Grain Boundary Precipitation Research Articles

In situ thermomechanical processing to avoid grain boundary precipitation and strength-ductility loss of age hardening alloys

To avoid grain boundary (GB) precipitation during aging, a new strategy of in situ thermomechanical processing for age hardening alloys was proposed. Specifically, high-density nanoscale precipitates were introduced into ultrafine grain (UFG) interiors of 7075Al alloy by equal-channel-angular (ECAP) processing at 250 °C for 8 passes, thus avoiding GB precipitation. Tensile test results indicated that the UFG 7075Al alloy exhibits superior mechanical properties (yield strength of 350 MPa, ultimate tensile strength of 500 MPa, uniform elongation of 18% and tensile ductility of 19%) compared with the UFG 1050Al counterpart (yield strength of 170 MPa, ultimate tensile strength of 180 MPa, uniform elongation of 2.5% and tensile ductility of 7%). Fracture surface morphology studies revealed numerous homogeneous micro shear bands in necking shrinkage areas of both UFG 7075Al and 1050Al alloys, which are controlled by cooperative GB sliding. Moreover, the introduction of nanoscale precipitates in UFG 7075Al matrix weakened the tendency of shear fracture, resulting in a higher tensile ductility and more homogeneous deformation. Different from the GB precipitation during postmortem aging, in situ thermomechanical treatment dynamically formed GBs after precipitation, thus avoiding precipitation on GBs.

Effect of Aging Time on Crushing Performance of Al-0.5Mg-0.4Si Alloy for Safety Components of Automobile

The effect of aging time on the crushing performance of Al-0.5Mg-0.4Si alloy used for safety components of automobile was investigated by tensile test and crush test. Moreover, the microstructure of the alloy was investigated by transmission electron microscopy (TEM). The results show that the localized deformation ductility index, ΔAabs, which is defined as the difference between total elongation and uniform elongation, of Al-0.5Mg-0.4Si alloy is 6.5%, 7.0% and 8.5%, respectively, after being aged at 210 °C for 1, 3 and 6 h, and this tendency is the same as that of the crushing performance. The spacing of grain boundary precipitates (GBPs) from TEM results are found to be 94.9, 193.6 and 408.2 nm after being aged at 210 °C for 1, 3 and 6 h, respectively, and this tendency is same to that of ΔAabs. A mechanism about the relation between the spacing of GBPs and the ductility index ΔAabs has been proposed based on localized deformation around GBPs. With the increase of GBPs spacing, the ΔAabs increases, and the crushing performance is improved.

Synergic grain boundary segregation and precipitation in W- and W-Mo-containing high-entropy borides

The structures of W- and W-Mo-containing high-entropy borides (HEBs) are systematically studied by combining atomic-resolution transmission electron microscopy imaging, electron diffraction, and chemical analysis. We reveal that W or W-Mo addition in HEBs leads to segregation of these elements to the grain boundaries (GBs). In the meantime, W- or W-Mo-rich precipitates also form along the GBs. Crystallographic analysis and atomic-scale imaging show that the GB precipitates in both W- and W-Mo-containing HEBs have a cube-on-cube orientation relationship with the matrix. With further strain analysis, the coherency of the precipitate/matrix interface is validated. Nanoindentation tests show that the simultaneous GB segregation and coherent precipitation, as a supplement to the grain hardening, provide additional hardening of the HEBs. Our work provides an in-depth understanding of the GB segregation and precipitation behaviors of HEBs. It suggests that GB engineering could be potentially used for optimizing the performance of high-entropy ceramics.

The role of one-dimensional magnetic nanoprecipitates on the magnetoresistance of soft magnetic alloys: Study of a melt-spun Cu–Co alloy

This work has investigated the microstructure and magnetic properties of supersaturated melt-spun Cu–10%Co alloys by means of magnetization, magnetotransport studies, electron microscopy and electron holography. Our results demonstrate that the microstructure of the annealed supersaturated alloys is constituted by two different types of Co-rich phases: spherical Co-rich precipitates as a result of spinodal decomposition, and Co rod-like precipitates, as a result of discontinuous grain-boundary precipitation. The aspect ratio of such precipitates is quite high, exhibiting magnetic anisotropy along their long axes, which generates global anisotropy of the alloy sample. We verified that the highest giant magnetoresistance occurs concurrently with an optimum discontinuous precipitate development. Electron holography reveals an antiferromagnetic coupling of the Co rods in the remanent magnetization state. The magnetization and magnetoresistance curves as a function of applied magnetic field are well described by an independent-moment approach considering the sum of the average magnetic moments of the spherical and the rod-like Co precipitates.

Effects of critical defects on stress corrosion cracking of Al–Zn–Mg–Cu–Zr alloy

The effects of critical defects on stress corrosion cracking (SCC) of Al–7.82Zn–1.99Mg–2.41Cu–0.12Zr alloy under under-ageing (UA), peak-ageing (PA) double-ageing (DA) and retrogression and reageing (RRA) were investigated. It is found that anodic dissolution of grain boundary precipitates (GBPs) for UA sample generates critical defects due to low Cu content of GBPs, whereas the dissolution of Al, Mg and Zn elements within the grain interior is responsible for the formation of critical defects for PA, DA and RRA samples. The high distribution continuity of GBPs promotes the continuous propagation of stress corrosion crack along grain boundary, leading to intergranular SCC for UA sample. Coarsened GBPs which are distributed discontinuously could hinder anodic dissolution along the grain boundary. As a result, the cracks propagate along the grain interior, causing transgranular SCC for DA sample. And large size and low density of matrix precipitates (MPTs) decease SCC susceptibility. Al7Cu2Fe particles promote the dissolution of the surrounding matrix, inducing local stress concentration, and thus leading to the generation of stress corrosion cracks.

Enhanced mechanical performance of grain boundary precipitation-hardened high-entropy alloys via a phase transformation at grain boundaries

Grain-boundary (GB) precipitation has a significant adverse effect on plasticity of alloys, which easily leads to catastrophic intergranular failure in safety-critical applications under high external loading. Herein, we report a novel strategy that uses the local stress concentration induced by GB precipitates as a driving force to trigger phase transformation of preset non-equiatomic high-entropy solid-solution phase at GBs. This in situ deformation-induced phase transformation at GBs introduces a well-known effect: transformation-induced plasticity (TRIP), which enables an exceptional elongation to fracture (above 38 %) at a high strength (above 1.5 GPa) in a GB precipitation-hardened high-entropy alloy (HEA). The present strategy in terms of “local stress concentration-induced phase transformations at GBs” may provide a fundamental approach by taking advantage of (rather than avoiding) the GB precipitation to gain a superior combination of high strength and high ductility in HEAs.

Dynamic Precipitation of Laves Phase and Grain Boundary Features in Warm Deformed FeCrAl Alloy: Effect of Zr

Warm compression deformation of Fe–13.5%Cr–4.7%Al–2.0%Mo–0.70%Nb–0.40Ta (wt%) (FeCrAl) and Fe–13.5%Cr–4.7%Al–2.0%Mo–0.45%Nb–0.40Ta–0.11Zr (wt%) (FeCrAl–Zr) ferritic stainless steel was performed by a thermal simulation machine Gleeble 3800 at 600 °C and strain rates of 0.01–10 s−1. Before deformation, all the samples were solution-annealed for 2 h at 1150 °C for FeCrAl alloy and 1250 °C for FeCrAl–Zr alloy. The strain rate has little or no effect on peak stress, and the precipitates in matrix or grain boundary precipitates (GBPs) have no difference in the samples deformed at the strain rate 0.01 s−1 and 1 s−1 both in FeCrAl and FeCrAl–Zr alloys. The addition of Zr increased the proportion of low-angle grain boundaries (LAGBs). The Laves phase in FeCrAl alloy precipitated uniform in the matrix, while in FeCrAl–Zr alloy Laves phase precipitated at grain boundary and formed GBP. The LAGBs and Σ3 coincident site lattice (CSL) grain boundary both increased in FeCrAl–Zr alloy, which possessed some beneficial properties such as high-temperature creep resistance to the Fe–Cr–Al alloy. More interesting, twins were created by warm deformation, which was difficult in typical bcc ferrite alloy. These results could be expected to provide guidance for subsequent warm working processes for the alloy.

Detecting Intergranular Phases in UNS N07725. Part II: Double-Loop Electrochemical Potentiokinetic Reactivation Test Validation

The formation of interconnected grain boundary (GB) phases in precipitation hardening (PH) nickel Alloy 725 (UNS N07725) compromises its mechanical properties and has been linked to hydrogen embrittlement failures. However, normative industry standards such as API 6ACRA are inadequate for differentiating between Alloy 725 batches with a critical extent of GB precipitation, creating the need for a complementary qualification method.In Alloy 725, intergranular phases produce a Cr and Mo depletion adjacent to GBs (i.e., sensitization). In Part I of this work, this impoverishment in Cr and Mo was used as the basis to develop a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test, optimized to quantify the extent of GB precipitation in Alloy 725. Part II explores the reproducibility and sensitivity of the new approach using three new independent commercial batches containing different extents of precipitation. Results were highly reproducible and confirmed the method’s sensitivity to detect small amounts of GB phases. Additionally, the DL-EPR procedure revealed clear microstructural inhomogeneities across the bars’ thickness, which increased with increasing bar diameter. This investigation strongly supports the validity of the new DL-EPR technique as a reliable and straightforward industry-friendly qualification method to quantify the extent of intergranular phases in Alloy 725.

Thermomechanical pretreatment of Al-Zn-Mg-Cu alloy to improve formability and performance during creep-age forming

Abstract Thermomechanical treatment is often conducted to improve the formability and performance of high-strength aluminum alloys simultaneously. This paper presents an improved creep-age forming (CAF) processing technology that combines thermomechanical pretreatment and CAF. The proposed processing technique is expected to enhance the forming efficiency and comprehensive properties of Al–Zn–Mg–Cu alloys subjected to CAF. Creep-aging behavior and properties of the 7150 aluminum alloy undergoing retrogression and re-aging (RRA) with 3 % pre-strain treatment was experimentally investigated in this study. Results obtained reveal greater creep deformation in thermomechanically pretreated samples than in those pretreated using RRA or 3 % pre-strain exclusively. Mechanical properties, electrical conductivity, and exfoliation corrosion of creep-aged samples with thermomechanical pretreatment were closely match those of the traditional RRA-treated sample. Microstructure observation discovered η2 phase in the creep-aged RRA pretreated sample in addition to conventional GP / η’ phase, η phase and coarse and discrete grain-boundary precipitates. Low concentration of solutes in matrix in RRA pretreated sample impeded dislocation motion less and pre-strain-induced dislocations increased the creep strain significantly. Moreover, hardening effect of η2 phase and dislocation hardening compensated the strength loss due to precipitation coarsening. The proposed thermomechanical pretreatment process provides an appropriate initial temper for CAF of Al–Zn–Mg–Cu alloys, thereby improving the creep formability and comprehensive properties remarkably. The findings of this study could aid in the development of CAF technology for Al–Zn–Mg–Cu alloys, especially in terms of manufacturing aviation components.

Microstructural evolution, precipitation behavior and mechanical properties of a novel Al–Zn–Mg–Cu–Li–Sc–Zr alloy

Microstructural evolution, precipitation behavior, mechanical properties and strengthening mechanism of a novel Al–Zn–Mg–Cu–Li–Sc–Zr alloy (7075-1.2Li–0.1Sc–0.1Zr alloy, wt%) were investigated. The strong tendency for preferential precipitation of Al3Li was observed. After single-stage aging (120 °C/48 h), the main precipitation phase was core–shell Al3(Sc, Zr, Li), consisting of Al3(Sc, Zr) core formed at homogenization (460 °C/32 h) and Al3Li-shell formed at subsequent aging. However, under two-stage aging (120 °C/16 h + 150 °C/48 h), co-precipitation of core–shell Al3(Sc, Zr, Li) and rod-shape η′ (MgZn2) happened, triggering the further improvement in microhardness of Al–Zn–Mg–Cu–Li–Sc–Zr alloy. The grain boundary precipitates (GBPs) and precipitate free zones (PFZs) existed in the alloys after both single-stage aging and two-stage aging. The strength of Al–Zn–Mg–Cu–Li–Sc–Zr alloy was gradually improved after two aging treatments, however, the ductility was continuously decreased. The strengthening mechanisms of Al3(Sc, Zr, Li) and η′ (MgZn2) at single-step and two-step aging treatments were both bypass strengthening.

Development of thin-gauge low iron loss non-oriented silicon steel

Microstructure, texture, inclusions and precipitates in Fe-2.97wt%Si non-oriented silicon steel during manufacture were investigated using Scanning Electron Microscopy (SEM), Organic Solvent Electrolysis and Electron Backscattered Diffraction(EBSD)techniques. The P10/400, P15/50 and B50 of thin-gauge non-oriented silicon steel with 0.3 mm in thickness were 13.85 W/kg, 2.38 W/kg and 1.66 T, respectively. Due to annealing of hot rolled band, the size of precipitates increased. The precipitates are mostly located at the grain boundaries in the annealed sheet, the main and average size of the grain-boundary precipitates were in the range of 30 ∼ 500 nm and 63.2 nm, respectively. The pinning force caused by 100 ∼ 300 nm particles at the grain boundaries was the largest, 70 ∼ 100 nm was second. During annealing of hot rolled band, the α*-fiber texture significantly developed and γ-fiber dropped dramatically. The γ-fiber texture and α*-fiber texture composed the main textures of annealed sheet. The texture randomization would give rise to better magnetic properties compared to the γ-fiber.

Investigations on reformed austenite in the microstructure of dissimilar super duplex/pipeline steel weld

This work investigates the reformed austenite in the super duplex stainless /pipeline steel dissimilar weld, fabricated using gas tungsten arc welding process (GTAW) employing super duplex filler (ER 2594). Multi-pass weld was fabricated at a constant heat input of 0.51 kJ/mm. Super duplex filler 2594 solidifies in ferritic mode and has low cracking susceptibility due to a low level of (P + S) impurities. Upon the weld metal cooling below the ferrite solvus, the austenite precipitation occurs, starting from the grain boundaries. Investigation reveals precipitation of grain boundary (GBA), Widmanstatten (WA), and Intergranular (IGA) austenite along with the minor presence of partially transformed austenite (PTA) in sequential order. The temperature ranges and morphology of each precipitated austenite has been discussed. The reformed austenite influences the mechanical properties of weld significantly. Based on the characteristics of different austenite, their influence on properties can be established. High chromium concentration of weld establishes its superior pitting corrosion resistance.

Microstructure, mechanical behaviour and strengthening mechanisms in Hastelloy X manufactured by electron beam and laser beam powder bed fusion

Understanding the microstructure and mechanical properties associated with different metal additive manufacturing techniques is crucial for its wide-scale acceptance in industries. In this work, we aim to understand the microstructural variations and mechanical properties of Hastelloy X (HX) manufactured by electron beam powder bed fusion (PBF-EB) and laser-based powder bed fusion (PBF-LB) process. The size and shape of melt pool and grains were analysed and correlated to final microstructure. The elevated powder bed temperature during PBF-EB was found to promote the formation of grain-boundary precipitates. The higher thermal gradient and cooling rate in PBF-LB resulted in higher tensile residual stress (σmax = 447 ± 10 MPa) within the parts in comparison to PBF-EB parts (σmax = 16 ± 13 MPa). The as-fabricated PBF-EB HX specimens showed a lower tensile strength of 590 MPa but a higher elongation of 60%, whereas its PBF-LB counterparts demonstrated a significantly higher tensile strength of 825 MPa but a lower elongation of 38%. This notable difference in the mechanical behaviour of PBF-EB and PBF-LB built HX was attributed to the columnar microstructure,<100> crystallographic texture and underlying strengthening mechanisms. Furthermore, the mechanical properties of PBF-EB and PBF-LB built HX specimens were predicted using multiple strengthening mechanisms, which demonstrated a good agreement with that of experimentally measured.

A comparative study of a solid solution hardening in carbon-alloyed FeMnCrNiCo0.95C0.05 high-entropy alloy subjected to different thermal–mechanical treatments

Different thermal–mechanical processing regimes were used to provide a precipitate-free austenitic structure in interstitial FeMnCrNiCo0.95C0.05 high-entropy alloy containing 1.07 at. % of carbon. Relative to interstitial-free FeMnCrNiCo alloy (a = 0.3599 nm), an expanded FCC crystal lattice (a = 0.3602–0.3603 nm) is typical of as-cast and solution-treated specimens, but grain-boundary carbides are also observed in them. Near complete carbon dissolution in austenite of FeMnCrNiCo0.95C0.05 high-entropy alloy was achieved utilizing two multistage thermal–mechanical processing regimes, which included the combinations of (i) a high-temperature anneal (25 h total), hot forging and cold rolling, and (ii) cold rolling with intermediate high-temperature anneals (6 h total). Despite the fact that both processing regimes provided high solid-solution hardening of austenite (a = 0.3611 nm) and complete dissolution of grain-boundary precipitates, some individual coarse carbides were identified in microstructure of the alloy. Both processing regimes enhanced the mechanical properties of the alloy.

Influence of Cu/Mg ratio on microstructure and mechanical properties of Al–Zn–Mg–Cu alloys

The relationship between microstructure evolution and mechanical performance of Al–Zn–Mg–Cu alloys with varying Cu/Mg ratios was discussed in detail under different two-step aging (T7X). The results showed that the undissolved second-phase particles including σ-Al7Cu2Fe and T-Al2Mg3Zn occurred in the quenched Al–Zn–Mg–Cu alloys. When the Cu/Mg ratio approaches 1.64, the S-Al2CuMg phase was also observed. Although the Cu/Mg ratio did not change the precipitation sequence of the Al–Zn–Mg–Cu alloys, it changed the coarsening rate and precipitate size distribution during aging process. The Al–Zn–Mg–Cu alloy with the lowest Cu/Mg ratio has the slowest coarsening rate and the highest strength (655 MPa). The fracture morphology of Al–Zn–Mg–Cu alloys changes from the typical brittle fracture to the intergranular-dominated mixed fracture mode or ductile fracture when the Cu/Mg ratios increases. Besides, the T79-aged alloys with varying Cu/Mg ratios have similar grain boundary precipitates (GBPs) and precipitate-free zones (PFZs). The T74-aged alloys with higher Cu/Mg ratio have more coarser GBPs and wider PFZs.

Effect of TIG remelting on microstructure, corrosion and wear resistance of coating on surface of 4Cr5MoSiV1 (AISI H13)

This study aims at surface modification of the 4Cr5MoSiV1 (AISI H13) steel through the TIG (tungsten inert gas) remelting process. TIG remelting process was conducted using the major parameters, i.e. 250 A welding current and 120 mm/min welding speed (3.0 mm electrode diameter), under the protection of Argon. After remelting, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to analyze phases, microstructure and element distribution, respectively. After TIG remelting on the surface of 4Cr5MoSiV1 steel, the phase transition from body-centred cubic (BCC) to face-centred cubic (FCC) has occurred, and a great amount of Cr3C2 segregated along the grain boundaries, which resulted in eutectic structures. The results illustrated that the TIG remelting layer has better corrosion resistance than the original steel, which is related to the grain boundaries with Cr3C2, and result in wider passivation. Further, the microhardness of the TIG remelting layer has been increased to a great extent, which has leaded to lower friction coefficient and better wear resistance. And then, the small punch tests exhibit the morphology of intergranular fracture at the remelting layer, and this situation testified the role of grain boundaries in this study. To summarize, in the study, including the phase transition, precipitation, and change of grain boundary, have improved the performance of the TIG remelt-ed 4Cr5MoSiV1 greatly, which can develop its innovative applications.

Multistage aging treatment influenced precipitate characteristics improve mechanical and corrosion properties in powder hot-extruded 7055 Al alloy

The precipitation evolution, mechanical properties and corrosion behavior of powder hot-extruded 7055 Al alloy subjected to T6, RRA, T76 and T74 aging treatments were investigated. The TEM and DSC results reveal that the dominant intragranular precipitates under those conditions are GP (I, II) + η' phase, GP (II) + η' phase, η' phase and η' + η phases, respectively. The quantitative characteristics (size, interspacing and volume fraction) show that the small volume fraction of intragranular precipitates limits the yield strength of the T6 alloy, while the enlarging of interspacing caused by the coarsening of intragranular precipitates is the main reason for the reduction of yield strength of over-aged alloys. The continuously distributed grain boundary precipitates (GBPs) with low Cu content in the T6 alloy are responsible for higher corrosion susceptibility, while the separated and coarsened GBPs with high Cu content formed in the deepening aging process contribute to the better corrosion resistance. At the same time, the drop in corrosion potential of Al matrix also reduces the corrosion driving force of GBPs with the aging degree deepening. The optimal comprehensive performance of the 7055 Al alloy can be obtained after RRA treatment, namely the highest tensile strength of 739.7 MPa and the acceptable corrosion resistance comparable to that of the T76 state.

Effects of Pre-stretching and Aging Treatments on Microstructure, Mechanical Properties, and Corrosion Behavior of Spray-Formed Al-Li Alloy 2195

In this study, different pre-stretching and aging processes were applied to spray-formed Al-Cu-Li alloy 2195. The effects of different processes on the microstructures, mechanical properties, and corrosion behaviors of the alloy were examined. The strengthening and corrosion mechanisms were investigated. The samples treated with pre-stretching and aging have higher strength than the only aged samples. However, the strength is not significantly improved with the increase in the amount of pre-stretching. The pre-stretching treatment can suppress the formations of grain boundary precipitates (GBPs) and precipitate-free zones (PFZ), and the intergranular corrosion (IGC) sensitivity of the alloy is therefore reduced. The increase in pre-stretching promotes massive precipitation of the T1 phase, which leads to the negative shift of open-circuit potential. The over-aging treatment after pre-stretching leads to the growth of the PFZ and GBPs as well as the coarsening of the T1 phase. These changes in microstructure cause a decrease in mechanical properties and the re-sensitivity to the IGC. Under the same pre-stretching deformation, the two-step aging treatment (110 °C/12 h + 160 °C/24 h) can result in more intensive precipitation of the T1 phase, which enhanced the strength to a higher level. Meanwhile, the two-step aged alloy was immune to the IGC. The evolution mechanism of IGC for the alloy with different pre-stretching and aging processes was proposed based on the TEM characterization and the observation of IGC morphology.

Quantitative investigation on deep hydrogen trapping in tempered martensitic steel

In this work, the correlation between different microstructural components and hydrogen trapping with high density in tempered niobium carbide (NbC)-precipitated martensitic steel was quantitatively investigated using a combination of electrochemical hydrogen permeation experiments and thermal desorption spectroscopy. The martensite lath and a high density of dislocations, which constitute the reversible hydrogen trapping sites, with a density of 2.24 × 1020 cm−3 in Fe-0.05C-1.10Mn-4.50Ni-0.50Cr-0.50Mo-0.05Nb wt.% martensitic steel. The dislocation with high density could disperse the hydrogen distribution. Furthermore, the uniformly distributed NbC nanoprecipitates, the high-angle grain boundaries, and the grain-boundary precipitates were found to act as irreversible hydrogen traps, with a density of 1.00 × 1020 cm−3. These deep hydrogen trapping sites could not only trap hydrogen irreversibly, but also can inhibit the accumulation of hydrogen. The interpretation of hydrogen trapping is significant to enhance the hydrogen embrittlement resistance of high-strength martensitic steels.

Influence of isothermal and non-isothermal aging treatments on microstructure and properties of Al-Zn-Mg alloy helical profile

Aging treatment was an important way to improve the mechanical properties of Al-Zn-Mg alloys. In this work, the Al-Zn-Mg (7 N01) alloy helical profile was extruded and treated by different kinds of isothermal and non-isothermal aging processes. It was found that the alloy had the peak strength at 39 h during the isothermal aging treatment. Its precipitation sequence was supersaturated solid solution (SSS) → GP zones → metastable η′ phase, where the stable η phase had not been formed. The peak strength of the alloy during the non-isothermal aging reached at 4-5 h, but the peak strength was lower than those with isothermal aging treatment. The maximum temperature Tmax of non-isothermal aging played a key role on the mechanical properties: the higher the maximum temperature was, the longer time was required, and the lower the strength and the hardness were. The grain boundary structure of the alloy with Tmax = 185 °C was similar to that with isothermal aging, and the grain boundary precipitate (GBP) was rarer than that with Tmax = 200 °C and Tmax = 215 °C, where there was no obvious precipitate free zone (PFZ) in the alloy, which was conducive to improving the corrosion resistance of the alloy.