Precipitate-free Zones Research Articles

Effect of grain structure and precipitate on tensile properties and low-cycle fatigue behaviors of 2A55 Al-Cu-Li alloy

The tensile properties and cycle fatigue behaviors of 2A55 Al-Cu-Li alloy plates were assessed along the rolling direction (RD) and transverse direction (TD). The stronger anisotropy in elongation attributes to the grain boundary and crystallographic texture, and decreases as the formation of precipitation-free zones. The cyclic hardening is caused by the interaction between dislocations and dislocations or non-shearable precipitates. The non-shearable T1 precipitates could be bypassed via dislocation cross-slip and then more multiple slip planes are activated, which causes more shearable precipitates could be cut, leading to the cyclic softening at higher strain amplitude.

Cavitation-resistant intergranular precipitates enhance creep performance of θ′-strengthened Al-Cu based alloys

Tensile and compressive creep properties of a quaternary Al-Cu-Mn-Zr (ACMZ) alloy and its commercial counterpart (Al-Cu-Mn-Zr with Ni, Co and Sb additions, RR350) are investigated at 300°C. At low stresses up to 30 MPa where diffusional creep dominates, creep resistance is the same in tension and compression and RR350 deforms more slowly than ACMZ, consistent with RR350 alloy's larger linear fraction of intergranular precipitates (Al7Cu2(NiFe) and Al9FeNi for RR350 vs. θ-Al2Cu for ACMZ) and a reduced fraction of precipitate-free zones near grain boundaries. At stresses between 30 and 80 MPa, dislocation creep with a stress exponent n ∼ 3 becomes rate-limiting in compression, which is expected to be controlled by θ′ precipitates within the grain bulk. By contrast, in tension, enhanced creep rate and higher apparent stress exponents are measured, consistent with cavitation at intergranular precipitates becoming increasingly dominant as the stress increases. In the dislocation creep regime, RR350 alloy is again more creep resistant than ACMZ alloy, which is related to three mechanisms (i) a reduced fraction of softer precipitate-free zones, (ii) more effective load transfer to intergranular precipitates, and (iii) reduced cavitation. A model for cavitation is applied to calculate tensile creep rates from compressive creep rates and the model successfully predicts the improved tensile creep resistance of the RR350 alloy. The present investigation underscores the importance of intergranular grain boundary precipitates, in addition to strengthening θ′ precipitates, in enhancing the creep resistance of Al-Cu alloys.

Influence of Grain Boundary Precipitates on Intergranular Corrosion Behavior of 7050 Al Alloys

The effects of the grain boundary precipitation on intergranular corrosion behavior were investigated by exfoliation tests and complementary techniques like scanning electron microscope (SEM), optical profilometry (OP), transmission electron microscope together with energy dispersive spectroscopy (TEM-EDS), and atomic force microscope (AFM). The results reveal the influencing mechanism of intergranular corrosion behavior from grain boundary precipitates (GBPs). The potential discrepancy between GBPs and adjacent areas induces corrosion cavity germination along the grain boundary. Furthermore, the increase of both active Mg and Zn content in GBPs improve the potential difference, which aggravates the intergranular corrosion cavity germination. However, the increment of noble Cu content in GBP is beneficial to reduce the potential difference. On the other side, the distribution of the continuous precipitates in the grain boundary region helps the initial corrosion cavities to connect, which improves the growth of intergranular corrosion cracks. Additionally, discontinuous GBPs and precipitation free zone (PFZ) hinder the spread and connection of intergranular corrosion cavities. Therefore, 7050 aluminum alloy forming different grain boundary precipitation characteristics after different aging treatments shows different resistance to intergranular corrosion: peak-aging < under-aging < over-aging.

Grain boundary decohesion in body-centered cubic Mg-Li-Al alloys

Body-centered cubic (BCC) Mg-Li alloy can be effectively strengthened by Al addition. However, brittle fracture degrades the ductility. Further improvement in mechanical properties requires a deep understanding of the fracture mechanism. In this work, we investigate the fracture behaviour of BCC Mg-Li-Al alloys and probe underneath mechanisms. Three factors are likely responsible for the grain boundary decohesion: narrow precipitation free zones provide dislocations initiation and slip pathway; shearing of intragranular precipitates by planar slip will cause large stress concentration to GBs at the end of slip bands; grain boundary precipitates of Al-rich ordered phases weaken the interatomic bonding among grains. Once the stress locally exceeds GB cohesion, cracking nucleates and propagates along/through GBs and intergranular/cleavage fracture takes place.

The Microstructure Evolution and Electrochemical Corrosion Behavior of 7A46 Aluminum Alloy in Different Quenching Conditions.

The quenching condition of aluminum alloy can affect the mechanical property and corrosion resistance of the profile. This paper is aimed at the low quench sensitivity of aluminum alloys. Scanning electron microscopy and transmission electron microscopy were used to analyze precipitate behaviors of the 7A46 aluminum alloy under different isothermal cooling conditions and microstructure evolutions of quench-induced precipitations. The effect of the different isothermal time on the corrosion resistance of the alloy, and the relationship between microstructure and corrosion resistance after quenching were revealed through electrochemical impedance spectroscopy and potentiodynamic polarization tests. Results show that corrosion sensitivity of the quenching-aged alloy is much higher than that of the double-aged (DA) alloy, and the corrosion resistance of the quenched alloy decreases firstly and then increases. Due to the high density of the matrix precipitates, the increased content of the impurity element, the discontinuity of the grain boundary precipitates and the widening of the precipitates free zone, the most serious degree of corrosion performance among the quenched alloys is 295 °C at 800 s, and the self-corrosion potential and self-current density is −0.919 V and 2.371 μA/cm2, respectively.

Obtaining heterogeneous α laths in selective laser melted Ti–5Al–5Mo–5V–1Cr–1Fe alloy with high strength and ductility

In this study, direct ageing treatments have been employed to improve the strength of selective laser melted near β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe (Ti-55511). The microstructure evolution and mechanical behavior of Ti-55511 alloy were systematically investigated. It was demonstrated that the high-density α laths were precipitated with the martensite and the dislocations in as-deposited Ti-55511 as nucleation sites. The alloy aged at or below 600 °C showed the ultimate tensile strength (UTS) as high as 1500 MPa, while the ductility is poor owing to the precipitation of needle-like α laths or isothermal ω phase. With the increase of ageing temperature, the tip curvature of α laths decreased, and all the α laths coarsened, as well as α phase precipitate-free zones (PFZ) are more pronounced, which leads to insufficient strength and ductility of aged Ti-55511. This, therefore, suggests a new duplex ageing treatment to regulating the microstructure and mechanical properties of Ti-55511 alloy. The alloy obtained heterogeneous α laths consist of micron-sized primary α laths and nano-sized secondary α laths, which retain a maximum elongation of 10% with the high UTS of 1264 MPa. The nano-sized secondary α laths are randomly distributed in the β grain and the width of PFZ decreases, which is helpful to improve the ductility of the alloy. In addition, the high density of twinning deformations was observed in the duplex aged Ti-55511, indicating an excellent ability to accommodate the deformation straining under high stress.

Effects of retrogression and re-aging treatment on precipitation behavior, mechanical and corrosion properties of a Zr+Er modified Al-Zn-Mg-Cu alloy

The microstructure, mechanical properties, and corrosion resistance of a Zr + Er modified Al-Zn-Mg-Cu alloy after different retrogression and re-aging (RRA) treatments were investigated. Results show that the main precipitates after different heat treatment change from GP zones + η’ + Al 3 (Er, Zr) phase (T6) to η’ + Al 3 (Er, Zr) phase (RRA180–30) and then to η + T phase (RRA200–30). The component, size, and distribution of precipitated phases would be affected by the regression time and temperature during RRA treatment. It is concluded that the uniformly dispersed precipitate phase in matrix and narrow precipitate-free zones (PFZs) could significantly improve the strength of Al-Zn-Mg-Cu alloy, and the grain boundary precipitated phase (GBPs) with moderate size and large interparticle distance could improve the alloy's corrosion resistance. Besides, a wider PFZ within a certain range is beneficial to improve the intergranular corrosion (IGC) performance. Therefore, it is important to accurately control the heating temperature and retrogression time during the retrogression stage to regulate the characteristics of precipitated phases and achieve the best balance of mechanical properties and corrosion resistance. • The precipitates size of RRA sample was larger and discontinuously distributed in GB. • Uniform precipitates in matrix and narrow PFZ are beneficial to the alloys' strength. • Corrosion resistance was improved with increasing retrogression temperature and time. • The wide PFZ and discontinuous GBP could improve the IGC resistance of the alloy.

Microstructure and stress corrosion cracking resistance of Al-6.5Zn-2Cu-1.5Mg-0.05Ti alloy modified by Cr addition

The influence of Cr addition on the microstructure and stress corrosion cracking behavior of Al-6.5Zn-2Cu-1.5 Mg-0.05Ti alloy was investigated by microscopy characterization and slow strain rate tensile tests. In Cr-containing alloys, E (Al 18 Mg 3 Cr 2 ) phases precipitated from the matrix, which could serve as energetically preferred heterogeneous nucleation sites for η (Mg(Zn, Cu) 2 ) phases. Due to the decrease of η’ (MgZn 2 ) strengthening phases and the formation of η’ (MgZn 2 ) precipitate free zones, the strength of Cr-containing alloys was reduced to some extent. However, the crack propagation could be suppressed by the discrete distribution of grain boundary precipitates and the inhibition of recrystallization, leading to the increase of the elongation and the stress corrosion cracking resistance. With the addition of 0.15 wt% Cr, the comprehensive performance of Al-6.5Zn-2Cu-1.5 Mg-0.05Ti alloy was improved. Compared with the base alloy, the susceptibility index of stress corrosion cracking ( I SCC ) decreased by 82% and the elongation increased by 42% at the expense of partial strength (less than 5%). • With the addition of 0.15 wt% Cr, the comprehensive performance of Al-6.5Zn-2Cu-1.5 Mg-0.05Ti alloy was improved. • E (Al 18 Mg 3 Cr 2 ) phases could serve as energetically preferred heterogeneous nucleation sites for η (Mg(Zn, Cu) 2 ) phases. • Crack propagation of Cr-containing alloys was suppressed by the discrete distribution of GBPs and the inhibition of recrystallization.

Evolution Process of Precipitate-Free Zones in a Mg-Gd Alloy During Creep

Evolution Process of Precipitate-Free Zones in a Mg-Gd Alloy During Creep

The Formation of Precipitate-Free Zones in Β-Quenched Tc18 Titanium Alloy During Two Phase Field Heat Treatment

The Formation of Precipitate-Free Zones in Β-Quenched Tc18 Titanium Alloy During Two Phase Field Heat Treatment

The Formation of Precipitate-Free Zones in Β-Quenched Tc18 Titanium Alloy During Two Phase Field Heat Treatment

The Formation of Precipitate-Free Zones in Β-Quenched Tc18 Titanium Alloy During Two Phase Field Heat Treatment

Multiscale electron microscopy investigation of grain boundary-mediated crack initiation in AA6451 during three-point bending

The microstructural origins and early behavior of crack formation in AA6451 samples under three-point bending has been investigated using a multiscale electron microscopy-based approach. Two different heat treatments were investigated: solution treated and naturally aged (T4) and artificially aged (T6). Electron backscatter diffraction and cross-sectional scanning electron microscopy imaging showed that, prior to crack formation, grain boundary ledges formed. These ledges formed adjacent to grain boundaries whose surface traces were oriented parallel to the bend axis and their planes forming angles below 70° with the sample surface. The grain boundary misorientation angle did not show any correlation with grain boundary ledge or crack formation. The sub-surface deformation fields associated with the grain boundary ledges were strongly affected with the precipitate state, with grain refinement and ultrafine grain formation occurring in artificially aged samples in the precipitate free zone and more diffuse deformation structures forming in the non-aged samples. In both cases, constituent particles were not found to have a significant influence on damage nucleation processes.

Role of Ti in the microstructure evolutions and mechanical properties of cast Al-2.5Li-1.5Cu-1Zn-0.5Mg(−0.2Zr) alloys

Transition elements are important alloying elements in aluminum alloys, whilst there was a blank of the role of Ti in cast Al-Li-Cu alloys. In this study, the impact of 0.1 and 0.2 wt% Ti additions on Al-2.5Li-1.5Cu-1Zn-0.5Mg(−0.2Zr) alloys has been investigated by microstructure characterizations and tensile tests. The optical microscopy results showed that the grains were significantly refined as the transition elements presented. There was little poisoning phenomenon in the alloys containing both Zr and Ti for the limited holding time during the smelting process. The age-hardening and mechanical properties were prominently enhanced with the addition of Ti and Zr after ageing at 150 ºC. The results of transmission electron microscopy showed that dispersed and dense θ՛ (Al2Cu) precipitates distributed in the alloy containing 0.2 wt% Ti, while T1 (Al2CuLi) phases dominated in the alloy containing 0.2 wt% Ti and 0.2 wt% Zr, accompanied by a moderate number of S՛ (Al2CuMg). The coarsening rates of δ՛(Al3Li) and δ՛-precipitation free zones (PFZs) decreased gradually with the content of transition elements increasing, but the case in the 0Zr2Ti alloy was abnormal for the interaction between θ՛ and δ՛. The best comprehensive mechanical properties of YS = 374 MPa, UTS= 527 MPa and EL = 7.1% could be obtained by 2Zr2Ti alloy after ageing at 150 °C for 64 h.

Effect of strain rate and sample thickness on mechanical properties of spray deposited and extruded Al–Zn–Mg–Cu alloy

The microstructure of spray deposited and extruded Al–Zn–Mg–Cu alloy after regression re-aging (RRA) treatment was investigated by transmission electron microscope and electron back-scatter diffraction. The influence of strain rate and sample thickness on the mechanical properties were evaluated. The results showed that the main precipitate phases in the RRA Al–Zn–Mg–Cu alloy included short rod-shaped η′, coarse rod-shaped η and G.P. zones with nanoscale dimensions. The yield and ultimate tensile strength of RRA alloys were determined by sample thickness and strain rate to regulate the mass fraction of precipitate phases and the restraint of internal grains by sample surfaces, the number of activated slip systems and dislocation density. The RRA Al–Zn–Mg–Cu alloy exhibited a mixed-mode fracture due to the nanoscale precipitate phases restraining grain boundary sliding, the precipitate-free zones and high angle grain boundary inducing crack nucleation and propagation, and the post-necking elongation producing cleavage planes. The strain hardening is conducted by deformation texture. The strain hardening exponent n and the strain rate sensitivity coefficient m were determined by the sample thickness. The deformation texture intensity of RRA alloy was related to the strain rate controlled by texture components. The high strain rate improved the content of Cu texture {112}<-1-11> to elevate the mechanical properties of RRA alloy.

Nanoscale corrosion investigation of surface nanocrystallized 7150 Al alloy in 3.5 wt% NaCl solution by using FIB-TEM techniques

Nanocrystalline grains were fabricated on the surface layer of 7150 Al alloy by using ultrasonic shot peening treatment. The nanoscale corrosion behavior of this surface nanocrystallized 7150 Al alloy in 3.5 wt% NaCl solution was studied by combining focused ion beam (FIB) and transmission electron microscopy (TEM) techniques. It was found that, compared with the untreated substrate alloy, pit corrosion is largely inhibited and intergranular corrosion (IGC) is eliminated on the surface nanocrystallized layer. The transition from IGC to uniform corrosion caused by ultrasonic shot peening can be attributed to the disappearance of precipitate free zone (PFZ) along grain boundaries.

Microstructural evolution and properties of electromagnetic cast-rolled novel Al–Li alloy under different heat treatment procedures

A large number of dislocations and dislocation tangles are induced in the Al–Li alloy after 5% pre-stretching deformation before aging treatment, which increases the density of the defects in α(Al) matrix and affects the competitive precipitation relationship and precipitation behavior of δ′ (Al3Li), S′ (Al2CuMg) and T1 (Al2CuLi) precipitates during the aging process. The increase of crystal defects such as dislocations provides superior nucleation sites for the heterogeneous nucleation and promotes the precipitation of T1 phase, greatly improving the mechanical properties of the alloy. The uniform and dense precipitation of T1 phase in the grain reduces the potential difference between the grains and the grain boundaries. Therefore, not only does it reduce the driving force of intergranular corrosion (IGC), but also decreases the difference of the corrosion rate between the grain boundary and intragranular. These are beneficial to slow down the corrosion process and promote uniform corrosion. The intermittently distributed grain boundary precipitates (GBPs) in the T8 treated alloy cut off the continuous corrosion channel at grain boundaries and hinder the IGC process. Furthermore, the width of the precipitation free zones (PFZs) in the T8 treated alloy is narrow. Hence, the alloy exhibits excellent IGC resistance and superior corrosion protection performance under T8 single aging treatment. The pre-aging process in low temperature before T8 single aging treatment greatly enhances the precipitation driving force of T1 phase and effectively promotes the precipitation of T1 phase in the subsequent high-temperature aging process, which further increases the strength and improves the IGC resistance of the alloy.

Effect of aging state on fatigue property of wrought aluminum alloys

The high-cycle fatigue (HCF) behaviors of wrought Al alloys are vitally important for their industrial application. To optimize the fatigue property of the wrought Al alloy, the effects of aging conditions on the fatigue properties of 7N01, 7075 and 2024 Al alloys were studied. The results show that the overaged samples have the lowest tensile strength among the three aging states, but their fatigue strength is the highest. Further investigations on the dislocation morphologies indicates that the dislocations are easy to accumulate and annihilate at the second phase boundaries for the overaged state, which reduces the fatigue damage degree at the grain boundaries. Besides, the discontinuous distribution of the grain boundary precipitates and the increased width of the precipitate-free zone also contributed to the improved fatigue property of the overaged state. Those results should be significantly important for the optimization of their fatigue properties and the industrial application of wrought Al alloys.

Microstructure and superior quenching sensitivity of a novel Sc, Zr alloyed Al-Zn-Mg-Cu alloy

This paper studies a novel Al-Zn-Mg-Cu-Mn-Sc-Zr alloy that exhibits outstanding quenching sensitivity with a quench-hardening depth of more than 150 mm. Jominy end quenching allows the alloy to obtain different quenching rates. The Brinell hardness of the aged alloy with a quenching rate of 27 °C/s is 154 HB; when the quenching rate drops to 1.5 °C/s, its hardness value loses about 8%, which is 142 HB. The quenching-induced equilibrium precipitates heterogeneously precipitated at the grain boundary and the incoherent phase interfaces (E phase, Al6(Mn, Fe) phase), but no heterogeneous precipitates are observed along the coherent L12-Al3(Sc, Zr) particles. After artificial aging, densely distributed nano-scaled atomic clusters and G.P zones, as well as grain boundary precipitation free zones (PFZs) were observed.

Modulation of Multiple Precipitates for High Strength and Ductility in Al-Cu-Mn Alloy.

The category and morphology of precipitates are essential factors in determining the mechanical behaviors of aluminum alloys. It is a great challenge to synthetically modulate multiple precipitates to simultaneously improve strength and ductility. In the present work, by optimizing the precipitations of the GP zone, θ’-approximant and θ’ phase for an Al-Cu-Mn alloy, a high tensile strength of 585 MPa with large elongation of 12.35% was achieved through pre-deformation and aging. The microstructure evolution pattern was revealed by detailed characterizations of scanning electron microscopy and transmission electron microscopy. It was found that such high tensile strength of the samples was due to a combination of strengthening by the high density of dispersive fine precipitates and dislocations, and the high elongation to failure was primarily attributed to the multimodal precipitates and elimination of precipitation-free zones along the grain boundaries. The strategy proposed here is a promising way of preparing ultra-strong Al-Cu-Mn alloys.

Characterization of Al3Zr precipitation via double-step homogenization and recrystallization behavior after subsequent deformation in 2195 Al-Li alloy

The processability and final mechanical properties of Al-Li alloys are strongly affected by the uniformity of solute element and the degree of recrystallization, which can be improved via homogenization treatments. It is essential to control the microstructure to achieve the desired properties. In this work, the optimal double-step homogenization (DH) parameters for as-cast 2195 Al-Li alloy were first determined as 460 °C/16 h + 515 °C/20 h according to the comparative analysis, which fitted well with the results predicted by the corresponding kinetics modeling. Compared with the conventional single-step homogenization (SH), DH not only prevented the occurrence of overburning but also provided a more favorable condition for Al3Zr dispersoids to precipitate uniformly. The size and number density of Al3Zr dispersoids in the DH sample are finer and higher than those in the SH sample based on quantitative statistics. Due to the sufficient driving force for nucleation, the precipitate free zone (PFZ) of Al3Zr dispersoids was much narrower after DH. As a result, the DH sample possessed a higher Zener drag pressure, leading to a remarkable inhibition on the continuous evolution of substructures. Therefore, the recrystallization behavior in the DH sample was impeded effectively during the subsequent solution treatment. These findings are expected to provide reliable theoretical guidance for the industrial processing of as-cast 2195 Al-Li alloy.