Grain Boundary Precipitation Research Articles

Effects of multi-stage aging treatments on the precipitation behavior and properties of 7136 aluminum alloy

The solutionized 7136 Al alloy was heat treated by one-step aging, two-step aging, retrogression and re-aging (RRA) and RRA plus further aging treatment, respectively. The influences of different aging treatments on the precipitation behavior, electrical conductivity and tensile properties were investigated. The results showed that one-step aging treatment forms continuous grain boundary precipitates (GBPs), coherent Guinier-Preston (GP) zone and semi-coherent η’ grain interior precipitates (GIPs). By two-step aging treatment, the GIPs are coarsened and partly transformed into incoherent η phase, which result in a reduction of strength but an enhancement of ductility and electrical conductivity. RRA treatment makes the GBPs to be discontinuous and forms precipitates free zone (PFZ) near grain boundary. The RRA alloy shows similar strength but higher electrical conductivity in comparison with one-step aging. When the RRA alloy further aged at 160 °C, the strength keeps almost constant until 10 h, while the electrical conductivity is increased with increasing aging time. The PFZ is widened and the GIPs become coarser by further aging at high temperature. By contrast, the PFZ is eliminated, accompanying with the formation of GP(II) as further aged at low temperature of 70 °C, therefore a higher strength and lower electrical conductivity are obtained.

Microstructure and mechanical properties of Cr-rich Co-Cr-Fe-Ni high entropy alloys designed by valence electron concentration

Cr element is considered to have strong solid solution strengthening effect in high entropy alloys (HEAs). However, the formation of brittle σ phase is enhanced with the increase of Cr content, which is harmful to the ductility of alloys. The valence electron concentration (VEC) is highly correlated with phase stability of HEAs, which is used to predict the formation of σ phase. The microstructure and mechanical behavior of Cr-rich non-equal molar Co-Cr-Fe-Ni HEAs were investigated. Results indicate the formation of σ phase could be suppressed with the high VEC, while lattice parameter tends to increase as Cr content increases, which enhances the metastability of Co-Cr-Fe-Ni system to accelerate grain-boundary precipitates of σ phase. Minor precipitates of σ phase strengthen the HEAs and do not cause serious embrittlement. Deformation-induced martensitic transformation (FCC →HCP) occurs during the tensile deformation of alloys with the VEC of 8.0 and 7.9, and the transformation induced plasticity effect is the main reason for good combination of strength and ductility.

Dynamic Precipitation Behavior of a Mg-Zn-Ca-La Alloy During Deformation

The dynamic precipitation behavior of a Mg-Zn-Ca-La alloy during deformation has been investigated. A large amount of fine precipitates was detected after deformation, indicating that dynamic precipitation occurred. The precipitates at the grain boundaries included Ca2Mg6Zn3 and Mg4Zn7, whereas the intergranular precipitates were mainly Mg4Zn7. Ca, which has a large atomic radius, preferred to segregate to grain boundaries, where it resulted in the formation of Ca2Mg6Zn3. Zn, which has a similar diffusion coefficient to Mg, resulted in Mg4Zn7 being distributed around the matrix. The stability of the Mg4Zn7 phase was enhanced in the as-extruded Mg-Zn-Ca-La alloy. The grain-boundary precipitates acted as effective obstacles to grain growth, giving rise to the formation of a refined microstructure. The intragranular Mg4Zn7 precipitates hindered the mobility of dislocations, improving the mechanical properties. Additionally, a high density of ultrafine nanoparticles was detected in the un-dynamically recrystallized (DRXed) region, being related to the high density of dislocations therein.

Revealing the evolution of microstructure, mechanical property and corrosion behavior of 7A46 aluminum alloy with different ageing treatment

Mechanical property and corrosion behavior of 7A46 aluminum alloy under peak-ageing (T6), double-ageing (DA) and pre-ageing (PA) followed by double-ageing (PA-DA) are systematically investigated. The hardness and strength of aged 7A46 alloy follows an order of T6>PA-DA > DA and the rank of corrosion resistance is PA-DA > DA > T6. The small size and dispersed distribution of matrix precipitates (MPs) lead to the high hardness and strength of T6 alloy, and low hetero-phase potential differences and less formation probability of corrosion galvanic couples enhance the corrosion resistance of PA-DA alloy. Intergranular corrosion (IGC) test results reveal that considerable potential differences promote the formation rate and the quantity of corrosive-batteries between the matrix and grain boundary precipitations (GBPs) or precipitate-free zones (PFZs). The exfoliation corrosion (EXCO) rate and the weight gain trend of corrosion products are related to the oxide film formation rate and corrosion products accumulation. Localized anodic dissolution and hydrogen embrittlement can induce the stress cracking corrosion (SCC).

Effect of W–Mo balance on long-term creep life of 9Cr steel

ABSTRACTThe effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture tr exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in tr with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe2(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening.

Simultaneously enhanced mechanical strength and inter-granular corrosion resistance in high strength 7075 Al alloy

The strength and corrosion resistance of 7000 series (Al-Zn-Mg-Cu) alloys seldom co-exist and are often trade-off with each other. In present study, an alternative surface nanocrystallization technique called sliding friction treatment (SFT) was adopted to successfully generate a gradient nanostructured layer on coarse-grained (CG) peak-aged AA 7075-T6 alloy sheet. The grain size on the topmost surface of SFTed sample was extremely reduced to nano-scale (average grain size: ∼56.2 nm). Excitingly, the SFTed 7075 possessed simultaneously improved mechanical strength and inter-granular corrosion (IGC) resistance relative to the CG counterpart. Specifically, both ultimate tensile strength (UTS) and yield strength (YS) of SFTed sample are even ∼ 40 MPa higher than those of untreated peak-aged CG sample. Meanwhile, the IGC test results demonstrate that the average maximum corrosion depth is considerably reduced from ∼179 μm to ∼40 μm after SFT treatment. The obviously refined grains account for the enhanced strength, while the improved IGC resistance should benefit from SFT-induced grain refinement, elimination of both grain boundary precipitates (GBPs) and precipitates free zones (PFZs) along the grain boundaries of SFT-induced nanocrystalline. Our results demonstrate that SFT processing offers a design pathway to fabricating 7000 alloys with modified strength-IGC resistance synergy.

Improved fatigue crack growth resistance by retrogression and re‐aging heat treatment in 7010 aluminum alloy

AbstractAircraft grade 7010 aluminum alloy was heat treated to two different conditions: (1) standard peak aging (T6) and (2) retrogression and re‐aging (RRA). The microstructures of these alloys were characterized by using transmission electron microscope. Fatigue crack growth rate (FCGR) tests were conducted using standard compact tension specimens, following ASTM standards. Tests were conducted at various stress ratios, R ranging from 0.1 to 0.7. The RRA‐treated alloy was observed to contain coarsened η′ (MgZn2) precipitates with higher inter‐particle spacing when compared with T6‐treated alloy. The grain boundary precipitates (GBPs) were also coarsened and discontinuous in RRA‐treated alloy as compared with continuous GBPs in T6 condition. The FCGR was lower and ΔKth was higher in RRA‐treated alloy compared with T6‐treated alloy at all the stress ratios investigated. Improved fatigue crack growth resistance in RRA‐treated alloy was correlated to the modified microstructure and enhanced crack closure levels.

Study on the optimizing mechanisms of superior comprehensive properties of a hot spray formed Al-Zn-Mg-Cu alloy

In this study, the microstructure of a hot spray formed Al-Zn-Mg-Cu alloy was observed, and the effects of ageing treatment parameters on the precipitation, the mechanical properties and corrosion behavior of the alloy were investigated. The as-deposited Al-Zn-Mg-Cu alloy had a fine equiaxed grain structure with low level of segregation and porosity. After hot extrusion and solution treatment, most of the coarse phases were dissolved, providing a favorable condition for subsequent ageing strengthening. The alloy was subjected to three different ageing treatments: peak-aging, over-aging, and non-isothermal retrogression and re-ageing (NRRA). Results showed that the alloy in peak-ageing condition possessed much better mechanical properties, but poorer corrosion resistance than that in the over-ageing condition. After NRRA treatment, however, great comprehensive performance was achieved for the alloy. In NRRA condition, the optimum combination of the size and number of strengthening phases was responsible for the high mechanical properties, whilst the separated grain boundary precipitates (GBPs) and the higher Cu content of the GBPs contributed to the better corrosion resistance.

TiFe Precipitation Behavior and its Effect on Strengthening in Solution Heat-Treated Ti-5Al-3.5Fe During Isothermal Aging

We investigated the TiFe precipitation behavior of solution heat-treated Ti-5Al-3.5Fe during isothermal aging, quantified the effect of precipitation on strengthening by evaluating the hardness, and compared it to the effect of Ti3Al precipitation in Ti-6Al-4V. TiFe precipitates formed both at grain boundaries (GBs) and within the grain matrices. Phase transformation from the β to α phase also occurred during isothermal aging; this transformation generated lamellar interphase boundaries between the transformed α phase and remaining β phase in prior β grains. These interphase boundaries enabled the formation of in-grain TiFe precipitates by acting as a nucleation site. GB precipitation did not require prior β → α phase transformation to generate nucleation sites (i.e., interphase boundaries), so TiFe precipitation could occur immediately upon isothermal aging. Thus, GB precipitation proceeded more quickly than in-grain precipitation; as a result, precipitates were larger and more spherical at the GBs than in grains. The strengthening behavior exhibited by TiFe precipitation differed obviously from that caused by Ti3Al precipitation in Ti-6Al-4V because of its differing precipitation kinetics and related microstructural evolution.

Controlling the grain boundary morphology and secondary γ′ precipitate size distribution in Ni-base superalloys

The size, morphology and fraction of each of the γ′ precipitate populations in polycrystalline γ- γ′ nickel-base superalloys are highly sensitive to the cooling rate from the solution temperature. As a result, thermal processing parameters are carefully selected to control precipitate populations in an effort to modify specific microstructural features that can impact resulting properties. The present investigation reports on how a stepped cooling rate was able to modify the microstructure of two experimental Ni-base superalloys. Cooling rates of 0.1 °C/s and 0.01 °C/s were maintained through the γ′ solvus temperature to a transition temperature ∼10 °C to 15 °C below the solvus. Following the transition temperature, a controlled cooling rate of 1 °C/s was applied to control the size and distribution of the secondary γ′ precipitates. For both experimental alloys, slow cooling rates through the γ′ solvus temperature resulted in the formation of coarse grain boundary precipitates while secondary γ′ precipitates formed upon the subsequent faster cooling rate of 1 °C/s. Coarse grain boundary γ′ precipitates resulted in higher amplitudes and wavelengths of serration along the boundary. In addition to cooling rate, the grain boundary misorientation was also found to influence the precipitation of grain boundary γ′ with larger precipitate populations found along high angle grain boundaries. Results from this study appear to support the concept that meso-scale engineering of grain boundary structures may potentially be used to enhance and optimize the high temperature properties of Ni-base superalloys.

Correlating the five parameter grain boundary character distribution and the intergranular corrosion behaviour of a stainless steel using 3D orientation microscopy based on mechanical polishing serial sectioning

Using 3D-orientation microscopy consisting of electron backscatter diffraction (EBSD) and serial sectioning by mechanical polishing we investigated the correlation between grain boundary (GB) character expressed by its 5-rotational parameters and the GB precipitation and intergranular corrosion (IC) behaviour on an AISI 304L stainless steel. From the measurements, a volume of approximately 4.8 × 106 μm3 containing 451 grains and approximately 1500 GBs was reconstructed. Dominance of {111} planes for Σ3n (n = 1, 2, 3) GBs was found. In general, the IC behaviour of random high angle grain boundaries (RHAGBs) and low-Σ coincidence site lattice (CSL) boundaries (Σ ≤ 29) does not show a large difference. The corrosion behaviour of low angle grain boundaries (LAGBs) is strongly dependent on the misorientation angles, while the crystallographic GB plane becomes dominant for the behaviour of low-Σ CSL GBs and RHAGBs. It was found that the corrosion resistance is related on the atom packing density (APD) of the GB plane. High corrosion resistance appears for GBs with high APD values (in particular for the (111) planes with a maximum APD value).

Precipitation behavior during hot deformation of powder metallurgy Ti-Nb-Ta-Zr-Al high entropy alloys

Refractory high entropy alloys with compositions of TiNbTa0.5Zr and TiNbTa0.5ZrAl0.2 were prepared by powder metallurgy. The alloys were hot deformed at 800 °C with a strain rate of 0.001s−1. The microstructural evolution, precipitation behavior and flow behavior at different compressive strain were investigated. Results show that the alloys are single BCC phase. The hot deformation induces precipitation both in grain boundaries and inside grains. The critical compressive strain for intragranular precipitation of TiNbTa0.5Zr and TiNbTa0.5ZrAl0.2 alloys is 20% and 10%, respectively. The precipitates are BCC phase and enriched with Nb and Ta, while the deformed matrix is enriched with Zr. A bi-modal grain size microstructure with micro-scaled intergranular precipitates and nano-scaled intragranular precipitates is achieved after being deformed to 50%. The precipitation leads to a significant flow softening during deformation. And after hot deformation, the TiNbTa0.5ZrAl0.2 alloy has a compressive yield strength of 1500 MPa with a high compressive strain of 18% at room temperature, mainly due to the bi-modal grain size microstructure.

Grain boundary precipitation behavior of δ-Ni3Nb (D0a) phase in a Ni-Nb-Fe ternary model alloy

Aiming at covering grain boundary (GB) with a geometrically close-packed (GCP) type intermetallic phase in Ni-based superalloys, GB precipitation behavior of the δ-Ni3Nb phase with the D0a structure in the A1 matrix is investigated using a model alloy Ni-12Nb-3Fe. The D0a phase precipitates on GB prior to grain interior (GI). The precipitation of D0a phase tends to take place in a continuous manner above its nose temperature (1373 K) and in a discontinuous manner below the temperature. The area fraction of a GB covered by the precipitate (ρ) significantly differs by GB in aging at 1423 K, i.e. between 0 and 80% in short time aging and between 30 and 100% in long time aging, while their average area fraction (ρ¯) increases up to about 75% after long time aging. The observed large difference in ρ is found to be enhanced in the growth stage of the precipitation. A crystallographic orientation analysis indicates that the difference may be caused by the geometry of the habit planes, {111} in the matrix phase, with respect to the GB plane; that is, in the case that one of the habit planes is nearly parallel to the GB plane, the D0a phase grows along the GB, resulting in high ρ, while the phase grows towards GI when any habit planes are inclined to the GB plane, resulting in low ρ. A multi-step heat treatment is proposed to suppress the growth of the precipitates towards GI.

Effect of Retrogression Heat Treatment Time on Microstructure and Mechanical Properties of AA7010

The effect of retrogression time during retrogression and re-aging (RRA) treatment of AA7010 is evaluated by performing tensile tests and characterizing the microchemistry of the grain boundary precipitates (GBPs) using transmission electron microscope coupled with the energy-dispersive spectroscopy. Retrogression time is evaluated so that the ultimate tensile strength of the RRA-treated sample is equal to that of the T6-treated sample and the grain boundary microstructure similar to that of the over-aged (T7451) condition. The investigation reveals that the sample retrogressed at 200 °C for 20 min has UTS of 586 MPa which is equivalent to that of the T6 sample and 11.5% higher than that of the T7451 condition. The fracture toughness of the RRA-treated sample was 41 MPa√m. Microstructure of the RRA-treated sample is similar to T7451, along the grain boundaries and in the grain interior similar to that of the T6-treated sample. Energy-dispersive spectroscopy confirmed the increment of Cu content on the GBP’s with increase in the retrogression time, which is expected to improve the stress corrosion cracking resistance of the alloy.

Effects of creep-aging parameters on aging precipitates of a two-stage creep-aged Al–Zn–Mg–Cu alloy under the extra compressive stress

Two-stage creep-aging characteristics of an Al-Zn-Mg-Cu alloy under the extra compressive stress are investigated. The effects of the second-stage creep-aging temperature (SSCAT) and compressive stress on precipitates are discussed. It is found that the main precipitates are η′ and η phases in the two-stage creep-aged alloy. With the increase of SSCAT, the size of intragranular aging precipitates (IAPs) rapidly increases, while its density decreases. Meanwhile, the density of IAPs first increases and then decreases with the increase of compressive stress. Additionally, the grain boundary precipitates (GBPs) discontinuously distribute on grain boundaries. The size of GBPs and the width of precipitate free zones (PFZs) increase with increasing the SSCAT, and the total volume of GBPs increases with the increase of compressive stress. Moreover, the sizes of IAPs under compressive stress are bigger than those under tensile stress, while their density is relatively smaller. Furthermore, the compressive stress can effectively inhibit the growth of GBPs and the broadening of PFZs. Compared with the creep-aged alloys under tensile stress, the sizes of the GBPs are smaller, and the PFZs is narrower in the creep-aged alloys under compressive stress.

Evolution of microstructure and tensile properties during the three-stage heat treatment of TA19 titanium alloy

This study reported the development of a three-stage heat treatment for TA19 titanium alloy to obtain a ternary microstructure consisting of equiaxed, lamellar and acicular α, which were successively acquired in the first stage (I-stage), second stage (II-stage) and third stage (III-stage) treatments. The content ratio among the equiaxed, lamellar and acicular α was tailored by controlling the I-stage and II-stage temperatures. Tensile test revealed that an increase of I-stage temperature improved both ultimate strength (UTS) and elongation (EL) due to the increased content of lamellar α. However, an excessively high I-stage temperature led to a very low content of equiaxed α, coarsening of β grain and precipitation of grain boundary α, which reduced the EL. An increase of II-stage temperature resulted in an increase of UTS and a decrease of EL, because the increased content of acicular α produced a number of α/β interfaces, which strengthened the alloy but was detrimental to plasticity. The fractography analysis indicated that majority of heat treated specimens exhibited a completely ductile fracture mode, while a mixed mode of brittle and ductile fracture was observed in specimens, which was subjected to an excessively I-stage temperature.

Precipitates and corrosion resistance of an Al–Zn–Mg–Cu–Zr plate with different percentage reduction per passes

7055 aluminum alloy plates with the same size were rolled by two processes: small percentage reduction per pass (PRPP) and large percentage reduction per pass, respectively. Meanwhile, the effect of PRPP on the precipitates and corrosion resistance of 7055 aluminum alloy plate was investigated. The mechanisms were analyzed and discussed by optical microscopy (OM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and electron back-scattered diffraction (EBSD) technique. Large PRPP can improve the corrosion resistance. For the plate rolled by small PRPP, the main precipitate is guinier–preston (GP) zone and continuous grain boundary precipitates (GBPs), while, for the plate rolled by large PRPP, the main precipitates are the GP zone and η′ precipitate, and the GBPs are discontinuous.

Influence of grain boundaries on the distribution of components in binary alloys

Based on the free-energy density functional method (the Cahn–Hilliard equation), a phenomenological model that describes the influence of grain boundaries on the distribution of components in binary alloys has been developed. The model is built on the assumption of the difference between the interaction parameters of solid solution components in the bulk and at the grain boundary. The difference scheme based on the spectral method is proposed to solve the Cahn-Hilliard equation with interaction parameters depending on coordinates. Depending on the ratio between the interaction parameters in the bulk and at the grain boundary, temperature, and alloy composition, the model can give rise to different types of distribution of a dissolved component, namely, either depletion or enrichment of the grain-boundary area, preferential grainboundary precipitation, competitive precipitation in the bulk and at the grain boundary, etc.

Structure, properties, and resistance to stress-corrosion cracking of a nitrogen-containing austenitic steel strengthened by thermomechanical treatment

The results of comparative studies of the structure, mechanical properties, and resistance to stresscorrosion cracking (SCC) in the chloride solutions of a Cr–Mn–Ni austenitic nitrogen-containing steel (20Cr–6Ni–11Mn–1.5Mo–N–V–Nb) produced with the use of different regimes of the high-temperature thermomechanical treatment (HTTMT) have been presented. An unfavorable effect of the grain-boundary precipitates of the nitride phase on the impact toughness and resistance to SCC has been found. It has been shown that the strengthening of nitrogen-containing steel upon HTTMT, which ensures an increase in the yield stress by 1.8 times compared to the austenitized state, does not decrease the resistance to SCC.

Corrosion behavior of ultrafine-grained AA2024 aluminum alloy produced by cryorolling

The objectives of this study were to produce ultrafine-grained (UFG) AA2024 aluminum alloy by cryorolling followed by aging and to evaluate its corrosion behavior. Solutionized samples were cryorolled to ~85% reduction in thickness. Subsequent aging resulted in a UFG structure with finer precipitates of Al2CuMg in the cryorolled alloy. The (1) solutionized and (2) solutionized and cryorolled samples were uniformly aged at 160°C/24 h and were designated as CGPA and CRPA, respectively; these samples were subsequently subjected to corrosion studies. Potentiodynamic polarization studies in 3.5wt% NaCl solution indicated an increase in corrosion potential and a decrease in corrosion current density for CRPA compared to CGPA. In the case of CRPA, electrochemical impedance spectroscopic studies indicated the presence of two complex passive oxide layers with a higher charge transfer resistance and lower mass loss during intergranular corrosion tests. The improved corrosion resistance of CRPA was mainly attributed to its UFG structure, uniform distribution of fine precipitates, and absence of coarse grain-boundary precipitation and associated precipitate-free zones as compared with the CGPA alloy.