Quenching sensitivity and heterogeneous precipitation behavior of AA7136 alloy

The quenching sensitivity of AA7136 alloy was investigated by time−temperature−property (TTP) diagrams, and the heterogeneous precipitation behavior during isothermal holding was investigated using scanning electron microscopy, scanning transmission electron microscopy and high resolution transmission electron microscopy. Based on 99.5% TTP diagram, the nose temperature is determined to be about 346 °C with the transformation time of about 0.245 s. The precipitation of η (MgZn₂), T (Al₂Zn₃Mg₃), S (Al₂CuMg) or Cu−Zn-rich Y phases can be found depending on isothermal holding temperature and time, and it is described in a time−temperature−precipitation diagram. The size and area fraction of isothermal holding induced phase particles increase, which results in the decrease of hardness of samples after aging. The quantitative contribution to loss of hardness by grain boundaries/subgrain boundaries and dispersoids in the matrix is discussed based on the amount of heterogeneous precipitation related to them.     

Phase precipitation behavior and tensile properties of as-cast Ni-based superalloy during heat treatment

The phase precipitation behavior and tensile properties of an as-cast Ni-based alloy, IN617B alloy, after solution heat treatment and long-term aging treatment were investigated. Ti(C,N), M₆C and M₂₃C₆ are the primary precipitates in as-cast microstructure. After solution heat treatment, most of carbides dissolve into the matrix except a few fine Ti(C,N) within grains. During long-term aging at 700 °C, the phase precipitation behaviors of the alloy are characterized as follows: (1) M₂₃C₆ carbides at grain boundaries (GBs) transform from film-like shape to cellular shape and gradually coarsen due to the decrease of the surface energy and element aggregation to GBs; (2) M₂₃C₆ carbides within grains have a bar-like morphology with a preferential growth direction [110] and have a cube-on-cube coherent orientation relationship with the matrix γ; (3) γ′ particles inhibit the coarsening of M₂₃C₆ within grains by constraining the diffusion of formation elements. Furthermore, the tensile strength of the alloy obviously increases, but the ductility significantly decreases after the aging for 5000 h. The alloy has a relatively stable microstructure which guarantees the excellent tensile properties during long-term aging.     

Microstructure evolution and deformation behavior of ultrafine-grained Al–Zn–Mg alloys with fine η′ precipitates

An aged Al–5Zn–1.6Mg alloy with fine η′ precipitates was grain refined to ～100 nm grain size by means of confined channel die pressing. Microstructure observations and mechanical tests were carried out to characterize the materials before and after various degrees of severe plastic deformation. Deformation processing enhanced the strength of the alloy, but limited its work hardenability. An analysis of deformation mechanisms revealed that plasticity proceeded by dislocation slip through ultrafine-grained cellular and subgrain arrangements. η′ precipitates strengthened the alloy by dispersion hardening, but retarded an increase in the strain rate sensitivity during grain refinement. The influence of η′ precipitates is discussed with respect to their effect on dislocation configurations and deformation mechanisms during processing of the alloy.     

Influence of Mg content on quench sensitivity of Al-Zn-Mg-Cu aluminum alloys

The influence of Mg content on quench sensitivity of Al-8.0Zn-x(=1.0-2.0)Mg-1.6Cu alloys was investigated by an end-quenching test method. The depths of age hardening layer referenced to hardness retention values (e.g., 90%) of the end-quenched samples were used to describe the quench sensitivity. The results showed that the depths of age hardening layer decreased with increase of Mg content. The amount of the equilibrium-state MgZn₂ (η) particles was the primary factor that determined the depths of age hardening layer. The η particles underwent two precipitation processes successively with decrease of cooling rate during quenching: precipitation on grain boundaries and precipitation inside the grains. The precipitation temperature peaks rose, and the ranges of the temperatures are extended with the increase of Mg content. A preliminary prediction has been made for the initial precipitation temperatures and the temperature peaks satisfying linear relation with Mg content (wt.%).     

Aging precipitation behavior and properties of Al–Zn–Mg–Cu–Zr–Er alloy at different quenching rates

The effect of quenching rate on the aging precipitation behavior and properties of Al–Zn–Mg–Cu–Zr–Er alloy was investigated. The scanning electron microscopy, transmission electron microscopy, and atom probe tomography were used to study the characteristics of clusters and precipitates in the alloy. The quench-induced η phase and a large number of clusters are formed in the air-cooled alloy with the slowest cooling rate, which contributes to an increment of hardness by 24% (HV 26) compared with that of the water-quenched one. However, the aging hardening response speed and peak-aged hardness of the alloy increase with the increase of quenching rate. Meanwhile, the water-quenched alloy after peak aging also has the highest strength, elongation, and corrosion resistance, which is due to the high driving force and increased number density of aging precipitates, and the narrowed precipitate free zones.     

Effect of continuous cooling on the decomposition processes in a Zn–1.6 wt.% Al alloy

The decomposition processes in a two-phase Zn–1.6 wt.% Al alloy, homogenized and quenched to room temperature through continuous cooling (1.5 K min⁻¹), have been studied by scanning electron microscopy. At the beginning of cooling, continuous precipitates were formed at grain boundaries and in grain interiors (α₀ phase, Zn-rich solid solution), with simultaneous eutectoid decomposition of non- stoichiometric intermetallic ZnAl (β phase). As the cooling proceeded, the density of continuous precipitates at grain boundaries and in grain interiors increased and at the same time the coarsening of eutectoid phase initiated. Although some precipitates at grain boundaries have grown by a ‘pucker mechanism', the occurrence of discontinuous precipitation was not observed during cooling. After a long period of aging, however, discontinuous precipitation cells were formed through basically two stages: (a) precipitation of a metastable Al-rich phase on the continuous precipitates formed at grain boundaries and (b) nucleation and growth of discontinuous precipitation cells within the metastable Al-rich phase.     

Improvement of strength and ductility of Al-Cu-Li alloy through cryogenic rolling followed by aging

To develop an improved approach in achieving an excellent combination of high strength and ductility, the solutionized Al-Cu-Li plates were subjected to rolling at cryogenic and room temperatures, respectively, to a reduction of 83%, followed by aging treatment at 160 °C. The results indicate that Al-Cu-Li alloys through cryogenic rolling followed by aging treatment possess better mechanical properties. Rolling at cryogenic temperature produces a high density of dislocations because of the suppression of dynamic recovery, which in turn promotes the precipitation of T₁ (Al₂CuLi) precipitates during aging. Such high density of T₁ precipitates enable effective dislocation pinning, leading to an increase in strength and ductility. In contrast, room temperature rolled alloys after aging treatment exhibit lower strength and ductility due to low density of T₁ precipitates in the grain interior and high density of T₁ precipitates around subgrain boundaries.     

Tailoring phase fractions of T’ and η’ phases in dual-phase strengthened Al-Zn-Mg-Cu alloy via ageing treatment

Hardness tests and transmission electron microscopy were used to investigate the strategy of tailoring the phase fraction of precipitates in an Al-Zn-Mg-Cu alloy strengthened by T’ and η’ phases. Different phase fractions of T’ and η’ phases are presented in samples subjected to either single or two stages of ageing treatments at 120 and 150 °C. For both types of ageing, the precipitation of η’ phase is found to be promoted by ageing at lower temperature and its phase fraction increases with prolonging ageing time at 120 °C; whereas the phase fractions of T’ and η’ phases almost remain constant during ageing at 150 °C. Besides, the strain fields produced by T’ and η’ phases were analyzed by using the geometric phase analysis technique, and on a macroscale the contributions of T’ and η’ phases to precipitation strengthening have been quantitatively predicted by combining the size, phase fraction and number density of precipitates.     

Distribution and evolution of aging precipitates in Al-Cu-Li alloy with high Li concentration

The evolution and distribution of the aging precipitates in 1460 Al-Li alloy with high Li concentration (2.14%, mass fraction) during T6 aging and two-step T8 (4% predeformation) aging were investigated through TEM. The aging precipitates include δ′ (Al₃Li) and T1 (Al₂CuLi) phases, of which the δ′ phases are formed first in grain interiors. A lot of δ′/GPI/δ′ composite precipitates in which GPI zones are flanked with a pair of δ′ phases, are formed at 145 °C of T6 aging, which are thermally stable. At 160 °C and 175 °C of T6 aging, many T1 phases nucleate first at subgrain boundaries and grain boundaries, and then form and grow within grains. As to the T8 aging, the δ′/GPI/δ′ composite precipitates are formed during the first-step aging at 130 °C for 20 h, which are thermally stable during the second-step aging at 160 °C. The plastic predeformation accelerates T1 nucleation within grains during the second-step aging at 160 °C.     

Effect of the Size of Al<Subscript>3</Subscript>(Sc,Zr) Precipitates on the Structure of Multi-Directionally Isothermally Forged Al-Mg-Sc-Zr Alloy

The effect of Al₃(Sc,Zr) dispersoids on the evolution of the cast Al-Mg-Sc-Zr alloy structure under multi-directional isothermal forging (MIF) has been investigated. The alloy, which has an equiaxed grain structure with a grain size of ~25 μm and contains dispersoids 5–10 and 20–50 nm in size after onestage (at 360°C for 6 h) and two-stage (360°C for 6 h + 520°C for 1 h) annealing, respectively, was deformed at 325°C (~0.65 T_m) up to cumulative strains of e = 8.4. In the initial stages of MIF, new fine (sub)grains surrounded by low-angle and high-angle boundaries (HABs) were formed near the initial grain boundaries. With increasing strain, the volume fraction and misorientation of these crystallites increased, which led to the replacement of a coarse-grained structure with a fine-grained one with a grain size of ~1.5-2.0 μm. Dynamic recrystallization occurred in accordance to a continuous mechanism and was controlled by the interaction of lattice dislocations and/or (sub)grain boundaries with dispersoids that effectively inhibited the migration of boundaries, as well as the rearrangement of lattice dislocations and their annihilation. The particle size and the density of their distribution significantly affected the parameters of the evolved structure; in an alloy with smaller particles, a structure with a smaller grain size and a larger HAB fraction developed.     

AZ91镁合金的挤压和析出硬化行为(英文)

挤压比为4:1,将铸态AZ91镁合金分别在250,300和350℃下进行挤压,随后进行析出硬化处理(T6)。经过热挤压和析出硬化处理后,铸态AZ91镁合金中粗大的和偏析Mg17Al12析出相被细化并均匀分布在α-镁基体中。在不同的挤压温度下合金中发生了部分或全部动态再结晶。经挤压后,该合金的极限抗拉强度从铸态的190MPa增加到570MPa。AZ91镁合金的时效硬化特征与晶粒尺寸有关。在250、300和350℃下以4:1的挤压比挤压该合金后,获得峰值硬度的时效时间分别为35、30和20h。SEM观察到在AZ91基体中存在均匀细小的Mg17Al12析出相。     

Precipitation sequences during quenching of the AA 7010 alloy

The precipitation sequences have been investigated during the cooling of an aluminium alloy AA 7010 from the solutionizing temperature. Interrupted quenching was performed at various temperatures and different holding times. The microstructure was then characterized at room temperature after the final fast cooling, using complementary experimental techniques.At high temperatures, the η phase nucleates on dispersoids and grain boundaries. The composition, as well as the growth rate, of coarse intragranular precipitates are measured. At lower temperature (typically 250 °C), the solid solution decomposition promotes the precipitation of the η phase first, and then of the S and T phases. At low temperature, the precipitation of the η’ phase is observed. In this domain, the growth kinetics is followed. Those observations have been reported in a nucleation diagram.     

Microstructure evolution and nitrides precipitation in in-situ Ti2AlN/TiAl composites during isothermal aging at 900 °C

Microstructures of Ti₂AlN/TiAl composites prepared by in-situ method were characterized in in-situ and aging treatment conditions and the nitride precipitation was investigated in Ti₂AlN/TiAl composites aged at 900 °C for 24 h after being heat treated at 1400 °C for 0.5 h. The in-situ composites consist of γ+α₂ lamellar colonies, equiaxed γ grains and Ti₂AlN reinforcements. Matrix with nearly fully lamellar structure formed after solution and subsequently aging treatment. With the increase of Ti₂AlN content, the nearly fully lamellar structure becomes instable for the aged composites. According to TEM study, fine Ti₂AlN precipitates are found to distribute at the grain boundaries of lamellar colony. Needle-like Ti₃AlN precipitates arrange in line with growing axis parallel to [001] direction of the γ-TiAl matrix and another needle-like Ti₃AlN precipitates with lager size distribute at the dislocations.     

Quantitative analysis of δ′ precipitation kinetics in Al–Li alloys

The Rietveld method has been applied to X-ray spectra in order to study the precipitated mass fraction of δ′ and δ in Al–Li alloys. The method allows us to obtain quantitatively the δ′ and δ precipitate mass fractions, and their evolution with aging time. Furthermore, this method also gives directly the cell parameter evolution of the matrix phase and indirectly the mean half radius of δ′ precipitates through an appropriate calibration curve. Experimentally, this calibration has been approached by previously studying the evolution of the mean half radius of δ′ by transmission electron microscopy (TEM). Thermoelectric power has also been shown to be a powerful technique to study the microstructural evolution of Al–Li alloys, being sensitive to the different stages of precipitation associated to the δ′ and δ phases. The comparison of the different experimental results allow us to stablish a clear difference between the precipitation kinetics and the hardening kinetics.     

6063铝合金的TTP曲线与淬火敏感性

采用中断淬火技术测定了6063铝合金的时间-温度-性能(TTP)曲线,透射电镜研究了6063铝合金的淬火敏感性.结果表明,6063铝合金的淬火敏感性低于6061和6082铝合金的,合金的鼻尖温度为360℃,淬火敏感区间为300～410℃.微观组织观察表明,在敏感区间内,β-Mg2Si平衡相优先在(AlxFeySiz)相上非均匀形核而析出,且在360℃鼻尖温度时的长大速度最快.平衡相的析出导致合金溶质原子的浓度下降,减少了时效时的β"强化相的数量,降低了强化效果.因此,对于6063铝合金大型材的淬火,一方面,在淬火敏感区间(410～300℃)应加大冷却速率以抑制平衡相的析出,从而获得较佳的时效强化效果;另一方面,适当减小从固溶温度到410℃以及低于300℃时的冷却速率,从而减小淬火应力.     

Dispersoid precipitation and process modelling in zirconium containing commercial aluminium alloys

Small dispersoid particles inhibit recrystallisation and are thus critical in controlling the grain structure of many high strength commercial aluminium alloys. A general, physical model has been developed for the precipitation of Al₃Zr dispersoids in aluminium alloys. The predictions of the model have been compared with results of an experimental investigation of Al₃Zr precipitation in 7050. The model has been shown to faithfully reproduce the distribution of dispersoids observed in this alloy, correctly predicting dispersoid free zones observed in interdendritic regions and at grain boundaries. Furthermore, the predicted precipitation kinetics agree well with experimental observation. The model has been used to study the effects of homogenisation conditions and alloy composition on dispersoid formation and has been shown to be a powerful tool for optimising the dispersoid distribution in 7xxx series aluminium alloys.     

淬火析出对Al-Zn-Mg-Cu合金时效行为的影响(英文)

采用硬度测试、光学显微镜(OM)、透射电镜(TEM)和差示扫描量热法(DSC)研究淬火和时效(T6、T7、RRA)对Al-Zn-Mg-Cu合金微观组织的影响。研究发现水淬合金经T6时效后的硬度最高。T7和RRA时效后样品的淬火敏感性相当,较T6时效的高1.2%。TEM观察表明,合金的淬火敏感性主要是由缓慢冷却时非均匀析出引起的。大量η相在再结晶晶粒内的Al3Zr弥散粒子和(亚)晶界上形核,而S和T相在有高密度位错和缺陷的亚结构区生成。时效后,平衡η相周围的η'相更加粗大。经T6、T7、RRA处理后,这些析出相的尺寸和形貌呈现出不同的特征。DSC结果与TEM观察结果一致。T6态的DSC曲线和T7、RRA态的不同,反映了不同的微观组织。     

Precipitation location of secondary phase and microstructural evolution during static recrystallization of as-cast Ti-25V-15Cr-0.3Si titanium alloy

The microstructural evolution and precipitation location of the secondary phase of an as-cast Ti-25V-15Cr-0.3Si titanium alloy were investigated via isothermal compression experiments and heat treatment. The average aspect (length-to-width) ratio, average area and size of the grains at different heat treatment temperatures and holding time were analyzed and the effects of deformation and annealing time on the grain area and size were considered. It was found that the grain size was strongly influenced by the height reduction and holding time. Grain growth was significant when annealing time increased from 10 min to 2 h at 950 °C and height reduction of 30%; however, grain growth was minimal at annealing time between 2 and 4 h. Many dispersion particles were observed to form in continuous chains; the precipitation location was confirmed to be along initial grain boundaries, and the dispersion particles were identified to be Ti₅Si₃ phase by TEM.     

Effect of aging time and temperature on the aging behavior in Sn containing AZ91 alloy

Effects of aging temperature and time on the aging behavior in AZ91 alloy and Sn containing AZ91 alloy (AZT915) have been investigated in the present study. The mode of precipitation, i.e. discontinuous and continuous precipitation in both alloys is strongly affected by the aging temperature. At low aging temperature of 403 K, only discontinuous precipitation occurs at the grain boundaries, whereas at high aging temperatures of 573 and 623 K only continuous precipitation occurs inside the grains. At intermediate temperature range (443 or 498 K) both discontinuous and continuous precipitation reactions occur. In AZT915, the Mg₂Sn particles at the grain boundary effectively reduce the available nucleation sites for discontinuous β precipitates, and slow down the movement of the grain boundary, resulting in suppression of discontinuous precipitation. In addition, increased local lattice strain by the presence of Sn in the α-Mg solid solution matrix accelerates the nucleation of the continuous precipitates at the early stage of aging treatment. Therefore, significantly higher peak hardness can be obtained within a shorter aging time in AZT915.     

Accelerated precipitation in the AFA stainless steel Fe–20Cr–30Ni–2Nb–5Al via cold working

The effects of cold work on the microstructural evolution during aging of a solutionized alumina-forming austenitic stainless steel, Fe–20Cr–30Ni–2Nb–5Al (at.%), were investigated using scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy. Cold work prior to aging at either 700 °C or 800 °C facilitated the heterogeneous precipitation of both Laves phase and B2-type NiAl precipitates. While often co-located after cold work, these particles were distinct. γ′-Ni₃Al precipitates were also observed in samples aged at 700 °C with 90% prior cold work. Compared to material that had not been strained, defects introduced by 50 and 90% cold work at 700 °C and 90% cold work at 800 °C not only caused a more rapid precipitation in the matrix but also an increase in the total volume fraction of precipitates as compared to material that had been simply aged.