Number Density Of Precipitates Research Articles

Achieving superior strength and ductility in 7075 aluminum alloy through the design of multi-gradient nanostructure by ultrasonic surface rolling and aging

Aluminum alloys were used in various fields due to their high specific strength and low density. However, the disadvantage of low strength limits its wider application. In this work, the effect of ultrasonic surface rolling process (USRP) and artificial aging on microstructure and mechanical properties of 7075 aluminum alloy were investigated in detail. The results indicated that a multi-gradient nanostructure (MGNS) layer, i.e., grain gradient and precipitation gradient, was generated on the surface of the alloy. The tensile strength of the aged-USRP sample (692 MPa) was about 63 % higher than that of the coarse-grained sample (425 MPa), and the plasticity was comparable to that of the coarse-grained specimen. The underlying mechanisms of grain refinements, precipitation behavior and their effects on tensile properties were quantitatively analyzed. The high strength of the aged-USRP sample originated from grain boundary strengthening, precipitation strengthening, dislocation strengthening and synergistic strengthening. The activation of geometrically necessary dislocations (GNDs) during tensile deformation improved the plastic coordination ability of different microstructures. Thus, excellent strength-ductility synergy of 7075 aluminum alloy was obtained. • Multi-gradient nanostructure was formed on the surface of 7075 Aluminum Alloy. • Gradient nanostructure of type and number density of precipitates formed. • The multi-gradient nanostructure enhanced strength-ductility synergy of the alloy.

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

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

Effect of Ag and Cu addition on the strength and electrical conductivity of Al-Mg-Si alloys using conventional and modified thermomechanical treatments

The effect of adding Ag and Cu on the strength and electrical conductivity of Al-Mg-Si conductor alloys was investigated using conventional and modified thermomechanical treatments. In the conventional thermomechanical treatment, the addition of Ag and Cu moderately increased the strength from 296 MPa to 305–316 MPa above the minimum required electrical conductivity (52.5% IACS) compared to that of the base alloy. However, the modification of the thermomechanical treatment could maximize the efficiency of Ag and Cu addition at strengths above 52.5% IACS, exhibiting that the strength was increased from 317 MPa to 341–348 MPa with the Ag and Cu additions. All alloys under the modified thermomechanical treatment (MTMT) showed a superior strength and electrical conductivity compared to their counterparts under the conventional thermomechanical treatment (CTMT), resulting in a wider window in the high end of strength and electrical conductivity. In addition, MTMT led to a shorter post-aging time to reach the minimum required EC, compared to CTMT. The precipitate characteristics under both thermomechanical treatment conditions were analyzed and quantified using differential scanning calorimetry and transmission electron microscopy (TEM). The TEM results revealed that the alloys under MTMT had a higher precipitate number density by more than three times compared to their counterparts under CTMT, leading to the higher strength levels in the MTMT alloys. Strength and electrical resistivity models were then applied to understand the multiple contributions of the main strengthening mechanisms and microstructure features to the mechanical strength and electrical conductivity. • Modified thermomechanical treatment is proposed to enhance both strength and electrical conductivity (EC). • The effects of Ag and Cu additions on the strength and EC in Al-Mg-Si conductor alloys are studied. • The efficiency of the Ag and Cu additions is maximized using the modified thermomechanical treatment in terms of strength and EC. • The strength and electrical resistivity models are applied.

Enhancing corrosion resistance of ZK60 magnesium alloys via Ca microalloying: The impact of nanoscale precipitates

Enhancing corrosion resistance of Mg-Zn alloys with high strength and low cost was critical for broadening their large-scale practical applications. Here we prepared solutionized, peak- and over-aged ZK60 alloys with and without microalloying Ca (0.26 wt.%) to explore the effects of nanoscale precipitates on their corrosion behavior in detail via experimental analyses and theoretical calculations. The results suggested the peak-aged ZK60 alloy with Ca addition showed improved corrosion resistance in comparison with the alloys without Ca, owing to the contribution of Ca on the refinement of precipitates and increase in their number density. Although the precipitates and Mg matrix formed micro-galvanic couples leading to dissolution, the fine and dense precipitates could generate “in-situ pinning” effect on the corrosion products, forming a spider-web-like structure and improving the corrosion inhibition ability accordingly. The pinning effect was closely related to the size and number density of precipitates. This study provided important insight into the design and development of advanced corrosion resistant Mg alloys.

Characterization of structure and hardness at aging of the A319 type aluminum alloy with Sn trace addition

The effect of 0.1 wt% (0.02 at%) Sn trace addition on the structure and phase composition of Al-Si-Cu based alloy has been studied using thermodynamic calculations (Thermo-Calc software) and experimental techniques (transmission electron microscopy (TEM), atom probe tomography (APT), hardness and specific electrical conductivity measurements). The Vickers hardness measurement made after aging at 175 °C revealed that the peak hardness of the Sn-containing alloy is about 22% higher than that of the Sn-free alloy (135 HV vs 115 HV). Moreover, the peak hardness is achieved in a much shorter aging time (4 h vs 16 h). Considerably finer precipitates of the θ′ phase (average length less than 60 nm and thickness 3–5 nm) with a substantially higher number density are observed in the Sn-containing alloy. Tiny, rounded particles most of which are associated with the θ′ phase precipitates are also observed. Atom probe tomography analysis confirmed that the observed nanoparticles consist of tin. Quantitative APT analysis revealed that the number density of the copper-containing precipitates is at least twice that of the tin-containing ones (7.6·104 vs 3·104 µm-3). APT data also revealed a clear evidence of noticeable dissolution of Sn (average concentration from 0.05 to 0.10 at%) and silicon (average concentration from 2.2 to 2.8 at%) at the core of the θ′ phase precipitates, which is consistent with the fact that Sn and to a lesser extent Si act as catalysts in the nucleation of this phase. The data obtained using the proxigram technique suggest that Sn atoms show no tendency to segregate at the coherent θ′/(Al) heterophase interface upon aging, whereas localized Sn segregation is observed at the semi-coherent interface. Both Sn-free and Sn-containing alloys in their peak aged states have been subjected to uniaxial compression tests at an elevated temperature of 250 °С. The results suggest that the yield strength of the Sn-containing alloy is about 1.5 times that of the Sn-free alloy (250 MPa vs 175 MPa). The revealed difference is very big and testifies to a high potential of the new alloys as materials for new-generation engine parts.

Laser powder bed fusion of an age-hardenable Mg-10Gd-0.2Zr alloy with excellent strength-ductility synergy

Generally, Mg-Gd alloys need to be firstly solution treated at relatively high temperatures and then artificially aged at low temperatures to achieve adequate strength, which costs dozens of hours. Herein a Mg-10Gd-0.2Zr (wt%) alloy was reported to achieve excellent strength-ductility synergy with the product of ultimate tensile strength and elongation of 2791.3 MPa × % just by laser powder bed fusion (LPBF). The excellent strength-ductility synergy is attributed to the novel multi-scale hierarchical microstructure containing heterogeneous grain structure, intragranular lamellae substructure and nano-sized precipitates. Abnormal precipitation of a large number of intragranular β 1 phases with blocky shape is first found in the as-built Mg-10Gd-0.2Zr alloy. The findings demonstrate that the unique heat and strain history during LPBF can be exploited to deliberately regulate the type, area number density, morphology and distribution of precipitates, thus controlling mechanical properties. • Mg-10Gd-0.2Zr (wt%) alloy prepared by LPBF achieves excellent strength-ductility synergy. • The multi-scale hierarchical microstructure leads to excellent strength-ductility synergy. • Abnormal precipitation of β 1 phases with blocky shape and unique distribution is found for the first time.

Precipitate number density determination in microalloyed steels by complementary atom probe tomography and matrix dissolution

Particle number densities are a crucial parameter in the microstructure engineering of microalloyed steels. We introduce a new method to determine nanoscale precipitate number densities of macroscopic samples that is based on the matrix dissolution technique (MDT) and combine it with atom probe tomography (APT). APT counts precipitates in microscopic samples of niobium and niobium-titanium microalloyed steels. The new method uses MDT combined with analytical ultracentrifugation (AUC) of extracted precipitates, inductively coupled plasma–optical emission spectrometry, and APT. We compare the precipitate number density ranges from APT of 137.81 to 193.56 × 1021 m−3 for the niobium steel and 104.90 to 129.62 × 1021 m−3 for the niobium-titanium steel to the values from MDT of 2.08 × 1021 m−3 and 2.48 × 1021 m−3. We find that systematic errors due to undesired particle loss during extraction and statistical uncertainties due to the small APT volumes explain the differences. The size ranges of precipitates that can be detected via APT and AUC are investigated by comparison of the obtained precipitate size distributions with transmission electron microscopy analyses of carbon extraction replicas. The methods provide overlapping resulting ranges. MDT probes very large numbers of small particles but is limited by errors due to particle etching, while APT can detect particles with diameters below 10 nm but is limited by small-number statistics. The combination of APT and MDT provides comprehensive data which allows for an improved understanding of the interrelation between thermo-mechanical controlled processing parameters, precipitate number densities, and resulting mechanical-technological material properties.Graphical abstract

Effect of high pressure heat treatment on the recrystallization and recrystallization texture of CC AA 3003 aluminum alloy

The hot band of CC AA 3003 aluminum alloy was cold-rolled to different reductions, and then annealed at 482 °C for 1 h under different pressures. The evolution of microstructure and texture was investigated. High pressure heat treatment can refine recrystallized grains and weaken pinning effect. The average grain size decreases from 205.17 μm to 73.54 μm with the increasing pressure from atmosphere pressure to 6 GPa. In the meanwhile, the aspect ratio increases from 0.18 to 0.72. Compared with atmosphere pressure, the fine precipitates (50–150 nm) disappear after pressurization, the number density of large precipitates increases and the average size of large precipitates decreases. The recrystallization textures are characterized by the weak P and R-cubeND components and a relatively strong M component under atmosphere pressure. The intensities of the P and M components increase with the increasing rolling reduction, while the R-cubeND component changes slightly. With the increase of pressure, the P and R-cubeND components strengthen, while the M component weakens significantly, nearly disappears. The hardness decreases slightly with the increasing rolling reduction in the sheets annealed under atmosphere pressure and increases slightly with the increasing rolling reduction after pressurization. In addition to that, the hardness increases slightly with the increasing pressure in the sheets with the same rolling reduction.

Utilization of Bayesian Optimization and KWN Modeling for Increased Efficiency of Al-Sc Precipitation Strengthening

The Kampmann and Wagner numerical model was adapted in MATLAB to predict the precipitation and growth of Al3Sc precipitates as a function of starting concentration and heat-treatment steps. This model was then expanded to predict the strengthening in alloys using calculated average precipitate number density, radius, etc. The calibration of this model was achieved with Bayesian optimization, and the model was verified against experimentally gathered hardness data. An analysis of the outputs from this code allowed the development of optimal heat treatments, which were validated experimentally and proven to result in higher final strengths than were previously observed. Bayesian optimization was also used to predict the optimal heat-treatment temperatures in the case of limited heat-treatment times.

Effect of Cu content on the precipitation behavior of Cu-rich and NiAl phases in steel

The nanoscale precipitation and the mechanical properties of the two CuNiAl-containing steels with different Cu contents aged at 500 °C were investigated in this study. Results of mechanical property indicated that the co-precipitation of Cu-rich and NiAl phases during aging contributed to the precipitation strengthening effects. The influence of Cu content on the evolution of precipitates was studied by atom probe tomography (APT). The results demonstrated that the increasing of Cu content enhanced the number density of precipitates and accelerated the precipitation process. The two steels showed two kinds of precipitation sequences on account of different Cu contents. The steel 1 with low Cu content exhibited a precipitation sequence of “supersaturated solid solution → NiAl → NiAl + Cu”, while the steel 2 with high Cu content presented the process of “supersaturated solid solution → Cu → Cu + NiAl”. After further aging, the precipitates of both steels were transformed into Cu/NiAl co-precipitates without isolated Cu-rich phases and isolated NiAl phases. • Hardness of the CuNiAl-containing steel can be improved by increasing the Cu content. • The increasing of Cu content enhances the number density of precipitates and accelerates the precipitation process. • The precipitation sequences of two steels with different Cu contents are clarified.

Synergistic enhancement of strength and ductility of cobalt-free maraging steel via nanometer-scaled microstructures

Maraging steel with ultra-high strength and good ductility is usually achieved via semi-coherent nanometer-sized precipitates and a dual-phase structure. In this work, we studied the effect of solution treatment parameters on the microstructural evolution and the mechanical properties of cobalt-free maraging steel, with high strength (1852 MPa) and satisfactory tensile elongation (11.5%) at room temperature was obtained. The nano-scaled precipitates in the martensite matrix were analyzed, in which the η-Ni3(Ti, Mo) precipitates were distributed in width of 7.1 nm and length of 19 nm. It was found that the needle-like Ni3Ti phase plays a predominant role in enhancing the strength of the maraging steel, while the ductility can be attributed to the nano laminated austenite structure, which could effectively prevent the propagation of microcracks in the martensite matrix. A heterogeneous microstructure of precipitates was formed in the martensite matrix. Besides, the size and number density of precipitates could be regulated by tuning the solution treatment conditions, in which a finer size and a larger number of precipitates could be gained by annealing at a high temperature for a short time.

Improved irradiation resistance of accident-tolerant high-strength FeCrAl alloys with heterogeneous structures

Post–neutron irradiation examination is performed on advanced accident-tolerant fuel (ATF) cladding iron-chromium-aluminum (FeCrAl) alloys with ∼10–13at. % Cr, ∼10–12 at. % Al, ∼1 at. % Mo, and minor alloying elements including Y irradiated to a damage level of 7 displacements per atom (dpa) at irradiation temperatures of 267–282 °C. A compositional dependency of the Cr and Al content is observed on the ratio of sessile and glissile dislocation loops, where the density of a⟨100⟩ type loops is somewhat higher than the a/2⟨111⟩ type loops. The α′ precipitate number density is inversely correlated to the starting Cr concentration of the alloys of interest. The irradiation to a higher dose of 7 dpa results in a higher density of dislocation loops and α′ precipitates for the same alloys at a lower irradiation dose, such as 1.8 dpa. In this work, the effect of α′ precipitates on the dislocation loop density is discussed, and the presence of α′ appears to inhibit the nucleation of loops. Compared with first-generation FeCrAl alloys, these advanced alloys with heterogeneous structure exhibit a lower Cr concentration in α′ precipitation at the same dose level; they act as weaker obstacles deviating from the primary hardening contribution from the mature α′. Hence, the overall irradiation-induced hardening decreases; our alloys show improved radiation resistance because of their stronger sink strengths. The results presented in this paper could provide insights for the design and optimization of ATF cladding materials for future fission and space applications.

Influence of cooling condition after solidification on T5 heat treatment response of hypoeutectic Al-7Si-0.4Mg casting alloy

The effect of the cooling conditions after solidification of a hypoeutectic Al-7Si-0.4Mg alloy on the precipitation behavior during T5 heat treatment (artificial aging immediately after casting) was investigated. Al-7Si-0.4Mg alloys with two different cooling conditions, mold cooling and water quenching, were respectively fabricated by holding the sample in a permanent mold throughout solidification and by placing the sample in room temperature water just after solidification was completed. TEM analysis of the as-cast alloys confirmed that mold cooling after solidification led to the formation of a small amount of clusters, Si and B′ in the mold which may have pre-precipitated during cooling, increasing the hardness and strength of the as-cast alloy. However, the hardenability of the alloy with mold cooling during T5 heat treatment is less pronounced than the alloy with water quenching. This suggests that the pre-precipitation in the alloy with mold cooling suppresses the aging hardening response, reducing the overall strength of the alloy with T5 heat treatment compared to that of the alloy with no pre-precipitation. Analysis of precipitation kinetics and number densities of precipitates shows that the pre-precipitation can interfere with the precipitation of β", otherwise promoting the precipitation of coarse β', U2 and Si, which reduced overall T5 precipitation hardenability. Therefore, for the T5 heat treatment condition, we suggest that the cooling conditions after solidification are a key factor affecting the best performance of precipitation hardening.

Interaction of transmutation products with precipitates, dislocations and grain boundaries in neutron irradiated W

Tungsten is the primary candidate materials for the high neutron flux, high temperature components of a future demonstration fusion reactor. Despite this, there is a lack of data on W under fusion relevant neutron doses and irradiation temperatures. In this study, single crystal and polycrystalline W samples irradiated at the High Flux Reactor (HFR) at 900 ∘C were characterised using Atom Probe Tomography (APT) and Scanning Transmission Electron Microscopy (STEM). Bulk chemical and isotopic concentration predictions were validated by analysing the mass spectrum from APT experiments. A post irradiation composition of W-1.26 ± 0.15 at.%Re - 0.08 ± 0.02 at.%Os - 0.01 ± 0.01 at.%Ta was measured in the single crystal sample, whereas W-1.09 ± 0.07 at.%Re - 0.08 ± 0.02 at.%Os - 0.01 ± 0.01 at.%Ta was measured for the polycrystalline. APT and STEM showed that a high number density of Re and Os rich precipitates had formed under neutron irradiation. These typically consisted of a core rich in Re and Os, surrounded by a less dense Re rich cloud. Multiple analysis methods were applied to investigate the composition of these clusters. APT showed that the centres of some of the precipitates had a rod shaped core which were rich in both Re and Os. Line profile analysis suggests that in the centre of the precipitates, the threshold composition for σ phase formation may have been reached, as has been observed in higher transmutation rate experiments. In addition, dislocations, sub grain boundaries and dislocation loops were all shown to be decorated with both Re and Os, in agreement with predictions from DFT simulations.

Characterization of dynamic precipitation behaviors accompanying dynamic recrystallization in an Mg-Al-Zn-RE alloy

In the present study, we investigated the dynamic precipitation (DP) of Mg17Al12 phase accompanying dynamic recrystallization (DRX) during the deformation of an Mg-Al-Zn-RE alloy. Hot compressions at various strains, strain rates and temperatures were followed by microstructural characterization via scanning electron microscope (SEM) and energy-dispersive X-ray spectroscope (EDS). The precipitation during heating and holding was avoided by proper treatment to guarantee initial microstructures without Mg17Al12 phase. During hot compression, the DP of Mg17Al12 particles with blocky shape accompanies the DRX process and mainly occurs on the DRXed grain boundaries. The volume fraction, number density and average size of Mg17Al12 precipitates depend on both temperature and strain rate, while the influence of temperature is more prominent for volume fraction. Phenomenological models bridging the number density and average size of precipitates with deformation conditions (temperature and strain rate) were established and validated. The pinning effect of DP on the grain growth makes DRXed grain size deviate from the values predicted by the classical function of the Zener-Hollomon parameter. Influenced by the sensitiveness of the pinning effect, temperature makes more sense in trimming DRXed grain size than strain rate. Vickers hardness tests were also conducted after hot compression. The influence of DP on microhardness lies in its effect on grain refinement.

Improvement of Measurement Method of Precipitate Number Densities in 9Cr Steel by Cs-corrected Transmission Electron Microscopy

Improvement of Measurement Method of Precipitate Number Densities in 9Cr Steel by Cs-corrected Transmission Electron Microscopy

Hardening and precipitation behavior of as-quenched Al–0.4Mg–1.0Si alloy with minor Sn addition at different aging temperatures

The effect of Sn on the hardening and corresponding precipitation behavior of as-quenched Al–0.4Mg–1.0Si alloy at different aging temperatures was investigated with hardness tests, differential scanning calorimetry analysis, and transmission electron microscopy. The hardness of the Sn-free alloy was higher than that of the Sn-added alloys during under-aging, but the tendency is reversed during over-aging. The effect of Sn on the hardening behavior of these alloys is not substantially affected by the aging temperature, i.e., the addition of Sn can improve resistance to over-aging softening at different aging temperatures. The reason for the resistance to over-aging softening by the addition of Sn is related to the increase in the number density of precipitates during under-aging and the decrease in precipitates size during over-aging.

The effect of pre-straining on the annealing-induced precipitation behavior of the icosahedral I-phase in an aluminum-based alloy

The current study focuses on the influence of pre-straining on the precipitation behavior of Al-1.0at.%Mn-1.7at.%Mg alloy. The Mn-bearing quasicrystalline-structured dispersoids of the I-phase precipitated heterogeneously at dislocations and grain boundaries during annealing at 300–350 °C. The hardness and in situ electrical resistance measurements during annealing of the as-cast and pre-strained samples confirmed the significant strain-induced acceleration in precipitation kinetics. Pre-straining increased the number density of the quasicrystalline-structured precipitates and the hardnening effect after annealing.

Uncovering the critical factor governing the fatigue crack propagation under the coexistence of multi-interfaces in AlCoCrFeNi2.1 eutectic high-entropy alloy

The AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs) with a typical dual-phase lamellar microstructure can reach a good balance of strength and ductility, which have gained great attention recently. However, their high-temperature microstructural evolution and corresponding fatigue-crack growth (FCG) behavior have not yet been reported. In the present work, the microstructure of the AlCoCrFeNi2.1 EHEAs aged at 900 °C for different durations was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). After that the FCG tests were conducted for the EHEAs with different microstructures to reveal the mechanism of fatigue-crack propagation. The results showed that the B2Ⅱ precipitates had a priority in the precipitation sequence during thermal exposure. Particularly, the EHEAs before and after thermal exposure exhibited a waved da/dN-ΔK curve, which should be ascribed to the coexistence of multi-interfaces driven frequent and excessive crack deflections during the FCG tests. In addition, long-term thermal exposure (for example for 200 h) led to a significant increase in FCG resistance. The inherent reason was revealed based on the evolution of high-angle grain boundaries, average size and number density of precipitates in the aged EHEAs. The data obtained will contribute to understand the EHEAs' high-temperature microstructural stability and provide a valuable way to enhance the FCG resistance.

The effect of pre-ageing/stretching on the ageing-hardening behavior of Al–Zn–Mg–Cu alloys correlated with Zn/Mg ratio

Pre-ageing and pre-stretching before artificial ageing can significantly affect the precipitation during the subsequent artificial ageing in Al–Zn–Mg–Cu alloys, and their effects are closely relevant to the alloy composition and Zn/Mg ratio due to the different predominate precipitates. The diverse effects of pre-ageing and pre-stretching on the mechanical properties and microstructures of Al–Zn–Mg–Cu alloys with five different Zn/Mg ratios have been systematically studied with transmission electron microscopy, hardness measurements and tensile tests. The results reveal that pre-ageing can effectively enhance the strength due to the improvement of number density of precipitates. Different pre-aging time causes similar improvement of strength but different elongation values in peak-aged alloys A2 (T′ phase strengthened) and A5 (η′ phase strengthened), which can be explained by different size and density of precipitates and PFZ width. The effects of pre-stretching on age-hardening behavior of the alloys with various compositions are significantly different. It results in a reduction in hardness in the lowest Zn/Mg ratio alloy A1 (T′ phase strengthened) due to accelerated coarsening of precipitates; on the contrary, it results in a remarkable improvement of the hardness of the highest Zn/Mg ratio alloy A5. Pre-stretching causes different strengthening effect in alloy A5, and the maximum enhancement takes place with 2% pre-stretching because the precipitates become denser and smaller. Additionally, both pre-ageing and pre-stretching are effective ways to reduce PFZ width by promoting the precipitation near the grain boundary.