AlCu alloys inoculated by in-situ nano-sized particles crystallized from Ni-Nb-Ti amorphous alloys were prepared by casting method in this work. Different compositions of Ni-Nb-Ti amorphous alloys significantly enhanced heterogeneous nucleation for α-Al and promoted obvious grain refinement of the AlCu alloys. It was found that 0.05 wt% Ni-Nb-Ti amorphous alloys addition caused a significant and efficient refinement on α-Al (from 200 μm to 39 μm) and θ′ phase (from 146.4 nm to 37.6 nm). The strength and ductility of the inoculated alloy were improved significantly at high temperature. The tensile strength and fracture strain of the alloy inoculated by 0.05 wt% Ni-Nb-Ti amorphous alloy for 20 min at 533 K under the strain rates of 10−1 s−1 were increased by 32.63% and 90.35% respectively, compared to those of the uninoculated alloy. The nano-sized NiTi particles contributed to the grain refinement of α-Al, and grain refinement shortened the solute atomic diffusion distance and refined θ′ phase during heat treatment, resulting in higher strength and plasticity at high temperature. The model developed for this new type AlCu alloy is found to be efficient in predicting the stress at 453 K–533 K under the strain rates of 10−4 s−1 to 10−1 s−1.

Efficient microstructure refinement of Al–Si–Mg alloy manipulated by nanocrystals formed by in-situ crystallization in melt

Al–Si–Mg alloys manipulated by in-situ synthesized nanoparticles have been prepared by casting method in this work. The addition of different contents of refiners promoted the obvious grain refinement of the Al–Si–Mg alloys. It was found that 0.10 wt% refiners addition caused a significant and efficient refinement on α-Al (from 1675 µm to 199 µm) and Mg2Si in the inoculated Al–Si–Mg alloys. The morphology of the eutectic Si in the inoculated Al–Si–Mg alloys transformed from needle-like structure to a fibrous one, and the mean size sharply decreased (from 26.56 µm to 11.89 µm) and increased a little with the increase of FeBSi amorphous refiners content. Tensile property tests showed that both strength and ductility of the alloys were improved by the addition of FeBSi amorphous refiners. The yield strength, tensile strength, fracture strain and elongation of the alloy inoculated with 0.10 wt% FeBSi amorphous refiners for 20 min were 226 MPa, 343 MPa, 18.4% and 16.7%, compared to those of the uninoculated alloy (170 MPa, 289 MPa, 13.7% and 12.2%), respectively. It was found that the Fe2B and Fe3Si nano-sized particles which were crystallized from FeBSi amorphous refiner contributed to the grain refinement of α-Al, resulting in higher strength. Moreover, grain refinement shortened the atomic diffusion distance and refines Mg2Si. The grain refinement and spheroidization of eutectic Si enhance uniform deformation during tension, and reduce the possibility of crack initiation and expansion, thus improving the plasticity.

Influence of pressure on interfacial microstructure evolution and atomic diffusion in the hot-press bonding of Ti-33Al-3V to TC17

Joining of TiAl to Ti alloy is a potential requirement for manufacturing the components with dual properties. In this paper, the hot-press bonding of Ti-33Al-3V to TC17 was implemented at different pressures and the temperature of 850 °C for 10 min with post bond heat treatment. The influence of pressure on interfacial microstructure evolution and atomic diffusion was analyzed in detail. The microstructure observation and phase composition analysis on the bond indicate that the metallurgical bonding of Ti-33Al-3V/TC17 is produced at pressures of 23–50 MPa, and an interfacial zone with a certain width forms between base alloys. A new α2 (Ti3Al) phase is generated in the interfacial zone. The influence of pressure on the metallurgical bonding of Ti-33Al-3V/TC17 includes two aspects. One is to promote the void shrinkage in the interfacial zone. The quantitative analysis of voids shows an increase in pressure induces a decrease in amount and size of voids. The highest bonding ratio of 96.3% can be obtained at the pressure of 50 MPa with post bond heat treatment. The second is to promote the atomic diffusion across bond line. The diffusion distances of Ti and Al atoms increase with an increase in pressure and the width of the interfacial zone shows a linear increasing law with the pressure. Moreover, a higher pressure induces a higher increment in atomic diffusion distance and width of the interfacial zone in the post bond heat treatment.

Thermal shock behaviors of Ti1−xZrxN coatings by accelerated test based on pulsed laser ablation

The degradation behaviors of Ti1−xZrxN coatings on SUS304 were investigated through an accelerated test method using a pulsed laser to induce thermal shock. The accelerated test was designed based on the Arrhenius model, and the changes in surface roughness were measured to determine the failure of the coating specimens. The predicted lifetimes obtained by a statistical analysis of the failure time data were increased by a factor of ~1.5 according to the increase in the Zr ratio from 10 to 50wt%. The increase of the lattice parameters and hardness for the as-deposited specimens indicated lattice volume expansion and distortion according to the Zr addition, respectively. Depth profiles for the coating (Ti, Zr) and the substrate (Fe, Cr) atoms revealed that the atomic diffusion distance by laser ablation decreased with lattice distortion. Delamination of the coating layer was only observed in the specimen with 10wt% of Zr after repeated thermal shock. The coating thickness decreased with surface cracks and was suppressed in the specimens with 30 and 50wt% of Zr by the increase of ZrN formation.

Formation Process of Joints Brazing with Amorphous Filler Metal

Abstract The formation, wetting and spreading process of the liquid amorphous filler metal were investigated. The results indicate that the eutectic phase of the amorphous filler metal is damaged and plenty of high melting-point copper solid solution and copper-nickel compound are generated due to the full solid-phase diffusion before melting. Therefore, relatively less liquid phase is produced by amorphous filler metal than crystalline filler metal and the liquid phase spreading and flow is weak. However, the crystalline filler metal generates a large amount of liquid phase. Spreading and flowing of the liquid phase is the main process of crystalline filler metal brazing. As the amorphous filler metal is very thin, the atomic diffusion distance is very short in the filler metal, which is quite favorable for the rapid dissolution into the base metals. The ordinary filler metal is relatively thick, and it tends to take longer time to dissolve than the amorphous filler metal. The thickness of the residual filler metal decreases with the increase of brazing temperature and dwell time. The thickness of the residual layer of amorphous filler metal is smaller than that of the crystalline filler metal under the same brazing temperature and dwell time.

621 新仮説「核/縁組織内における拡散寄与型原子空孔の枯渇」の提唱(ワークショップ 粉末成形・焼結の最前線)

The rate of peritectoid reactions such as FeSi(ε: core)+Fe_2Si_5(α: matrix) → 3FeSi_2(β: rim) in Fe-66.7at%Si alloy are well known to be generally extremely slow. Such extremely slow rate has been said to be because the reaction is a solid state reaction where atomic diffusion rate is extremely slow, or because the atomic diffusion distance of the reactant atoms from the matrix to the reaction interface of core/rim, i.e., the thickness of the rim increases with the progress of the reaction. On the other hand, we have proposed that such slow rate is supposed to be also due to the exhaustion of the diffusion-contributable vacancies in core/rim structure. In this report, the concept and applications of this hypothesis will be explained.

Effect of intermixing on self-propagating exothermic reactions in Al/Ni nanolaminate foils

Exothermic reactions can self-propagate rapidly in multilayered foils, and the properties of these reactions depend strongly on the heat of reaction, the average atomic diffusion distance, and the degree of intermixing at the layer interfaces prior to ignition. By performing low-temperature anneals on sputter-deposited Al/Ni nanolaminate foils, the thickness of the intermixed region between layers was increased and its effects on the heats and velocities of reactions were measured. The intermixed region consisted of the metastable Al9Ni2 phase while the final phase of the foil was Al3Ni2. Analytical and empirical models were used to predict reaction velocities as a function of bilayer thickness and intermixing thickness, and the predictions are in good agreement with the experimental results. Increasing the average thickness of the intermixed region from 2.4 to 18.3 nm reduced the reaction velocity for all of the foils but was most significant for the foils with bilayer thicknesses less than 25 nm. The results indicate that the reaction velocity can be separated into two distinct regimes. The first regime occurs for thicker bilayers in which the average atomic diffusion distance is large. In this regime, reaction temperatures are high and reducing the bilayer thickness increases the reaction velocity. The second regime occurs for thinner bilayers where reaction velocity is dominated by the reduction in available energy due to intermixing. In this regime, reducing bilayer thickness results in a decrease in reaction velocity.