Precipitate-free Zones Research Articles

Compressive creep behavior and microstructural evolution of sand-cast and peak-aged Mg–12Gd–0.4Zr alloy at 250 °C

Magnesium (Mg) alloys with high concentrations of Gd additions are known to exhibit high strength and good creep resistance at elevated temperatures. However, the main mechanisms including microstructural evolution and dislocation configurations for clarifying the low creep rates of Mg–Gd–Zr alloys are still controversial. The present work investigates the compressive creep behavior of both the sand-cast and peak-aged Mg–12Gd–0.4Zr (wt.%) alloys at a fixed temperature of 250 °C under the applied stress range of 60–100 and 80–120 MPa, respectively. It is revealed that β' and β'F precipitates distribute alternately forming precipitate chains in Mg–Gd–Zr alloys. Furthermore, β'+β'F precipitate chains led to the honeycomb-like structure in the sand-cast alloy during the creep process, improving its creep resistance to some extent. Nevertheless, weakly strengthening β1 precipitates occurred and detrimental β phases coarsened, which weakened the creep performance of the sand-cast alloy. However, the slight improvement of creep resistance in the peak-aged Mg–12Gd–0.4Zr alloy can be mainly attributed to the formation of precipitate-free zones (PFZs) and also the premature coarsening of β'+β'F precipitate chains. During creep, the cross slip of basal and prismatic <a> dislocations become the dominant creep mechanism for the sand-cast alloys. By contrast, the cross-slip of basal <a> dislocations and pyramidal <c + a> dislocations is the dominant creep mechanism for the peak-aged alloys, which was arrested by precipitates then strengthening the creep resistance of peak-aged alloys.

Enhanced age-hardening of an Al–5Mg–2Zn–1Cu alloy by pre-aging combined with pre-straining

In this work, the synergistic effect of pre-aging and pre-straining on the enhanced age-hardening response of an Al–5Mg–2Zn–1Cu alloy has been systematically investigated using high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). After pre-aging, only Guinier Preston (GP) zones uniformly distribute in the α-Al matrix. Moreover, dislocations induced by pre-straining can promote the nucleation and growth of T″ and T′ precipitates. With the synergy of pre-aging (80 °C/5 h) and pre-straining (∼4%) treatments, the alloy after second-aging at 130 °C for 12 h exhibits a giant increase of yield strength (∼182 MPa) in comparison to the as-solutionized alloy, reaching ∼348 MPa. The above results show that pre-aging and pre-straining are effective in accelerating the hardening response of Al–Mg–Zn–Cu alloys. Especially, this extraordinary age-hardening response depends strongly on the high number density and uniform distribution of the GP zones induced by pre-straining, acting as preferential nucleation sites for T″ phase and T′ phases during subsequent aging treatment. In addition, it proves that pre-aging and pre-straining can effectively control the precipitation free zones (PFZs) size, relieving the strain concentration in PFZs and improving the comprehensive mechanical properties.

Effect of substituting Gd with 3 wt% Nd on high temperature creep behaviors of peak-aged Mg-10Gd-0.4Zr casting Mg alloy

In this paper, the high temperature creep behaviors of peak-aged Mg-10Gd-0.4Zr and Mg-7Gd-3Nd-0.4Zr casting alloys were investigated. The results showed that substituting Gd with 3 wt% Nd promoted precipitation of the β′ phase with higher number densities and larger aspect ratios, which led to the higher tensile mechanical properties of the peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy at both room and elevated temperatures. The peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy had better tensile creep properties at temperatures of 200 °C–225 °C. However, the peak-aged Mg-10Gd-0.4Zr casting alloy showed better creep resistance when the creep temperature ≥250 °C. The creep mechanism for the peak-aged Mg-10Gd-0.4Zr casting alloy was dislocation climb assisted by grain boundary diffusion under all experimental conditions, while the creep mechanism of the peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy transformed from dislocation glide at 200 °C–225 °C into dislocation cross-slip above 250 °C. Due to the faster transformation from β′ phase into β1 phase in peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy, the blocking effect of precipitation on dislocation movement was weakened. And a larger number of basal < a > dislocations accumulated in the grain boundary region, leading to precipitate free zones (PFZs) widening more obviously in the peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy, and which well explained the reason why the creep resistance of peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy deteriorated rapidly above 250 °C.

Effect of various non-isothermal aging on properties and microstructure of 7055 aluminum alloy

This study investigated the effect of various non-isothermal aging processes on the mechanical properties and corrosion resistance of 7055 aluminum alloy extruded bars. The investigation utilized electrical conductivity, mechanical properties, local corrosion resistance and slow strain rate tensile test combined with microstructure characterization techniques such as metallographic microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the NIA-2 (120 °C→200 °C/30 min→120 °C, v = 30 °C/h) process significantly improves the strengths of the alloy compared to the T73 temper, the tensile strength and yield strength increase by 53.0 MPa–632.5 MPa and by 64.7 MPa–605.4 MPa respectively. Additionally, the maximum depths of intergranular corrosion (IGC) and exfoliation corrosion (EXCO) decrease from 80.6 μm to 97.7 μm–71.4 μm and 89.5 μm respectively. Therefore, the NIA-2 process can achieve both high strength and excellent corrosion resistance. Furthermore, the study shows that the sizes of grain boundary precipitates (GBPs) and matrix precipitates (MPs) increase with the sequence of NIA-1 (120 °C→200 °C→120 °C, v = 30 °C/h), NIA-2, and NIA-3 (120 °C→200 °C→120 °C, v = 20 °C/h) processes. The atomic percentages of Zn, Mg, and Cu in GBPs also vary with the different processes. The increases in the sizes of GBPs, MPs, and the widths of precipitation-free zones (PFZs), as well as the segregations of GBPs components, are primarily responsible for the improved mechanical properties and corrosion resistance of the alloy.

Effects of natural aging on the artificial aging kinetics and responses of Al–5Mg–3Zn–1Cu (wt.%) alloy

Effects of natural aging (NA) on artificial aging (AA) behavior of Al–5Mg–3Zn–1Cu alloy were studied from the perspectives of kinetics and microstructure evolution. The results show that short-term NA (24 h) has marginal effects on AA at any temperature, as well as long-term NA (>240 h) on low-temperature AA (120 °C), but long-term NA can accelerate aging kinetics and alleviate severe hardening ability attenuation at high temperatures (>150 °C). Analysis of the precipitates evolution demonstrates that such attenuation is primarily attributable to coarsening and reduction of intragranular precipitates and is also slightly related to the formation of precipitate-free zone (PFZ). Three main factors cause the unfavorable microstructure in NA-free alloy at high temperatures AA: (1) Insufficient nucleation of intragranular precipitates induced by weakened nucleation driving force, larger critical nucleation radius, and rapid vacancy annihilation; (2) The nucleated precipitates are prone to coarsening; (3) The precipitation near grain boundary is obstructed due to the depletion of vacancies and solutes. Clusters formed during NA can inhibit the unfavorable microstructure by delaying the annealing out of excess vacancies, promoting the nucleation of precipitates, and elevating the activation energy for precipitation coarsening, thereby enhancing the hardening potential. This work is instructive for the formulation of heat treatment strategies for Al–Mg–Zn–Cu alloys with high Mg/Zn ratios.

Unusual secondary precipitation within the primary precipitation free zone substantially enhances the ductility of Al–Zn–Mg–Cu alloy

The formation of unusual secondary precipitation zones (SPZ) within the primary precipitation free zones (PFZ) effectively reduces the PFZ width, associated with finer grain boundary precipitates (GBP) and precipitates in the matrix. Such a microstructure remarkably facilitates the uniform distribution of dislocations within the matrix during the deformation process, and inhibits the strain concentration due to the dislocation tangle within the PFZ. These result in a significant improvement in the ductility of Al–Zn–Mg–Cu alloys, while retain comparable ultimate tensile strength as if treated in T6 temper.

Long-term natural-aging-induced alleviation of attenuation of artificial-aging hardenability in an Al-5Mg-3Zn-1Cu (wt.%) alloy

Effects of long-term natural aging (NA, 2400 h) on the artificial aging (AA) responses of Al-5Mg-3Zn-1Cu alloy at various temperatures (120–175 °C) were scrutinized. The results show that the hardenability of the alloy without NA (NA-free) deteriorates seriously at high AA temperatures (>150 °C). Such decay of AA hardenability is primarily induced by the larger and sparser intragranular precipitates and is also associated with the wide precipitation-free zone. Compared with NA-free alloy, long-term natural pre-aging improves the nucleation conditions in the early stage of subsequent AA, thus preventing these unfavorable microstructures and thereby mitigating the decay of hardening potential.

Effects of heat treatments on the microstructure and localized corrosion behaviors of AA7075 aluminum alloy

Five different heat treatments were applied to AA7075 aluminum alloy, resulting in distinct localized corrosion behaviors in 5 wt% NaCl solution. Some treatments exhibited higher intergranular corrosion (IGC) susceptibility due to the anodic nature of grain boundaries (GBs) caused by Cu depletion in the precipitate free zones (PFZs) along the GBs. Cu-enrichment and an increase in intervals between η-phase grain boundary precipitates (GBPs) did not improve IGC resistance. The open circuit (OCP) and galvanostatic (GL) tests could distinguish differences in corrosion behaviors, suggesting the potential use of GL testing as a faster method for predicting localized corrosion behaviors.

Enhanced strength and ductility in sand-cast Al-Li-Cu-Mg-Zr alloy via synergistic microalloying with Sc and Ti

Cast Al-Li alloys show attractive potential for application in the field of manufacturing large-size and complex parts as a result of profound weight reduction and good fluidity. However, the ductility and strength of sand-cast Al-Li alloys are relatively poor due to the low cooling rate of sand casting. In this study, Sc and Ti were added to sand-cast Al-Li-Cu-Mg-Zr casting alloy to investigate the microstructure evolution and tailor the final performance for the first time. The results showed that grain decreased from 189 µm to 56 µm and the needle Cu-rich phase increased in the vicinity of grain boundaries as the total element content of Sc and Ti increased. After aging at 175 °C, the enrichment of the T1 and S′ phases, the finer δ′, the higher number density of core-shell composites, and the narrowed δ′-precipitation free zones (δ′-PFZs) were stimulated by Sc and Ti, leading to a significant increase in strength of the peak-aged alloy. The SZT alloy exhibits the best yield strength of 359 MPa and ultimate tensile strength of 452 MPa with an adequate elongation of 4.6 %. In addition, a Li-rich zone was observed and intersected with T1 phase, which provided a new precipitation path for Li atom diffusion and nucleation of T1 precipitates. The present study suggests that microalloying with Sc and Ti is expected to be an effective strategy to simultaneously improve the strength and ductility of sand-cast Al-Li alloys.

Designing ultra-strong and ductile hierarchical titanium alloys via interstitial solute-mediated multi-morphologic α-nanoprecipitates

In conventional duplex titanium (Ti) alloys, the soft primary α-precipitates (αp) are beneficial to ductility with the loss of high yield strength, while the continuous α-precipitates at grain boundaries (GBs, αGBs) often accompanied with α precipitate-free zones (α-PFZs) renders the strain incompatibility for limited ductility. Here, to overcome this longstanding issue, we utilize interstitial (O and N) atoms to notably strengthen the α-phase, and simultaneously architect the multi-morphologic secondary α-precipitates (αs) so as to improve the strain compatibility nearby GBs, thus develop a Ti-4.1Al-2.5Zr-2.5Cr-6.8Mo (wt.%) alloy with ultra-high yield strength of ∼1580 MPa and good ductility of ∼8.2%. The phase boundaries between the hard α-precipitates and the soft β-matrix not only act as dislocation barriers for strengthening, but also as sustainable dislocation sources for ductilizing to achieve a good combination of strength and ductility in our Ti alloys. This strategy not only sheds light on the understanding of the strength-ductility synergy of duplex Ti alloys, but also offers an available pathway to design ultra-strong and ductile alloys.

Extrusion weld seam and mechanical properties of high-strength Mg-Gd-Y-Zn-Mn alloy hollow profile fabricated via porthole die extrusion

A high-strength Mg-8.8Gd-3.8Y-1.0Zn-0.7Mn (wt%) alloy hollow profile with complex cross-section was successfully fabricated through porthole die extrusion, and its weld seam was analyzed for microstructure and element distribution to investigate the impact of solid-state bonding. The weld seam displayed a homogeneous magnesium grain structure and a precipitation-free zone. The diffusion behavior of alloying elements during solid-state bonding significantly affected the microstructure of the weld seam, with Gd and Y atoms diffusing towards the outside of the weld region. The hollow profile exhibited favorable mechanical properties, with an ultimate tensile strength (UTS) of 470 MPa, a tensile yield strength (TYS) of 309 MPa, and an elongation (EL) of 7.4%, and showed a minor negative effect from the weld seam.

The Effects of Double-Step Homogenization on Precipitation Behavior of Al3Zr Dispersoids and Microstructural Evolution in 2196 Aluminum Alloy

The effects of double-step homogenization processes on the precipitation of Al3Zr dispersoids and the dissolution of the primary phases of 2196 aluminum alloy were studied by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was revealed that first-step homogenization facilitates the nucleation of Al3Zr, while second-step homogenization results in the dissolution of the primary phases and the growth of Al3Zr dispersoids. The nanosized θ’ precipitates formed in the first-step homogenization are dissolved after the second-step homogenization. The optimum homogenization process was selected as 400 °C/10 h + 520 °C/24 h, which effectively dissolves the primary phases and promotes the formation of refined distribution of Al3Zr dispersoids. This phenomenon is mainly caused by the highest nucleation rate of the Al3Zr phase at 400 °C. While reducing the heating rate of the homogenization process can increase the number density of the Al3Zr dispersoids and reduce the precipitate free zone (PFZ), it does not alleviate the inhomogeneity of the Al3Zr precipitation. These results are expected to be meaningful for tailoring the industrial homogenization processing of as-cast Al-Cu-Li alloy.

Microstructural characterization and mechanical properties of 6061 aluminum alloy processed with short-time solid solution and aging treatment

In order to reduce the cost of hot stamping of aluminum alloys and develop high-strength aluminum alloy parts for light-weight car body, short-time solid solution (SS) followed by shot-time artificial aging process was implemented for 6061 aluminum alloy with salt bath furnace and silicone oil bath. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM-EDS) and high-resolution transmission electron microscope (HR-TEM) were used to characterize the microstructural evolution in details. The holding time for SS can be reduced by 80 % with short-time SS treatment because of the decreased vacancy generation energy and effective boundary areas. Peak aging can be obtained at 200 °C for only 60 min, and the microstructure was characterized by a lot amount of acicular nano-size β’’ phase in the coherent with matrix, while precipitate-free-zone (PFZ) also appeared along the grain boundaries. All tensile properties such as 327 MPa yield strength, 362 MPa tensile strength and 13.1 % elongation meet the 6061 T6-standard. The micro stress introduced by superheated degree along with short-time treatment should be attributed to the accelerated diffusion of magnesium and silicon atoms, which greatly affects the diffusion hermodynamics and kinetics, especially nearby the grain boundaries.

Effect of trace Sc on high-temperature creep performance of spray-deposited Al-Zn-Mg-Cu alloy

In this work, the effect of trace Sc on the mechanical property and high-temperature creep performance of the spray-deposited Al-Zn-Mg-Cu alloy was systematically investigated. The experimental results showed that the microstructure of the deposited and extruded alloy was greatly refined by the addition of Sc. For the Sc-containing alloy, precipitates within grains and the width of precipitate free zone (PFZ) are similar with those of the Sc-free alloy, whereas the area fraction of the precipitates at the grain boundaries is dramatically decreased. Both the mechanical property and high-temperature creep performance can be simultaneously improved by the addition of Sc. After 0.2 wt% Sc was added, the ultimate tensile strength of 831 MPa and elongation of 10% can be obtained, while the steady-state creep rates reached 1.1 × 10−6/h and 1.8 × 10−5/h at 80 °C and 120 °C, respectively. Based on the microstructure observation, amount of fine and dispersed Al3(Sc, Zr) secondary phase particles were generated by the addition of Sc. It indicated that the Sc microalloying can not only bring about additional strengthening effect, but also inhibited the dislocation migration and coarsening of precipitates at high temperature, leading to the enhanced mechanical and creep resistance performance.

Evolution of the microstructure and mechanical properties of Sanicro 25 austenitic stainless steel after long-term ageing

A newly developed heat-resistant austenitic steel, Sanicro 25 is currently considered the leading candidate material for an advanced ultra-supercritical installation. The test material was subjected to long-term ageing (up to 30,000 h) at 700 and 750 °C, after which investigations into the microstructure, identification of precipitates, and testing of mechanical properties were conducted. Sanicro 25 had an austenitic microstructure with annealed twins and numerous large primary NbX and Z-phase precipitates in the as-received condition. It was found that the long-term ageing of the steel resulted in numerous precipitation processes. For example, M23C6 carbides, Laves, σ and G phases occurred at the grain boundaries. However, Z-phase precipitates, ε_Cu particles, and Laves phase were observed inside the grains. At the same time, compound complexes of precipitates based on the primary Z-phase precipitates were revealed in the microstructure. The ageing process increased the particle size of M23C6 carbides and the σ phase. After longer ageing times, a precipitate-free zone (PFZ) near the grain boundaries was observed. The precipitation processes initially lead to an increase in the strength properties of the steel. However, after 5000 h, an over-ageing effect was observed at 750 °C, which was not observed at 700 °C.

Effect of dislocation-induced aging precipitate bands on creep resistance of Mg-Gd-Y-Zr-Ag alloy

This work investigates the effect of dislocation array induced β’ precipitation bands on the creep behavior in wrought Mg-8.9Gd-1.8Y-0.5Zr-0.2Ag (wt%) alloy. The alloy with β’ precipitate bands shows significantly enhanced creep resistance ranging from 448 to 548 K. Pipe diffusion-controlled grain boundary sliding (GBS) is suggested to be the main steady creep mechanism below 498 K. Cross-slip is suggested to be the main steady creep mechanism above 498 K. The characterization indicates that the precipitate bands separate the matrix into multiple units within the grains and restrict the long-distance slip of the dislocations. Bands with high thermal stability limit the diffusion of vacancies across the grain and prevent precipitation free zone (PFZ) broadening on the grain boundaries. For the alloy without precipitate bands, basal slip is initiated before the formation of β’, and it is easy to form broad PFZs on grain boundaries. Although cross-slip is still unavoidable at 523 K, these results provide guidance for improving the creep resistance of the deformed Mg-Gd series alloys.

On the Competition between Intergranular and Transgranular Failure within 7xxx Al Alloys with Tailored Microstructures.

7xxx aluminium series reach exceptional strength compared to other industrial aluminium alloys. However, 7xxx aluminium series usually exhibit Precipitate-Free Zones (PFZs) along grain boundaries, which favour intergranular fracture and low ductility. In this study, the competition between intergranular and transgranular fracture is experimentally investigated in the 7075 Al alloy. This is of critical importance since it directly affects the formability and crashworthiness of thin Al sheets. Using Friction Stir Processing (FSP), microstructures with similar hardening precipitates and PFZs, but with very different grain structures and intermetallic (IM) particle size distribution, were generated and studied. Experimental results showed that the effect of microstructure on the failure mode was significantly different for tensile ductility compared to bending formability. While the tensile ductility was significantly improved for the microstructure with equiaxed grains and smaller IM particles (compared to elongated grains and larger particles), the opposite trend was observed in terms of formability.

Corrosion behavior and mechanism of 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet in sea water acidified accelerated test

The microstructure and corrosion properties of a novel 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet were investigated in comparison with the traditional 4004/3003/4004. It revealed that, the microstructure after brazing of two aluminum brazing sheets from outer to center were both residual Al-Si eutectic, precipitate-free zone (PFZ), band of dense precipitates (BDP) and core material, respectively. However, the alloying element distribution and the types and amounts of second phases were quite different. The sea water acidified accelerated test showed that 4004/Al-Mn-Cu-Mg-Si/4004 exhibited typical exfoliation corrosion, while the corrosion quickly spread deep into the core material of 4004/3003/4004. The formation of Cu concentration gradient and the strip distribution of Al-Mg-Cu-Si precipitates in the BDP region contributed to the exfoliation corrosion of 4004/Al-Mn-Cu-Mg-Si/4004. The corrosion behaviors of Al-Mn-Cu-Mg-Si and AA3003 core materials are both in form of intergranular corrosion (IGC)，and the later was more severe than the former. For Al-Mn-Cu-Mg-Si core material, high-density fine dispersed cathodic α-Al(Mn, Cu)Si precipitates promoted the formation of a profusion of micro-galvanic cells in the Al matrix, which would retard any effects between the grain boundary precipitations and the adjacent relatively PFZs. Hence, the IGC susceptibility of Al-Mn-Cu-Mg-Si core material was decreased. Besides, less large-sized α-AlFeMn(Cu)Si phases in core material also reduced the localized corrosion attack. At last, the corrosion in depth direction of 4004/Al-Mn-Cu-Mg-Si/4004 was retarded, compared to its counterpart.

The formation of precipitate-free zones in β-quenched TC18 titanium alloy during two phase field heat treatment

The formation of α precipitate-free zones (PFZs) has great influence on fatigue strength, tensile ductility and fracture toughness of titanium alloys. In this paper, the formation process of PFZ and the relationship between PFZ, β grain boundary and α phase in TC18 titanium alloy were investigated through β phase field deformation and α + β phase field heat treatment. It was found that the formation of PFZ was affected by the misorientation of β grain boundaries and the morphology of grain boundary α phase (αGB). The thick and continuous αGB phase would precipitate at low angle β grain boundaries (LAGBs), and led to the depletion of α phase stable elements near the β grain boundaries, which further resulted in the formation of PFZ. At high angle β grain boundaries (HAGBs), both discontinuous αGB phase and grain boundary Widmanstätten α phase (αWGB) can precipitate to occupy β matrix, no obvious PFZ. The pre-deformation in β phase field can increase the content of HAGBs and promote the formation of discontinuous αGB phase. In this case, the area of PFZ can be effectively reduced.

The role of the ω phase on the microstructure evolution of precipitation and mechanical property in Ti–3Al–5Mo–4Cr–2Zr–1Fe

This study investigates the effect of the ω phase on the microstructure evolution and mechanical properties of the Ti–3Al–5Mo–4Cr–2Zr–1Fe (Ti-35421) near β titanium alloy. The study employed a solution treatment at 850 °C followed by two aging treatments, namely, Double Aging (DA) and Isothermal Annealing (IA). The results show that the DA alloy had finer precipitation and fewer Precipitation-Free Zones (PFZs) than the IA alloy. While the Ultimate Tensile Strength (UTS) of the DA alloy was higher (1376 MPa) than that of the IA alloy (1272 MPa), its plasticity was lower. On the other hand, the IA alloy exhibited a better balance between UTS (1272 MPa) and ductility (8.12%). The nucleation mechanism of DA was attributed to the presence of the α stable element Al, which diffused in the microstructure and aggregated in the ω phase, creating favorable nucleation sites for the precipitation of the secondary α phase during low temperature aging. As a result, the PFZs decreased, and the dispersion distribution strengthened the β matrix structure. Conversely, the high aging temperature during IA made it difficult to form the ω phase, resulting in coarser precipitation and increased PFZs as the temperature increased. This paper revealed the influence of ω on the relative precipitation during the phase transition, the microstructure and properties of the Ti–3Al–5Mo–4Cr–2Zr–1Fe, providing valuable insights for optimizing its heat treatment process.