Over-aging Stage Research Articles

Synergistic effects of Ag and Sc addition on superior thermal stability in Al-Mg-Si-Cu alloy

Al-Mg-Si-Cu-Ag-Sc alloy with superior thermal stability has been designed, and a new mechanism for enhancing thermal stability was suggested in this study. The complex addition of Ag and Sc and the increased Cu content significantly enhanced the thermal stability of the Al-Mg-Si-Cu alloy by preventing the growth of over-aged precipitates. Particularly, Ag addition only delayed hardness degradation at the over-aging stage, whereas Sc addition lowered both the precipitation kinetics during the early and over-aging stages. The different roles of Ag and Sc create a synergistic effect so that excellent thermal stability can be obtained by the complex addition of Ag and Sc. The role of Ag in enhancing thermal stability generally depends on the conventional mechanisms of disordering and solute segregation. However, since the amount of Sc detecting was too low to have a sufficient influence on the thermal stability of the precipitate, the presence of Sc solute in the matrix was predicted. Solute analysis of the matrix revealed that Sc enhanced the thermal stability by disturbing Cu diffusion to Cu-containing precipitates. The strong Sc-Cu interaction enabled the temporary catching of Cu atoms by forming an Sc-Cu pair. Therefore, Sc not only enhances thermal stability but also delays age-hardening kinetics because the formation of Cu-containing precipitates is disturbed by the delaying mechanism depending on Sc-Cu pair formation. Additionally, the observation that the complex addition of Ag and Sc accelerated the formation of Mg-rich clusters during early stage aging provides minor evidence for the enhanced thermal stability of the Ag-Sc added Al-Mg-Si-Cu alloy. This is because Mg-rich clusters may be related to precursors of Cu-containing precipitate due to their high Cu contents and high Mg composition of Q′ and L phases.

Precipitation behavior and performance evolution of cold-rolled cu-Ti-Fe alloy during heat treatment

The CuTi alloy is extensively utilized for its high strength, excellent elasticity, and processability. Heat treatment processes are crucial for affecting the microstructure and properties. The effects of different aging processes on the microstructure and properties of cold-rolled Cu-Ti-Fe alloy were investigated, and the heat treatment parameters were optimized. The results show that the cold-rolled Cu-Ti-Fe alloy exhibits excellent comprehensive performance at 450 °C for 2 h, with the hardness of 342.2 HV, the electrical conductivity of 16.1 % IACS, and the tensile strength of 1051 MPa. The aggregation of solute atoms occurs in the early stages of aging. The uniformly distributed β'-Cu4Ti phase precipitates at peak aging, which has a coherent relationship with the matrix. The precipitation of Ti atoms enhances the electrical conductivity of the alloy, and the movement of dislocations is prevented by precipitates, increasing the strength. During the over-aging stage, the precipitates transform into β-Cu4Ti phases, losing complete coherency with the matrix. The coarsening of precipitates leads to the softening of the Cu-Ti-Fe alloy. Theoretical calculation results indicate that the thermal diffusion ability of solute atoms is the strongest and precipitates completely when the alloy aged at 450 °C. The precipitation strengthening mechanism plays a significant role in improving the strength.

Aging hardening and precipitation evolution of Mg-3.0Sm-1.4Nd-0.4Zn-0.5Zr alloy

Precipitation-hardenable Mg-Sm-Nd-Zn-Zr alloy is a high-performance magnesium alloy with great potential for aviation applications. A more comprehensive understanding of the precipitation evolution will help fully exert its mechanical properties. The hardness initially rose and then fell with alloy aging to 100 h at 200 °C. A peak hardness of 84.1 HV was achieved at the aging time of 20 h, with a 39.7 % increase in hardness compared to the unaged alloy. The primary precipitates were the GP zones and β′ phase at the under-aged stage. As the aging time was extended to 20 h, a more abundant β′ phase precipitated in the matrix, and the alloy achieved the peak-aged stage. Eventually, the over-aging stage came with the aging process continuing, where the β′ phase coarsened severely and further transformed into the β1 phase. When the aging temperature was raised to 350 °C, even under a short aging time of 1 h, the coarse β1 phase formed rapidly, and the β phase could also nucleate and grow directly on the β1 phase. This work offers a guide for developing aging treatment processes for Mg-RE alloys.

Coarsening mechanism of over-aged δ-Ni2Si nanoscale precipitates in Cu-Ni-Si-Cr-Mg alloy

δ-Ni2Si precipitate is the crucial strengthening phase in Cu-Ni-Si alloy, and its coarsening behavior and mechanism impose dramatic effects on mechanical properties. In this paper, Cu-3Ni-0.7Si-0.1Cr-0.05 Mg (wt%) alloy was prepared to systematically investigate the coarsening behavior and elucidate the coarsening mechanism of δ-Ni2Si precipitate during over-aging treatment. Featured core-shell structure was observed via careful characterization during coarsening of δ-Ni2Si phase. Specifically, the core structure was stable δ-Ni2Si, while the shell structure was a metastable interface composed of Cu, Ni and Si elements. At later over-aging stage, the core-shell structure completely transformed into a stable and coarsened δ-Ni2Si phase being coherent with the Cu matrix. It is confirmed for the first time that the coarsening process of δ-Ni2Si precipitate can be interpreted by the trans-interface-diffusion-controlled (TIDC) coarsening theory. The core-shell structure was caused by the disparity between the diffusion activation energy and diffusion rate of Ni, Si, and Cu from the perspective of diffusion kinetics. Moreover, the presence of trace amounts of Cr and Mg components exhibited little effect on the coarsening process of δ-Ni2Si. During over-aging, tensile strength dramatically decreased owing to diminishing precipitation strengthening effect from coarsening of δ-Ni2Si precipitates.

Effect of Ag and Ge on the precipitation behavior and resultant properties of AA6061 alloys

This study investigated the effects of Ag and Ge on the precipitation behavior and resultant properties of AA6061 alloys using hardness measurements, electrical conductivity tests, transmission electron microscopy (TEM), and atom probe tomography (APT). Trace addition of Ge can increase the electrical conductivity and thermal stability, but slightly deteriorates the hardness during the early- and peak-aging stages. Trace addition of Ag can enhance the hardness during aging, especially in the over-aging stage, but it deteriorates the electrical conductivity. For the Ag-added alloys, the Ag atoms provide more nucleation sites and are substantially incorporated into the clusters, suggesting the formation of various types of precipitates with increasing aging time. During the peak- and over-aging stages, the Ag atoms not only segregate but also incorporate into the precipitates, resulting in the disordering of the precipitates and suppression of the coarsening of the precipitates. For the Ge-added alloys, the formation and evolution of the clusters are suppressed by Ge addition, owing to the high binding energy between the Ge and Mg atoms. With the aging time increasing, the Ge atoms are incorporated into the precipitates and suppress the ordering process of the precipitates, resulting in better thermal stability of the Ge-added alloys in the over-aging stage. The addition of both Ag and Ge shows potential for elevated temperature component applications.

Revisiting Precipitation kinetics in Mg-Zn alloy – a multi-characterization and modeling study

This work aims to provide new insights into understanding precipitation kinetics in Mg-Zn alloys. First, the evolution of Zn solute, precipitate size and volume fraction is characterized using a combination of thermoelectric power, X-ray diffraction, atom probe tomography and transmission-electron microscopy in an Mg-1.7at.%Zn alloy when aged at 150 °C and 200 °C. The experimental results are then supported by a precipitation model based on classical nucleation and growth theories for rod and plate-shaped precipitates. It is found that rod-shaped precipitates are prevalent in the early aging stage, while plate-shaped precipitates dominate the over-aging stage. The kinetics of both precipitates are depicted in detail to represent this competition. Finally, a time-temperature-transformation diagram for these two types of precipitates is proposed for this system.

Effect of Mg Content and Cooling Rate After Solidification on the Mechanical Properties of T5-treated Al-Si-Mg Alloy

The effects of Mg content and cooling rate on the T5 treatment of Al-Si-Mg alloys have been investigated using Avrami kinetics and transmission electron microscopy. The alloys were super-heated at 800 oC for 1 hr followed by degassing treatment at 720 oC for 15 min, and poured into a metallic mold preheated to 250 oC. The as-cast alloys were then T5-treated at a temperature of 190 oC. As the Mg content and/or cooling rate increased, the strength of the T5-treated alloys increased while the elongation decreased over the entire aging time. However, some T5 treatment conditions were effective to at enhancinge elongation as well as strength. The alloy with low Mg content showed excellent elongation in the early stage of aging. B, but as the aging time increased, h. However, the elongation decreased rapidly so that there was showed no significant difference from with the alloy with high Mg content in the over-aging stage. This was due to a coarse and agglomerated precipitate structure, which were likely to be formed by the rapid precipitation and agglomeration of the β′ phase. The alloy with a high cooling rate exhibited superior strength with similar elongation over the entire aging time compared to the slowly cooled alloy. The reason for the decrease in strength for in the slowly cooled alloy was attributed to the coarsening of precipitates and suppression of the formation of fine precipitates during cooling.

Microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging

The microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging were studied using hardness testing, conductivity testing, differential scanning calorimetry (DSC), and high resolution transmission electron microscopy (HRTEM). The results show that the precipitation sequence of the Al-Mg-Si alloy during initial aging can be represented as: supersaturated solid solution → spherical Mg/Si clusters → needle-like Guinier Preston (GP) zone → β″. Clusters are completely coherent with the Al matrix. The GP zone with relatively complete independent lattice parameters that differ slightly from the Al matrix parameters, is oriented along the direction of <111>Al and lying on {111}Al plane. The strength of the Al-Mg-Si alloy is greatly enhanced by the three-dimensional strain field that exists between the β″ phase and the two {200}Al planes. After aging at 170 °C for 6 h, the hardness reaches the peak of 127 HV and remains for a long time. At this stage, the electrical conductivity keeps relatively stable due to the formation of coherent precipitates (Mg/Si clusters/GP zones) and the reduction in solute atom concentration in the Al matrix. The severe coarsening and decreased number density of the β″ phase during the over-aging stage result in a significant decrease in the hardness.

Microstructure changes and enhanced properties of an automotive AlMgSi alloy improved by micro-alloying La-addition

Micro-alloying of rare earth elements can improve the properties of Al alloys, but the mechanisms how they change the microstructures and therefore the properties of the alloys are still generally unclear. Using scanning electron microscopy and atomic-resolution transmission electron microscopy, here we have revealed the microstructural differences between two AlMgSi alloys with and without La-addition, and investigated the impacts of La-addition on specific microstructure-property relationships of the material. It is shown that La-addition has three significant effects on the microstructures and the properties of AlMgSi alloys. Firstly, La-addition changes the types of precipitates formed during aging. Most of β“ precipitates formed in the peak-aged alloy with La-addition are polycrystalline β” precipitates. In the over-aging stage of this alloy, the dominant precipitates formed are the β”/U2, β”/β’ and β’/U2 composite precipitates, rather than the expected β’ precipitates. Secondly, La-addition induces the formation of numerous FCC AlSiLa intermetallics, which significantly refine the grain size and enhance the strength and the ductility of the alloy. Thirdly, La-addition may enhance the electrical conductivity of the age-hardened alloy by reducing the solute atoms in the Al-matrix.

Precipitation behaviors and property variations of Cu-3.0 wt% Ti fabricated by a novel short-processing non-vacuum heating-cooling combined mold continuous casting

A Cu-3.0 wt% Ti alloy is fabricated by a novel non-vacuum short-process based on heating-cooling combined mold (HCCM) continuous casting in this study. Similar properties including tensile strength, hardness and electrical conductivity are obtained by this newly developed process, compared to its counterpart alloy prepared by conventional vacuum smelting. The tensile strength, total elongation and conductivity of the HCCM prepared Cu-3 wt% Ti alloy were 895 MPa, 13% and 13.00% IACS, respectively, after 95% deformation and aging treatment at 450 °C for 1 h. The cast materials fabricated by HCCM method can be directly cold rolled and aged after solution treatment without hot deformation such as hot forging. Through the HCCM continuous casting method, not only the Cu-Ti alloy can be fabricated under non-vacuum conditions, but also the fabrication process can apparently shortened by shortening the solid solution treatment time. Similar precipitation process is also found in the Cu-3.0 wt% Ti alloy prepared by this method. The precipitation of the ordered, metastable and coherent β'-Cu4Ti phase with body-centered tetragonal (bct) structure are formed in the early stage of aging treatment, which is transformed into stable β-Cu3Ti with an orthogonal structure in the over-aging stage. The precipitation hardening of β'-Cu4Ti phase, i.e., Orowan strengthening effect, is believed to be the leading strengthening mechanism in this alloy.

Effect of Heat Treatment on Mechanical Properties and Fracture Behavior of Al-7.0Si-0.3Mg Alloy at Low Temperature

In order to investigate the effect of heat treatment on the mechanical properties of the Al-7.0Si-0.3Mg alloy at 20–60 °C under different heat treatment processes, the tensile mechanical properties of Al-7.0Si-0.3Mg at low temperature after heat treatment were explored. The microstructure of Al-7.0Si-0.3Mg was observed by scanning electron microscopy (SEM) and transmission electron microscopy. The results show that the resistance of the dislocation movement in α-Al increased in the low-temperature condition, which is beneficial for the number of Si phase fractures that increase to enhance the tensile strength and weaken the elongation of the Al-7.0Si-0.3Mg alloy. After the solution treatment, the particle size of the Si phase reduced, while the morphology became rounded. In the early stage of aging, a GP region is generated inside the α-Al. With the prolonging of aging time, the acicular β″ phase is formed and then grows into rod-shaped β′. In the overaging stage, β′ eventually grows into flaky β. Due to the different linear expansion coefficients of the α-Al and the Mg2Si phase in the Al-7.0Si-0.3Mg alloy, the α-Al is squeezed by the Mg2Si phase under the effect of low-temperature shrinkage.

Fabrication of High-Strength Mg–Gd–Nd–Zn–Sn–Zr Alloy via Extrusion and Aging

A high-strength Mg–9Gd–3Nd–1Zn–1Sn–0.5Zr (wt%) alloy bar was successfully fabricated by extrusion and following aging two-steps processes, during which the alloy′s microstructures were determined to investigate the refinement mechanism during extrusion and to reveal the morphology precipitation behavior of the as-extruded alloy after aging. Based on the characterization of the mechanical properties, the strengthening mechanism of the alloy under different treatment processes had been discussed. Our results showed that the greater the extent of extrusion, the finer the grains and eutectic phase, and crushing and dynamic recrystallization are the refinement mechanisms of the alloy in narrow-zone. After peak-aged treatment at 200 °C, a large number of spherical β′ phase and lamellar precipitated phase had been observed, which replaces the amorphous β" phase of the alloy was under-aged. The ultimate tensile strength, yield strength, and elongation were 462.12 MPa, 391.97 MPa, and 4.2% respectively, and the increase number of the β′ phase and long-period stacking ordered (LPSO) phase were the reason why the strength of the alloy reached the highest value. At the over-aging stage (96 h), the size of the lamellar precipitates and the LPSO was larger than that of the peak-aged, and the spherical β′ phase appeared in peak-aging stage decreased significantly, which reduced its performance.

Strain aging characterization and physical modelling of over-aging in dual phase steel

This study presents an integrated work of experimental investigations and physical modeling of bake hardening (BH) response in dual-phase steel (DP). A DP steel with a martensite volume percentage of 22% was produced by intercritical annealing followed by quenching in brine. Aging experiments with up to 5% pre-straining were carried out in the temperature range of 100 to 220 °C, for 2 to 2·104 min at temperature. The DP steel was characterized using light optical, scanning electron and transmission electron microscopy. The pinning effect of the dislocations was revealed by atom probe tomographic analysis. The increase in the yield strength accompanying the aging phenomenon, measured using tensile tests, showed a two-step increase, followed by an over-aging stage. The dependence of the time-interval of each stage on the pre-strain value and aging temperature was analyzed. A physical-based model for interpreting the over-aging stage in DP steel was developed. A new concept for over-aging, correlating it to carbon-diffusion from ferrite to martensite due to the gradient in the chemical potential at the interface between the two phases, was introduced. This diffusion causes a partial dissolution of the already formed Cottrell atmosphere/carbide precipitates. Finally, the time required for the onset of over-aging, calculated using physical simulations, was compared with experimental results showing a good matching between experiment and simulation.

Nanoscale precipitates evolution and strengthening mechanism of the aged Cu-Mg-Fe-Sn-P-Y electrical contact wire

The evolution and strengthening effect of nanoscale precipitates of the aged Cu-Mg-Fe-Sn-P-Y alloy were investigated via aging treatment in the 400–480°C temperature range and the 10–480min aging time range. Nanoscale γ-Fe particles uniformly distributed in the matrix caused dispersion strengthening in the peak-aging stage and transformed to α-Fe at the over-aging stage. The Mg3P2 secondary phase hindered the appearance of coarse Fe3P and formed definite orientation relationship (OR) of cubic to cubic with the Cu matrix after 480min aging, <110>Cu||<110>Mg3P2, (002)Cu||(008)Mg3P2. The optimal aging parameter for the Cu-Mg-Fe-Sn-P-Y alloy is 60% cold deformation and aged at 460°C for 20min. The corresponding tensile strength is 599MPa with the conductivity of 71.1%IACS and the elongation of 6.9%. Compared with the current contact wire, the ultimate tensile strength and conductivity of the studied alloy increased by 20% and 15%, respectively.

Effect of Ag addition on the precipitation evolution and interfacial segregation for Al–Mg–Si alloy

The effect of Ag addition on the precipitation evolution and interfacial segregation for Al–Mg–Si alloys was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) and density functional theory (DFT) calculation. At the early aging stage, Ag atoms could enter clusters and refine the distribution of these clusters. Then, Ag atoms preferentially segregate at the GP zone/α-Al and β"/α-Al interfaces at the peak aging stage by the replacement of Al atoms in FCC matrix. With prolonging aging time, Ag atoms generally incorporate into the interior of β" precipitate, facilitating the formation of QP lattice (a hexagonal network of Si atomic columns) and the local symmetry substructures, Ag sub-unit (1) and Ag sub-unit (2). At the over-aged stage, the Ag sub-unit (1) and Ag sub-unit (2) could transform to the β′Ag (i.e. β′Ag1 and β′Ag2.) and Q′Ag unit cells, respectively. All the precipitates at the over-aging stage have a composite and disordered structure due to the coexistence of different unit cells (β′Ag1, β′Ag2, Q′Ag and β′) and the non-periodic arrangement of Ag atoms within the precipitate. In the equilibrium stage, the incorporated Ag atoms in the precipitates release into the α-Al matrix as solute atoms or form Ag particles. In general, Ag atoms undergo a process of “segregate at the precipitate/matrix interface → incorporate into the interior of precipitate → release into the α-Al matrix” during the precipitation for Al–Mg–Si–Ag alloys. Besides, Ag segregation is found at the interfaces of almost all metastable phases (including GP zone, β″, β′/β′Ag phase) in Al–Mg–Si–Ag alloys. The Ag segregation at the β′/α-Al interface could increase the length/diameter ratio of β′ phase and thus promote the additional strengthening potential of these alloys. These findings provide a new route for precipitation hardening by promoting the nucleation and morphology evolution of precipitates.

Effect Of The Application Of Constant And Variable Temperatures at the Overaging Stage in the Mechanical Properties of Trip780 Steel

The effect of the application of constant temperatures (isothermal plateau) and variables (cooling ramp) during the overaging (OA) stage of the continuous annealing cycle in the microstructural characteristics and mechanical properties of a TRIP steel of the class of 780 MPa was investigated. It was simulated six overaging conditions in the equipment Gleeble. The microstructures presented variable fractions of ferrite, bainite, retained austenite and a small martensite portion. All the conditions achieved the minimum strength of 780 MPa it is possible to highlight the conditions for initial OA temperature of 440 °C where presented the best results of mechanical properties and typical TRIP steels behaviors.

The relation between aging temperature, microstructure evolution and hardening of Custom 465® stainless steel

The microstructure and hardness of Custom 465® precipitation hardened stainless steel were characterized following 4 hours aging at temperatures ranging from 482°C to 648°C. Dilatometry measurements and thermodynamic calculations were used to understand the mechanism of martensite-to-austenite reversion. Three major stages of aging were observed. The first stage is the peak hardening at aging temperatures of 480–510°C, where both η-Ni3Ti precipitation and austenite reversion begin independently of each other. The second stage is the initial stage of overaging observed above 538°C as reduction in hardness is due to a slight increase in both the precipitate dimensions and the reverted austenite volume fraction. The third, progressive overaging stage occurs above 593°C in which a dramatic hardness decrease is caused by a significant increase in both precipitate dimensions and reverted austenite volume fraction. Transmission Kikuchi diffraction orientation mapping revealed that globular austenite randomly nucleates at martensite grain boundaries, while acicular austenite nucleates along martensite lath boundaries. Thermodynamic calculations indicated that the dissolution of η-Ni3Ti precipitates does not govern the austenite reversion, although both phases are in competition for Ni. While the thermodynamic calculation predicts increase in the fraction of austenite with increase in aging temperature, the fraction of austenite found at room temperature in aged samples does not vary when the aging temperature, in which austenite reversion occurs, is increased above 593°C. This is explained by the change in composition of the austenite as a function of aging temperature.

Effect of Cu content on precipitation and age-hardening behavior in Al-Mg-Si-xCu alloys

The effect of varying the Cu content of Al-Mg-Si-xCu (x = 0.5, 1.0, 2.5 and 4.5, in wt.%) alloys on precipitation and their age-hardening behavior was investigated through hardness measurements, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The results indicated that aging at 165 °C, the peak aging hardness increased from 109 to 218 HV with the Cu content increasing from 0.5 to 4.5 wt%. Similarly, there was an increase in the peak aging time with Cu content. However, in over-aging stage, the hardness of 4.5Cu alloy decreased more rapidly than that of 0.5, 1.0 and 2.5Cu alloys. Furthermore, an increase in Cu content brought about an apparent change in the main precipitate of the peak-aged samples from needle-shaped β″ phase in 0.5Cu alloy to: needle-shaped β″ phase and granular Q′ phase in 1.0Cu alloy, granular Q′ phase and θ′ platelets in 2.5Cu alloy, and θ′ platelets with a small amount of Q′ phase in 4.5Cu alloy.

Dry wear behavior of ultra-high strength Cu–10Ni–3Al–0.8Si alloy

Cu–10Ni–3Al–0.8Si alloy exhibited desirable physical and mechanical properties and wear resistance for bearing applications. An electrical conductivity of 18.1% IACS, hardness of 309.3HV, tensile strength of 1004.2MPa, coefficient of friction of 0.58 and volume wear rate of 4.69×10-6mm3/mm were obtained with over-aged sample. Adhesion and oxidation were observed to be the two main mechanisms under dry wear condition. The wear resistance of the experimental alloy increased with aging time until the over-aging stage due to the increment of micro-hardness. Adhesion was observed to be the dominant wear mechanism for the cold-rolled sample while tribo-oxidation dominated for the over-aged sample. The latter was attributed to the effect of precipitates on the mechanically mixed layer of the wear sample.

Mechanical Property and Intergranular Corrosion Sensitivity of Zn-Free and Zn-Microalloyed Al-2.7Cu-1.7Li-0.3Mg Alloys

The influence of 0.72 pct Zn addition on the tensile properties of Al-2.7Cu-1.7Li-0.3Mg alloys was investigated. Their intergranular corrosion (IGC) dependence on aging [T6 type at 423 K (150 °C) and 448 K (175 °C) and T8 type at 423 K (150 °C)] time was studied. An IGC diagram associated with aging process was established. The addition of 0.72 pct Zn enhanced the strength of the Al-Li alloy with T6 type aging at 448 K (175 °C). With aging process, the corrosion mode of the T6-aged Al-Li alloys was changed in the following order: pitting and local IGC (initial aging stage), general IGC (underaging stage), local IGC (near peak-aging stage), and pitting (overaging stage) again. The IGC depth was increased first and then decreased with aging time extension. The corrosion potential change of grains and the microstructure variation were used to explain the IGC sensitivity of the Al-Li alloy with different tempers. Meanwhile, 0.72 pct Zn addition decreased the IGC sensitivity of the Al-Li alloy, especially the T6-aged Al-Li alloy.