Coherent Precipitates Research Articles

Effect of grain size on precipitation and microstrain of nanocrystalline NiTi alloys

The effect of aging treatment on the microstructure, phase transformation behavior and mechanical properties of nanocrystalline NiTi alloy was studied. The nanocrystalline NiTi alloys with different grain sizes were acquired by cold drawing followed by annealing at 350–500 °C for 10 min. The annealed samples were aged at 250–400 °C for 48 h. The Ti3Ni4 precipitates were found in aged nanocrystalline NiTi alloys. In the sample with smaller average nanograin size, the precipitates were found at the edge of grain boundaries and little lattice strain was shown in R phase matrix. In the sample with larger average grain size, the precipitates were found in the nanograins and exhibited a coherent interface with the matrix. The nanocrystalline R phase NiTi matrix exhibited a significant compressive stress at the end of the coherent precipitate. The coherent precipitates in sample aged at 250 °C after annealing at 500 °C suppress the stress-induced R → B19′ phase transformation and increased the upper plateau stress. The precipitation in sample aged at 250 °C after annealing at 350 °C unable to suppress the martensitic transformation effectively.

Coherent to semi-coherent transition of precipitates in Van der Merwe epitaxial films

Abstract The interplay of stresses of two coherent systems, specifically a coherent precipitate within an epitaxial film (in Van der Merve growth mode), gives rise to an interesting scenario; wherein the configurational effects and the strain energy landscape are enriched. This leads to an interdependent evolutionary pathway for the coherent to semi-coherent transition. The transition of the film-substrate interface to the semi-coherent state occurs beyond a critical thickness ( t c ) which arises from energy minimization. Using the model example of the precipitation of NbH0.5 precipitate in a Nb/Sapphire epitaxial system, the critical radii ( r film ∗ ) for model geometries have been computed. 3D finite element models have been used to compute the stress state and energetics of precipitates growing in an epitaxial film. The following important observations can be made based on the simulations. (1) The epitaxial stresses can lead to the stabilization of the coherent precipitate. (2) The transition to a semi-coherent film interface, characterized by an array of interfacial misfit dislocations, can energetically favor the semi-coherent state of the precipitate over the coherent state. The magnitude of r film ∗ depends on the spatial coordinates within the film. (3) The presence of Nb–H coherent precipitates can stabilize the coherent state of the film interface.

Ultrahigh strength – ductility in Ti–6Al–4V composites with high-activity graphene-induced in-situ TiC and coherent nanophases

Achieving ultra-high strength and ductility of titanium alloys possesses great potential for structural applications in the aerospace and military, yet there is a great challenge for breaking the trade-off barrier of these two properties. In this study, large elongation of ∼10 % and ultra-high tensile strengths of 1510 MPa were obtained in a titanium-based composite. We designed this superior composite based on nanoscale coherent αʹʹ precipitation within near-equiaxed β-Ti as well as micro-scale TiC network architectures along grain boundaries. These in-situ formed triangular αʹʹ coherent precipitates and TiC mainly contributed to the strengthening, while the extremely large elongation resulted from coherent β/αʹʹ interfaces and strain-induced coherent αʹʹ nanotwins. We demonstrated that this composite was easily fabricated using a simple powder metallurgy followed by a hot rolling process. This work can contribute to the design of duplex titanium-based composites as well as other structural materials with exceptional mechanical properties for broad applications.

Thermal Stability and Strengthening Effect of Coherent Precipitates in a (FeCoNi)92Al2.5Ti5.5 High Entropy Alloy

Thermal Stability and Strengthening Effect of Coherent Precipitates in a (FeCoNi)92Al2.5Ti5.5 High Entropy Alloy

On the theory of nucleation of coherent inclusions in irradiated crystals

Based on a critical analysis of the existing model for the effect of excess vacancies and interstitials on the nucleation of coherent particles under irradiation, a new nucleation model is developed that uses the Brailsford-Bullough model for the steady state occupation probabilities of vacancies and interstitials at the particle interface, recently refined by the author. It is shown that, depending on the surface tension γ of a coherent particle, the nucleation mechanism can be qualitatively different. In the case of a relatively small γ, an instability can arise in the irradiated solid solution leading to the barrier-free nucleation of coherent precipitates. In the opposite case of a relatively large γ, the classical one-dimensional theory of homogeneous nucleation is applicable. In order to eliminate the uncertainty in the magnitude of surface tension (from the literature) and to better understand the underlying mechanisms of coherent particle nucleation in irradiation tests, additional atomistic studies are recommended.

Superior tensile properties at cryogenic temperature induced by dual-graded structures in medium entropy alloys

The dual-graded structure with both gradients of grain size and coherent L12 precipitation was applied to a (CoCrNi)96Al3Ti3 medium-entropy alloy for achieving superior tensile properties. Simultaneous improvement on yield strength and uniform elongation is observed in the dual-graded structure at cryogenic temperature, compared to those at room temperature. Stronger hetero-deformation-induced hardening and higher density of geometrically necessary dislocations are observed at cryogenic temperature in both single-grain-size-graded and dual-graded structures. The hardening mechanisms at room temperature are characterized by low density of deformation twins and stacking faults (SFs) on one slip system, while are dominated by high density SFs at two slip systems and formation of Lomer-Cottrell (L-C) locks at cryogenic temperature for the single-graded structure. The hardening mechanisms at both room and cryogenic temperatures are revealed by high density of SFs and L-C locks in the dual-grade structure. These SFs and L-C locks can provide strong hardening themselves as dynamic Hall-Petch effect on one hand, and can interact with coherent L12 precipitates for intense precipitation hardening on the other hand. The better tensile properties for the dual-graded structure at cryogenic temperature should be attributed to a special SF-induced plasticity, i.e., the formation of SF networks and their interactions with nanoprecipitates.

Effects of electrochemical corrosion on retrogressed and different reaged friction stir welded AA7075-AA6082 dissimilar welds

ABSTRACT The research is primarily to study the microstructure, tensile and electrochemical corrosion behaviour of friction stir welded AA7075-T651-AA6082-T6 dissimilar welds, subjected to retrogression and reaging at 280⁰C for 5 min followed by aging at 110⁰Cfor 1 , 5 , 10 and 24 h respectively. The AA7075-AA6082 welds were made using optimum parameters such as feed rate 80 mm/min, tool rotation speed 980 rpm, and tilt angle 2 o . The alloys of 100 × 50 × 6mm³ plates were welded with a conical tapered tool pin profile. Further, the welds were scanned for potentiodynamic polarisation with a 3.5 percentNaCl solution. Optical metallographic (OM) images were revealed to determine morphology andcorrelate with the measured corrosion rate. Here, the dissimilar friction stir weld AA7075-AA6082-aged for 5 h had an ‘Ultimate tensile strength’ (UTS) of 269.73N/mm2 due to coherent precipitates. Moreover, at 24 h, the lower tensile strength of 226.06 N/mm2 was observed, due to coarser precipitates across the AA7075-AA6082 weld cross-section. Due to over-aged coarse precipitates, the corrosion rate of 6082BM and weld metal had increased to 5.110e + 003mpy and 1.010e + 003mpy. Under other aged conditions, the corrosion rate was decreased below 0.001mpy.

Investigation of the precipitation behavior and its role on creep deformation and failure mechanisms at high temperatures for a turbine disk alloy

The creep deformation behavior of a turbine disk alloy at high temperatures was studied, and the role of the multi-scale precipitates on the creep deformation and failure mechanisms was revealed. The turbine disk alloy shows a typical microstructure with the micron-scale MC carbides, submicron delta phase, together with nanoscale γ″ and γ' precipitates distributed in the matrix after hot working and aging treatment. The MC carbides are mostly formed from the solidification process due to enrichment of Nb in the inter-dendritic areas. The creep life decreases from 763.5 h to 5.7 h when the creep temperature increases from 953 K to 1023 K at 500 MPa. The fracture surfaces exhibit a transgranular fracture pattern accompanied with the formation of substantial microvoids. Slipping occurs in both unidirectional and bidirectional modes during deformation. The microvoids along the grain boundaries are generated through decohesion of δ precipitates from the matrix or the interaction between slip traces and grain boundaries. When the intragranular MC carbides interact with the slip traces, microcracks can be formed by particle fracturing. The dense distribution of particles along the grain boundaries will result in a significantly higher critical stress required for microvoid nucleation, whereas the non-coherent particles inside the grains, particularly the large size MC carbides, can decrease the critical shear stress for microcrack formation to as low as 88.9 MPa, thereby contributing to alloy rupture. For the coherent precipitates, a comprehensive strengthening effect of 389.12 MPa is achievable by the synergistic precipitation of γ″ and γ', which primarily serve to enhance and coordinate intragranular deformation during creep.

Insights into precipitation characteristics and strengthening mechanisms in a low Cu/Mg ratio Al-Cu-Mg-Ag alloy

Using aberration-corrected scanning transmission electron microscopy, a new fully coherent precipitate platelet designated X′′, with a flexible atomic layer FCC structure, was identified in a low Cu/Mg ratio Al-Cu-Mg-Ag alloy. The emergence of the X′′ phase is primarily attributed to the preferential segregation behavior of Mg and Cu atoms towards the {111}α Ag layers. The strengthening mechanism of the X′′ phase is principally driven by the coherency strengthening, yielding efficient reinforcement (99.8 MPa increment per 1 % volume fraction). After dislocation cutting, the X′′ phase maintains coherence with the matrix, minimizing local strain accumulation at the interface. The mechanical tests show that the alloy with the precipitated X′′ phase, despite its lower strength compared to the alloy with the precipitated Ω phase, exhibits superior ductility and work hardening. The study holds promising implications for designing novel Al-Cu-Mg-Ag alloys with a desirable strength-ductility balance.

The Influence of Heat Treatment on Microstructure and Properties of Selective Laser Melting CuCrZr Alloy

The effects of different heat treatment processes on the properties of selective laser melting (SLM) CuCrZr alloy are investigated toenhance its overall performance. Solution treatment (ST) results in recrystallized grains, leading to an increase in electrical conductivity to 45.36% IACS and a decrease in tensile strength to 221.8 MPa compared to SLM CuCrZr alloys. Solution aging treatment (SAT) further improves the conductivity to 83.12% IACS and the tensile strength to 335.6 MPa. The optimal combination of electrical conductivity (72.82% IACS) and tensile strength (611.3 MPa) is achieved through direct aging treatment (DAH) at 480 °C for 2 h. With increasing aging temperature and time, the electrical conductivity initially increases rapidly and then plateaues, while the tensile strength exhibits an initial increase followed by a decrease. The proportion of precipitated phase gradually increases with longer aging time. Coherent precipitation occurs between the phase and the matrix at 480 °C for 120 min, while at 480 °C for 360 min, both face‐centered cubic and body‐centered cubic precipitates coexist with an incoherent relationship with the matrix. Overall, DAH proves effective in enhancing the properties of SLM CuCrZr alloy, with precipitation strengthening serving as the primary mechanism for improving its performance.

Effect of Ti/Nb/Ta addition on the γ/γ' coherent microstructure in low-density and high-strength Co-Al-W-Mo-based superalloys

Coherent precipitation of cuboidal γ'-Co3(Al,W) nanoparticles in face-centered-cubic (FCC)-γ matrix is of great significance for improving high-temperature mechanical properties of Co-based superalloys. The present work developed a series of low-density Co-based superalloys in light of the cluster composition formula of [Al1-(Co,Ni)12]((Al0.5(Ti/Nb/Ta)0.5W0.5)(Mo0.5Cr0.5Co0.5)), where the addition of Ti, Nb, and Ta is mixed with an equimolar ratio. It is found that these designed alloys with different combinations of Ti/Nb/Ta, Ti/Nb, and Ti/Ta possess the coherent microstructure of cuboidal γ' nanoprecipitates in the FCC-γ matrix. The microstructural evolution of coherent γ/γ' during aging at 1173 K shows that these superalloys exhibit higher thermal stability at high temperatures. Even after aging for 1000 h, there do not exist any other precipitated phases on grain boundaries, except the coarse γ' precipitates. Also, the coarsening rate constants of cuboidal γ' nanoprecipitates in these alloys are very low (K = 5.76–6.03 nm3/s), which is mainly ascribed to a moderate lattice misfit (ε = 0.28 %-0.45 %) between γ and γ'. The stable γ/γ' microstructure renders the alloys with prominent mechanical properties, as evidenced by the high yield strength of σYS = 438–445 MPa at 1143 K. A large amount of stacking faults appear after compressive deformation and Lomer-Contrell dislocation locks are also formed due to the reaction of partial dislocations of stacking faults. Moreover, the microhardness (285–320 HV) in each alloy keeps almost constant with the aging time. Besides, these superalloys have a relatively lower density (8.67–8.89 g/cm3), among which the alloy containing Ti0.25Ta0.25 also exhibits a much higher γ' solvus temperature (1361 ± 2 K) than those of the existing Co-Al-W-based superalloys.

Enhancing ambient and elevated temperature performance of hypoeutectic Al–Ce cast alloys by Al3(Sc,Zr) precipitate

This study explored the consequences of incorporating Sc and Zr into hypoeutectic Al–9Ce cast alloys, specifically investigating their influence on microstructure and mechanical properties. The findings demonstrated the significant reduction in the grain size of the Al–9Ce alloy while successfully maintaining the distinctive shape of the eutectic Al11Ce3 phase through the incorporation of Sc and Zr additions. During aging treatments, Al3(Sc,Zr) coherent precipitates formed both at the interface between the α-Al and Al11Ce3 phases and within the α-Al matrix. Remarkably, this led to optimal hardness achieved within a short duration of 3 h at 350 °C. Peak-aged quaternary Al–9Ce–xSc–yZr alloys showed significantly better tensile strength than the binary Al–Ce alloy at both ambient and elevated temperatures. Overall, the study underscored promising prospects of Al–Ce–Sc–Zr alloys for use in high-temperature applications, as they exhibited enhanced mechanical properties.

Ta-induced strengthening of CoCrNi–AlTi medium-entropy alloys via nanoscale heterogeneous coherent precipitate

Nano-precipitate strengthened medium-entropy alloys (MEAs) with good strength elongation combinations are crucial for structural applications. This study demonstrated Ta-induced strengthening of CoCrNi–AlTi MEAs using nanoscale heterogeneous coherent precipitates. The density of coherent nanolamellar phases in (CoCr0.5Ni1.5)86Al10Ti4 alloys can be increased with recrystallisation heat treatment, although this enhances the ductility of the alloy by up to 26.57 % the strength attained is only 733 MPa. However, addition of 2 at. % of Ta to (CoCr0.5Ni1.5)86Al10Ti4 MEAs enhances the mechanical properties, and the resulting yield strength, ultimate tensile strength, and acceptable elongation are 1385 MPa, 1690 MPa, and 7.44 %, respectively at room temperature. Thus, Ta-addition and aging treatments increase the yield stress by 89 %. This is attributed to the coexistence of nanoscale lamellar, heterogeneous, and continuous precipitates in the incompletely recrystallized grains. Therefore, this study serves as a reference for developing correlations between the microstructure evolution and property design of L12-γ′ precipitation-strengthened MEAs.

Improving ductility by coherent nanoprecipitates in medium entropy alloy

Controlling precipitates in spatial density and size distribution is essential for tailoring the microstructure and mechanical properties through precipitation hardening. We herein obtained heterogeneous grain structures with coherent L12 nanoprecipitates in (CrCoNi)94Al4Ti2 medium entropy alloy (MEA) by annealing and aging. Additional pre-aging leads to a high spatial density and more random distribution nucleation sites of the coherent L12 nanoprecipitates. The pre-aging doubled the ductility without apparently sacrificing the strength. Transmission electron microscopy (TEM) revealed that, in pre-aged MEA, finely dispersed L12 nanoprecipitates with higher spatial density were sheared by dislocation, promoting planar slips, which favors geometrically necessary dislocations (GNDs) piling up to increased hetero-deformation-induced (HDI) stress and work-hardening. Stacking faults, Lomer-Cottrell locks, and 9R structures were formed in aged and pre-aged MEA after tensile deformation. The formation of these defects enormously enhanced strain hardening by blocking dislocation movements and accumulating dislocations. Moreover, a higher frequency of interactions between defects and coherent L12 nanoprecipitates can be observed in the pre-aged MEA due to the more randomly distributed L12 nanoprecipitates, substantially increasing ductility. This work demonstrates a new route to achieving a super strength-ductility combination of single-phase FCC high entropy alloys by nanoscale coherent precipitation strengthening.

Unravelling the mechanisms of copper precipitation-induced strengthening in austenitic stainless steels: An atomistic approach

Copper addition in austenitic stainless steel is known to provide good high-temperature strength through precipitation strengthening. However, the mechanism of such strengthening and its overall contribution to strength in austenitic stainless steels have not been adequately investigated. The present work employs molecular dynamics (MD) simulation to investigate the interaction between an edge dislocation and copper precipitate in austenitic stainless steel. The mechanism controlling the strength was found to be trailing partial detachment for smaller precipitates up to a maximum radius of 3 nm, whereas leading partial detachment controls the strength for larger precipitate sizes, irrespective of the inter-precipitate spacing. Besides, modulus strengthening was identified as the primary strengthening contributor for the coherent Cu precipitate in the present alloy, which was subsequently aligned with the theoretical predictions by the existing Russell-Brown (R-B) model, adopting some modifications. The discrepancy between the simulation results and the original R-B model is attributed to the interaction between the two partial dislocations in presence of the precipitate. The modified R-B model was then used in combination with Thermo-calc and DICTRA predictions to estimate the strength of Cu-added austenitic stainless steel. However, the predictions overestimated the experimental data due to the model's inability to account for the random distribution of precipitates. To address this limitation, a subsequent discrete dislocation dynamics (DDD) simulation was conducted, incorporating a random distribution of Cu precipitates. Finally, the DDD simulation results demonstrated a good match with the experimental results, which can be further extended to other alloy systems as well.

Bulk ultrafine grained microstructures with high thermal stability via intragranular precipitation of coherent particles

Producing ultrafine-grained (UFG) microstructures with enhanced thermal stability is an important yet challenging route to further improve mechanical properties of structural materials. Here, a high-performance bulk UFG copper that can stabilize even at temperatures up to 750 °C (∼ 0.75Tm, Tm is the melting point) was fabricated by manipulating its recrystallization behavior via low alloying of Co. Addition of 1 wt.%–1.5 wt.% of Co can trigger quick and copious intragranular clustering of Co atoms, which offers high Zener pinning pressure and pins the grain boundaries (GBs) of freshly recrystallized ultrafine grains. Due to the fact that the subsequent growth of the coherent Co-enriched nanoclusters was slow, sufficient particles adjacent to GBs remained to inhibit the migration of GBs, giving rise to the UFG microstructure with prominently high thermal stability. This work manifests that the strategy for producing UFGs with coherent precipitates can be applied in many alloy systems such as Fe- and Cu-based, which paves the pathway for designing advanced strain-hardenable UFGs with plain compositions.

Strong and ductile niobium-based refractory alloy via deformable zirconia nanoparticles

The use of dispersed precipitates is an effective method for strengthening metals, but it typically results in a reduction in ductility. A potential way to overcome this strength-ductility trade-off is through the incorporation of coherent nanoscale deformable precipitates. Here, we present an approach of designing and fabricating a niobium-based (Nb-5W-2Mo-1Zr, Nb521) alloy with copiously and semi-coherently dispersed nanoscale zirconia (ZrO2) precipitates through laser powder bed fusion followed by heat treating. The intergranular ZrO2 precipitates at grain boundaries underwent tetragonal-to-monoclinic martensitic phase transformation and deformation twinning, while the intragranular ZrO2 precipitates within the matrix experienced deformation twinning and substantial plastic deformation during tension. The combination of transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) from the nanoscale ZrO2 precipitates provides sufficient capacity to accommodate strain and relieve stress concentration. Additionally, the intragranular ZrO2 precipitates effectively piled up incoming dislocations from the Nb matrix to form high-density dislocation networks during tension, which interacted with each other via the Orowan mechanism and finally resulted in its significant plastic deformation. These mechanisms contribute to the exceptional tensile properties of the Nb521 alloy, including an ultimate strength of 628.7 ± 2.3 MPa and a ductility of 17.8 ± 0.6% with a superior work-hardening capacity (∼230 MPa), achieving a record of strength and ductility combination for the Nb521 alloys fabricated by additive manufacturing to date. This strategy represents a promising approach for designing dispersion-strengthened refractory alloys with excellent synergy of strength and ductility.

Enhancing the Yield Strength of Casting Eutectic High-Entropy Alloys via Coherent Precipitates

Enhancing the Yield Strength of Casting Eutectic High-Entropy Alloys via Coherent Precipitates

Facile formation of a superalloy-like microstructure in Ni77Ga23 alloys by rapid solidification processing

Ni77Ga23 alloys were undercooled and rapidly solidified using glass fluxing treatment. An in situ diagnosis of dendrite growth velocity during rapid solidification processing suggests crystallization of a disordered solid solution from low to high undercoolings. While this diagnosis is consistent with an X-ray diffraction analysis, a neutron diffraction analysis suggests the formation of Ni3Ga phase in addition to the disordered solid solution. An EBSD analysis shows the formation of an equiaxed microstructure at a high undercooling. A HAADF-STEM study of equiaxed grains reveals a superalloy-like substructure, where cuboidal coherent precipitates of Ni3Ga phase are dispersed in the matrix of the disordered solid solution. A HRTEM study further reveals disordered nanodomains in the lattice of Ni3Ga phase. It is concluded that rapid solidification processing can lead to facile formation of a superalloy-like microstructure without an aging treatment.

Sources of predictability of synoptic‐scale rainfall during the West African summer monsoon

AbstractNumerical‐model‐based forecasts of precipitation exhibit poor skill over northern tropical Africa when compared with climatology‐based forecasts and with other tropical regions. However, as recently demonstrated, purely data‐driven forecasts based on spatio‐temporal dependences inferred from gridded satellite rainfall estimates show promise for the prediction of the 24‐hr precipitation occurrence rate in this region. The present work explores this potential further by advancing the statistical model and providing meteorological interpretations of the performance results. Advances include (a) the use of a recently developed correlation metric, the Coefficient of Predictive Ability (CPA), to identify predictors, (b) forecast evaluation with robust reliability diagrams and score decompositions, (c) a study domain over tropical Africa nested in a considerably enlarged spatio‐temporal domain to identify coherent propagating features, and (d) the introduction of a novel coherent‐linear‐propagation factor to quantify the coherence of propagating signals. The statistical forecast is compared with a climatology‐based benchmark, the European Centre for Medium‐Range Weather Forecasts (ECMWF) operational ensemble forecast, and a statistically postprocessed ensemble forecast. All methods show poor skill within the main rainbelt over northern tropical Africa, where differences in Brier scores between the different approaches are hardly statistically significant. However, the data‐driven forecast outperforms the other methods along the fringes of the rainbelt, where meridional rainfall gradients are large. The coherent‐linear‐propagation factor, in concert with metrics of convective available potential energy and convective instability, reveals that high stochasticity in the rainbelt limits predictability. At the fringes of the rainbelt, the data‐driven approach leverages coherent precipitation features associated with propagating tropical weather systems such as African Easterly Waves.