Formation Of Precipitate-free Zones Research Articles

A comparative study on cryogenic formability and paint baking property of AA7075 alloy sheets tempered at fast retrogressed and pre-aged conditions

Forming at cryogenic temperatures provides a new approach to the formability improvement of 7000-series high-strength aluminum alloys. However, the cryogenic forming process based on solution-treated (W) or peak artificial aged (T6) alloys cannot meet the demands of autobody manufacturing for good formability, high post-formed strength and high efficiency simultaneously. The fast retrogression (FR) treatment of AA7075-T6 and pre-aging (PA) treatment of AA7075-W are elaborately designed to tailor a pre-hardened AA7075 alloy, which has good cryogenic formability and relatively high initial strength, and can be further strengthened by paint baking in high efficiency. The results show the optimum fast retrogression temperature for AA7075-FR is 400 °C, at which the sample can acquire a cryogenic elongation of 29.2% and obtain 90% of initial T6 sheet tensile strength by the combined effect of cryogenic pre-deformation and subsequent paint baking treatment. The optimum pre-aging temperature for AA7075-PA is 120 °C, at which the sample can acquire a cryogenic elongation of 31.0% and exceed 96% of tensile strength of AA7075-T6 after paint baking treatment. The comparative study demonstrates that the supersaturated solid solution matrix with an appropriate proportion of GP zones and η′ phases, but few Cr-containing dispersoids and precipitate free zones, is an ideal pre-hardened microstructure to get a good combination of cryogenic formability and final strength. The precipitation of GP zones and η′ phases is prior to the formation of precipitate free zones during pre-aging of AA7075-W, making the pre-aged hardening cryogenic forming process more suitable for automobile industry.

Researches on the formation of precipitate-free zones with different characteristics and corresponding correlation with creep fracture for the peak-aged Mg-10Gd-0.4Zr casting alloy

In this paper, the precipitate-free zones (PFZs) formed during creep testing in the peak-aged Mg-10Gd-0.4Zr casting alloy were carefully distinguished by different characteristics, the corresponding formation mechanisms and relationship with creep fracture were systematically investigated. The results showed that the PFZs approximately perpendicular to the stress direction can be classified into type I PFZs with one precipitate array and type Ⅱ PFZs with two precipitate arrays around grain boundary. Type I PFZs can be further classified into type IA and type IB according to the width difference between adjacent grains. The adjacent grains of type IA PFZs had a big difference in the plastic deformability, while type IB PFZs were with the similar plastic deformability. The different plastic deformability influenced the degrees of dislocation pile-up and accumulation on the two sides of precipitate array, which led to the formation of type IA PFZs with significant width difference. The type Ⅱ PFZs formed in the tertiary creep stage and had serrated grain boundary, which may be attributed to the strain-induced grain boundaries migration of type IB PFZs caused by localized stored energy difference between adjacent grains. Although the width of PFZs with different characteristics was roughly the same, the micro-crack almost initiated and propagated along the type IA PFZs, which was resulted from the large stress concentration. By contrast, type Ⅱ PFZs retarded micro-void formation and lowered micro-crack propagation rate due to the serrated grain boundary.

Influence of heat treatment on the microstructure and creep response of additively manufactured cobalt superalloy Mar M509

ABSTRACT This work explores the influence of various post-build heat treatment procedures on the microstructural characteristics and creep response of laser powder bed fusion (L-PBF) Mar M509 superalloy. Besides MC-type carbides formed primarily during the L-PBF process, the obtained results indicate the influence of heat treatment on the nucleation of M6C and M7C3 carbides due to elemental segregation. Short-term creep-rupture tests have been conducted at 900°C/110MPa on eight specimens, each subjected to a specifically individual heat treatment procedure. Rupture times are found to be sensitive to the heat treatment applied and vary from ~178 to ~641 hours. Detailed microscopic characterisation of ruptured specimens highlights the synergistic influence of oxidation and M7C3/M6C precipitate-free zone formation on macroscopic intergranular failure facilitated by grain boundary sliding along favourably oriented small grains. Besides notable grain growth, the presence of a serrated grain boundary structure with relatively less MC-type carbide coarsening is found to offer the best creep resistance among the eight L-PBF specimens. Further improvement of the elevated temperature performance of the alloy is seen to require a focus on the L-PBF process conditions to optimise the morphology of MC carbides, as they form.

Effects of Rapid Quenching on Grain Boundary Microstructure and Mechanical Properties of an Al-Mg-Si-Cu Alloy.

Precipitate free zones (PFZs) near grain boundaries generally soften alloys. The quenching rate after solution treatment is an important factor influencing the width of PFZs in Al-Mg-Si-Cu alloy. This study explored the effects of high quenching rates on the grain boundary microstructures and mechanical properties of an Al-Mg-Si-Cu alloy. Samples of various thickness were quenched in water at room temperature and in ethylene glycol at -40 °C, respectively. The results showed that the rapidly quenched samples at -40 °C exhibited better comprehensive mechanical properties than the water-quenched samples. Transmission electron microscopy studies revealed the rapidly quenched samples had wider PFZs, shorter intragranular precipitates, and larger grain boundary precipitates (GBPs) than water-quenched samples. It is proposed that when the quenching rate exceeds the critical cooling rate, e.g., in water quenching or rapid quenching, the formation of PFZs is controlled by the solute depletion mechanism rather than the vacancy depletion mechanism. The nucleation and growth of GBPs thus lead to the depletion of solute atoms, resulting in wider PFZs rather than thinner PFZs according to previous knowledge. This research provides valuable insights into the application of rapid quenching technology for modifying alloys' microstructures and properties.

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.

Anisotropy of Al-Li alloy plate and its heredity effect in mechanical property distribution of spun-dome

The mechanical properties and grain structures of 2A55 Al−Li alloy plate with 20 mm in thickness after T6-aging at 165 °C for 24 h were investigated. The aging precipitates, grain structures and mechanical property distribution along the circumferential and axial directions in the T6-aged spun-dome with 1 m in diameter spin-deformed through the 2A55 Al−Li alloy plate with 20 mm in thickness were studied. The precipitates in the T6-aged dome include a great number of T1 (Al2CuLi) precipitates and a small number of δ′ (Al3Li) and θ′ (Al2Cu) precipitates. The distribution of T1 precipitates is uneven, which is characterized by dense distribution at subgrain boundaries and the formation of precipitate-free zone (PFZ) at high angle grain boundaries. The T6-aged plate possesses almost same strength along a same direction, but there exists anisotropy with the lowest strength along 45° direction and the highest strength along the rolling direction. The T6-aged dome possesses great mechanical property discrepancy along a same circumferential direction and axial direction, which is correlated with the variation of the angle of the dome circumferential direction and axial direction to the rolling direction and the resulting different textures and grain aspect ratio at different locations. There exists a heredity effect in the grain structures and mechanical properties from the original hot-rolled plate to the subsequent spin-deformed dome.

Quantified effect of quench rate on the microstructures and mechanical properties of an Al–Mg–Si alloy

It is well known that lower quench rate leads to the formation of precipitate free zones (PFZs) adjacent to grain boundaries in aging hardening materials such as Al–Mg–Si alloys. However, the combined effect of PFZs and intragranular precipitates on the yield strength of Al–Mg–Si alloys is not systematically understood. To clarify this, an Al–Mg–Si alloy was water/oil/air quenched after solid solution treatment, then aged at 180 °C for 6 h. It was found that a lower quench rate led to coarser precipitates and wider PFZs. The effects of PFZs and precipitates on the yield strength of the Al–Mg–Si alloy were separated by quantitative characterization of multi-scale microstructures and mechanical simulations. Wide PFZs were found to have a notable effect on yield strength, amounting over half the strengthening effect of intragranular precipitates. This effect increases as the width of PFZ increases, due to more serious strain localization caused by PFZs. A corresponding equation has thus been proposed to describe the deleterious effects of PFZs on yield strength of Al–Mg–Si alloys.

Improved strength and ductility of rapidly solidified 2195 alloy by pre-rolling combined with double aging and interrupted aging

Pre-rolling combined with double aging (DAT8) and interrupted aging (IAT8) were carried out to improve the mechanical properties of rapidly solidified 2195 Al–Cu–Li alloy. For the DAT8 treatment (4% pre-rolling + 120 °C/12 h + 165 °C/16 h), low aging temperature (120 °C) in the first stage forms abundant δ′ phase, which grows into well-dispersed T1 plates in the second stage (165 °C). The IAT8 treatment (165 °C/2 h + 4% pre-rolling + 65 °C/5 day + 165 °C/16 h) first produces some small GP zones, which are connected to dislocations generated by pre-rolling. Subsequent long-term low-temperature aging (65 °C/5d) results in the enrichment of Cu, Mg, Ag and Li atoms on dislocations, significantly enhancing the precipitation driving force of the T1 phase. The results show that DAT8 and IAT8 treatments simultaneously improve the strength and ductility of the alloy. The increased strength is mainly due to the dense precipitation of the T1 phase, generating higher precipitation strengthening, as well as the introduction of high dislocation density, yielding strain strengthening. The improved ductility is attributed to the fine and well-dispersed T1 phase, decreasing the incongruous deformation between precipitates and matrix, and suppressing the formation of precipitation free zone (PFZ), weakening local stress concentration at sub-grain boundaries.

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

The Effect of Quench Parameters on Self-Piercing Rivet Joint Performance in a High Strength Automotive 6111 Aluminum Alloy

<div class="section abstract"><div class="htmlview paragraph">The process parameters to manufacture structural aluminum alloys are critical to their ductility. In particular, quench rate after solution heat treatment impacts the extent of grain boundary precipitation and the formation of precipitate free zone (PFZ) during later artificial aging. Cu-containing 6XXX alloys used for high strength automotive applications are quench sensitive as the Cu addition leads to Q-phase precipitation at grain boundaries, resulting in loss of ductility, which can negatively affect downstream manufacturing steps such as automotive joining and forming processes. Self-piercing rivet (SPR) joining, is a single step, spot joining process used to mechanically connect sheet materials together in automotive body structures. Ductility has been identified as an important metric of material rivet-ability or the ability to make a successful, crack-free SPR joint.</div><div class="htmlview paragraph">Through a full-scale industrial trial on a high strength 6111 aluminum alloy - varied quench rates ranging from slow to fast have been used to produce different grain boundary precipitation and PFZ characteristics. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize grain boundary precipitation and PFZ width as a function of quench rate. Multiple mechanical characterizations, including SPR joint evaluations were done as a function of quench rate. It is shown that uniform and total elongation metrics are not suitable to understand rivet-ability. Instead, ductility metrics from both bending and reduction of area measurements from tensile tests were found to correlate with SPR joint quality.</div></div>

Enhanced age hardening response and precipitation evolution of elastic stress aged Mg–Zn alloys

How to improve the age hardening response of Mg alloys in economical and effective ways is an important and interesting issue. The age hardening response and precipitation evolution of Mg–Zn alloys with and without external elastic stress are investigated. Microstructural observations of stress-free aged samples show that the low hardness is mainly due to the low number density and relatively small aspect ratio of β1′ precipitates, while a large number of nano-scale strengthening β1′ precipitates appear in the α-Mg matrix at the early stage of stress aging. In addition, the aspect ratio of β1′ precipitates in the stress aged samples is evidently higher than that of the stress-free aged samples at the same peak aged stage. Abundant dislocations introduced by the applied stress act as nucleation sites of precipitates and rapid diffusion path of solutes, which are mainly responsible for the remarkable increment in hardness and promoted kinetics. Precipitate-free zones ( PFZs) are observed in both samples with and without stress. In the stress aged samples, the formation of PFZs is mainly controlled by the motion of dislocations, which is different from the solute-vacancy depletion mechanism in the stress-free samples. Results imply that the external stress plays a similar role to that of commonly used methods in reducing the size, increasing the density, and promoting the precipitation kinetics, but has a negligible stress-orienting effect on the precipitate evolution.

Investigation of Structure Formation During Ageing of High-Strength Pseudo-β Titanium Alloy VT47

The morphology and temperature-time ranges of α-phase particle precipitation during ageing of metastable β-titanium alloy VT47 are established. The incubation period of α-particle nucleation in the temperature range of 350–650°С is determined by metallographic analysis. The effect of temperature-time parameters of ageing and conditions of secondary α-phase precipitate-free zones formation are studied in sheets of VT47 alloy. It is demonstrated that high-temperature ageing is characterized by precipitatefree zone formation, and for low-temperature ageing there are areas impoverished in precipitates. An increase of ageing duration up to 16 hours makes it possible to reduce the volume fraction significantly the volume fraction of precipitate-free zones over the whole range of the ageing temperatures studied.

Dislocation arrays, precipitate bands and free zones in forged Mg-Gd-Y-Zr alloy

Dislocation arrays are extensively generated in the hammer forged and then annealed Mg-8.3 Gd-2.6 Y-0.4 Zr (wt. %) alloy. Microscopy characterizations reveal a high thermal stability of these dislocation arrays. During ageing treatment, dislocation arrays act as preferred sites for precipitation of β' phase, promoting the formation of the bamboo-like precipitate bands, and resulting in a depletion of solutes nearby and the formation of precipitate free zones (PFZs) along these bands. The forged samples after annealing and then ageing at an optimized temperature of 265 °C for different periods exhibit a similar strength and strain hardening but an increased elongation with ageing time, suggesting that the precipitate bands act as strong barriers for the motion of dislocations and twins while PFZs can effectively enhance deformability of Mg-Gd-Y-Zr alloy.

Creep-induced microstructural evolution in a nickel-based superalloy designed for advanced ultra-supercritical boilers

Long-term microstructural stability is a major concern for candidate materials used in advanced ultra-supercritical (A-USC) power plants. In the present work, precipitation behavior of a nickel based superalloy designed for A-USC boilers, with a base composition of Ni-25Cr-20Co, was investigated during long-term thermal exposure under different stresses at 1043 K. The results showed that the coarsening of γ′ precipitates in the grain interior had occurred during thermal exposure in the presence and absence of stress. The microstructure around grain boundary was altered remarkably under stress. The precipitate-free zones (PFZs) commonly existed near the grain boundary in the gauge sections of specimens after long-term creep. Coarsened and elongated particles in the PFZs were identified as γ′-Ni3 (Ti, Al) precipitates by chemical composition analysis and diffraction pattern. Furthermore, the morphology and distribution of MC-type carbides have no significant changes observed due to their good stability, but discontinuous M23C6 appeared under stress, due to formation of PFZs resulting in dissolution of the adjacent M23C6. Besides, the creep-rupture mechanism was discussed to explore a potential approach for improving creep properties.

Effect of heat treatment variables on the formation of precipitate free zones (PFZs) in Mg-8Al-0.5Zn alloy

The present study is focussed on investigating the effect of heat treatment variables on the formation of precipitate free zones (PFZs) in the AZ80 Mg alloy. The effect of solutionizing temperature, quenching media, aging temperature and time on the PFZ development has been systematically examined. Scanning electron microscopy and nanoindentation were employed to characterize the PFZ regions. Electron probe micro analysis (EPMA) was used to examine the role of solute depletion in the formation of these zones. Moreover, the estimation of vacancy concentration profiles near the grain boundary was used to investigate the role of vacancy depletion. The solute depletion is found to be the dominant mechanism in the development of PFZ regions as compared to vacancy.

Modeling of particle coarsening and precipitation free zones

Previous simulation studies of the effect of volume fraction on the precipitate coarsening process have treated precipitates as point sources or sinks of solute atoms. These studies have demonstrated that, although the average particle size varying with time as is still valid, the coarsening rate constant is an increasing function of volume fraction. In this study we extend these simulation methods to model particle coarsening near grain boundaries. An essential feature of the new model is the solute depletion that occurs at grain boundaries. The simulation results are shown to quantitatively reproduce the following aspects of particle coarsening near grain boundaries in certain alloys: (a) precipitate free zones (PFZs) form near grain boundaries, (b) the width of PFZs increases with time and is proportional to the square root of time, (c) particles at the edge of PFZs are larger than those inside the grain. This novel model is shown to be well suited to describe particle coarsening near grain boundaries. In addition, it reinforces the credibility of the theories built into our mathematical model, i.e., the formation of PFZs near grain boundaries is caused by diffusion of solute atoms.

Nano-scale precipitation and phase growth in Mg-Gd binary alloy: An atomic-scale investigation using HAADF-STEM

In the present study, nanoscale precipitation and phase growth in Mg-15Gd(wt%) binary alloy are comprehensively studied by atomic-resolution high angle annular dark field - scanning transmission electron microscopy (HAADF-STEM). Various nanoscale precipitate structures, including βH, β′, βT′, βM and β′′-like structure, assembled by the “basic unit” as building block, are observed, reconstructed and discussed in detail. In the early stage of precipitation, the precipitate growth basically follows y (nm)=1.53x−2.68 (nm) and protrusions start to grow when the size of a precipitate reaches a threshold value. Long-range diffusion of Gd solute preferentially proceeds along <101¯0>Mg directions and protrusions are formed. A large number of fine β′ precipitates exist and act as the key strengthening structures. The formation of precipitate-free zones in the over-age condition is attributed to the intertwining of precipitates.

Effects of uniaxial creep ageing on the mechanical properties and micro precipitates of Al-Li-S4 alloy

In the present work, effects of creep ageing (CA) on the mechanical properties and micro precipitates of Al-Li-S4 alloy was investigated. Compared with single stress-free ageing (SFA), experimental results show that CA has significantly improved the mechanical properties of Al-Li-S4 alloy, including hardness, strength and elongation. Meanwhile, the peak-ageing time was also prolonged under CA treatment. When specimens were continuously placed under an exterior stress of 240MPa after CA treatment at 153°C for 25h, more T1(Al2CuLi) nano-precipitates developed and further contributed improvement in mechanical properties. It is mainly attributed to the exterior stress that promoted to form T1 nano-precipitates. Moreover, CA enabled to restrict both the coarsening of precipitates at grain boundaries and the formation of precipitation free zone. In addition, fractographic examination of SFA-treated and CA-treated Al-Li-S4 tensile specimens were carried out, respectively. Through controlling the formation process of fine T1 nano-precipitates during CA treatment, it can facilitate the development of novel microstructures in Al-Li-S4 alloys with certain promising mechanical properties.

Effect of Cold Work on Creep Rupture Strength of Alloy263

Creep rupture strength and the microstructure change during creep deformation of pre-strained Alloy263 were investigated. Creep rupture tests were conducted at 1023, 1073 K at stress range from 120 to 250 MPa. Creep strength of the pre-strained samples was higher than that of the non-strained samples. However, rupture strain of the pre-strained samples was much lower than that of non-strained samples. In the pre-strained samples, Ni₃(Al, Ti)- γ’ and M₂₃C₆ inside of the grains precipitated finer than the non-strained samples compared at the same creep time. At grain boundaries, the grain boundary shielding ratio covered by M₂₃C₆ carbide showed almost same value among each samples. However, diameter of the M₂₃C₆ particle decreased in pre-strained samples. Furthermore, dynamic recrystallization was promoted and precipitation free zone (PFZ) was formed around Ni₃Ti-&eta; phase at grain boundary. These observations show that increase in creep-strength of pre-strained sample was due to increase in precipitation strengthening in the grain by fine precipitation of γ’ and M₂₃C₆. In addition, resistance against crack propagation at grain boundary increased by the fine precipitation of grain boundary M₂₃C₆ even though formation of PFZ and promotion of the dynamic recrystallization. It is estimated that Orowan-stress of pre-strained samples was 1.7 times higher than non-strained samples. It is considered that these strengthening effects overcome the weakening effects in the pre-strained samples.