Pipe Diffusion Research Articles

Microstructure and impression creep characteristics Al-9Si-xCu aluminum alloys

The effects of 1.5, 2.5 and 3.5 wt.% Cu additions on the microstructure and creep behavior of the as-cast Al-9Si alloy were investigated by impression tests. The tests were performed at temperature ranging from 493 to 553 K and under punching stresses in the range 300 to 414 MPa for dwell times up to 3000 seconds. The results showed that, for all loads and temperatures, the Al–9Si–3.5Cu alloy had the lowest creep rates and thus, the highest creep resistance among all materials tested. This is attributed to the formation of hard intermetallic compound of Al2Cu, and higher amount of α-Al2Cu eutectic phase. The stress exponent and activation energy are in the ranges of 5.2- 7.2 and 115 -150 kJ/ mol, respectively for all alloys. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.

Activation Energy Calculations for the Portevin–Le Chatelier Effect in Nimonic 263 Superalloy

The Portevin–Le Chatelier (PLC) effect in the Nimonic 263 superalloy was investigated by tensile test in a wide temperature range, from room temperature to 1033 K (760 °C), and at strain rates between 0.1 and 4 × 10–4 s–1. Types A, B, and C serrations were observed depending upon the test temperatures and strain rates. The activation energy (Q) for serrated flow was determined by employing various methodologies for T < 723 K (450 °C), where a normal PLC effect was observed. The value of Q was found to be independent of the method employed. The average Q value of 70 KJ/mol was found to be in agreement with that for diffusion of substitutional solutes such as Cr and Mo in a Ni matrix by pipe diffusion. At temperatures ranging from 723 K to 923 K (450 °C to 650 °C), type C serrations and an inverse PLC effect were noticed, which may be caused by unlocking process.

An examination of strain ageing in a Mg–Y–Zn alloy containing Gd

Dynamic strain ageing has been observed on straining a Mg–Y–Zn–Gd alloy in both the quenched and the annealed states, where long-period-structural-order phase and solution or precipitation of Gd is found. Deformation mechanisms are analysed in terms of solute diffusion to mobile dislocations, the role of excess vacancies and the presence of a Taylor dislocation network. In quenched material, the operation of multiple deformation modes leads to the creation of a three-dimensional (Taylor) dislocation network such that pipe diffusion along forest dislocations can readily lead to pinning of a moving dislocation. The many planar precipitates present in annealed material inhibit non-basal flow such that the Taylor dislocation network does not form and bulk diffusion of solute is required for flow serrations to appear.

Impression creep behavior of extruded Mg–SiCp composites

Impression creep experiments were performed on pure magnesium and its composites reinforced with 5, 7.5, 10 and 15vol% of 10-μm SiCp fabricated by powder metallurgy and extrusion processes. Tests were conducted on planes perpendicular to the extrusion direction, in the temperature range 423–473K and under stress level in the range 150–250MPa. Analysis of data showed that for all loads and temperatures magnesium reinforced with 10vol% SiCp had the lowest creep rates, and thus, the highest creep resistance among all materials tested. The improved properties of the composite materials were discussed based on various strengthening mechanisms. The stress exponents of 7.0–7.9 and activation energies of about 90kJmol–1, which are close to that for dislocation-pipe diffusion in Mg, were almost the same for all tested materials. These creep parameters imply that the dominant creep mechanism could be dislocation climb controlled by pipe diffusion.

Thermodynamic and kinetic modeling of oxide precipitation in nanostructured ferritic alloys

The mechanical properties and radiation tolerance of nanostructured ferritic alloys rely on a dense population of nanometer-scale Y–Ti oxides. The stability of these nano-oxides during extended service is critical in high temperature applications. Here, a model framework is developed for the thermodynamics and kinetics of Y–Ti oxide nucleation, growth and coarsening. The model, which is based upon available thermodynamic and kinetic data as well as key density functional theory calculations, shows that nano-oxide nucleation and growth are highly driven and that pipe diffusion is the dominant mode of their coarsening, in agreement with previous analyses of experimental high temperature data. The model predicts that the nano-oxides are thermally stable for 80 or more years below 1175K. This analysis also provides insights into the effect of O and Ti on nano-oxide sizes, and on optimization of alloy microstructure.

Effects of liquid viscosity on flows inside and outside a bubble diffuser pipe

Effects of liquid viscosity on flows inside and outside a bubble diffuser pipe were investigated in this study. Glycerol–water solutions and air were used for the liquid and gas phases, respectively. A circular pipe having five aeration holes on the topside and larger opening on the bottom side was placed in a rectangular water tank. The pipe diameter was 20mm and the hole diameter was 5mm. The liquid viscosity μL ranged from 1 to 100mPas and the total gas flow rate QIN ranged from 3×10−4 to 7×10−4m3/s. The flows inside and outside the pipe were observed using a high-speed video camera and the gas flow rates from each aeration hole were measured by capturing bubbles generated from each hole. The liquid entered into the pipe through the aeration hole and the opening, resulting in wavy flow pattern with intermittent slugging. The conclusions obtained are as follows: (1) the increases in QIN and μL decrease the frequency of slugging disturbing aeration, so that the gas flow rate from each hole becomes uniform at high QIN and μL, (2) the main triggers of slugging are interfacial waves at low μL and interface deformation caused by liquid drops falling from the aeration holes into the pipe at high μL, (3) a slugging criterion for inviscid fluids proposed by Mishima and Ishii (1980) is applicable not only to low μL liquids but also to high μL liquids because the wave growth rate is much larger than the viscous damping rate even at high viscosities tested in the present experiments, and (4) the Davidson–Schüler correlation (Davidson and Schüler, 1960) gives reasonable estimations of the bubble diameter not only for low viscosity liquids but also for high viscosity liquids, provided that the influence of slugging is small.

Mechanisms of the formation of morphological features of micropipes in bulk crystals of silicon carbide

New morphological features of dislocation micropipes in silicon carbide single crystals have been investigated experimentally and theoretically. It has been shown that the cylindrical shape of micropipes is unstable, and the cross-sectional sizes of the micropipes change along their axes. The experimental results have been obtained by measuring X-ray phase-contrast microscopy images of micropipes in highly coherent synchrotron radiation. The cross-sectional sizes of the micropipes have been determined using computer simulation of their images. A theoretical model of the transformation of dislocations into micropipes due to nonequilibrium processes of pipe diffusion and coagulation of vacancies on dislocations parallel to the crystal growth axis has been proposed.

The stress dependence of olivine creep rate: Implications for extrapolation of lab data and interpretation of recrystallized grain size

Based on measured values for the stress exponent, n≈3.5, combined with the empirically determined relationship between dislocation density and stress (ρ∝σ1.37) and an analysis of diffusion kinetics in olivine, we conclude that silicon pipe diffusion limits strain rate in the dislocation creep regime. Furthermore, assuming that steady state recrystallized grain size is set by a dynamic balance between strain energy density (associated with dislocations) and surface energy density (associated with grain boundaries), the resulting dependence of recrystallized grain size on stress accurately describes experimental observations when the empirical dislocation density versus stress relationship is accounted for (d∝1/σ1.37). The improved physical understanding of the stress dependence of creep rate provides justification for incorporation of experimentally derived flow laws into models of geodynamical process and grain size evolution. These lab constraints combined with independent analyses of the stress dependence of mantle viscosity based on geophysical data provide bounds on rheological properties such as the yield stress of the lithosphere.

Effect of Li content on the indentation creep characteristics of cast Mg–Li–Zn alloys

Creep behavior of the hcp Mg–4Li–1Zn (LZ41), hcp/bcc Mg–8Li–1Zn (LZ81), and bcc Mg–12Li–1Zn (LZ121) cast alloys was studied by long-term Vickers indentation testing in the temperature range of 423–498 K and under constant load of 5 N. It was established that increasing Li content decreased the creep resistance at all test temperatures. This was stemmed from the effect of Li on the crystal structure of the alloys, the increment of which increased the volume fraction of the bcc β-phase. This phase activated non-basal slip and facilitated the slip-assisted deformation, and thus, resulted in lower strength and creep resistance. The average stress exponents of about 7.0, 4.5, and 4.2 and activation energies of 90.0, 91.7 and 98.2 kJ/mol were obtained for the LZ41, LZ81, and LZ121 alloys, respectively. It is suggested that the operative creep mechanism is dislocation pipe diffusion in the LZ41 alloy, and pipe-diffusion-controlled dislocation viscous glide in the LZ81 and LZ121 alloys.

Plastic deformation and ductility of magnesium AZ31B-H24 alloy sheet from 22 to 450 °C

Mg alloy AZ31B-H24 sheet was investigated through microstructural characterization and mechanical testing. Tension tests were conducted across a broad range of strain rates for temperatures from 22 up to 450°C, as were gas-pressure bulge tests for several forming pressures at 350°C. Tensile data were used to calculate flow stresses, tensile elongations, activation energies for creep and the Lankford coefficient, or R-value. The AZ31B material exhibits strong plastic anisotropy (R=7) at room temperature because of its basal crystallographic texture. Plastic anisotropy decreases with increasing temperature but demonstrates no sensitivity to strain rate until recrystallization occurs. Upon recrystallization, the R-value becomes sensitive to strain rate and continues decreasing with increasing temperature until it reaches a minimum (R=1–3) near approximately 300°C. Plastic anisotropy at these high temperatures is greatest at the fastest strain rate. This sensitivity to strain rate is attributed to a competition between dislocation-climb creep, which produces anisotropic flow and dominates at fast strain rates, and grain-boundary-sliding creep, which produces isotropic flow and dominates at slow strain rates. The mechanistic understanding developed for plastic flow in AZ31B was implemented in a material constitutive model for deformation at 350°C. A new aspect of this model is the inclusion of dislocation pipe diffusion as a potential accommodation mechanism for dislocation-climb creep. This model was validated against independent gas-pressure bulge test data through the predictions of a finite-element-method simulation, and the model provided quite accurate predictions of the experimental data.

Atomistic simulation of hydrogen dynamics near dislocations in vanadium hydrides

Kinetics of interstitial hydrogen atoms near dislocation cores were analyzed by atomistic simulation. Classical molecular dynamics method was applied to model structures of edge and screw dislocations in α-phase vanadium hydride. Simulation showed that hydrogen atoms aggregate near dislocation cores. The spatial distribution of hydrogen has a planner shape at edge dislocation due to dislocation partialization, and a cylindrical shape at screw dislocation. Simulated self-diffusion coefficients of hydrogen atoms in dislocation models were a half- to one-order lower than that of dislocation-free model. Arrhenius plot of self-diffusivity showed slightly different activation energies for edge and screw dislocations. Directional dependency of hydrogen diffusion near dislocation showed high and low diffusivity along edge and screw dislocation lines, respectively, hence so called ‘pipe diffusion’ possibly occur at edge dislocation but does not at screw dislocation.

Novel Bi-containing Sn–1.5Ag–0.7Cu lead-free solder alloy with further enhanced thermal property and strength for mobile products

In this study, the low Ag-content Sn–1.5Ag–0.7Cu (SAC157) lead-free solder was modified for advance electronic components with minor additions of Bi. The microstructure, melt properties and tensile behavior after and before compositional modifications were investigated. From microstructures evaluation, 1wt% Bi addition to SAC157 significantly enhanced the solid solution effect of Bi and refined needle-like Ag3Sn and plate-like Cu6Sn5 particles as well as extended the eutectic area, which raised the mechanical strength and Young’s modulus to about 1.5 times of SAC157 solder. With increasing Bi addition to 3wt%, the cubic-shaped Bi precipitates and enlarged eutectic area cause a dramatic increase of ultimate tensile strength, yield strength and Young’s modulus to about 2.2–2.6 times of the SAC157 alloy, although the elongation was maintained at the SAC157 level. The peak flow stress during deformation can be correlated with temperature and strain rate by a hyperbolic-sine equation. The activation energy for SAC157 alloy was increased from 58.2kJ/mol to 101.7 and 109.0kJ/mol when doped with 1.0 and 3.0wt% of Bi, indicating the change of dominant deformation mechanism from dislocation pipe diffusion to lattice self-diffusion. Besides, the addition of Bi not only reduced the solidus temperature (Tonset) and eutectic temperature (Tm), but also decreased the undercooling even though the pasty rang is slightly increased.

Study of Coupling Numerical Flow Field Simulation of Low-Pressure Last Stage Exhaust Passage in Steam Turbine

The model of coupled exhaust hood with condenser throat and the model of coupled exhaust hood, condenser throat with last stage were simulated based on the turbulence model Realizable k-ε. Calculated results show that due to the ignoring of the inlet swirl in the model coupled exhaust hood with condenser throat, the flow field is symmetrical and the pressure loss is small. Due to the influence of last stage, in the model of coupled exhaust hood, condenser throat with last stage, the flow field of the inlet of the exhaust hood is uneven, and the vortexes changed more complex, resulting in the increase of the pressure loss of each part and a greater influence in the diffuser pipe. The proportion of pressure loss of diffuser pipe in total pressure loss increases from 0.086 to 0.358, and there is a 70% decline of proportion of pressure loss in volute and condenser throat. In addition, the proportion of the pressure loss in volute is the largest one in these two coupled models. So more attention should be paid in the influence of the last stage, and weaken the vortexes in the volute when designing or optimizing the exhaust passage of steam turbine.

Creep of Al–0.2Sc–0.04Zr alloys after different cold-rolling and ageing combinations

The tensile creep of Al–0.2Sc–0.04Zr (wt%) after pre-ageing+cold-rolling+ageing (A+CR+A) and cold-rolling+ageing (CR+A) was comparatively studied in the temperature range from 225 to 300°C. The results show that both A+CR+A and CR+A apparently improve the creep resistance. Comparing to A+CR+A, a much higher creep resistance can be obtained by CR+A. The creep threshold stress at 300°C for CR+A is 50MPa, which is 117% higher than that (23MPa) for A+CR+A; furthermore, it decreases linearly with increasing temperatures. The creep of Al–0.2Sc–0.04Zr after both treatments can still be attributed to the dislocation creep with the existence of threshold stresses. Nevertheless, the much smaller creep activation energy (0.50–0.67eV) than that of pure Al may be associated with fine grain sizes and/or pipe diffusion inside grains.

Mechanical Deformation Behavior of Sn-Ag-Cu Solders with Minor Addition of 0.05 wt.% Ni

The aim of the present work is to develop a comparative evaluation of the microstructural and mechanical deformation behavior of Sn-Ag-Cu (SAC) solders with the minor addition of 0.05 wt.% Ni. Test results showed that, by adding 0.05Ni element into SAC solders, generated mainly small rod-shaped (Cu,Ni)6Sn5 intermetallic compounds (IMCs) inside the β-Sn phase. Moreover, increasing the Ag content and adding Ni could result in the change of the shape and size of the IMC precipitate. Hence, a significant improvement is observed in the mechanical properties of SAC solders with increasing Ag content and Ni addition. On the other hand, the tensile results of Ni-doped SAC solders showed that both the yield stress and ultimate tensile strengths decrease with increasing temperature and with decreasing strain rate. This behavior was attributed to the competing effects of work hardening and dynamic recovery processes. The Sn-2.0Ag-0.5Cu-0.05Ni solder displayed the highest mechanical properties due to the formation of hard (Cu,Ni)6Sn5 IMCs. Based on the obtained stress exponents and activation energies, it is suggested that the dominant deformation mechanism in SAC (205)-, SAC (0505)- and SAC (0505)-0.05Ni solders is pipe diffusion, and lattice self-diffusion in SAC (205)-0.05Ni solder. In view of these results, the Sn-2.0Ag-0.5Cu-0.05Ni alloy is a more reliable solder alloy with improved properties compared with other solder alloys tested in the present work.

Assessment of substitutional self-diffusion along short-circuit paths in Al, Fe and Ni

The kinetics of self-diffusion along grain boundaries and dislocations (pipe diffusion) is analyzed in Al, α- and γ-Fe and Ni. The available experimental information is critically reviewed. The suggested values given in the present work are assessed with focus on consistency and applicability to thermo-kinetic simulation. The proposed data are evaluated between room temperature and the melting point.

Direct measurement of hydrogen dislocation pipe diffusion in deformed polycrystalline Pd using quasielastic neutron scattering.

The temperature-dependent diffusivity D(T) of hydrogen solute atoms trapped at dislocations-dislocation pipe diffusion of hydrogen-in deformed polycrystalline PdH(x) (x∼10(-3) [H]/[Pd]) has been quantified with quasielastic neutron scattering between 150 and 400 K. We observe diffusion coefficients for trapped hydrogen elevated by one to two orders of magnitude above bulk diffusion. Arrhenius diffusion behavior has been observed for dislocation pipe diffusion and regular bulk diffusion, the latter in well-annealed polycrystalline Pd. For regular bulk diffusion of hydrogen in Pd we find D(T)=D(0)exp(-E(a)/kT)=0.005exp(-0.23 eV/kT) cm(2)/s, in agreement with the known diffusivity of hydrogen in Pd. For hydrogen dislocation pipe diffusion we find D(T)≃10(-5)exp(-E(a)/kT) cm(2)/s, where E(a)=0.042 and 0.083 eV for concentrations of 0.52×10(-3) and 1.13×10(-3)[H]/[Pd], respectively. Ab initio computations provide a physical basis for the pipe diffusion pathway and confirm the reduced barrier height.

The microstructure and creep characteristics of cast Mg–4Zn–0.5Ca and Mg–4Zn–0.5Ca–2RE alloys

The effect of 2wt% Ce-rich rare earth (RE) addition on the microstructure and creep behavior of the Mg–4Zn–0.5Ca (wt%) alloy was investigated by impression creep tests in the temperature range of 423–498K and under stresses in the range of 250–800MPa for dwell times up to 3600s. Addition of RE was shown to significantly improve the creep resistance of the base alloy. Formation of the thermally stable (MgZn)12RE and Mg17RE2 particles was responsible for such an improvement. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.024<(σ/G)<0.038 and 0.031<(σ/G)<0.045 for the base- and RE-containing alloys, respectively. In the low-stress regime, stress exponents were in the range 5.0–5.2 and 5.1–6.2, and activation energies were found to be 65±4 and 57±7kJmol−1 for the base and RE containing alloys, respectively. The activation energies were stress dependent and this implied that the stress assisted dislocation climb controlled by pipe diffusion is the rate-controlling mechanism. In the high-stress regime, stress exponents were in the range 10.8–11.7 and 12.1–14.0, and activation energies were 105±20 and 85±18kJmol−1 for the base and RE containing alloys, respectively. Power-law creep was invalid in this region and activation energies were stress dependent, suggesting that the stress-assisted dislocation climb controlled by lattice diffusion, accelerated due to high concentration of vacancies resulting from applying high stresses, is the rate-controlling mechanism.

Simulation of Cu Precipitation in the Fe-Cu Binary System

Cu precipitation in steel has been investigated numerous times. Still, a consistent simulation of the nucleation, growth and coarsening kinetics of Cu precipitates is lacking. Major reason for this is the fact that Cu precipitation involves complex physical interactions and mechanisms, which go beyond the classical precipitation models based on evaporation and absorption of precipitate-forming monomers (atoms). In the present work, we attempt a comprehensive modeling approach, incorporating coalescence results from Monte Carlo simulation, prediction of the nucleus composition based on the minimum energy barrier concept, diffusion enhancement from quenched-in vacancies, dislocation pipe diffusion, as well as the transformation sequence of Cu-precipitates from bcc-9R-fcc. Our simulations of number density, radius and phase fraction coincide well with experimental values. The results are consistent over a large temperature range, which is demonstrated in a TTP-plot.

Creep behavior of AS41 alloy matrix nano-composites

Nano-alumina reinforced AS41 magnesium alloy matrix composites with 2wt% and 5wt% Al2O3 were fabricated by combined stir casting and ultrasonic processing in order to distribute the nano-alumina particles uniformly in the melt. Indentation creep tests were performed on the nano-composites and the as cast AS41 alloy at four different temperatures of 448K, 473K, 498K, and 523K and three different stresses of 109.2MPa, 124.8MPa and 140.4MPa. The creep curves obtained in these tests show that creep resistance increased with the addition of nano-Al2O3 into the magnesium alloy, and nano-composites with 5wt% Al2O3 exhibited highest creep resistance. The values of obtained stress exponent range from 3 to 3.5, 3.55 to 6, and 4 to 6.5 for the as-cast AS41 alloy, and AS41/2wt% Al2O3, and AS41/5wt% Al2O3 composites, respectively. Thus, dislocation creep is the dominant creep mechanism. The calculated activation energies for as-cast AS41, AS41/2wt% Al2O3 and AS41/5wt% Al2O3 are approximately 66.17kJ/mol, 83.77kJ/mol, and 88.3kJ/mol, respectively. The activation energy values for the creep of composites are close to that of the pipe diffusion (92kJ/mol) of magnesium.