Early Stages Of Precipitation Research Articles

Machine learning-augmented modeling on the formation of Si-dominated Non-β″ early-stage precipitates in Al-Si-Mg alloys with Si supersaturation induced by non-equilibrium solidification

Recent experiments have shown that Al-Si-Mg alloys solidified under high cooling rates may lead to the nucleation of Si-enriched clusters that are remarkably different from the conventional Mg-Si co-clusters (e.g. β″ particles), and yet the responsible mechanism remains to be elucidated. Here we tackle the problem using a multiscale modeling framework that integrates atomistic modeling, energy landscape sampling, and lattice-based kinetic Monte Carlo (kMC) simulation. The migration energy barriers for vacancy-mediated diffusion amid complex local chemical environments are predicted on-the-fly using a surrogate machine learning model. We discover that the actual vacancy-Si migration barriers are much lower than those assumed in the classic linear interpolation approximation. Such a strong deviation from conventional wisdom, in conjunction with differing Si solute composition, can lead to a great variety in the nucleated early-stage precipitates. More specifically, a high-level supersaturation of Si solute (i.e.xSi/(xSi+xMg)>0.75) would lead to an unexpectedly high enrichment of Si in the nucleated clusters with the Si:Mg ratio up to 5∼6; while at a lower-level supply of Si solute the Mg-Si co-clusters (i.e. Si:Mg ratio around 1∼2) are nucleated instead. These findings provide a viable explanation for the diverse types of early-stage precipitates observed in various experiments, from Si-enriched precipitates in high-pressure die cast Al alloys to β″ particles in conventional casting and/or heat-treated alloys. The implications of our findings are also discussed.

Dual-scale clusters enabling exceptional mechanical properties of a CoNiCr-based MP159 superalloy at 650 °C

Prior or early-stage precipitation products, e.g., GP zones and chemical short-range ordering, attract tons of interests recently due to their extremely small size and notable hardening effects. In this work in a commercial CoNiCr-based MP159 superalloy, we, for the first time, simultaneously introduced two kinds of these products, i.e., the atomic-scale L11 cluster, and the nano L12 cluster with 2.7 nm in size, by simply removing the standard annealing process and directly conducting the intermediate 650 °C tension. The ultra-dense co-existence of these dual-scale clusters generates over 200 MPa hardening effect at the specific 650 °C when compared to the standardly-annealed MP159 alloy that contains only larger (∼6.0 nm) L12 precipitates. High-angle annular dark field-transmission electron microscopy associated with atom probe tomography evidence that firstly, the atomic-scale cluster is of the L11 type, similar to the one observed in the NiCoV medium entropy alloy. And secondly, the composition of the nano L12 cluster is abnormally close to (Ni,Co)3Ti, whose equilibrium crystalline structure is otherwise D024. This likely leads to a high anti-phase boundary energy hence high shearing resistance for dislocations.

Remote Effects of double Typhoons on Record-breaking Rainfall: A case Study in North China

During 18–22 July 2021, an extreme precipitation event occurred in Henan, China, with maximum daily precipitation reached 657 mm, broking their historical records and resulting in severe disaster. This heavy rainstorm occurred with the background of two typhoon (In-fa and Cempaka) activities, but resent studies have only considered the effects of individual typhoons, ignoring their combined effects. The Dynamic Recycling Model and Weather Research and Forecasting Model were applied to investigate the contributions of two typhoons on water vapor transportation. Two transportation paths were determined: the eastern path for In-fa and the southwestern path for Cempaka. These paths contributed up to 78% of the total water vapor over Henan. Two typhoons jointly led to extreme precipitation, and Cempaka showed greater impact than In-fa on the vapor transportation. The precipitation was 39.0% of that in the control test when only Cempaka existed and 34.4% when only considering In-fa. The sensitivity results also indicate that Cempaka mainly induced the early stage of precipitation, while In-fa controlled the later stage. In addition to the effect of water vapor transportation, In-fa also influenced the local atmospheric stability. The pseudo equivalent potential temperature near the surface was 6 K higher when only In-fa existed relative to the scenario in which no typhoon was considered, indicating that In-fa strengthened the convective instability. Therefore, both Cempaka and In-fa contributed to this extreme precipitation event. Cempaka transferred more water vapor than In-fa, and In-fa acted to increase the instability. Their joint contribution amplified the precipitation intensity beyond the cumulative impact of individual effects.

Study of Interface-Related Mechanisms in the Early Stage Precipitation of σ Phase in Hyper Duplex Stainless Steels

This work aims to computationally explore the diffuse prerequisites for σ phase precipitation in high-alloyed duplex steels. The diffusion control for the early precipitation stage of the σ phase was studied by the Calphad thermodynamic modeling combined with the DICTRA software. Different nucleation sites, α/γ, and α/α and γ/γ grain-boundaries, were studied using thermodynamic and mobility databases developed by the present authors in previous works for high-alloyed duplex stainless steel. Simulation results were compared to experimentally obtained microstructures of aged samples. The study of the concentration profiles of the elements Cr, Mo, and Ni revealed that the precipitation is most favorable within the α phase due to fast mobility and strong thermodynamic driving force favoring the formation of the σ phase. Based on the present results, the role of the different heterogeneous sites for the precipitation of the σ phase in hyper duplex steel is discussed.

Comparison of Intra-Event Characteristics of Hydrogen and Oxygen Stable Isotopes between Rainfall and Throughfall and the Effects of Pre-Event Precipitation

The isotopic composition of precipitation provides valuable information about its source and transportation. However, raindrops interact with vegetation before reaching the earth’s surface, leading to isotopic changes in the infiltrating water. Comparing isotopic composition between rainfall and throughfall helps to understand canopy processes and their impact on isotopic variation. Based on observational data collected during the periods of July 2019, July–August 2020, and July–August 2021 in a planted forest located in the southwest monsoon region of China, this study examines hydrogen and oxygen isotopes in rainfall and throughfall at event and intra-event scales, and investigates the effects of pre-event precipitation (PEP) on the isotopic composition. The results indicate that during the initial stage of precipitation, δ18O was enriched in rainfall and it presented a dilution effect gradually, while the d-excess exhibits a low initial value followed by an increasing trend. The difference in δ18O between throughfall and rainfall initially increased and subsequently converged around 0, whereas the difference in d-excess experiences a decreasing phase, followed by an increasing phase, and finally a decreasing phase. Canopy interception led to a lag effect during the early stage of precipitation; the forest exhibited higher water vapor content compared to open land in the intermediate stage, which reduced the degree of non-equilibrium fractionation in throughfall, and the flow pathway enhanced in the later stage. Evaporation processes become more prominent as precipitation intensity weakens. The rainfall and throughfall were influenced by distinct meteorological factors in different precipitation events, and the role of the forest canopy varied across different precipitation periods. PEP was found to augment the intercept and slope of the linear relationship between the H-O isotopic composition of throughfall and rainfall. This pre-event effect also contributes to heightened fluctuations in the δ18O and d-excess values during subsequent precipitation events. The findings contribute to understanding water dynamics, vegetation interception, and mechanisms governing water input in forested areas during precipitation events, which provides valuable insights for analyzing factors influencing water movement in forest ecosystems.

Kinetic simulation of early-stage precipitation behavior in Mg-RE binary alloys during aging process

The precipitation behavior of magnesium-rare earth (Mg-RE) alloys plays a crucial role for their properties. However, the precipitation happens at small-length and long-time scales, making it challenging to be analyzed by state-of-the-art experimental techniques. Here, we combine the advantages of both molecular dynamic force fields on describing atom interactions and Monte Carlo method on describing diffusive events to develop an embedded atom method (EAM) potential based kinetic Monte Carlo (KMC) model. Using the proposed model, we simulated the formation and evolution of Y clusters in Mg-Y alloy formed by the vacancy mechanism, and rationalize the simulation results using aberration-corrected scanning transmission electron microscopy characterize. We conducted a systematic analysis of the atomic structure, the evolution kinetics and path of the Y cluster by tracing Y atoms and comparing with density function theory (DFT) calculations. Our work reveals that, all solute columns in a same cluster trend to grow along the [0001]Mg direction synchronously. The method presented is not only used for the Mg-Y alloy but also other Mg-RE alloys such as Mg-Gd as illustrated in the last part of the paper.

The evolution of precipitates in an Al–Zn–Mg alloy

The precipitation sequence in Al–Zn–Mg alloys has been subject to many revisions over the years as more structural details of the early-stage Guinier-Preston (GP) zones and precipitates have been uncovered. To further investigate this, a 7003 aluminium alloy naturally aged for one year was subjected to artificial ageing at 140 °C to investigate the evolution of early-stage precipitates. Scanning transmission electron microscopy was coupled with atom probe tomography and hardness measurements to characterise the precipitate structure and chemistry of the alloy at different stages in the heat treatment. The naturally aged condition contained a dense population of GPI zones, co-existing with a smaller distribution of η′ precipitates. After artificial ageing for 10 min, the hardness decreased from 121 HV to 88 HV. The number densities of clusters were similar in the two conditions, while the average size of the clusters had increased during the first 10 min of artificial ageing. The average Zn/Mg ratio of the clusters decreased from 2.0 to 1.7 and the clusters no longer exhibited the GPI zone atomic structure, indicating that the GPI zones had dissolved. Concurrently, the fraction of precipitates having η′ structure increased from 2% to 18%. During further ageing, the hardness increased again, reaching a peak after 5 h. In this condition, η′ was the dominant phase, co-existing with η1, η2 and T′ phases. Atomically resolved scanning transmission electron microscopy images in addition to precession electron diffraction patterns of the disputed T′ phase are presented and compared to literature.

Elastic Strain Relaxation of Phase Boundary of α' Nanoscale Phase Mediated via the Point Defects Loop under Normal Strain.

Irradiation-induced point defects and applied stress affect the concentration distribution and morphology evolution of the nanophase in Fe-Cr based alloys; the aggregation of point defects and the nanoscale precipitates can intensify the hardness and embrittlement of the alloy. The influence of normal strain on the coevolution of point defects and the Cr-enriched α' nanophase are studied in Fe-35 at.% Cr alloy by utilizing the multi-phase-field simulation. The clustering of point defects and the splitting of nanoscale particles are clearly presented under normal strain. The defects loop formed at the α/α' phase interface relaxes the coherent strain between the α/α' phases, reducing the elongation of the Cr-enriched α' phase under the normal strains. Furthermore, the point defects enhance the concentration clustering of the α' phase, and this is more obvious under the compressive strain at high temperature. The larger normal strain can induce the splitting of an α' nanoparticle with the nonequilibrium concentration in the early precipitation stage. The clustering and migration of point defects provide the diffusion channels of Cr atoms to accelerate the phase separation. The interaction of point defect with the solution atom clusters under normal strain provides an atomic scale view on the microstructure evolution under external stress.

Radiogenic Nd in bioapatite from rare earth elements rich deep sea sediments from Central Indian Oceanic Basin and its implication in material sources

Rare Earth Element and Yttrium (REY)-rich deep sea sediments have been estimated to be a highly important potential resource. However, the main material sources that supply the huge amount of REY resources, as well as geochemical procedure determines the distinct REY patterns of these sediments, remain unclear. To fill this research gap, in this study, three sediment cores (GC02, GC04, and GC11) collected from the Central Indian Ocean Basin (CIOB) and a core (GC03) from eastern of the 90° E Ridge (NER) were sampled for mineral identification and elemental (major, trace, and rare earth elements) analyses. Fish teeth and detrital minerals were isolated from the sediments, and their 87Sr/86Sr and 143Nd/144Nd ratios were measured. Accumulation of REY is revealed in three cores of GC02, GC04, and GC11 from CIOB with mean value of 511.93 μg/g, 614.79 μg/g, and 1036.14 μg/g, respectively. Most REY rich sediment exhibit obvious to slight negative Ce abnormal, slight positive Eu and Y abnormal. The detrital fraction of the REY rich sediments exhibit less radioactive εNd values of − 8.36295 and more radioactive 87Sr/86Sr of of 0.71651. On the contrary, the fish teeth exhibit less radiogenic 87Sr/86Sr ratio of 0.70915 but more radiogenic εNd values of − 5.13511 than those of the sea water of the Indian Ocean. The results reveal that the REY-rich sediments in the CIOB are characterized by multiple material sources including weathering continental products, alteration of volcanogenic glasses, hydrothermal and marine authigenic materials. Distinct volcanogenic materials input are obvious in the CIOB evidenced with radiogenic 143Nd/144Nd ratios of the fish teeth, the wide occurrence of clinoptilolite and high Te contents contained. The volcanogenic related hydrothermal materials are then indicated to provide most part of REY for the fish teeth, which contribute the most amount of REY of the sediments. However, the terrestrial input of REY cannot be neglected, which indicates the REY-rich deep sediments were formed under multi-source supply system. The study indicated that continuous exchange of Sr between fish teeth and ambient pore water after deposition, and ambient pore water was concluded to contain an abundance of released radiogenic terrestrial Sr. However, the exchange of Nd between fish teeth and bottom sea water ceased after buried in depth. Nd ratios on the fish teeth recorded the more radioactive hydrothermal material carried by bottom seawater at the early stage of precipitation.

Modeling precipitation kinetics in multicomponent alloys during deformation

A new mean-field modeling tool is developed to correctly tackle the problem of precipitation during deformation. The model is an extension of a previously developed Langer–Schwartz–Kampmann–Wagner precipitation modeling approach for multicomponent alloys. The latter is now integrated together with Thermo-Calc software and with physically based equations describing the evolution of the dislocation density during hot-working. New equations for the nucleation barrier and the critical size have also been added to correctly simulate the early stages of precipitation. The model is applied to hot compression tests, for which experimental information was available. The model shows the importance of accounting for the overall precipitation kinetics, and not only for constant values of the precipitate volume fraction or size, to investigate the mechanical property evolution. This is due to a complex interplay between the defects and the precipitates’ evolution driven by a competition between thermodynamics and kinetics. Finally, it is highlighted how the strain rate used affects the final microstructure of the material.

Investigation of Strain-Induced Precipitation of Niobium Carbide in Niobium Micro-Alloyed Steels at Elevated Temperatures

Two steels with a base composition of Fe-0.2C-0.8Mn-1.2Cr (wt%) but with different niobium (Nb) contents (0.02 and 0.03 wt%) were employed to study the effect of precipitate evolution on the softening resistance in the austenite region under elevated temperature deformation. The thermomechanical procedure was executed by a deformation-dilatometer and involved double deformation processes with 25% strain at a 0.25 s−1 strain rate at 900, 925, 950, and 1000 °C. The softening ratios, reflecting the competition between recrystallization and precipitation, were evaluated. The results indicated that both steels showed better softening resistance at 900 °C than at other temperatures. However, the softening ratio of 0.03 wt% Nb-containing steel (Steel 3N) rose after 100 s at 900 °C, while 0.02 wt% Nb-containing steel (Steel 2N) maintained a low softening ratio within 300 s at 900 °C, indicating that Steel 3N was relatively non-durable. A microstructural characterization showed that in the Steel 3N sample deformed at 900 °C, recrystallization occurred more strongly than for Steel 2N after a 1000 s holding time. A follow-up analysis then showed that Steel 3N treated at 900 °C revealed a faster coarsening of the carbides than Steel 2N even in the early stage of precipitation, evidencing that Steel 2N exhibited a lower softening resistance at 900 °C.

Early-stage clustering and precipitation behavior in the age-hardened Al–Mg–Zn(-Cu) alloys

T-Mg32(AlZnCu)49 phase-strengthened Al–Mg–Zn–Cu alloys have shown a promising age-hardening response. However, the age-hardening mechanism underlying the different heat treatments is not clear during the early-stage precipitation. Additionally, the lack of information on the early-stage precipitation impedes the development of this series alloys. In this study, the formation of the Guinier-Preston (GP) zone and T′-phase of the novel Al–Mg–Zn(-Cu) alloys during the early-stage precipitation has been investigated using transmission electron microscopy and atom probe tomography analysis. Investigation revealed that the Cu content and thermal treatment had significant effects on the microstructure evolution of the alloys. GP zones were observed to act as sites for the formation of the T′-phase. The higher number density of Cu-incorporated GP zones led to higher strength and bake-hardening response after pre-aging. Occasionally, some GPI zones were observed to dissolve into the Al matrix without further transforming into T′ precipitates during natural aging (NA). Pre-aging prior to NA facilitated stable GP zone formation and mitigated the negative effect of NA. Furthermore, the precipitates were grown to an appreciable size after single-step aging at 180 °C, which adversely affected the mechanical properties of the novel alloys. The present findings provide an efficient guideline for designing the T-phase in the crossover Al–Mg–Zn-(Cu) alloys.

Co-precipitated Ni/Mn shell coated nano Cu-rich core structure: A phase-field study

This work is to clarify such pending issues as the formation paths, diversity patterns, segregation modes and magnetic effect of Ni/Mn shell-coated Cu-rich particles in Fe-15at. %Cu-1at. %Mn-1at. %Ni alloy by using the phase-field method. It is found that three nucleation mechanisms (classical nucleation, non-classical nucleation and instability decomposition) can coexist in early stage of precipitation, and four kinds of structural patterns of the co-precipitates will appear during the precipitation, growth and coarsening. The Cu-rich nanoparticles gradually transform from bcc-α-Cu to fcc-γ-Cu, the Ni/Mn shell maintains bcc structure and may be precipitated on the surface of bcc-α-Cu, or fcc-γ-Cu, or both surfaces in a heterogeneous nucleation mode and runs through the whole aging stage. The Ni/Mn shell inhibits the crystal structure transformation of bcc-α-Fe matrix, the internal magnetic energy and coherent elastic strain energy also inhibit phase separation and crystal structure transformation. Therefore, we can obtain the characteristic microstructure of different aging stages by adjusting the aging time, magnetic field and elastic field conditions, so as to obtain different microstructure and corresponding mechanical properties. This work is applicable to multicomponent Fe–Cu based alloys and core–shell structural materials.

Monitoring the precipitation of the hardening phase in the new VDM® Alloy 780 by in-situ high-temperature small-angle neutron scattering, neutron diffraction and complementary microscopy techniques

The hardening phase precipitation process plays an important role in the development of new Ni-base superalloys. In the present work, we apply a powerful combination of complementary characterization techniques to characterize in-situ the γ’ precipitation in the new VDM® Alloy 780. During the whole heat treatment process, in-situ time-of-flight (TOF) neutron diffraction (ND) unambiguously identified the γ’ phase as well as its weight fraction and the misfit with the matrix while in-situ small-angle neutron scattering (SANS) provided precise precipitates’ size analysis. Atom probe tomography (APT) and scanning electron microscopy (SEM) provided detailed microstructural characterization and chemical composition of the phases necessary for a proper neutron scattering data evaluation. This contribution reveals more detailed information on the nucleation, growth, and Ostwald ripening processes starting from the early precipitation stage in bulk samples using the complementary microstructure investigation methods. The nucleation and growth kinetics of precipitates at 720 °C depend on heating rates and the size distribution obtained in the pre-heating history of the sample. A subsequent heat treatment step at 620 °C, typically used in Ni-base superalloys, does not lead to similar progressive precipitation or growth. The expected matrix-diffusion-controlled Ostwald ripening process of the γ’ precipitates was in-situ monitored by SANS on a full precipitation hardened sample at expected operating temperatures (750 °C) showing slower coarsening kinetics than other reported Ni-based superalloys.

PosgenPy: An Automated and Reproducible Approach to Assessing the Validity of Cluster Search Parameters in Atom Probe Tomography Datasets

Abstract One of the main capabilities of atom probe tomography (APT) is the ability to not only identify but also characterize early stages of precipitation at length scales that are not achievable by other techniques. One of the most popular methods to identify nanoscale clustering in APT data, based on the density-based spatial clustering of applications with noise (DBSCAN), is used extensively in many branches of research. However, it is common that not all of the steps leading to the selection of certain parameters used in the analysis are reported. Without knowing the rationale behind parameter selection, it may be difficult to compare cluster parameters obtained by different researchers. In this work, a simple open-source tool, PosgenPy, is used to justify cluster search parameter selection via providing a systematic sweep through parameter values with multiple randomizations to minimize a false-positive cluster ratio. The tool is applied to several different microstructures: a simulated material system and two experimental datasets from a low-alloy steel . The analyses show how values for the various parameters can be selected to ensure that the calculated cluster number density and cluster composition are accurate.

Synthesis of Nano-Oxide Precipitates by Implantation of Ti, Y and O Ions in Fe-10%Cr: Towards an Understanding of Precipitation in Oxide Dispersion-Strengthened (ODS) Steels.

The properties of oxide dispersion-strengthened steels are highly dependent on the nature and size distribution of their constituting nano-oxide precipitates. A fine control of the processes of synthesis would enable the optimization of pertinent properties for use in various energy systems. This control, however, requires knowledge of the precise mechanisms of nucleation and growth of the nanoprecipitates, which are still a matter of debate. In the present study, nano-oxide precipitates were produced via the implantation of Y, Ti, and O ions in two different sequential orders in an Fe-10%Cr matrix that was subsequently thermally annealed. The results show that the oxides that precipitate are not necessarily favoured thermodynamically, but rather result from complex kinetics aspects related to the interaction between the implanted elements and induced defects. When Y is implanted first, the formation of nanoprecipitates with characteristics similar to those in conventionally produced ODS steels, especially with a core/shell structure, is evidenced. In contrast, when implantation starts with Ti, the precipitation of yttria during subsequent high-temperature annealing is totally suppressed, and corundum Cr2O3 precipitates instead. Moreover, the systematic involvement of {110} matrix planes in orientation relationships with the precipitates, independently of the precipitate nature, suggests matrix restriction effects on the early stages of precipitation.

Coherent nature of speleothems petrographic and isotopic proxies for climate change: A case study from northwestern Himalaya

Speleothems are widely used terrestrial archives for the reconstruction of regional paleoclimatic conditions using mainly stable oxygen isotopes. However, the isotopic signals are quite vulnerable to post‐depositional recrystallization, and in the absence of detailed petrographic characterization, the validity of the isotopic dataset remains elusive. The present study is an attempt to bridge the gap between speleothem physical and geochemical proxies, their interrelationship and applicability, to reconstruct the regional climatic conditions in the northwestern Himalayas. We have identified different zones of depleted/enriched oxygen isotopes linked with the enhanced/decreased regional rainfall with strong/weak Indian summer monsoon. Prograding growth laminae and distinct columnar crystalline texture in calcite are associated with depleted oxygen isotopes. This suggests ample water availability and a higher drip rate in the cave, and further approves the wetter regional climate. Whereas, retractional growth laminae with dark micrite and microspar point towards slower drip rate and high evaporation associated with decreased rainfall. The presence of microspar in the studied stalagmite may indicate possible diagenetic alteration. However, oxygen isotopes data is unable to detect these small‐scale changes. Nevertheless, a coherency exists between isotopes and petrographic proxies for regional climate variability on a larger scale, and diagenesis in the studied sample possibly occurred during the early stage of precipitation. Therefore, isotopic studies coupled with detailed petrography can better ascribe paleoclimate conditions, and minimize the chances of erroneous interpretation because of diagenetic alterations of carbonate.

Quantifying early-stage precipitation strengthening of Al–Mg–Zn(-Cu) alloy by using particle size distribution

A new generation of aluminum alloys, strengthened by the T-Mg32(AlZn(Cu))49 phase, has been created in the last decade based on the 5xxx series Al–Mg alloy, modified with Zn/Cu alloying elements used in 7xxx series alloys. The precipitation strengthening behavior in early stage artificial aging has been an obstacle to the microstructural design and processing control owing to the small size of T-precipitates. The present study aims to quantify the early stage precipitation strengthening behavior, including pre-aging and bake-hardening of the newly developed Al–Mg–Zn alloy with different Cu contents (0 wt% and 0.5 wt%), by particle size distribution via transmission electron microscopy and atom probe tomography. The rapid strengthening response strongly depends on the number density and constitution of Guinier-Preston zone acting as preferential nucleation sites for the precursor of T-phase during the short-term bake-hardening treatment and is significantly increased by the addition of Cu. The coexistence of different types of phases during artificial aging is categorized into clusters, Guinier-Preston zones, and T′ precipitates based on microstructural characterization. The yield strengths of the alloy were quantified by adding the strengths of three individual distributions, wherein the precipitation strengthening was evaluated according to the different types of precipitates. These are in reasonable agreement with the reported experimental results. This study sheds light on predicting the precipitation strengthening of alloys using a combination of microstructural observations.

Phase-field simulation of evolution kinetics of second γ′ phase in Ni–Al alloy under tensile stress

The precipitation and evolution of secondary γ′-Ni3Al phase under the tensile stress in Ni-17at. %Al alloy were studied by using phase-field simulation. The coarsening of secondary γ′ phase via the Ostwald ripening and coalescence coarsening is revealed in system of bimodal γ′ precipitates, and the concentration inhomogeneity inside of the primary and secondary γ′ phase is clarified under the influence of tensile stress. The γ′ phase presents a high concentration at the boundary regions of γ′ phase under the tensile stress, the tensile stress induces a high diffusion potential and drives the directional diffusion of Al, the Al concentration in γ′ phase reduces parallel to the tensile stress, while it rises in direction perpendicular to the tensile stress, which produces the N-type rafting. In addition, the tensile stress accelerates the dissolution of small γ′ phase, results in the increase of the volume fraction and radius of primary γ′ phase, while that of secondary γ′ phase decreases. The cube of average radii of primary and secondary γ′ phase follows a linear relationship with time in the early stage of precipitation, while a sharp rise happens for large tensile stress at later coarsening stage.

Micro-addition of Fe in highly alloyed Cu-Ti alloys to improve both formability and strength

Cu-Be alloys provide excellent electrical and mechanical properties, but present serious health hazards during manufacturing. Among alternative alloys, the Cu-Ti system has the highest yield strength; however, Ti cannot be easily solutionized at concentrations above 4 wt%, resulting in a relatively low formability. In this study, Cu-xTi-yFe (x = 3, 5, 6 wt% and y = 0, 0.3 wt%) alloys were studied after both solution-annealing and age-hardening through mechanical testing and microstructure analysis. Micro-additions of Fe kept high concentration of Ti in solid solution (up to 6 wt%) after water quenching and suppressed the classical “wave-like” early-stage precipitation. Instead, a new dispersion of nano precipitates was observed. This behavior results in doubling the ductility in the solution annealed state (up to 48% elongation), together with maintaining a very high strength after ageing (up to 975 MPa) from precipitation of metastable nano α-Cu4Ti. This study shows that Fe micro-additions, when combined with a higher amounts of Ti (6 wt%), enables the production of Cu-based alloys combining high formability and strength, providing an excellent alternative to Cu-Be in mechanical applications.