Precipitate Microstructure Research Articles

Effect of vanadium on the microstructure and kinetics of discontinuous precipitation in Cu–3.2Ti–0.2Fe alloy

The microstructure and kinetics of discontinuous precipitation in Cu–3.2Ti–0.2Fe– x V ( x = 0, 0.1, 0.2 and 0.3 wt.%) alloys aged at 723, 773, 823 and 873 K were studied by using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS), with the help of CALculation of PHAse Diagram (CALPHAD) method and Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. The volume fraction of Laves_C14 was increased with raising V content, leading to the effective grain refinement during casting and the inhibiting grain growth in solution treatment process. The activation energies of Cu–3.2Ti–0.2Fe– x V ( x = 0, 0.1, 0.2 and 0.3 wt.%) alloys were 196.4, 172.9, 147.4, 154.1 kJ mol −1 , respectively. The discontinuous precipitation was promoted and controlled by grain boundary diffusion in the early stages at low temperature, while it was inhibited by bulk diffusion at high temperature or in the later stages at low temperature. In addition, the peak-aged hardness was improved with proper amount of V addition due to the strengthening of Laves_C14 and grain refinement, as well as the inhibition of discontinuous precipitation.

High-temperature age-hardening behavior of Al–Mg–Si alloys with varying Sn contents

The effects of Sn content on the age-hardening behavior and peak aging precipitates of Al–Mg–Si alloys at 250 °C were investigated by hardness testing, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The results indicated that the peak hardness of the experimental alloys initially increased and then decreased with Sn content increasing from 0 to 0.15 wt.%, and it reached a maximum value of 108 HV in the alloy containing 0.1 wt.% Sn. All the Sn-added alloys exhibited higher peak hardness than the Sn-free alloy. Microstructure observations revealed that the enhanced peak hardness of the Sn-added alloys derived from the refinement of precipitates and their increase in number density. The underlying mechanisms of varying Sn addition contents on the precipitate microstructure were discussed.

Aging hardening and precipitate behavior of a solution-treated Mg-6Zn-4Sn-1Mn (wt.%) wrought Mg alloy

Mg-Zn-Sn-Mn alloy is a new type of age hardening wrought Mg alloy. However, the precipitation behavior of the alloy is still not well understood. A solution-treated Mg-6Zn-4Sn-1Mn alloy is selected as the experimental alloy in this paper. The hardening behavior, precipitate microstructure and precipitation sequence of the solution-treated alloy during single aging and double aging at 180 °C are systematically studied by means of XRD, OM, SEM, TEM, EDS and microhardness. The results show that four types of precipitated phases are successively precipitated in the experimental alloy during the aging treatment, namely, G.P. zone parallel to the (0001)α base plane, rod-like β1′(MgZn2) phase perpendicular to the (0001)α base plane, disc-like β2′ (MgZn2) phase parallel to the (0001)α base plane, and irregular β' (Mg2Sn or Mg3Sn) phase. The habitual plane of most β' phases is the (0001)α basal plane, and a small part of the β' phase forms a certain angle with the (0001)α basal plane. A large number of finely dispersed G.P. zones can provide the heterogeneous nucleation sites for the β1′ phase during the pre-aging stage, which promotes the nucleation of the precipitates during the second aging, so the peak aging hardness (87HV) of the double-aged alloy is much higher than that of the single-aged alloy (74HV), and the peak aging time (6 h) during second aging of the double aging is shorter than that (12 h) of single aging. The possible precipitation sequence of Mg-6Zn-4Sn-1Mn alloy during the isothermal aging treatment can be obtained, that is, α-Mn→β1′ (MgZn2)→β (MgZn) in the Mn-rich zone, and SSSS→G.P. zones→β1′(MgZn2)→β2′(MgZn2)→β' (Mg2Sn or Mg3Sn)→(β (MgZn) and β (Mg2Sn)) in Zn-rich and Sn-rich zone.

Enhanced high-temperature age-hardening behavior and mechanical properties of Al–Mg–Si alloys via microalloying with Cd

The effects of trace (0.05 at.%) Cd addition on the high-temperature (250 °C) age-hardening behavior, precipitate microstructure and room-temperature tensile properties of an Al-0.8Mg-0.8Si (wt.%) alloy were investigated. The results showed that the Cd addition not only accelerated the precipitation kinetics but also increased the peak hardness of Al–Mg–Si alloys. The quantitative analysis of the peak-aged precipitates based on (high-resolution) transmission electron microscopy images indicated that the Cd addition refined the size of needle-shaped precipitates and increased their number density. High-angle annular dark-field scanning transmission electron microscopy characterizations revealed that the Cd addition stimulated the heterogeneous nucleation of β" on the early formed nanosized Cd-rich precipitates and Cd-containing QP lattices, resulting in a much-refined precipitate microstructure. Furthermore, the Cd addition also enhanced the room-temperature tensile properties of the peak-aged alloy. The Cd-added alloy exhibited higher yield strength, ultimate tensile strength, and total elongation than the Cd-free alloy, which was due to the refined precipitate microstructure and narrowed precipitate-free zone along the grain boundaries.

Investigations of the precipitation microstructure in alloys, hard metal composites and powders using SANS

Investigations of the precipitation microstructure in alloys, hard metal composites and powders using SANS

Microstructural features in additively manufactured EUROFER97 components

By uncoupling the manufacturability from the design process, additive manufacturing of RAFM steels like EUROFER97 can open significant design freedom for components in fusion reactors, e.g., breeding blankets and divertor applications.As mechanical properties of RAFM steels are based on their specific microstructure and high number density of precipitates, good control of the microstructure of additively manufactured parts has to be ensured. As additive manufactured components are known to possess unique microstructures compared to EUROFER97 from standard technologies, the aim of this paper is to investigate additive manufactured EUROFER97 components and the influence of post-processing steps on their microstructure and in particular on the formation of precipitates.This paper covers the technological fabrication process of EUROFER97 by selective laser melting (SLM), including the production and chemical analysis of pre-alloyed EUROFER97 powder. In the initial state after fabrication, SLM-EUROFER97 components exhibit a bimodal, anisotropic microstructure with large ferritic grains instead of the desired homogeneous, fully martensitic microstructure. Further, neither M23C6 nor MX precipitates were detected in the as-built condition.A heat treatment including hot isostatic pressing, austenitization, quenching and tempering, allows achieving a fully martensitic, uniform microstructure as well as the formation of M23C6 and MX precipitates with similar size and volume density as in conventionally produced EUROFER97. In combination with the chemical analysis of the raw material, impurities stemming from the atomization process forming oxides were detected. However, due to their small size, those are not expected to influence the performance of SLM-processed EUROFER97. The comparison of the as-built condition and the condition after hot isostatic pressing and heat treatments reveals the necessity of appropriate post-processing to tailor the microstructure to one comparable to conventional EUROFER97.

The influence of the temperature on the mechanical properties of Sanicro 25 steel

Austenitic steel 22Cr25NiWCoCu (Sanicro 25) is one of the newest and most promising steels for use in supercritical and ultra-supercritical power units. High resistance of Sanicro 25 steel to corrosion and oxidation in the steam atmosphere at up to 700 °C is ensured by chromium content at 21.5-23.5 wt%. Also, higher chromium content provides less mass decrement during high-temperature corrosion at 700 °C. Currently, the Sanicro 25 steel is characterised by the highest creep strength among the commercially used creep-resistant stainless steels. In this work, the study of precipitation processes and microstructure stability in the delivery state and after the ageing process in time up to 20,000 h at 750 °C was performed. Identification of secondary phases was performed using electron diffraction on thin films using transmission electron microscopy and X-ray phase composition analysis. Long-term ageing depending on time and temperature showed that there are four secondary phases in Sanicro 25: M23C6, MX, NbCrN, σ-phase and Cu_rich. The precipitation and increase in the size of secondary phase particles during ageing results in a decrease in matrix saturation with alloying elements, which leads to a reduction in hardening by solid solution mechanism. These processes and effects result in changes in mechanical properties. The alloy overageing effect observed for the ageing temperature of 750 °C results in obtaining strength properties similar to those of the steel in the as-received condition.

Effects of aging treatment on the evolution of precipitated phase in CoCrFeNiTi0.6 high entropy alloys

In this study, the effect of aging time on the microstructure of precipitates in a face-centered cubic (FCC) CoCrFeNiTi0.6 high-entropy alloy (HEA) was investigated. The microstructure of precipitates was analyzed by SEM and TEM. The results showed that the CoCrFeNiTi0.6 alloy exhibited a typical dendritic structure, whose phase consisted of an ordered FCC solid-solution matrix (nanoscale L12-(Ni, Co)3Ti type particles) and intermetallic phases at 900 °C for different aging times. Meanwhile, the intermetallic compounds were composed of (Cr, Fe)-rich σ phases with a tetragonal structure and (Ni, Ti)-rich η phase with a long period stacking ordered (LPSO) structure. Additionally, the influence of aging time on the shape and structure of γ' precipitates was revealed. It was noticeable that the morphological transformation of nanoscale γ' precipitates evolved from an initially spheroidal to spheroidal-like shapes, following cuboidal-like shapes with flat edges, and finally, γ' precipitates with cuboidal shape were formed simultaneously with the appearance of the split with increasing aging time. Furthermore, the morphological evolution of γ' precipitates was quantitatively analyzed using a characteristic parameter's value of precipitates based on lattice misfit between the matrix and precipitates. This research would provide guidance for designing the precipitate structure in the L12-strengthened HEA and facilitate to establish the relationships between the microstructure of the precipitates and properties.

Cyclic stress-strain behaviour of particle-hardened cast aluminium alloys: Microstructure sensitive modelling of the effect of non-shearable precipitates

The influence of precipitate microstructure on the cyclic stress-strain behaviour is investigated for two cast aluminium alloys with different copper content AlSi7Cu0.5Mg0.3 and AlSi7Cu3.5Mg0.1. Over-ageing occurs in these alloys during service in critical areas of cylinder heads used in automotive industry. To analyse the relationship between changes in microstructures and tensile and cyclic mechanical behaviours, heat treatment conditions are chosen in order to provide a large spectrum of microstructure changes and a large range of yield strength. The microstructures are examined using transmission electron microscopy and quantitative estimates of precipitate distributions are made. Tensile tests and cyclic strain controlled tests with an incremental loading procedure are used to characterize the monotonic and cyclic stress-strain behaviour of both alloys. These measurements are then used to identify a new microstructure-sensitive constitutive model. A mean field constitutive model is proposed using a visco-plastic framework with particular attention to the Bauschinger effect. Microstructure sensitivity is obtained considering the accumulation of dislocations and Orowan loops averaging the main features of precipitate distributions. Only a fairly small number of parameters has to be identified from cyclic or tensile tests, all the other constants coming from data published in the literature. Model predictions for other conditions are in good agreement with experiments. Application to other data published in the literature and description of loops formation and annihilation during cycling are shown.

Modelling the influence of process parameters on precipitate formation in powder-bed fusion additive manufacturing of IN718

In this work, we have developed a two-scale (finite element + phase field) model for simulating the precipitate evolution in IN718 in powder-bed fusion additive manufacturing. The thermal simulations at the scale of the component are resolved at the level of a single stripe of the scan path. This resolution allows to model various process parameters (or scan strategies) in complex geometry domains, here demonstrated by an impeller, and predicts the complete thermal history within each finite element layer. The predicted thermal history is in-full passed to multiple phase-field simulation representative volume elements (RVEs) that explicitly model the evolution of the γ′ and γ″ precipitates in various locations of the impeller. We simulate the precipitate microstructures in different regions of the impeller and demonstrate the influence of process parameters on the volume fraction, size, and shape of the precipitates. Although no precipitates are found to form during the standard printing conditions (for instance EOS M290), we show that changing the process parameters such as stripe width or chamber temperature may induce significant precipitation in the as-built component.

Microstructural effects and mechanical response of microalloyed ferrite and austenite subjected to metal forming at extremely different rates of deformation

The mechanical behaviors and microstructures of severely deformed microalloyed austenitic M_A and ferritic M_F steels were analyzed in wires drawn from wire rods (6.5 mm in diameter) down to the final diameter of 0.75 mm. The deformation induced microstructures in the cold drawn wires under “static” (low speed wire drawing) and “dynamic” (high speed wire drawing) conditions were compared and analyzed. The analysis has been focused on a detailed characterization of the effects of microalloying on the dislocation substructure development and the in-situ recrystallization. The correlations between mechanical behavior and dislocation substructure development with increasing deformation reveals that precipitation strengthening and microstructure refinement are the dominating mechanisms that control the mechanical behaviors of wires, especially in those subjected to deformation under dynamic loading conditions. An approach has been presented to model the evolution of work hardening process during complex deformation. Additional advantage of the presented approach is its capability of tracking evolution of dislocation density during the deformation process. The changes of dislocation density calculated for both materials show similar behaviour, but higher values of the dislocation densities were observed in the specimens of M_F than in the case of M_A.

Effects of radiation damage and precipitate distribution on micro-pillar compression testing of irradiated CuCrZr

The results of small-scale mechanical tests are convoluted by the so-called size effect, whereby materials appear stronger when the scale of the test is reduced to the order of microns or less. The dimensional range over which this occurs has been shown to be linked to a change in sample microstructure, such as the addition of defects induced by irradiation. To investigate this response, a CuCrZr alloy was subjected to proton irradiation and mechanically tested using micro compression of pillars with a range in size. It was found that irradiation defects dominate over the extrinsic size effect and the sensitivity to differences in precipitate microstructure was also somewhat reduced, suggesting that size-independent results could be obtained from much smaller test volumes in irradiated material compared to their non-irradiated counterparts. Finally, comparison was made between the increase in yield strength predicted by models and the experimentally measured values to establish the key parameters driving the strengthening behaviour.

Designing [formula omitted]-strengthened Al-Cr-Fe-Ni-Ti complex concentrated alloys for high temperature applications

For alloys with nanoprecipitate/matrix microstructures, the lattice coherency between the two phases plays an important role in determining the mechanical performances at high temperature. In this work, we systematically investigate the AlxCr13.3Fe71.5-xNi11.2Ti4 (x=8, 10, 12, 14, 16 at%) complex concentrated alloys with an aim to enhance the lattice coherency between the BCC matrix and L21 precipitate. The precipitate microstructures, lattice misfit and the resultant coherency strain evolution are comprehensively studied. With increasing Al from 8 at% to 16 at%, the interfacial structure gradually transforms from semicoherent to fully coherent interface by the decrease in precipitate size and lattice misfit, which leads to the stronger elastic interaction between the matrix and precipitate. Meanwhile, the crystal structure of precipitate is slightly distorted from cubic to tetragonal. The yield strength of alloys at room temperature continuously increases with the addition of Al by the solid-solution strengthening and precipitation hardening effect. Here, the mechanism of precipitation hardening changes from Orowan process to coherency strengthening. Interestingly, the strengthening effect by Al addition is further amplified in the tensile test at 700 °C. The higher degree of lattice coherency and the distorted structure of the precipitate resulting from the Al addition lead to the effective strain transfer and the stronger precipitate, respectively. Therefore, the Al16Cr13.3Fe55.5Ni11.2Ti4 alloy exhibits an excellent combination of yield strength (400.8 MPa) and ultimate tensile strength (572.9 MPa). These values are much higher than those of the previously reported nanoprecipiate-strengthened alloys, suggesting that the alloy is highly promising as high temperature structural applications.

The precipitation behavior and its effect on mechanical properties of cold-rolled super-ferritic stainless steels during high-temperature annealing

The precipitation behavior, microstructure evolution and mechanical properties of a cold-rolled 27.6Cr–3.7Mo–2Ni ferritic stainless steel during high temperature annealing treatment were investigated. Experimental results demonstrated that the cold rolled microstructure was characterized by orientation-dependent heterogeneity, and many shear bands were found in the <111>//ND (normal direction) orientation grains. The elongated deformation microstructure gradually changed into equiaxed grains and three kinds of interphases including σ-phase, χ-phase and Laves phase were observed as the annealing temperature increased from 850 to 1000 °C. After annealing below 1000 °C, the σ-phase and χ-phase were observed to be located at grain boundaries, near the TiN particles as well as along with shear bands in grains, whereas the Laves phase distributed dispersedly in grains. After annealing at 1000 °C, both the σ-phase and χ-phase were almost completely dissolved, while the nanoscale Laves phases at grain boundaries were dominant. The grain boundary Laves phase precipitation at high temperature was attributed to the Nb segregation induced by cold rolling deformation. The σ-phase precipitation kinetics was calculated based the Johnson–Mehl–Avrami equation, and the fastest precipitation kinetics appeared at 850 °C with the activation energy of ~176.7 kJ·mol−1. The Vickers hardness, ultimate tensile strength and yield strength values decreased while the elongation value increased with increasing annealing temperature. The breakage pattern transformed from brittle to ductile fracture as the annealing temperature increases from 850 to 1000 °C.

Linking mechanical properties to precipitate microstructure in three Al-Mg-Si(-Cu) alloys

Abstract The mechanical properties of age hardenable Al alloys depend strongly on the precipitate microstructure. This work has investigated the relationship between properties such as strength and ductility and the distribution of precipitates, using three Al-Mg-Si(-Cu) alloys (Cu ≲ 0.1 at.%). A range of ageing conditions was examined in order to understand the effect of an evolving precipitate microstructure, and the results were used as input for strengthening models. The mechanical properties were obtained by tensile tests and microstructure characterisation was attained by transmission electron microscopy. The results showed that minor changes to the Si, Mg, and Cu additions – the total addition (at.%) kept approximately equal – had a significant impact on material properties, with corresponding changes in the precipitate microstructure. On the peak strength plateaus differences as large as 35 MPa in yield strength were measured between the strongest and the weakest alloy, obtained as 410 MPa and 375 MPa, respectively. Higher material yield strength correlated well with a refined precipitate microstructure comprising higher number densities of smaller precipitates. Differences with respect to material ductility first appeared after moderate overageing of the alloys, showing negative correlation with material strength. At significantly overaged conditions the differences in strength exceeded 100 MPa, demonstrating large differences with respect to the thermal stability of these materials, which has important consequences for alloys exposed to elevated temperatures under in-service conditions. The highly comprehensive body of data presented here should serve as a valuable reference in the development of precipitation and strengthening models for the Al-Mg-Si-Cu system and will hopefully incite further investigations on the topics covered.

Modeling peak-aged precipitate strengthening in Al–Mg–Si alloys

Strengthening by needle-shaped β′′ precipitates is critical in Al–Mg–Si alloys. Here, the strengthening is studied computationally at the peak-aged condition where precipitate shearing and Orowan looping are usually considered to have equal strengths. Pseudo-random precipitate microstructures are constructed based on experimental precipitate dimensions and volume fractions at peak aging. A Discrete Dislocation Dynamics method is then adapted to compute the Critical Resolved Shear Stress (CRSS) for Orowan looping of dislocations moving through the non-shearable precipitate field. The CRSS for Orowan looping is determined by a typical in-situ precipitate spacing that is smaller than the average spacing and by the dislocation core energy within a radius of ≈5b, a factor rarely considered. The matrix misfit stresses, volume fraction, and precipitate shape have small effects on the CRSS. With microstructure and property details introduced as faithfully as possible, the CRSS for Orowan looping using atomistically-calibrated core energies at room temperature is nonetheless ≈33% higher than experiments. This suggests that precipitate shearing controls strength, and analyses of (i) forces acting on the precipitates, (ii) misfit stresses inside the precipitates, (iii) first-principles results for the relevant precipitate fault energies, and (iv) simulations that mimic precipitate shearing indicate a shearing CRSS closer to experiments. Thus, Orowan looping only sets an upper bound for the CRSS even at peak aging, and further quantitative progress requires detailed modeling of precipitate shearing.

Influence of γ’ precipitate on deformation and fracture during creep in PM nickel-based superalloy

FGH96 is a powder metallurgy nickel based superally used for turbine disk of aero-engines. In the present study FGH96 alloy with four different γ′ precipitate microstructures were produced via solution heat-treatment with different cooling rates, and the maximum cooling rate reached 400 ​°C/min which was a super cooling rate for Nickel-based superalloy. The creep tests were conducted for PM FGH96 alloy under the testing condition of 704 ​°C and 690 ​MPa. The relationship between the creep properties and the distribution of γ’ precipitate was established. The creep mechanism was analyzed by using TEM and ACTEM, and the dislocation movement was studied at the atomic scale. The creep strain rate was calculated through a physically based crystal slip model established based on crystal plasticity. The calculated results were consistent with the test ones, illustrating the validity of the model. The fracture mechanism was also investigated, and the results showed that the creep cracks generated on the surface due to the oxidation. It was observed that the cracks propagated in different ways depending on the different average diameters of ​γ′ ​precipitate. With the decrease of the average ​γ’ ​precipitate size, the critical shearing stress increased and the resistance of the dislocation slipping increased. The fracture mechanism for the primary stage transformed from intragranular to intergranular due to the change of dislocation slipping.

Influence of Ni4Ti3 precipitation on martensitic transformations in NiTi shape memory alloy: R phase transformation

Precipitation of Ni4Ti3 is effective in tuning martensitic transformations (MTs) in NiTi shape memory alloys (SMAs). However, several fundamental issues concerning the influence of Ni4Ti3 on MTs remain unclear. In this study, the microstructure evolution process during precipitation and its influence on B2→R MT are investigated using computer simulations based on the phase field method. In particular, the roles played by Ni concentration gradient in the B2 matrix and internal coherency stress fields associated with different precipitate microstructures are analyzed in detail. It is found that Ni concentration gradient in the B2 matrix created by Ni4Ti3 precipitates alters significantly local martensitic start temperature and the overall MT behavior, while the internal coherency stress field associated with the Ni4Ti3 precipitates dictates the structure of martensitic domains via variant selection. The latter effect makes it promising to develop Invar alloys via stress-ageing to tailor the precipitate microstructure. These findings provide fundamental insights into the mechanism controlling the MT behavior in precipitate-bearing SMAs and provide guidance for the design of thermomechanical treatment for desired precipitate microstructures and the corresponding MT behavior.

H-phase precipitation and its effects on martensitic transformation in NiTi-Hf high-temperature shape memory alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

Phase field model for multiphase alloys under arbitrary thermal history: An application to IN718 super-alloy

Currently, it is still a significant challenge to predict the microstructural evolution of multiphase alloys in complex manufacturing processes such as joining, welding and additive manufacturing due to their complex thermodynamics and thermal history. In this work, we present a phase-field framework, aiming to predict the evolution of precipitate microstructure under arbitrary thermal history. Our model incorporates nucleation, growth and transformation by taking into account interfacial, chemical and elastic energies. In order to quantitatively describe the nucleation of phases, we leverage on experimental time-temperature-transformation data to parameterize the classical nucleation model. For demonstration, we employ super-alloy IN718, where thermodynamic and kinetic data is obtained using CALPHAD method and major precipitate phases, i.e. γ′,γ″ and δ and their variants are considered. The predicted continuous-cooling-transformation diagram is consistent with data existing in literature, thus validating the approach. We further predict the precipitate evolution under rapid thermal cycling to mimic a typical additive manufacturing process.