Types Of Precipitates Research Articles

Time-resolved X-ray absorption spectroscopy on Al–Cu alloys – from solute copper to stable precipitates

Although binary aluminium alloys seem to be uninteresting and well known, some aspects of their precipitation sequence – especially the early stages immediately after quenching – are still not well understood. Since the Al–Cu system is the basis for many ternary and quaternary high-strength alloys with application in the aviation sector, it is important to understand this binary system in detail. This problem is here tackled by a unique combination of differential scanning calorimetry and X-ray absorption fine structure measurements, where relaxed atomic coordinates for simulation of the spectra have been obtained by ab initio calculations. Thereby, it is possible to attribute any exo- or endothermal peak to a certain type of precipitate, even though formation and dissolution regions have a large overlap in this system. This unique combination of experimental and numerical methods allows one to determine the local atomic environment around Cu atoms, thus following the formation and growth of Guinier–Preston zones, i.e. Cu platelets on {100} planes, during the precipitation process.

Mass Transfer Phenomena during Electrodialysis of Multivalent Ions: Chemical Equilibria and Overlimiting Currents

Electrodialysis is utilized for the deionization of saline streams, usually formed by strong electrolytes. Recently, interest in new applications involving the transport of weak electrolytes through ion-exchange membranes has increased. Clear examples of such applications are the recovery of valuable metal ions from industrial effluents, such as electronic wastes or mining industries. Weak electrolytes give rise to a variety of ions with different valence, charge sign and transport properties. Moreover, development of concentration polarization under the application of an electric field promotes changes in the chemical equilibrium, thus making more complex understanding of mass transfer phenomena in such systems. This investigation presents a set of experiments conducted with salts of multivalent metals with the aim to provide better understanding on the involved mass transfer phenomena. Chronopotentiometric experiments and current-voltage characteristics confirm that shifts in chemical equilibria can take place simultaneous to the activation of overlimiting mass transfer mechanisms, that is, electroconvection and water dissociation. Electroconvection has been proven to affect the type of precipitates formed at the membrane surface thus suppressing the simultaneous dissociation of water. For some electrolytes, shifts in the chemical equilibria forced by an imposed electric field generate new charge carriers at specific current regimes, thus reducing the system resistance.

Experimental and numerical simulation analysis on creep crack growth behavior of CLAM steel

The experiment and numerical simulation were performed to analyze the creep crack growth behavior of China Low Activation Martensitic (CLAM) steel under temperatures ranging from 500 °C to 600 °C with a step of 50 °C. The creep crack growth micro-mechanism under multiaxial state of stress was investigated with the compact tension (CT) specimens. The creep crack growth rates were simulated based on the Nikbin-Smith-Webster (NSW) model. The difference of the crack propagation behavior is correlated with the evolution of two types of precipitates: Cr-rich M23C6 and W-rich M6C phases. The coarsening of M23C6 carbides and the precipitation of M6C phase particles lead to dissolving of the solid solution elements and deteriorating the creep properties during the testing processes.

Aging Response and Precipitation Behavior after 5% Pre-Deformation of an Al-Mg-Si-Cu Alloy.

In this study, Al-1.00 Mg-0.65 Si-0.24 Cu alloy was solution heat-treated, water-quenched, and then pre-deformed for 5% before aging. The peak hardness and yield strength of the pre-deformed sample with subsequent artificial aging were similar to that of a T6 condition sample. It was also found that the pre-deformation treatment could inhibit the negative influence of natural aging to some degree. After seven days of natural aging, the pre-deformed sample obtained better peak hardness and yield strength upon artificial aging than the sample without pre-deformation. In addition, the pre-deformation treatment could reduce 50% of the artificial aging time to reach the peak aging condition compared with T6 treatment. For the peak aged condition in the pre-deformed sample, transmission electron microscopy (TEM) observation found two types of precipitates exhibited along the dislocations besides the β″ precipitates in the Al matrix. Both precipitates had disordered atomic arrangements on the ordered subcell (Si network). The disordered precipitates occupied a number of Mg and Si atoms, resulting in less β″ precipitates formed during artificial aging at 180 °C.

Development and Characterization of a Novel Ti-Modified High-Si Medium-Mn Steel Possessing Ultra-High Strength and Reasonable Ductility After Hot Rolling

The present work systematically correlates the strength–ductility relationship of a Ti-Si-modified Fe-Mn-Al-C steel with its microstructural characteristics such as grain size, type and morphology of precipitates, and the transformation-induced plasticity (TRIP) effect during hot deformation. The Ti-Si-modified multicomponent steel developed through melting and casting route is subjected to thermo-mechanical processing such as hot forging (at 1373 K) and subsequent hot rolling (at 1173 K). An excellent combination of ultra-high tensile strength (UTS ~ 1700 ± 20 MPa), reasonable ductility (elongation ~ 11%) and high work hardening behavior (n ~ 0.89) is achieved in the hot-rolled specimen as compared to the hot-forged one (UTS ~ 824 ± 9 MPa, n ~ 0.07) with negligible change in the elongation. The better tensile properties of the hot-rolled specimen in contrast to the hot-forged one are due to the combined effects of grain refinement during rolling, twin–twin interactions, precipitation strengthening by mixture of hexagonal structured Ti3(Al,Si)C2 and Mn-Al-Si-rich carbide precipitates and most significantly the enhanced TRIP effect. The lower critical resolved shear stress value of martensitic transformation instigates the TRIP effect during tensile testing which resulted in the increased hardness and strength of the hot-rolled specimen. The above observation offers a strong support to the proposition that TRIP effect is the dominant plasticity-enhancing mechanism activated during the deformation of the low-stacking fault energy (~ 12.2 mJ/m2) Si-Ti-modified, medium-Mn multicomponent steel employed in the present investigation.

Effect of misorientation on microstructures and properties of a bicrystal alloy

Effect of mathematical misorientation on microstructures and properties of bicrystal superalloy RR2086 have been investigated. Prior to thermal exposure, precipitates of modified RR2086 bicrystals (BXs) at grain boundaries were mainly MCs. After the exposure, a reasonable amount of discontinuous and blocky M23C6 precipitates which were embedded in a γ′ layer along the grain boundary were found at grain boundaries of the modified RR2086 BXs. This type of precipitates is known to be ductile and creep-resistant. No cellular M23C6 appeared during exposure. This structure results in improved creep performance of bicrystals of high-angle boundaries in the modified BXs compared to their base counterparts. The modified RR2086 BXs exhibited higher grain boundary creep resistance and higher grain boundary ductility of creep fracture. This is attributed to the grain boundary strengthening of the MC and M23C6 precipitates, the mechanical keying effect due to their coherence with one neighbouring grain, which effectively suppresses grain boundary sliding. The γ′ layer also played an important role in improving creep resistance of the grain boundaries. The formation of a γ′ layer along grain boundaries was initiated by the conversion of M23C6 in modified BXs.

Quantitative study of the effect of stress on the precipitation in an Al-Cu-Mg-Ag alloy single crystal

Al-1.96Cu-0.24Mg-0.18Ag alloy single crystal with (2¯15) plane orientation has been used to investigate the effect of external stresses (50, 100, 134, 160 and 200 MPa) on the precipitation behavior of Ω, θ′ and S precipitates. The microstructure of the aged specimens was observed by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and scanning transmission electron microscopy (STEM). X-ray elemental mapping under high-angle annular dark-field (HAADF) was used to analyze the elements in the precipitates. TEM images showed that the amount and distribution of the three types of precipitates are different in specimens aged under different external stresses. Along with the SAED and STEM images, the type, amount and distribution of Ω, θ′ and S precipitates were quantitatively measured. Compared to the results on stress-free aged specimen, the amount of Ω precipitates decreases in stress aged samples, while that of the S precipitates increases, and the amount of θ′ precipitates is constant with low value. The orienting effect of stress on Ω precipitates gets serious at 100 MPa, but it decreases at 200 MPa. The orienting effect of stress on S precipitates is quite similar to that in the case of the Ω phase. As the competition of acquiring Mg and Cu atoms between S and Ω precipitates, the orienting effect of stress on S precipitates is serious from 100 to 160 MPa. Both the orienting effect of stress on Ω and S precipitates decreased because of the inhibiting effect of dislocations at 200 MPa. The orienting effect on θ′ variants is different from that of Ω precipitates. It is serious at 50 MPa and gets weaker at 100–200 MPa.

Microstructural evolution and precipitation strengthening in a new 20Cr ferritic trial steel

Microstructural evolution and precipitation strengthening of the newly-developed 20Cr steel were investigated in present work. Three types of precipitates, including G-phase (Ni16Ti6Si7, intermetallic silicide), Laves phase (Fe2Ti, intermetallic compound) and carbide (TiC), were observed and their crystal structures were resolved with combined electrochemical phase extraction, X-ray diffraction and transmission electron microscopy analysis. G-phase particles were observed in the grains and Laves phase occurred at grain boundaries while carbides were present at both of the two sites. A temperature-dependency sketch map for these precipitates was also plotted to illustrate the characteristics of G-particles below 750 °C, Laves phase at 850 °C~1050 °C and carbide below 1150 °C. In particular, G-phase precipitated and had the cubic-cubic orientation with the ferritic matrix, thus showing a strong aging hardening characteristic. Nanodispersion of these G-particles greatly enhanced the yield strength, which was estimated to be up to ~1700 MPa. Finally, theoretical calculations for phase equilibria, critical radius and precipitation strengthening helped to understand the precipitation thermodynamics and strengthening mechanisms for developing high-performance steel by making use of G-phase.

Effect of trace amounts of added Sc on microstructure and mechanical properties of 2055 aluminum alloy

The effects of Sc additions and isothermal aging on the microstructure and mechanical properties of 2055 aluminum alloy have been investigated. The isothermal aging temperatures investigated were 175 °C and 200 °C. Results show that the aging temperature does not affect the type of precipitates in both Sc-free and Sc-added 2055 Al alloys. While remarkable changes in the size and number density of T1 (Al2CuLi) precipitates have been observed. For samples aged at 175 °C for 8 h, the average length of T1 precipitates was increased from 39.9 nm to 45.6 nm, the number density decreased from 5.77 × 1022 m−3 to 3.68 × 1022 m−3 after Sc addition. On the contrary, when samples were aged at 200 °C for 8 h, the length decreased from 59.89 nm to 43.32 nm and number density increased from 2.13 × 1022 m−3 to 3.00 × 1022 m−3 by adding trace Sc. Slight deterioration of tensile strength and yield strength with higher ductility was founded at the aging temperature of 175 °C for 8 h due to the larger size and lower number density of T1 precipitates along with the formation of a W (AlCuSc) phase during the casting or homogenization processes. When aging temperature increased to 200 °C, tensile strength and yield strength were significantly improved by Sc addition without losing ductility. Highly dense fine T1 precipitates take responsible for the strength improvement, and to some extent it compensated the strength loss caused by the formation of W phase. A “vacancy-prison” mechanism was adopted to explain the effect of Sc additions on the precipitation of T1 phase as a function of aging temperature.

Tensile properties of mechanically alloyed Zr added austenitic stainless steel

A mechanically alloyed austenitic stainless steel (MA304LZ) was produced from pre-alloyed SUS304L powder with a small amount of Zr addition. The yield stress of MA304LZ was more than 3 times larger than that of SUS304L or 316L, while total elongation was reduced to about one third of the conventional steels. Microstructure analysis revealed an average grain size of 0.42 µm in MA304LZ and about 34/30 µm in SUS304L/316 L. In MA304LZ, two types of precipitates were observed; inhomogeneously distributed fine precipitates with an average size of 6.0 nm and homogeneously distributed coarse precipitates (d > 20 nm) with an average size of 47 nm. The strengthening mechanism of MA304LZ was discussed on the bases of Hall-Petch and Orowan equations, and the strengthening of MA304LZ was attributed mostly to refined grains. The dislocation barrier strength factor, α, is estimated to be 0.277 for the Zr-rich precipitates in MA304LZ.

Precipitation hardening effects on extension twinning in magnesium alloys

Precipitation is an efficient method to strengthen metallic materials. While precipitation hardening effects on dislocation slip have been studied extensively in the past, the influence of precipitates on twinning mediated plasticity and the development of corresponding hardening models that account for twin-precipitate interactions have received less attention. Here, the interaction of {10–12} extension twin boundaries (TBs) in pure magnesium with precipitates of plate-, sphere- and rod-like shapes is studied using molecular dynamics (MD) simulations. We find that TBs that engulf precipitates are absorbed by the precipitate-matrix interfaces, and the precipitates are neither twinned nor sheared but deform elastically leading to their rotation. TBs can pass small precipitates (length ≈ 20 nm) and remain intact. In contrast when TBs are interacting with large precipitates (length ≈ 50 nm), basal dislocations or stacking faults nucleate from the interfaces, causing local plastic relaxation. The stress field around a plate-like precipitate as calculated in the MD simulations suggests that a strong back-stress is imposed on the TBs. We then coarse grain these mechanisms into an analytical mean field model of precipitation hardening on twinning in magnesium alloys, which is based on the energy conservation during the TB-precipitate interaction. The model is in good agreement with the current MD simulations and published experimental observations. The hardening model shows that spherical precipitates have the strongest hardening effect on twinning, basal and prismatic plate-like precipitates have a medium effect while rod-like precipitates exert the weakest influence. We also find that most types of precipitates show a stronger hardening effect on twinning mediated plasticity than on basal dislocation slip. Finally, prismatic plate-like precipitates are predicted to have reasonable hardening effects on both twinning and basal slip. These results can help guiding the development of magnesium alloys with enhanced strength and ductility.

Microstructure and properties of V–5Cr–5Ti alloy after hot forging

In this work, the microstructure of V–5Cr–5Ti alloy after hot forging was analyzed by X–ray diffraction, optical microscopy, scanning electron microscopy, and transmission electron microscopy. After hot forging, the microstructure of V–5Cr–5Ti alloy was broken. An axial grain was formed through recrystallization, and the average grain size was approximately 80 μm. With increasing annealing temperature, the grain size increased from 80 μm to 150 μm. The precipitates elongated to form band structures. During annealing, the band structure changed into several small grains enriched in the precipitates. Some short strip–like precipitates were distributed near the grain boundary irregularly. The two types of precipitates in the V–5Cr–5Ti alloy after hot forging were as follows: a) short–strip precipitate (VTi)2(CON) with FCC structure and lattice parameter of 0.4220–0.4259 nm and b) elliptical–shaped precipitate (VTi)2(CON) with FCC structure and lattice parameter of 0.4252–0.4347 nm. The two precipitate types during annealing were as follows. The first type was a rectangular precipitate (VTi)2(CO) with FCC structure and lattice parameter of 0.4186–0.4270 nm. The second type was an elliptical–shaped precipitate (VTi)5(CO) with FCC structure and lattice parameter of 0.4246–0.4344 nm.

Thermodynamic analysis of 6xxx series Al alloys: Phase fraction diagrams

Microstructural evolution of 6xxx Al alloys during various metallurgical processes was analyzed using accurate thermodynamic database. Phase fractions of all the possible precipitate phases which can form in the as-cast and equilibrium states of the Al-Mg-Si-Cu-Fe-Mn-Cr alloys were calculated over the technically useful composition range. The influence of minor elements such as Cu, Fe, Mn, and Cr on the amount of each type of precipitate in the as-cast and equilibrium conditions were analyzed. Phase fraction diagrams at 500 ?C were mapped in the composition range of 0-1.1 wt.% Mg and 0-0.7 wt.% Si to investigate the as-homogenized microstructure. In addition, phase fraction diagram of Mg2Si at 177 ?C was mapped to understand the microstructure after final annealing of 6xxx Al alloy. Based on the calculated diagrams, the design strategy of 6xxx Al alloy to produce highest strength due to Mg2Si is discussed.

Identification and Visualization of Fine Precipitates in a Cr-Mo Steel Using SEM Contrasts

A precise characterization of fine precipitates is crucial because these precipitates control mechanical properties of steel materials. Different types of carbide precipitates appear in heat resistant Cr-Mo steels. Their identification requires characteristic X-ray and diffraction analysis because it is usually not possible to differentiate carbides from their size and morphology alone. Latest scanning electron microscopes that are equipped with multiple image detectors can give rich information on microstructures. Thus, in this paper, we study the conditions where secondary electron (SE) and/or backscattered electron image contrast will clarify the types of precipitates in a common 2.25Cr-1Mo steel. A mechanically polished bulk specimen was observed by systematically changing the SEM the acceleration voltages and working distances. We have found that low accelerating voltages with a short working distance enables differentiation of four types of carbides and AlN precipitate in this steel. A small penetration depth of primary electrons and selective acquisition of SE/BSE is the key to extracting material contrast. This technique will allow quantification of size, distribution and area fraction of each precipitate for a large area using a bulk specimen.

Microstructural and mechanical properties assessment of the P91 A-TIG weld joints

In the current work, the effects of CeO2 and MoO3 based activating flux on heat input, weld bead geometry, and angular distortion during single pass A-TIG welding of the P91 steel were evaluated and compared with the conventional TIG welding. The A-TIG weldments developed using CeO2 and MoO3 based activating flux were also undergone for microstructure, microhardness, tensile test, and Charpy impact test study. It was observed that, the heat input increases in presence of activating fluxes at same welding input variables namely welding current, welding speed and arc length. An increase of 200% and 300% in joint penetration was obtained with CeO2 based flux and MoO3 based flux, respectively as compared to the conventional TIG weldments. Microstructure study reveals the presence of lath martensite, prior austenite grain boundaries along with MX (M: Nb, V, and X: C, N) and M23C6 (M: Fe, Mo, Mn, and Cr) type of precipitates in different zones of the weldments. Tensile strength of the A-TIG weldments was found better than that of the base metal. However, the Impact toughness of the A-TIG weldments was found to very low than the base metal.

EBSD characterization of boundary types after discontinuous precipitation in a grit-blasted nickel-based superalloy

In the present work, direct and detailed insights with respect to the statistical information on boundary type of discontinuous precipitates and their advancing reaction fronts in a Re-containing single crystal superalloy DD6 have been obtained by EBSD. With the annealing time prolonging, the reaction fronts proceeded into the original matrix for gradual release of grit-blast induced stress. The average recrystallized grain diameter and the average depth of recrystallization were respective 0.83μm and 3.98μm for the specimen annealed at 1050°C for 10min, which, after 16h, had grown to the average values of 3.22μm and 31.17μm. The reaction fronts of discontinuous precipitation have been confirmed to be high-angle grain boundaries, but do not always correspond to CSL type. Besides the role of strain energy dissipation for recrystallization, the formation of twin structure has no direct relationship with the proceeding of reaction fronts. The information of boundary type of discontinuous precipitation and its advancing reaction fronts would provide new insight into the cellular recrystallization behavior.

Co-existences of the two types of β′ precipitations in peak-aged Mg-Gd binary alloy

In this work, an atomic-scale investigation on the phase structure in the Mg-Gd alloy during isothermal ageing at 230 °C was conducted by means of Cs-corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). Two types of precipitates including the long-structure β′ (denoted as βL′) and short-structure β′ (denoted as βS′) are found to co-exist in the peak-aged Mg-11.5 wt% Gd alloy, which is largely different from the previous reports that only βS′ or βL′ precipitate in the peak-aged Mg-Nd, Mg-Y and Mg-Gd binary alloys. According to the micrographs observed in the three directions of [0001]α, [112¯0]α and [101¯0]α, both the βL′ and βS′ precipitates are confirmed to exhibit an ordered orthorhombic structure and have a same composition of Mg7Gd. The βL′ phase exhibits the truncated lenticular shape, while the βS′ phase are found to exist in the bridging structure between the βL′ main bodies. Moreover, four different types of βL′/βS′ interface structures can also be observed.

Characterization of precipitates in an Al-Zn-Mg alloy processed by ECAP and subsequent annealing

Experiments were conducted to examine the influence of equal-channel angular pressing (ECAP) and post-ECAP annealing on the microstructures of an Al-Zn-Mg alloy. The results show that precipitates, mainly of the η′, η (MgZn2), T (Al20Cu2Mn3) and E (Al18Mg3Cr2) phases, are fragmented to fine spherical precipitates during ECAP processing for 4 and 8 passes. After post-ECAP annealing at 393 and 473K for 20h, precipitates with larger sizes lie primarily along the grain boundaries and finer particles are evenly distributed within the grains. Increasing the numbers of ECAP passes from 4 to 8 leads to an increase in the volume fraction of the finer precipitates in the ECAP-processed and annealed alloy. After 4 passes and heat treatment at 473K, the precipitates are slightly larger compared with the alloy processed under identical conditions and annealed at 393K. Nevertheless, significant coarsening is evident after processing for 8 passes and increasing the annealing temperature from 393 to 473K. Different types of precipitates are effective in impeding grain growth during the post-ECAP annealing even at 473K for 20h. In addition, η precipitates form within the T and E phases after both ECAP and post-ECAP annealing.

Effects of carbon and nitrogen on precipitation and tensile behavior in 15Cr-15Mn-4Ni austenitic stainless steels

Precipitation behavior and tensile properties have been investigated for 15Cr-15Mn-4Ni austenitic steels containing 0.2wt% carbon and/or 0.2wt% nitrogen. During aging at 600–1000°C, precipitates such as σ, M23C6, and Cr2N were formed and time-temperature-precipitation diagrams were constructed. The type, size, and density of precipitates depended on the carbon and nitrogen contents. The nitrogen addition suppressed carbide formation while the effect of carbon addition on nitride precipitation was comparatively small. The evolution of tensile properties after aging is explained by the changes in solid solution strengthening and deformation modes caused by the precipitation. The precipitation of M23C6 slightly decreased yield strength and elongation while it increased ultimate tensile strength. On the other hand, the precipitation of Cr2N hardly affected tensile properties due to its low volume fraction.

Creep properties, creep deformation behavior, and microstructural evolution of 9Cr-3W-3Co-1CuVNbB martensite ferritic steel

Creep deformation behavior and microstructure evolution of G115 steel were systematically investigated for temperatures of 625–675°C under uniaxial tensile stress of 120–220MPa. The relationship between minimum creep rate and applied stress followed the Bird–Mukherjee–Dorn (BMD) equation. The modified BMD equation was proposed using threshold stress to elucidate the actual creep deformation mechanism. The values of the threshold stress were determined to be 177.8, 91.4 and 87.6MPa at 625, 650, and 675°C, respectively. The true creep activation energy and the true stress exponent were 275.76kJ/mol and 6, respectively. Thus, the dominant creep deformation mechanism was identified as dislocation climb. Three types of precipitates can be revealed after creep deformation: W-rich Laves, Nb-rich MX, and Cu-rich phases. The creep damage of G115 steel after creep deformation was characterized by martensite cracks and martensite fractures owing to the hardness and brittleness of the lath martensite structure. Further, a dense array of deep and equiaxed dimples appeared in the central region of fracture surfaces under the tested creep conditions. Ductile fracturing was the main fracture mechanism during creep deformation.