Pipe Diffusion Research Articles

Deformation Enhanced Diffusion in Aluminium Alloys

Deformation introduces defects such as dislocations and excess vacancies that can strongly influence diffusion rates in aluminium alloys. This is of great importance in understanding the effect of deformation on processes such as precipitation hardening, which can be accelerated by orders of magnitude by the defects introduced. In this work, a simple and widely used classical model has been employed to explore the effect of process variables on diffusion enhancement due to deformation-induced excess vacancies and dislocations. It is demonstrated that in aluminium alloys, the strain rates and temperatures used in processing and testing encompass a range of regimes. At low strain rate or high temperature, a steady state becomes established in which the deformation enhancement depends on strain rate but not strain. Conversely, at high strain rate or low temperature, deformation enhancement is insensitive to strain rate but increases with strain. For all conditions, the effect of excess vacancies is much stronger than the direct effect of pipe diffusion along dislocations. The predicted deformation-induced vacancy concentration can exceed that expected after rapid quenching by an order of magnitude, but the lifetime of the deformation-induced vacancies is much shorter. Finally, limitations of the classical model and suggestions for improvement are proposed.

Effects of Re on microstructure evolution of nickel-based single crystal superalloys

The high temperature creep and long-term aging tests of the second-generation (DD6) and the third-generation (DD9) single crystal superalloys with different Re contents were carried out. The effects of Re, dislocations, TCP phases and their interaction on creep properties and microstructure evolution were researched. The third-generation single crystal superalloy has the more compact and uniform two-phase structure and the better creep properties. The rafting rate in dendrite is higher than that in interdendrite. The γ' phases connect into a stripline, the γ phases on the strip line are truncated, and the TCP phase is precipitated along the stripline. The element Re carries out the pipe diffusion and the vacancy diffusion at the high temperature. The TCP phase precipitates in the region with many dislocations and vacancies. Re hinders the movement of dislocations and promote the precipitation of TCP phases. The movement of dislocations promotes the diffusion of Re and the precipitation of TCP phases. TCP phases promote the diffusion of elements and accelerate the rafting of γ' phases.

The Effect of the Distribution Head Tilt and Diffuser Variants on the Evenness of Sowing Rye and Oat Seeds with a Pneumatic Seed Drill

Due to the sustainable development of agriculture machines with large working widths of 4–6 m or even 9–12 m are increasingly often used for agrotechnical operations. The sowing machinery whose working widths are much bigger than the width of the seed box is equipped with a pneumatic system for transporting seeds from the seed box to coulters. One of the structural elements that affect the sowing accuracy in such seed drills is the distribution head with a diffuser. This article is about research on the influence of the distribution head deviation from the vertical position and constructional variants of the diffuser (the number of diffusion rings and the configuration of their position in the diffuser pipe, which is the distance between them) on the accuracy of distribution of a stream of rye and oat seeds (a seed-and-air stream), which differ in physical characteristics. The main elements, i.e., the innovative stream distributor in the head and the diffusion rings were made using an original design and the rapid prototyping method. The research proved that a change of 0–10° in the angle of the distribution head deviation from the vertical position significantly affected the sowing quality of oat seeds only. The position (density) of the diffusion rings in the lower section of the diffuser (near the supply elbow) was the most effective for both oat and rye seeds, where the average values of the coefficient of variation were 5.31% and 4.62%, respectively. The research results can be used to redesign the construction of the diffuser of the seed drill distribution head so as to reduce the resistance of transport of the seed-and-air mixture in order to improve seed sowing evenness.

A comparative study on the effects of Gd, Y and La rare-earth elements on the microstructure and creep behavior of AZ81 Mg alloy

Impression creep tests were performed to study the effects of separate additions of 1 wt% Gd, Y, and La rare-earth (RE) elements on the microstructure and creep characteristics of a cast AZ81 Mg alloy. Constant punching stresses in the range 200–700 MPa were applied to the specimens at temperatures in the range of 423–523 K. The results revealed that the RE-containing alloys had lower creep rates in comparison with the base AZ81 alloy, the effect which was more significant in the AZ81+1La alloy. The enhanced creep resistance of the RE-containing alloys is a consequence of; (i) formation of Al2Gd and Al2Y particles with cubic morphologies, as well as Al11La3 particles with acicular-branched morphology, (ii) reduction in volume fraction of the β‒Mg17Al12 phase, and (iii) solid solution strengthening of Al in the Mg matrix. The studied alloys had stress exponents and activation energies in the range of 4.2–6.7 and 83–118 kJ/mol, respectively. The observed declining trend of creep-activation energy with increasing stress indicates the competition of two simultaneous mechanisms of lattice- and pipe-diffusion controlled dislocation climb. Dislocation climb controlled by dislocation pipe diffusion, and the climb of edge dislocations were found to be prevalent at high and low stresses, respectively.

Dislocation Pipe Diffusion and Solute Segregation during the Growth of Metastable GeSn

Controlling the growth kinetics from the vapor phase has been a powerful paradigm enabling a variety of metastable epitaxial semiconductors such as Sn-containing group IV semiconductors (Si)GeSn. In addition to its importance for emerging photonic and optoelectronic applications, this class of materials is also a rich platform to highlight the interplay between kinetics and thermodynamic driving forces during growth of strained, nonequilibrium alloys. Indeed, these alloys are inherently strained and supersaturated in Sn and thus can suffer instabilities that are still to be fully elucidated. In this vein, in this work the atomic-scale microstructure of Ge0.82Sn0.18 is investigated at the onset of phase separation as the epitaxial growth aborts. In addition to the expected accumulation of Sn on the surface leading to Sn-rich droplets and sub-surface regions with the anticipated equilibrium Sn composition of 1.0at.%, the diffusion of Sn atoms also yields conspicuous Sn-decorated filaments with nonuniform Sn content in the range of ~1 to 11at.% . The latter are attributed to the formation and propagation of dislocations, facilitating the Sn transport toward the surface through pipe diffusion. Furthermore, the interface between the Sn droplet and GeSn shows a distinct, defective layer with Sn content of ~22at.%. This layer is likely formed by the expelled excess equilibrium Ge as the Sn solidifies, and its content seems to be a consequence of strain minimization between tetragonal Sn-rich and cubic Ge-rich equilibrium phases. The elucidation of these phenomena is crucial to understand the stability of GeSn semiconductors and control their epitaxial growth at a uniform composition.

Dissociated dislocation-mediated carbon transport and diffusion in austenitic iron

Dislocation-solute interaction plays fundamental roles in mechanical properties of alloys. Here, we disclose the essential features of dislocation-carbon interaction in austenitic Fe at the atomistic scale. We show that passage of a Shockley partial dislocation in face-centered cubic iron is able to move carbon atoms on the slip plane forward by one Burgers vector, revealing a novel dissociated dislocation-mediated transport mechanism. This mechanism is induced by shear, which is distinct from the normal thermally activated diffusion process. Furthermore, we show that there exists a fast diffusion channel with significantly reduced diffusion energy barrier in the partial dislocation core, which is highly localized and directional. These inherent geometrical features are crucial for understanding the dependence of the diffusivity of dislocation pipe diffusion on the character of dislocations; most importantly, they can result in unbalanced pinning effect on the leading and trailing partials in a mixed dislocation, consequently facilitating stacking fault formation and deformation twinning. This explains the controversial effects of carbon on deformation twinning observed in various alloys. Our findings pave the road to tune mechanical properties of materials by manipulating dislocation-interstitial interaction.

Microstructural evolution and creep properties of Mg-4Sn alloys by addition of calcium up to 4 wt.%

The effects of addition of calcium up to 4 wt.% on the microstructure and creep properties of Mg-4Sn alloys were investigated by the impression creep test. Impression creep tests were performed in temperature range between 445 and 475 K under normalized stresses σ / G (where σ is the stress; G is the shear modulus) between 0.0225 and 0.035. Optical microscopy and scanning electron microscopy were used to study the microstructure of samples. It is observed that the addition of Ca more than 2 wt.% suppresses less stable MgSn 2 phase, and instead forms more thermally stable phases of Ca-Mg-Sn and Mg 2 Ca at the grain boundaries which improve the creep resistance of Mg-4Sn alloys. According to the stress exponents (6.04< n <6.89) and activation energies (101.37 kJ/mol< Q <113.8 kJ/mol) which were obtained from the impression creep tests, it is concluded that the pipe diffusion climb controlled dislocation creep is the dominant creep mechanism.

Quantitative analysis of the influences of pre-treatments on the microstructure evolution and mechanical properties during artificial ageing of an Al–Cu–Li–Mg–Ag alloy

The quantitative effects of pre-treatments, namely pre-stretching and natural ageing (NA), on the precipitation sequence, precipitation kinetics and mechanical properties of an Al–Cu–Li–Mg–Ag alloy during artificial ageing were systematically investigated, aiming to elucidate the mechanisms by which these effects proceed. At the early ageing stage, pre-stretching could inhibit the formation of GP zones by restricting the Mg–Cu co-clustering, and could result in a higher nucleation rate of the T1 precipitates due to an increase in the quantity of dislocations covered by solute segregations. The introduction of NA after pre-stretching could further increase the number density of T1 by providing more nucleation sites, which evolved from the much more Mg–Ag co-clusters forming during the NA process. With prolonging ageing time, the precipitation kinetics for T1 were strongly enhanced by pre-stretching due to a more efficient nucleation of T1 and a more rapid growth rate of this phase, which was attributed to the combination of the suppression effect on the GP zone precipitation and the pipe diffusion effect derived from the introduced dislocations. When applying NA after pre-stretching, a further acceleration of the precipitation kinetics was observed, which was related to the formation of much more T1 nuclei. At the peak-aged condition, both the two pre-treatments could not only refine the precipitates but also result in a higher T1 volume fraction. Mechanisms of the strength changes induced by the pre-treatments under various ageing conditions were discussed in detail in view of these experimental results.

High temperature creep behavior and creep microstructure evolution of T6 state Mg–15Gd alloy

The creep properties of Mg alloys are critical for their high temperature applications. But for Mg–Gd alloys, there is a lack of systematic research on the creep behavior and microstructure evolution above 250 °C. In this study, the high temperature tensile creep behavior and microstructure evolution of the T6 state Mg-15Gd (wt.%) metal mold casting alloy were investigated at temperatures from 235 to 300 °C and stresses from 50 to 90 MPa. The experimental alloy was composed of α-Mg matrix, elliptic shape β′ precipitates, cuboid-shaped GdH2 phase and a small amount of M5Gd phase. It was found that the creep strain and creep rate of the experimental alloy increased but the creep life decreased with increasing the creep temperature and the applied stress. At a fixed creep temperature of 260 °C, the calculated stress exponent n value was about 2.8 in the applied stress range of 50–90 MPa, suggesting that the creep deformation may be controlled by dislocation slip. Meanwhile, under a defined creep stress of 50 MPa, the calculated activation energy Q value of the alloy was about 108 kJ mol−1 in the temperature range of 235–300 °C, indicating that the pipe diffusion may be a dominant affecting factor. During the creep process, the α(Mg) grain size coarsened and the quantity, size and type of the precipitated phases changed with three-stage: β′ (cbco) →β1 (fcc) →β (fcc). Observation of creep microstructure evolution under 50 MPa and 260 °C confirmed that the creep mechanism was dislocation glide and the creep fracture mechanism was brittle transgranular cleavage fracture.

On the early stages of precipitation during direct ageing of Alloy 718

The Ni-based superalloy Alloy 718 is used in aircraft engines as high-pressure turbine discs and must endure challenging demands on high-temperature yield strength, creep-, and oxidation-resistance. Nanoscale γ′- and γ″-precipitates commonly found in duplet and triplet co-precipitate morphologies provide high-temperature strength under these harsh operating conditions. ‘Direct ageing’ of Alloy 718 is an attractive alternative manufacturing route known to increase the yield strength at 650 °C by at least +10%, by both retaining high dislocation densities and changing the nanoscale co-precipitate morphology. However, the detailed nucleation and growth mechanisms of the duplet and triplet co-precipitate morphologies of γ′ and γ″ during the direct ageing process remain unknown. We provide a correlative high-resolution microscopy approach using transmission electron microscopy, high-angle annular dark-field imaging, and atom probe microscopy to reveal the early stages of precipitation during direct ageing of Alloy 718. Quantitative stereological analyses of the γ′- and γ″-precipitate dispersions as well as their chemical compositions have allowed us to propose a qualitative model of the microstructural evolution. It is shown that fine γ′- and γ″-precipitates nucleate homogeneously and grow coherently. However, γ″-precipitates also nucleate heterogeneously on dislocations and experience accelerated growth due to Nb pipe diffusion. Moreover, the co-precipitation reactions are largely influenced by solute availability and the potential for enrichment of Nb and rejection of Al+Ti.

The mechanism of dynamic strain aging for type A serrations in tensile flow curves of Fe-18Mn-0.55C (wt.%) twinning-induced plasticity steel

To elucidate the mechanism of dynamic strain aging (DSA) causing serrations in the tensile curves of an fcc austenitic Fe-18Mn-0.55C (wt.%) twinning-induced plasticity (TWIP) steel, many tensile tests were performed by varying both tensile temperature (203−323 K) and initial strain rate (ε˙ini = 1 × 10−2 − 1 × 10−4/s). At the ranges of tensile temperature and ε˙ini adopted in this study, only type A serrations appeared, and a critical engineering strain (ec) for serrations decreased with increasing tensile temperature and with decreasing ε˙ini. For the short-range diffusion model based on C-Mn complex, the activation energy value (QreC) for the reorientation of C in C-Mn complex was calculated by ab-initio simulation. The QreC value was ~2.4 eV for the fcc austenite matrix, and 0.60 eV and 0.18 eV for the hcp stacking fault depending on diffusion path. There was no intersection between staying time (ts) and reorientation time (tre) calculated using the QreC values. This indicates that DSA is not caused by the reorientation of C-Mn complexes. In addition, ec revealed no dependency on the concentration of vacancy (Va). Therefore, DSA causing type A serrations is not explained by short-range diffusion models based on C-Mn and C-Va complexes. DSA is elucidated by the dislocation arrest model, which belongs to the long-range diffusion model. The measured activation energy (0.85 eV) corresponding to the activation energy (0.57−1.00 eV) for dislocation pipe diffusion of C and C-segregated dislocations in atom probe tomography (APT) maps support the occurrence of long-range C diffusion.

Hybrid quantum/classical study of hydrogen-decorated screw dislocations in tungsten: Ultrafast pipe diffusion, core reconstruction, and effects on glide mechanism

The interaction of hydrogen (H) with dislocations in tungsten (W) must be understood in order to model the mechanical response of future plasma-facing materials for fusion applications. Here, hybrid quantum mechanics/molecular mechanics (QM/MM) simulations are employed to study the ⟨111⟩ screw dislocation glide in W in the presence of H, using the virtual work principle to obtain energy barriers for dislocation glide, H segregation, and pipe diffusion. We provide a convincing validation of the QM/MM approach against full DFT energy-based methods. This is possible because the compact core and relatively weak elastic fields of ⟨111⟩ screw dislocations allow them to be contained in periodic DFT supercells. We also show that H segregation stabilizes the split-core structure while leaving the Peierls barrier almost unchanged. Furthermore, we find an energy barrier of less than 0.05 eV for pipe diffusion of H along dislocation cores. Our quantum-accurate calculations provide important reference data for the construction of larger-scale material models.

Effects of Strain Rate and Temperature on Tensile Properties, Deformation and Dynamic Strain Ageing Behavior of Ni-Base Superalloy Superni 263

Tensile studies on Superni 263 superalloy were carried out by employing various strain rates and temperatures in the range of (298–923 K), to explore the tensile deformation and fracture characteristics. It is found that Superni 263 alloy was sensitive to strain rate and peak value in yield strength was obtained at intermediate strain rate 1.3 × 10−3 S−1 and 673 K. The alloy exhibited serrated plastic flow, anomalous variations in tensile properties, minimum in ductility, work hardening rate and negative strain rate sensitivity which are the important manifestations of dynamic strain ageing (DSA) in the intermediate temperature range. The species responsible for the occurrence of DSA were identified as Cr and Mo with the activation energy measurements of 68–74 kJ/mol, indicating the pipe diffusion in austenite matrix. The alloy exhibited mixed ductile and intergranular mode of fracture in the entire temperature range. Few grains exhibited transgranular quasi-cleavage facets in the DSA range.

Effects of tempering on the mechanical and corrosion properties of dual phase steel

The mechanical and corrosion behaviors of low carbon dual phase (DP) steel were studied based on different tempering conditions. By increasing the time and temperature of tempering, the ultimate tensile strength (UTS), the hardness, and the corrosion current density (icorr) in the polarization curves decrease while the total elongation and the diameter of the semicircular arc in the Nyquist plots increase. Tempering treatment resulted in the appearance of the yield point phenomenon and an initial rise in the yield stress (YS). While the UTS values were related to hardness through a single relationship for both tempered and DP steels, two separate trends were observed for the relation of the YS and hardness. The latter was related to the high work-hardening rate of DP steels that increased the hardness during indentation. The tempering kinetics were also studied and the average value of the Avrami exponent (n) was determined as 0.65, which was interpreted as precipitation on dislocations (theoretical n-value of 0.667). The tempering activation energy was determined as ∼ 40 kJ/mol, which was indicative of the rapid diffusion of carbon atoms along the dislocation core (pipe diffusion), and hence, it was concluded that the kinetics of tempering is essentially controlled by carbon atom diffusion.

Effect of Ca addition on microstructure and impression creep behavior of cast AZ61 magnesium alloy

Abstract Microstructure and creep properties of AZ61 alloy containing 1 and 3 wt.% Ca were investigated. The creep properties were examined using impression method under different stresses between 200 and 500 MPa at the temperature ranging from 423 to 491 K. The microstructure of AZ61 alloy contains α(Mg) matrix and Mg17Al12 intermetallic phases. It is shown that adding Ca to AZ61 alloy reduces the amount of Mg17Al12 phase via forming (Mg, Al)2Ca phase; furthermore, increasing the Ca content to 3 wt.% leads to the formation of (Mg, Al)2Ca phase, as well as the elimination of the Mg17Al12 phase. Creep properties of AZ61 alloy are improved with the Ca addition. The improvement in creep properties is attributed to the reduction in the amount of Mg17Al12 phase and the formation of (Mg, Al)2Ca phase with high thermal stability. According to the obtained creep data, it is concluded that the pipe diffusion−climb controlled dislocation creep is the dominant creep mechanism and Ca addition has no influence on this mechanism. The effect of pre-deformation on the creep properties of AZ61+3%Ca alloy reveals that the creep resistance of the alloy depends on the continuity of (Mg, Al)2Ca phase. It is decreased by reducing the phase continuity.

Serrated Flow Behavior of Hot-Rolled Fe-6.5wt.%Si Sheet with Layered Structure

The microstructures and mechanical properties of the hot-rolled Fe-6.5wt.%Si sheet are analyzed. The microstructure of the hot-rolled sheet is layered along the thickness direction. The surface exhibits fine and equiaxed grains, whereas the center part shows coarse and elongated grains with a &lt;101&gt; fiber texture along the rolling direction. Serrated flow behavior is observed during tensile deformation of both the hot-rolled sheet and its center samples at 350 °C; thus, the serrated flow of the hot-rolled sheet is mainly attributed to the serration of the center part. The analyses of dislocation configurations, ordered structures, and crystal orientation show that the serrated flow behavior results from the interaction of solutes with mobile dislocations. Mobile dislocations are pinned by combining parallel forest dislocations with the pipe diffusion of solution atoms. This study provides a new perspective for the deformation mechanism of the Fe-6.5wt.%Si alloy.

Dislocation facilitated surface precipitation in an Al–Cu–Mg thin foil

The preferential surface precipitation of ‘tooth-like’ Cu-rich nanoprecipitates in an Al-Cu-Mg thin foil after long time natural ageing was revealed by high resolution transmission electron microscopy and electron tomography. These surface nanoprecipitates show an average cross-sectional diameter of ~6 nm, and some of them have core-shell structures. The dislocation facilitated solute diffusion, including dislocation assisted pipe diffusion and dislocation movement accelerated solute diffusion, was considered to be the dominant manner for solute diffusion from bulk interior to sample surface during natural ageing. The enrichment of Cu rather than Mg in surface nanoprecipitates was attributed to the superior Cu-vacancy binding energy, which can well facilitate Cu diffusion to dislocations. This result highlights the surface effect on element redistribution in nanoscale metallic alloys during long term performance, and provides a guidance for the development of metallic nanodevices.

Leveraging prior strain rates effect during stress relaxation of Sn–1.7Ag–0.7Cu lead-free alloys with microalloying of Te and Co for microelectronics applications

Prior strain rate history during stress relaxation (SR) behavior is one of the fundamental issues adapting the design of solder joints. In this paper, the SR tests were performed on Sn–1.7Ag–0.7Cu (SAC177) solder with Te and Co addition at the elastic region. The SR of SAC177 solders occurred at high prior deformation history could induce unexpected relaxation resistance depending on the qualified proportions of Te and Co microalloying. The global strain rate \(\varepsilon^{\cdot}_{\text{p}}\) of three solders was found to follow the power-law with stress-sensitivity n of 5.8–7.6. The activation energies Q of 58.15–78.23 kJ mol−1 were achieved from SR results at initial stress σo of 10 MPa. An evolved SR model based on logarithmic single barrier model is intended in the present study to predict and confirms the SR mechanism. Microstructure assessment shows that the SR behaviors are controlled by pipe diffusion assisted with dislocation climb. According to the proposed relaxation model, the extent of SR and SR rate of SAC177 solder were found to be decreased markedly with increasing prior strain rate, Q value and Te/Co addition. Moreover, significant enrichment in the elastic compliance of SAC177 solders was noticed after Te/Co addition due to increasing the ductility, which has good predictions for electronics applications.

Creep behavior of heat resistant Al–Cu–Mn alloys strengthened by fine (θ′) and coarse (Al20Cu2Mn3) second phase particles

In this work, creep behaviors of two Al–Cu–Mn alloys (Alloy-A: Al-1.7 wt%Cu-0.3 wt%Mn, Alloy-B: Al-2.2 wt%Cu-0.8 wt%Mn) were investigated at temperature 448 K–523 K under applied stress of 30–50 MPa for Alloy-A and 40–70 MPa for Alloy-B, respectively. Alloy-A contains small amount of fine (θʹ, <500 nm) precipitates and Alloy-B contains a great amount of coarse (T-Al20Cu2Mn3, 500nm-1μm) particles. Alloy-B exhibits much slower creep rates (as compared to Alloy-A) at every temperature up to an applied stress of 60 MPa. Creep mechanisms are discussed in terms of stress exponent and activation energy. It is observed that fine particles (θʹ) strengthened alloy exhibits an increased value of stress exponent with temperature while the coarse (T-Al20Cu2Mn3) particles strengthened alloy exhibits decreased values. Also, both the alloys show two regimes of creep mechanisms in the selected stress and temperature range, 498 K being the transition temperature. The fine particles (θʹ) strengthened alloy shows a transition from dislocations interaction process (due to small amount of particles) to dislocation climb mechanism with an activation energy of 110 kJmol-1 (at 50 MPa). While, the coarse particles (T-Al20Cu2Mn3) strengthened alloy shows a transition from grain boundary pipe diffusion mechanism to boundary sliding with 51 kJmol-1 of activation energy (at 65 MPa). Furthermore, the creep activation energy of the coarse particles strengthened alloy is decreased with the applied stress. The constructed constitutive equations show a good agreement between the experimental and simulated steady state creep rates, with an error less than 10% for Alloy-A and Alloy-B.

Dislocation dynamics formulation for self-climb of dislocation loops by vacancy pipe diffusion

It has been shown in experiments that self-climb of prismatic dislocation loops by pipe diffusion plays important roles in their dynamical behaviors, e.g., coarsening of prismatic loops upon annealing, as well as the physical and mechanical properties of materials with irradiation. In this paper, we show that the dislocation dynamics self-climb formulation that we derived in (Niu et al., 2017) is able to quantitatively describe the properties of self-climb of prismatic loops that were observed in experiments and atomistic simulations. This dislocation dynamics formulation applies to self-climb by pipe diffusion for any configurations of dislocations, and is able to recover the available models in the literature for rigid self-climb motion of small prismatic loops. We also present implementation method of this self-climb formulation. Simulations performed show evolution, translation and coalescence of prismatic loops as well as prismatic loops driven by an edge dislocation by self-climb motion and the elastic interaction between them. These results are in excellent agreement with available experimental and atomistic results. We have also performed systematic analyses of the behaviors of a prismatic loop under the elastic interaction with an infinite, straight edge dislocation by motions of self-climb and glide.