Grain Boundary Precipitation Research Articles

Effects of Aging Treatments on the Age Hardening Behavior and Microstructures in an Al-Mg-Si-Cu Alloy

In this study, we investigated the effects of modified aging treatments on the microstructures and hardness in a commercial 6016 Al alloy through hardness tests and transmission electron microscopy (TEM) observations. The results demonstrate that many fine needle-like β″ phases contribute to the high hardness of peak-aged (T6) alloys. Over-aging treatments lead to the precipitation of lath-like β′, β″/disordered, or B′/disordered composite phases. Moderate over-aging treatment results in the coarsening of grain boundary precipitates (GBPs) and widening of the precipitate-free zone (PFZ), while heavy over-aging treatment triggers the re-precipitation of Cu-containing GBPs and increases the number density of GBPs. A retrogression and re-aging (RRA) treatment precipitates β″, lath-like β′, and disordered phases, while a two-step aging (T78) treatment precipitates β″, B′, and disordered phases. Both the T78 and the RRA treatments lead to the coarsening of GBPs and the widening of PFZs. The decreased hardness during over-aging treatments is attributed to a combination of coarsening intragranular precipitates and/or wider PFZs. The T78 and RRA tempers achieve 95.5% and 94% of the hardness values of the T6 treatment, respectively. The hardness values of the RRA and T78 treated alloys are related to the finer nano-sized precipitates formed during the high temperature process. These precipitates can compensate for the loss of hardness caused by the increase in the widths of the PFZs and the coarsening of the matrix precipitates. The relationship between the hardness and microstructures such as PFZs and precipitates in the matrix during various heat treatments is elucidated.

Tailoring precipitate distribution in 2024 aluminum alloy for improving strength and corrosion resistance

For the traditional peak-aged (PA) AA2024 alloy, the formation of large S-phase precipitates within the grains, wide precipitate-free zones (PFZs) near the grain boundaries (GBs), and continuous distribution of grain boundary precipitates (GBPs) can be observed. As a result, the PA alloy exhibits relatively high strength but poor corrosion resistance. However, with the application of cyclic plasticity treatment, high-density 1–2 nm clusters form within the matrix, and no PFZs form near GBs. In this study, this treatment yields the optimal balance between strength–elongation characteristics and corrosion resistance. By combining cyclic plasticity and ageing heat treatment with different heating rates, the nanoscale clusters play a crucial role as heterogeneous nucleation sites, resulting in the formation of finer and higher number density of S precipitates within the matrix. Additionally, the presence of these clusters reduces the formation of GBPs and minimizes the width of PFZs. Consequently, compared to the traditional PA sample, this approach achieves a significantly higher yield strength (increased by 46 %) and ultimate tensile strength (increased by 18 %), along with superior corrosion resistance. Although the influence of ageing heat treatment with different rates on mechanical properties is not significant, it notably affects the formation of GBPs and corrosion resistance. Specifically, a slower heating rate leads to an increase in the spacing between adjacent GBPs, resulting in improved corrosion resistance. In summary, cyclic strengthening, as a novel method for alloy strengthening, when combined with ageing heat treatment, modulates the distribution of S precipitates within the matrix and GBs. This optimization maximizes the effects of precipitation strengthening and breaks the inverse relationship between strength and corrosion resistance.

Anisotropic growth of nano‐precipitates governed by preferred orientation and residual stress in an Al‐Zn‐Mg‐Cu alloy

Through an understanding of diffusion, precise control of the size distribution of nano-precipitates can be essential to developing superior properties in precipitation-strengthened alloys. Although a significant influence of crystallographic orientation on the diffusion process is known to exist in low-symmetry hexagonal close-packed alloys, such anisotropic diffusion is still unidentified in high-symmetry cubic alloys. In this work, we reveal the diffusion-controlled coarsening induced anisotropic growth process of nano-precipitates in an Al-Zn-Mg-Cu alloy. Our experimental and theoretical studies demonstrate that with an increase in the residual stress, the diffusion-controlled coarsening rate is slow along the 〈112〉 fiber texture in the alloy matrix with smaller grain sizes. As such, we find that the diffusion activation energy will be increased along the preferred orientation with largest residual stress, which leads to a reduced diffusion-controlled coarsening rate. Specifically, we demonstrate that the increase in the volume fraction of nano-precipitates originates from the rapid grain-boundary controlled coarsening of the grain-boundary precipitates. Based on these results, an underlying microstructural design strategy is proposed, involving the crystallographic orientation, the residual stress and the grain boundaries to manipulate the precipitate size distribution in this class of alloys.

Enhanced corrosion performance by controlling grain boundary precipitates in a novel crossover Al-Cu-Zn-Mg alloy by optimizing Zn content

The effect of Zn contents on grain boundary precipitates (GBPs), intergranular corrosion (IGC), and stress corrosion cracking (SCC) of crossover Al–Cu–Zn–Mg alloys was investigated. The increase in Zn contents accelerates the age-hardening response, and both matrix precipitates (MPs) and GBPs are transformed from S phase to S and η phases, while the width of the precipitates free zones (PFZs) first increases and then decreases. The narrowed PFZ width, coarse and discontinuous GBPs were able to inhibit anodic dissolution resulting in enhanced IGC resistance. The η phase inhibits anodic dissolution leading to an enhanced IGC resistance. First-principles calculations demonstrate that the GBPs surface with Mg termination planes shows a lower work function than that of Al leading to preferential anodic dissolution. The evolution of GBPs and PFZ widths changes the mechanism of SCC susceptibility for different Zn contents alloys from anodic dissolution and hydrogen embrittlement to anodic dissolution.

Interfacial strain driven nucleation and growth of grain boundary phases

Secondary phases precipitation in grain boundaries (GBs) are ubiquitous presence in multicomponent polycrystalline materials, and play a crucial role in determining the materials performances. However, the atomistic formation mechanism remains a matter of conjecture since the GB phase is difficult to well characterize. Here, using a typical Mg-rare-earth binary alloy as the model system, we systematically investigated the precipitation behaviors of nano-sized secondary phases in a partially coherent tilt GB. Atomic-scale observations combined with theoretical calculations show that GB elastic strain minimization drives the segregation of solutes to the interfacial strain zones, and the selective substitution of solutes resulting in the nucleation of face-centered-cubic (FCC) phase from the bi-crystalline hexagonal closed-packed (HCP) lattices. Interphase boundary strain driven solute diffusion plus atomic shuffling can realize the growth of parent phase {11¯00}HCP lattices to the {110}FCC atomic-planes. These findings not only confirm a novel HCP to FCC phase transition pathway, but also provide a mechanistic insight into the nucleation and growth of GB phases.

Mitigation effects of over-aging (T73) induced intergranular corrosion on stress corrosion cracking of AA7075 aluminum alloy and behaviors of η phase grain boundary precipitates during the intergranular corrosion formation

The stress corrosion cracking (SCC) behaviors of over-aged and peak-aged AA7075 aluminum alloys in a 5 wt% NaCl solution were compared. Even though the over-aging process induced intergranular corrosion (IGC), it decreased the SCC susceptibility of the material. Grain boundaries (GBs) with misorientation angles less than 30° and the grain matrix were considered factors that reduced the IGC propagation rate, thereby preventing the development of SCC in the over-aged material. η phase grain boundary precipitates (GBPs) underwent de-alloying, forming Cu-enriched particles during the IGC formation. However, the cathodic properties of these particles are not the key factors determining whether the IGC will occur or not.

Inhibiting segregation enabled outstanding combination of mechanical and corrosion properties in precipitation-strengthened aluminum alloys

Segregation of solutes to grain boundary (GB) is the dominant restriction on enhancement of mechanical properties and corrosion resistance during aging in precipitation-strengthened aluminum alloys. Here, we innovatively introduce the cyclic deformation by the vibration during aging to eliminate the GB segregation, resulting in the formation of the narrow precipitate-free zone (PFZ) widths near the GB, as well as the fine and discontinuous grain boundary precipitations (GBPs). Compared with the traditional peak-aging, the 2014 aluminum alloy treated by thermal cooperative vibration aging (TCVA) exhibits a superb combination of impact toughness and corrosion resistance, and retains the strength and ductility. In addition, the atomic simulations show that TCVA generates numerous vacancies near the GB, but does not change the dislocation density with the increase of cycle time. These results indicate that the vacancy significantly promotes the nucleation and growth of precipitates in the vicinity of GB, resulting in the narrow PFZs and fine discontinuous GBPs. The present work provides the fundamental knowledge and method to inhibit the equilibrium segregation of solutes to GB during aging, and further realizes the precipitation-strengthened aluminum alloys with excellent mechanical and corrosion properties.

Effect of Grain Structure and Quenching Rate on the Susceptibility to Exfoliation Corrosion in 7085 Alloy.

The influence of grain structure and quenching rates on the exfoliation corrosion (EXCO) susceptibility of 7085 alloy was studied using immersion tests, optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The results show that as the cooling rate decreases from 1048 °C/min to 129 °C/min; the size of grain boundary precipitates (GBPs); the width of precipitate-free zones (PFZ); and the content of Zn, Mg, and Cu in GBPs rise, leading to an increase in EXCO depth and consequently higher EXCO susceptibility. Meanwhile, there is a linear relationship between the average corrosion depth and the logarithm of the cooling rate. Corrosion cracks initiate at the grain boundaries (GBs) and primarily propagate along the HAGBs. In the bar grain (BG) sample at lower cooling rates, crack propagation along the sub-grain boundaries (SGBs) was observed. Compared to equiaxed grain (EG) samples, the elongated grain samples exhibit larger GBPs, a wider PFZ, and minor compositional differences in the GBPs, resulting in higher EXCO susceptibility.

Simultaneous enhancement of strength and ductility by combining heterogeneous microstructure and nanoprecipitates

Developing advanced metallic materials with high strength and ductility is an eternal theme, however, the strength and ductility are generally considered to be conflicted to each other. Here, a simultaneous enhancement of strength and ductility is achieved in a commercial Al6061 alloy by combining the heterogeneous microstructure (composed of recovered zones and partially recrystallized zones) and nanoprecipitates. This unique microstructure is produced by the cold-drawing (∼61%) and pre-torsion (∼360°) followed by low-temperature ageing treatments. When the ageing temperature decreases from ∼300 °C to ∼150 °C, the nanoprecipitates changes from the coexistence of the needle-like β′ and rod-shaped β′′ to exclusive granular β′′ with refined size and increased volume fraction, which leads to the remarkable improvement of yield strength from σy∼176.0 MPa to σy∼340.2 MPa. Concurrently, the evading of precipitate-free zones (PFZs), the suppression of the grain-boundary precipitates and the formation of extremely fine nanoprecipitates (<5 nm) contribute to the unexpected enhanced uniform elongation (from εu∼5.3–5.6%). Furthermore, the hetero-deformation induced (HDI) strengthening and work hardening by the heterogeneous microstructure are also beneficial for the synergistic enhancement of the strength and ductility.

Effects of Rapid Quenching on Grain Boundary Microstructure and Mechanical Properties of an Al-Mg-Si-Cu Alloy.

Precipitate free zones (PFZs) near grain boundaries generally soften alloys. The quenching rate after solution treatment is an important factor influencing the width of PFZs in Al-Mg-Si-Cu alloy. This study explored the effects of high quenching rates on the grain boundary microstructures and mechanical properties of an Al-Mg-Si-Cu alloy. Samples of various thickness were quenched in water at room temperature and in ethylene glycol at -40 °C, respectively. The results showed that the rapidly quenched samples at -40 °C exhibited better comprehensive mechanical properties than the water-quenched samples. Transmission electron microscopy studies revealed the rapidly quenched samples had wider PFZs, shorter intragranular precipitates, and larger grain boundary precipitates (GBPs) than water-quenched samples. It is proposed that when the quenching rate exceeds the critical cooling rate, e.g., in water quenching or rapid quenching, the formation of PFZs is controlled by the solute depletion mechanism rather than the vacancy depletion mechanism. The nucleation and growth of GBPs thus lead to the depletion of solute atoms, resulting in wider PFZs rather than thinner PFZs according to previous knowledge. This research provides valuable insights into the application of rapid quenching technology for modifying alloys' microstructures and properties.

Segregation-induced grain-boundary precipitation during early stages of liquid-metal embrittlement of an advanced high-strength steel

Segregation-induced grain-boundary precipitation during early stages of liquid-metal embrittlement of an advanced high-strength steel

Grain boundary microstructure-property relationships in the cast & wrought Ni-based superalloy René 41 with boron and carbon additions

Cast & wrought Ni-based superalloys are required for manufacturing high-performance components in gas turbine engines. Their excellent high-temperature properties and corrosion resistance are brought about by their complex microstructure of a γ-matrix, intra- and intergranular γ' precipitates, carbides, and borides. The most highly-alloyed superalloy grades provide the high-temperature strength required for the design of next generation gas turbine engines but their processability remains challenging due to grain boundary (GB) cracking. A common mitigation approach is the addition of B and C to improve GB cohesion and strength. However, their detailed roles in GB segregation and precipitation and the link to GB properties during processing remains largely unexplored.We provide a systematic study on decorated GBs in the cast & wrought Ni-based superalloy René 41 with B and C additions. Tensile testing reveals that B additions reduce its ductility due to GB cracking but slow down grain growth during heat treatments more effectively, than C additions. GB-M2B, M6C, M23C6, and GB-γ' precipitates decorating GBs are sequentially precipitated. Their structures and compositions are shown to be unaffected by B and C additions. However, significant differences in the interfacial segregation of solutes are revealed. B is captured by GB-M2B and therefore unavailable to improve GB cohesion. C depletion and enrichment of solutes at interphase boundaries are identified as origin for the reduced ductility and thus, GB cracking. The findings are summarized as microstructural model and advance the detailed understanding of the processing – microstructure – properties relationships of cast & wrought Ni-based superalloys.

Improving the stress corrosion resistance of 7075 matrix composites through combination of nanoparticle incorporation and retrogression re-aging treatment

7xxx aluminum matrix composites reinforced with ceramic nanoparticles are expected to possess advantageous comprehensive performance. However, further applications of these composites remain in doubt because it is likely that introducing these extrinsic phases could impair the resistance to stress corrosion crack (SCC). In this work, we used 7075 as the matrix, and TiB2 nanoparticles as the reinforcement, to study the influences of the retrogression and re-aging (RRA) treatment and nanoparticle incorporation on the SCC behavior of 7075 aluminum alloy. The intergranular corrosion test (IGCT) and slow strain rate test (SSRT) results show that both TiB2 and RRA are conducive to the improvements in the SCC resistance. The positive effects of RRA can be two-fold: (i) the lattice distortion is alleviated during the RRA temper; (ii) the coarse and discrete grain boundary precipitates (GBPs). The accumulation of active solute elements, however, also occurs at the grain boundaries, leading to severe intergranular corrosion. Surprisingly, some coarse TiB2 particles larger than the thickness of particle-free zones (PFZs) could obstruct the mass transportation, reducing the susceptibility to intergranular corrosion. In conclusion, the strength and SCC resistance of 7075 alloy can be concurrently improved by the combination of particle incorporation and RAA treatment.

Effect of Cu content on intergranular corrosion and exfoliation corrosion susceptibility of Al−Zn−Mg−(Cu) alloys

The effect of Cu content on intergranular corrosion (IGC) and exfoliation corrosion (EXCO) susceptibility of Al−Zn−Mg−(Cu) alloys was investigated by electrochemical test, immersion test, electron backscattered diffraction, optical microscope, scanning electron microscope, scanning transmission electron microscope and scanning Kelvin probe force microscope. As Cu content increases from 0 to 2.6 wt.%, IGC susceptibility increases, while EXCO susceptibility first increases and then decreases, reaching the maximum at Cu content of 1 wt.%. With the increase of Cu content, the area fraction of recrystallized grains increases, and the aspect ratio of recrystallized grains first decreases and then increases; moreover, the Volta potential difference between grain boundary precipitates (GBPs) and the matrix increases, making GBPs more susceptible to corrosion. The initiation and propagation of IGC and EXCO are discussed mainly based on the features of GBPs, grain structure, and the Volta potential difference between GBPs and the matrix.

Failure Investigation on Failed Towing Arm of an Aircraft: A Case of Improper Material Processing and Failed Repair Welding of a Cast Al-Cu-Si Type Aluminium Alloy

Present work describes the metallurgical failure investigation of a failed towing arm/bar. Aircraft attached with this incident travelled a distance of 2 km with normal towing speed (8 km/hr). The component is said to have broken while turning to right from taxi track and then turning left about 15-20o. The mating fracture pieces of the failed towing arm fork along with bent towing pin were analyzed for failure investigation. Parent material of the towing arm has been found to be a cast Al-Cu-Si alloy closely matching with the alloy 295 used in T6 condition. Pin is made of mild steel. Failure of the towing arm fork has been initiated by weld solidification cracking of the weld region at close to the fusion line. Liquation cracks at heat affected zone (HAZ) have aggravated the situation further by weakening the microstructure and facilitating the crack propagation. Grain boundary precipitates of Al-Cu-Fe type in HAZ have facilitated the propagation of failure along with the liquation cracking. On the other hand, Al-Fe-Mn type of precipitates sitting along the grain boundaries in weld area seems to have not facilitated the failure initiation unlike the solidification cracking. Improper material processing has led to the weakening of the bulk microstructure and thus the component itself, by introduction of various defects (pores, grain fall out, thick Al-Cu-Fe type of continuous grain-boundary precipitates and cracking). Weld repairing has been found to be a failure in Al-Cu-Si system in the present component.Keywords: Towing arm, cast Al-Cu-Si, repair welding, hot cracking, solidification cracking, liquation cracking, intergranular fracture, Al-Cu-Fe grain boundary precipitates, Al-Fe-Mn grain boundary precipitates.

Effect of various non-isothermal aging on properties and microstructure of 7055 aluminum alloy

This study investigated the effect of various non-isothermal aging processes on the mechanical properties and corrosion resistance of 7055 aluminum alloy extruded bars. The investigation utilized electrical conductivity, mechanical properties, local corrosion resistance and slow strain rate tensile test combined with microstructure characterization techniques such as metallographic microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the NIA-2 (120 °C→200 °C/30 min→120 °C, v = 30 °C/h) process significantly improves the strengths of the alloy compared to the T73 temper, the tensile strength and yield strength increase by 53.0 MPa–632.5 MPa and by 64.7 MPa–605.4 MPa respectively. Additionally, the maximum depths of intergranular corrosion (IGC) and exfoliation corrosion (EXCO) decrease from 80.6 μm to 97.7 μm–71.4 μm and 89.5 μm respectively. Therefore, the NIA-2 process can achieve both high strength and excellent corrosion resistance. Furthermore, the study shows that the sizes of grain boundary precipitates (GBPs) and matrix precipitates (MPs) increase with the sequence of NIA-1 (120 °C→200 °C→120 °C, v = 30 °C/h), NIA-2, and NIA-3 (120 °C→200 °C→120 °C, v = 20 °C/h) processes. The atomic percentages of Zn, Mg, and Cu in GBPs also vary with the different processes. The increases in the sizes of GBPs, MPs, and the widths of precipitation-free zones (PFZs), as well as the segregations of GBPs components, are primarily responsible for the improved mechanical properties and corrosion resistance of the alloy.

Unusual secondary precipitation within the primary precipitation free zone substantially enhances the ductility of Al–Zn–Mg–Cu alloy

The formation of unusual secondary precipitation zones (SPZ) within the primary precipitation free zones (PFZ) effectively reduces the PFZ width, associated with finer grain boundary precipitates (GBP) and precipitates in the matrix. Such a microstructure remarkably facilitates the uniform distribution of dislocations within the matrix during the deformation process, and inhibits the strain concentration due to the dislocation tangle within the PFZ. These result in a significant improvement in the ductility of Al–Zn–Mg–Cu alloys, while retain comparable ultimate tensile strength as if treated in T6 temper.

The evolution of dual-phase core–shell structure and mechanical properties induced by Co addition in as-cast high-entropy intermetallic

In this work, three as-cast L12-type HEIs with different content of Co element were carefully examined combined with TEM, SEM and XRD technologies. The main matrix of three alloys was L12 ordered phase, and Si,Ti-rich grain-boundary precipitate phase generated with the addition of Co. In Co1 alloy, distinct dual-phase core–shell structure was found within grain due to the precipitation of extra FCC disordered secondary phase, which leads to a better strength-plasticity combination (compressive strength of 3441 ± 50 MPa, and fracture strain of 46.0 ± 1.5%) than Co0 and Co0.5 alloy at room temperature. With the increase of temperature, particular anomalous yield effect was discovered in Co1 alloy. It is clear that SFs became main deformation substructure in Co1 alloy at elevated temperature, and the interaction between non-coplanar SFs motivated the generation of L-C Locks, which assists the K-W Locks to pin dislocations movement and enhance the yield strength from room temperature to about 650 ℃. In addition, the interaction between SFs resulted in the formation of nano-spaced SF network, which contributes to the dynamic Hall-Petch effect. Finally, excellent high-temperature yield strength was achieved in as-cast Co1 alloy, which is higher than room-temperature yield strength until 950 ℃.

Effects of heat treatments on the microstructure and localized corrosion behaviors of AA7075 aluminum alloy

Five different heat treatments were applied to AA7075 aluminum alloy, resulting in distinct localized corrosion behaviors in 5 wt% NaCl solution. Some treatments exhibited higher intergranular corrosion (IGC) susceptibility due to the anodic nature of grain boundaries (GBs) caused by Cu depletion in the precipitate free zones (PFZs) along the GBs. Cu-enrichment and an increase in intervals between η-phase grain boundary precipitates (GBPs) did not improve IGC resistance. The open circuit (OCP) and galvanostatic (GL) tests could distinguish differences in corrosion behaviors, suggesting the potential use of GL testing as a faster method for predicting localized corrosion behaviors.

Reducing stress corrosion cracking susceptibility of high-strength aluminum alloy and its fastener by a novel electromagnetic shocking treatment

A novel electromagnetic shocking treatment (EST) is put forward to reduce the stress corrosion cracking (SCC) susceptibility of high-strength aluminum alloys and its fasteners by selectively modifying grain boundary (GB) microstructure. The effect of EST on the microstructure, mechanical properties and SCC susceptibility of T73-Al7075 alloy and its high-lock nuts has been investigated. Mechanical properties, electrochemical performance and constant strain SCC test have been carried out for T73-Al7075 alloy, microhardness test and SCC test under wet-dry cyclic condition have been carried out for its high-lock nuts, as well as microstructure characterizations via SEM, EBSD and TEM have been carried out for T73-Al7075 alloy. Comparing to the received sample, the strength of EST sample basically keep consistent while the elongation increases slightly, besides, the electrical conductivity and electrochemical corrosion performance improves. The SCC susceptibility of T73-Al7075 alloy and its high-lock nuts have been reduced after EST. Microstructure characterization shows that the grain morphology and matrix intragranular precipitate distribution of the EST sample keep almost the same as those of the received sample, which is consistent with the almost unchanged mechanical properties of alloy sample. However, EST induces the transformation of grain boundary precipitates (GBPs) and the formation of stripy/wavy GB as observed experimentally, which indicates that EST promotes nonlinear GB wetting or pre-melting and even grain bridging. The phase transformation and GB complexion transition induced by EST are main factors attributing to the obvious improvement of SCC resistance of T73-Al7075 alloy and its high-lock nuts. It can be deduced that anodic dissolution cracking will be relieved because of the dissolution and coarsening of GBPs. Hydrogen induced cracking and crack propagation can be relieved with the occurrence of grain bridging. They are both beneficial to improve SCC resistance of Al7075 alloys. This work provides a novel EST method to targetedly embellish GB microstructure, optimize GB complexion and connectivity, and then enhance the service performance of solid alloys and even finished parts.