Distribution Of Precipitates Research Articles

Laser powder bed fusion of a new high gamma prime Ni-based superalloy with improved weldability

This study reports the crack-free fabrication of a new high gamma prime precipitation hardened Ni-based superalloy by Laser Powder Bed Fusion (LPBF) process. In the as-built condition, scanning electron microscopy and electron backscattered diffraction analysis indicated the presence of elongated columnar grains with an orientation along 〈1 0 0〉 direction. Following the sub-solvus heat treatment cycle determined using Thermo-Calc software, crack-free microstructure with columnar grains was observed along 〈1 0 0〉 orientation. High-resolution scanning electron microscope analysis revealed a bimodal distribution of gamma prime precipitates and a uniform distribution of carbide precipitates in the heat-treated samples. As a result of heat treatment, hardness increased from 365 HV to 508 HV. The heat-treated samples also displayed excellent yield strength, ultimate tensile strength, and elongation of 939 MPa, 1368 MPa, and 23% respectively at room temperature. Mechanical testing at elevated temperatures confirmed the yield strength, tensile strength, and elongation reached 80 MPa, 106 MPa, and 34% respectively at 1121 °C. The stress rupture test of the heat-treated samples revealed good creep properties with rupture times of 559 h and 75 h, respectively, at 927 °C and 982 °C, for stresses of 152 MPa and 138 MPa.

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.

Precipitation-dependent corrosion analysis of heat treatable aluminum alloys via friction stir welding, a review

Although the various advantages of novel Friction stir welding (FSW) process; the weld surfaces are subjected to various serious problems such as lower corrosion resistance, high susceptibility to stress corrosion cracking and poor joint fatigue strength due to complex material flow and severe plastic deformation during the welding process. Corrosion behaviour of friction stir welded (FSWed) precipitate strengthening Al alloys have significant impact on metallurgical and electrochemical properties of structures. In FSW of precipitate strengthening Al alloys the localized heat input and severe plastic deformation creating appreciable changes in microstructure and modifies the microchemistry and metallurgical characteristics of precipitates. The heterogeneous distribution of precipitates and precipitate free zones (PFZs) along the grain boundaries leads to variation in electrochemical behaviour across the weld zones and hence increasing the susceptibility of weld surface to various corrosion attacks such as intergranular corrosion, pitting corrosion, exfoliation corrosion, stress corrosion cracking and galvanic corrosion. However, the corrosion resistance of FSWed joints can be improved either by reducing the size or redissolve the coarsened precipitates within the joint or modifying the microchemistry by controlling the size, location and distribution of precipitates which largely determine the corrosion rate of the weld surfaces. Consequently, it is imperative to address the influence of material modifications during FSW on corrosion behaviour of weld surface. This review paper addresses the precipitate dependent corrosion behaviour of FSWed joints of heat treatable/precipitate strengthening Al alloys and the various effective methods either to reduce or eliminate the effect of corrosion attack in FSWed joints of heat treatable Al alloys.

Xenon plasma focussed ion beam preparation of an Al-6XXX alloy sample for atom probe tomography including analysis of an α-Al(Fe,Mn)Si dispersoid

A Xe plasma Focussed Ion Beam instrument was used to prepare in-situ specimens for Atom Probe Tomography from a bulk sample of an aluminium 6XXX alloy. The nature and distribution of precipitates and solute clusters observed in the alloy are not observed to differ between standard electropolishing methods and Xe plasma preparation. Enabled by site specific specimen preparation, analysis of an α-Al(Fe,Mn)Si dispersoid shows segregation at the phase boundary and in the shell of the dispersoid.

Interaction of stress relaxation aging behavior and microstructural evolution in Inconel 718 alloy with different initial stress status

The two-stage aging treatment with different initial loading stresses were carried out in the Inconel 718 alloy after solution annealing process to investigate the stress relaxation and age hardening behaviors on basis of microstructural evolution. The result showed that the stress relaxation behavior mainly occurred in the first aging stage and the corresponding mechanism changed from diffusion creep with initial loading stress of 100 MPa into dislocation slip with initial stress of 600 MPa. Simultaneously, the influence of initial stress on yield strength was counteracted balanced the creep recovery and dislocation hardening, resulting in nearly constant value if yield strength. During the secondary aging stage, the strength of the purely aged sample is 3% higher than that of the high initial stress sample, and the precipitated phase morphology distribution is more uniform. The stress relaxation occurred in the secondary aging stage was negligible, irrespective of initial stress status. In condition, the application of initial stress gave rise to the precipitation behavior, which presented faster growth and coarsening with increasing initial stress value. As a comparison, the distribution of precipitates in sample without loading stress relatively more homogeneous and revealed denser, leading to higher yield strength compared with other samples with initial stress.

Directional Solidification of AlSi7Fe1 Alloy Under Forced Flow Conditions: Effect of Intermetallic Phase Precipitation and Dendrite Coarsening

A forced flow was experimentally shown to influence the solidification microstructure of metal alloys by modifying the coarsening/ripening law. In some technical alloys (AlSi7Fe1), this flow effect can also be significantly suppressed due to the formation of intermetallic precipitates (β-Al5FeSi) that can block the flow in the mushy region. The forced flow was induced by a rotating magnetic field (RMF). Herein, a three-phase volume-average-based solidification model is introduced to reproduce the above experiment. The three phases are the melt, the primary solid phase of columnar dendrites, and the second solid phase of intermetallic precipitates. The dynamic precipitation of the intermetallic phase is modelled, and its blocking effect on the flow is considered by a modified permeability. Dendrite coarsening, which influences the permeability, is also considered. The RMF induces a strong azimuthal flow and a relatively weak meridional flow (Ekman effect) at the front of the mushy zone during unidirectional solidification. This forced flow reduces the mushy zone thickness, induces the central segregation channel, affects the distribution of the intermetallic precipitates, and influences dendrite coarsening, which in turn modifies the interdendritic flow. Both interdendritic flow and the microstructure formation are strongly coupled. The modelling results support the explanation of Steinbach and Ratke—the formed intermetallic precipitates (β-Al5FeSi) can block the interdendritic flow, and hence influence the coarsening law. The distribution of β-Al5FeSi is dominantly influenced by the flow-induced macrosegregation. The simulation results of the Si and Fe distribution across the sample section are compared with the experimental results, showing good simulation–experiment agreement.Graphic During alloy solidifications the flow can influence the mushy zone by inducing macrosegregation, modifying the solidification microstructure, and influencing the formation of intermetallic precipitates. The resulting microstructural features can in turn affect the melt flow by changing the flow intensity and flow pattern. A three-phase volume-average-based solidification model is introduced to study the flow-solidification interaction, and hence to improve the knowledge on the formation mechanism of intermetallics and their effect on solidification. (a) Schematic for the flow pattern and formation of different phases; (b) experiment–simulation comparison of macrosegregation (Fe) across the diameter of as-solidified sample.

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.

Evolution of texture and precipitates in Al–Cu–Li alloy during deformation and heat treatment and its effect on anisotropy

Al–Li alloy offersa great advantage over traditional alloys and it is the ideal material for today’s aerospace industry. Therefore, here we examined the Al–Cu–Li alloy as the third generation of Al–Li alloy. Deformation and heat treatment on the texture and precipitate type and distribution of Al–Cu–Li alloy were studied by mechanical properties test, XRD, and TEM analysis. The effect of texture and precipitate on the plane anisotropy of yield strength was discussed. The results showed thatBrass texture was mainly formed in the deformation process. On the one hand, increasethe pass deformation or cross rolling,the total volume fraction of the texture (S{123}〈634〉, G{011}〈100〉, {011}〈011〉) was increased, which lowered thesheet anisotropy. While on the other hand, the δ′ phase precipitated rapidly during solution treatment and easily caused a coplanar slip, increasing plane anisotropy. However, owing to the difference distribution of the aged precipitated T1 on the two group of {111} habit plane, the strengthening effect of the Al–Cu–Li alloy sheet was weak at 45° direction, but stronger atthe rolling direction (0°) and transverse direction (90°).

Microstructure and Mechanical Properties of a Medium-Mn Steel with 1.3 GPa-Strength and 40%-Ductility.

Steel designs with superior mechanical properties have been urgently needed in automotive industries to achieve energy conservation, increase safety, and decrease weight. In this study, the aging process is employed to enhance the yield strength (YS) by tailoring the distribution of V-rich precipitates and to improve ductility by producing high volume fractions of recrystallized ferrite in cold-rolled medium-Mn steel. A reliable method to acquire ultra-high strength (1.0–1.5 GPa), together with ductility (>40%), is proposed via utilizing non-recrystallized austenite and recrystallized ferrite. Similarly to conventional medium-Mn steels, the TRIP effect, along with the mild TWIP effect, is responsible for the main deformation mechanisms during tensile testing. However, the coupled influence of precipitation strengthening, grain refinement strengthening, and dislocation strengthening contributes to an increase in YS. The studied steel, aged at 650 °C for 5 h, demonstrates a YS of 1078 MPa, ultimate tensile strength (UTS) of 1438 MPa, and tensile elongation (TE) of 30%. The studied steel aged at 650 °C for 10 h shows a UTS of 1306 MPa and TE of 42%, resulting in the best product in terms of of UTS and TE, at 55 GPa·%. Such a value surpasses that of the previously reported medium-Mn steels containing equal mass fractions of various microalloying elements.

Effect of electron beam energy densities on the surface morphology and tensile property of additively manufactured Al-Mg alloy

Al-Mg alloys samples, after wire arc additive manufacturing based on cold metal transfer, have uneven microstructure, heterogeneous distribution of precipitate and elements, and even nonuniformity mechanical properties. Electron beam surface treatment (EBST) can effectively improve this phenomenon to some degree. In this study, the equipment of SOLO automatic with the electron beam is firstly adopted to develop surface treatment on wire-arc-additive- manufacturing Al-Mg specimens. Three electron beam energy densities (5, 10, 15 J/cm2) were set to investigate the effect of electron beam densities on surface microstructure, phase composition, elements distribution and tensile property of Al-Mg samples. As the irritation energy density increasing, the clarity of grain boundaries was heightened, the element Mg was evaporated furtherly and the thickness of the modification layer increased. Whereas, the surface microstructure morphology is relatively even and extending in the depth of intergranular cracks was prevented when the EBED is 10 J/cm2. Different EBST processes did not affect the phase composition of WAAM-CMT fabricated Al-Mg samples. However, the crystallization and tensile strength of Al-Mg alloy are optimum at the condition EBED = 10 J/cm2.

Microstructure, mechanical properties, cytotoxicity, and bio-corrosion of micro-alloyed Mg–xSn–0.04Mn alloys for biodegradable orthopedic applications: Effect of processing techniques

Magnesium alloys have great potential for load-bearing biodegradable orthopedic implants. The mechanical properties and corrosion resistance depend on alloy composition and processing technique. Presently, the effect of processing techniques on the properties of Mg alloys containing small additions of Sn and 0.04Mn is investigated. Mg–xSn–0.04Mn alloys (x = 0.25 and 1.53wt%) were prepared by casting, heat treatment, hot extrusion, and hot rolling. Microstructure was investigated by optical microscopy, SEM, and XRD. The type, size, and volume fraction of precipitates were found to play the major role in determination of mechanical properties and rate of dissolution in Hank’s solution. A considerable rise in strength, ductility, and reduction in dissolution rate were obtained after thermomechanical treatment based on grain size, volume fraction, and distribution of precipitates. Extruded Mg–1.53Sn–0.04Mn alloy exhibited the best combination of strength, ductility and biodegradability by exhibiting a strength of 219.6 MPa, elongation of 7.9%, and corrosion rate of 0.71 mm/y, indicating that it is indeed a promising candidate for biodegradable orthopedic applications.

Improved mechanical properties by nanosize tungsten-molybdenum carbides in tungsten containing hot work die steels

AISI H13 die steel with substitution of tungsten for vanadium (H13–W) was produced through vacuum induction melting and forging, followed by 1050°C-quenching and 600°C-tempering. Through 10–40 h of 600°C-tempering, H13–W exhibits superior red hardness as compared to H13 steel. The increments of hardness (4-6HRC) as well as yield strength were deducted to the precipitated M6C-type carbides with sizes ranging from 10 nm to 400 nm. Two classifications of carbides are identified. Type I of carbides has the nano size of 8–50 nm, and type II of undissolved primary carbides ((Fe1.34Mo1.62 Cr2.95W0.09)C) has the bigger size of 100–400 nm. The yield strength increments (Δσy) were modeled by a bivariate distribution of nano and undissolved precipitates. The calculated major strengthening effects from type I nano carbides in H13–W/H13 steels were close to the deduced values by Hall-Petch theory, they were 1053 MPa and 965.8 MPa, respectively. According to the calculation on equilibrium solubility of precipitations of MoC/Mo2C and WC/W2C, it was testified that MoC, Mo2C, WC and W2C were wholly dissolved into austenite at 1050 °C of quenching, followed by precipitating in large quantities in the form of nanosize carbides at 600oC-tempering.

On the microstructural origins of improvements in conductivity by heavy deformation and ageing of Al-Mg-Si alloy 6101

This work has examined an Al-0.54Mg-0.38Si (at.%) conductor alloy (6101) subjected to two different thermomechanical processing routes. Conventional solution treatment, quenching, and artificial ageing (170 °C) was compared to a process applying solution treatment, quenching, pre-ageing (7 h at 170 °C), 50% thickness reduction by cold-rolling, and re-ageing at 170 °C. Re-ageing after rolling caused a rapid increase in electrical conductivity. After 2 h re-ageing the rate of change in conductivity had slowed to a level comparable to that of the undeformed material after the same total ageing time. From this point on the deformed material maintained a 2–3%IACS gain in conductivity with further ageing. It is shown that the improvement in conductivity could largely be explained by two precipitate formation mechanisms, leading to increased solute depletion of the Al matrix in the deformed material, which was quantified by atom probe tomography. Clear differences between the deformed and undeformed material were also seen in precipitate distributions as shown by transmission electron microscopy results. The findings and the discussion presented are of importance to future alloy and process development for Al alloy conductor materials.

Tailoring microstructure in sintered Cu-Cr-Nb-Zr alloys for fusion components

High temperature, creep resistant heat sink materials represent a critical need for plasma facing components in future fusion reactors. In this study, we employ direct current sintering (often referred to as spark plasma sintering) to produce a Cu-Cr-Nb-Zr (CCNZ) alloy from gas-atomized feedstock powder with tailored precipitate distributions for enhanced stability and creep resistance. Microstructure was characterized by synchrotron X-ray diffraction, small angle X-ray scattering, and electron microscopy techniques. We report a multi-modal precipitate distribution containing submicron Cr (~493 nm) and Cr2Nb (~90 nm) precipitates at grain boundaries and a high density of nanoscale Cr (~8 nm) precipitates homogeneously distributed through the Cu matrix. By comparing the as-sintered and aged microstructures, precipitation kinetics are discussed in the context of dislocation networks due to the high sintering pressures biasing precipitate formation and the role of subsequent recovery and recrystallization. Due to the presence of the multi-modal precipitate distribution, the sintered CCNZ alloy exhibited a high hardness of 133.2 HV while retaining an appreciable thermal conductivity of 298.4 W/m·K and electrical conductivity of 74.6% relative to the International Annealed Copper Standard.

Pore-scale investigation on reactive flow in porous media considering dissolution and precipitation by LBM

The reactive transport process in oil gas reservoirs is complicated due to the coupling between dissolution and precipitation. In the present study, based on lattice Boltzmann method, the coupled dissolution and precipitation process in randomly generated porous media by quartet structure generation is investigated at the scale of geological pores. The fluid velocity, temperature and concentration in porous media are solved by multicomponent flow lattice Boltzmann model, multicomponent thermal lattice Boltzmann model and mass transport lattice Boltzmann model, respectively. The solid phase is updated by volume of pixel method. The results show that the dissolution reaction rate has a positive impact on the consumption of reactants. For different precipitation reaction rates, there exists an optimal value resulting in abundant precipitate and sparse solid distribution. Reaction equilibrium constant affects reaction rates slightly but influence the solid morphology heavily. The coupled dissolution and precipitation process accelerates with the increase of Peclet number, but the precipitate produced increases with the decrease of Peclet number.

Low-carbon advanced nanostructured steels: Microstructure, mechanical properties, and applications

Low-carbon advanced nanostructured steels have been developed for various structural engineering applications, including bridges, automobiles, and other strength-critical applications such as the reactor pressure vessels in nuclear power stations. The mechanical performances and applications of these steels are strongly dependent on their microstructural features. By controlling the size, number density, distribution, and types of precipitates, it is possible to produce nanostructured steels with a tensile strength reaching as high as 2 GPa while keeping a decent tensile elongation above 10% and a reduction of area as high as 40%. Besides, through a careful control of strength contributions from multiple strengthening mechanisms, the nanostructured steels with superior strengths and low-temperature impact toughness can be obtained by avoiding the temper embrittlement regime. With appropriate Mn additions, these nanostructured steels can achieve a triple enhancement in ductility (total tensile elongation, TE of ~30%) at no expense of strengths (yield strength, YS of ~1100 to 1300 MPa, ultimate tensile strength, UTS of ~1300 to 1400 MPa). More importantly, these steels demonstrate good fabricability and weldability. In this paper, the microstructure-property relationships of these advanced nanostructured steels are comprehensively reviewed. In addition, the current limitations and future development of these nanostructured steels are carefully discussed and outlined.

Improving creep resistance of Al-12 wt.% Ce alloy by microalloying with Sc

The near-eutectic Al-12 wt.% Ce alloy has been alloyed with 0.4 wt.% Sc to improve its creep resistance. A bidirectional influence between Sc and Ce has been identified: (i) the Ce element suppresses the Sc partition between the liquid and solid phases during solidification, as implied by the reduction of Sc microsegregation in the as-cast ingot; (ii) the Sc atoms migrate towards the Al11Ce3 interface to form Al3Sc thin layer or particles upon aging. The former effect promotes the spatial uniformity of Al3Sc precipitate distribution in the Al matrix and the latter one introduces additional lattice misfit strain to the Al11Ce3/Alα phase boundary inhibiting matrix dislocations to climb over the Al11Ce3 lamellae. Both effects can improve the creep resistance of the aged Al-12Ce-0.4Sc alloy showing a high tensile stress threshold for dislocation creep of ∼ 60 MPa at 300 ∘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.

Investigation on the Stress Relaxation Aging Behavior of 2195 Al-Li Alloy

2195 Al-Li alloy is famous for high strength, excellent fatigue strength and good stress corrosion resistance, which is widely used in the manufacture of high-performance aerospace components. The aim of this study is to validate how the stress relaxation aging behavior effect on the mechanical properties of 2195 Al-Li alloy. Through mechanical property test, the research was found that the performance after stress relaxation aging is higher than artificial aging (AA). In addition, the analysis of scanning electron microscopy SEM and TEM revealed that dislocations should be introduced by the stress relaxation aging process, which is more conducive to the precipitation of the T1 phase and strengthened the material with prolong ageing time. The results show that stress relaxation aging can significantly promote the precipitation of the T1. Therefore, this paper sheds new light on how SRA can improve mechanical properties and that SRA make better improve the distribution of precipitates in the grain boundary.

Understanding the interaction of extension twinning and basal-plate precipitates in Mg-9Al using precession electron diffraction

Precession electron diffraction is used to characterize the interaction between {101¯2} tensile twins and basal plate-like precipitates in a post-deformed, precipitate-dispersed Mg-9Al micropillar. We observed a heterogeneous distribution of precipitates in the micropillar sample, which enabled the study of the different stages involved in twin-precipitate interactions. We show that twin nucleation was promoted, taking place on the surface, as well as from the interior of the micropillar. Twin tip propagation and twin growth were hindered by the precipitates. Twin tips either were arrested by precipitates and new twins formed on the other side of the precipitate, or continued to grow around the precipitate without the re-nucleation events. During twin thickening, the precipitates did not significantly alter the shear-dominant twin boundary migration mechanism, as evidenced by the relatively flat twin boundaries around the partially embedded precipitates. However, the twin boundary migration was retarded by the precipitates, especially in regions confined by closely-spaced precipitates.