Supersaturated Solid Solution Research Articles

Developing age-hardenable Al-Zr alloy by ultra-severe plastic deformation: Significance of supersaturation, segregation and precipitation on hardening and electrical conductivity

The aluminum-zirconium (Al-Zr) alloys are of interest because of their superior thermal stability and good electrical conuctivity, but their main drawback is their rather low hardness (<75 Hv) even after severe plastic deformation (SPD). These alloys, which are actually composites of Al and Al3Zr intermetallics (D023 tetragonal structure), do not show age-hardening behavior due to the immiscibility of Zr in Al. In this study, ultra-SPD with shear strains up to 40,000, realized by high-pressure torsion (HPT), is employed to generate supersaturated solid solution in an Al-Zr alloy. Subsequent aging leads to unusual precipitation of AlZr precipitates at grain boundaries (Bf orthorhombic structure) and of coherent metastable Al3Zr precipitates within grain interiors (L12 cubic structure). It is found that the supersaturation of Zr in Al by ultra-SPD is controlled by an ultrafast dynamic diffusion which is comparable to surface diffusion, while the formation of precipitates by static aging is controlled by lattice and pipe diffusions. These microstructural features provide exceptional increase in hardness up to 148 Hv, great thermal stability up to 523 K and reasonable electrical conductivity up to 35 %IACS. Detailed analysis shows that ~30% of this hardening is caused by precipitation strengthening, while extra hardening is achieved due to nanograin formation, grain-boundary segregation and dislocation accumulation. This study introduces a rational approach to produce novel age-hardenable Al alloys with high thermal stability and reasonable electrical conductivity by application of ultra-SPD.

The Zr alloying effect on microstructure evolution and mechanical properties of nanostructured Al-Zr alloyed films

Al-Zr nanocrystalline alloy films with Zr contents range from 0 to 8.3 at.% were prepared by magnetron sputtering. The microstructure evolution, mechanical behaviors and strengthening mechanism of films were studied. All the films formed a supersaturated solid solution structure of Zr in the Al lattice. With the increasing of Zr contents, the grain size of films decreased rapidly. When the Zr content increases to 8.3 at.%, the grain size was reduced to about 20 nm. The added Zr atoms tended to segregate at grain boundaries in the Al-Zr alloy films. The hardness of the film gradually increased from 2.5 GPa of pure Al to 5.4 GPa of Al-8.3 at.% Zr film. The strengthening of Al-Zr alloy film comes from three aspects, grain refinement strengthening, solid solution strengthening and nanocrystalline solution pinning strengthening. Among the three strengthening factors of the Al-Zr alloy films, the contribution of fine grain strengthening is the largest, about 70%, followed by solid solution strengthening, 15–20%. The contribution of nanocrystalline solution pinning strengthening is the weakest, less than 10%.

The evolution of microstructure and properties of a Cu–Ti–Cr–Mg–Si alloy with high strength during the multi-stage thermomechanical treatment

Abstract A Cu–Ti–Cr–Mg–Si alloy with the super-high strength and high electrical conductivity was designed, and the effects of multi-stage thermomechanical treatment on the microstructure and properties of the alloy were investigated. The alloy treated by the multi-stage thermomechanical treatment had a yield strength of 1072 MPa, tensile strength of 1144 MPa, elongation of 6.1% and electrical conductivity of 24.1 %IACS. During the thermomechanical treatment, the nanoscale β′–Cu4Ti particles, Cr precipitates and submicro-particles that mainly consisted of Cu, Ti, Cr and Si elements formed, leading to the high strength of the alloy. β′–Cu4Ti phase with body-centered tetragonal structure had a classic orientation relationship with copper matrix: [001]Cu//[001]β' and [3-10]Cu//[100]β'. The super-high strength of the designed alloy was mainly attributed to the precipitation strengthening, sub-structure strengthening, solid solution strengthening and work hardening. Electrical conductivity increased during aging due to the decomposition of supersaturated solid solution. Compared with direct aging treatment, multi-stage thermomechanical treatment with the stepped temperature-reduction further improved the mechanical and electrical properties of the alloy.

Nanodispersoids of the quasicrystalline I-phase in Mn- and Mg-bearing aluminum-based alloys

The current study compares Al-Mn and Al-Mn-Mg alloys’ precipitation behavior in a temperature range of 350–450 °C. Mn-bearing nanodispersoids of the quasicrystalline-structured I-phase precipitate during the decomposition of supersaturated Al-based solid solution in both alloys. In the binary Al-Mn alloy, quasicrystalline-structured dispersoids precipitate only in the grain boundaries. The Mg addition does not change the dispersoids’ composition but significantly accelerates the precipitation kinetics. The Mg addition provides the precipitation of high-density distributed I-phase nanodispersoids in the body of the grains. The annealing temperature increase results in the I-phase transformation to a cubic α-phase, with both phases coexisting in the alloy structure.

An experimental and numerical study of feasibility of a novel technology to manufacture hot stamping dies with pre-constructed tube network

The cooling system is a critical element in tooling for hot stamping high-strength aluminium alloys, for which very high quenching rates are required to ensure a supersaturated solid solution state in formed parts. To enhance cooling, ducts within the tools should be close and conformal to the surface die profile. Currently, ducts with curved profiles are made by drilling short straight lengths in die segments which are clamped together to form a complete die, which is expensive and hard to achieve the shape of duct with best cooling performance. To address these disadvantages, a novel method which enables efficient manufacture of conformal cooling systems by embedding a network of tubular cooling ducts within a cast matrix is presented in this paper. The feasibility of the proposed method of making ducts in the hot stamping die is demonstrated. Both experimental and computer-based die quenching tests using heated aluminium test pieces were undertaken to determine the cooling performance of a laboratory-scale tool set with cooling ducts. Simulations using the validated FE model were performed to investigate the effects of cross-sectional geometry, material and duct layout, on the quenching performance of the tools. It was found that ducts made of mild steel perform sufficiently well to make the use of high conductivity copper unnecessary. For a flat die surface, square section ducts provided highest cooling rates in comparison with circular and diagonal ones. The uniformity of die temperature increases with the decreased distance between neighbouring ducts, which indicates that a minimal gap is recommended without deteriorating tool strength. The developed tooling technology has the potential to provide a low-cost, highly efficient method of making conformal cooling ducts because the hot stamping die of high-strength aluminium alloy panels requires larger dimension, greater complexity and higher quenching rates.

Simulation of Microstructure Evolution in Metal Materials under Hot Plastic Deformation and Heat Treatment

The development of modern computational techniques and equipment enables one to perform high-precision calculations of complex processes for industry, including metallurgy. This review has classified the basic physical and mathematical models of structure formation during heat and deformation treatment. The Kocks–Mecking–Estrin model describing the dislocation structure at the initial stage of hot plastic deformation has been analyzed. The models of dynamic, metadynamic, and static recrystallization kinetics based on the Johnson–Mehl–Avrami–Kolmogorov equation have been considered. The models of the kinetics of phase transformation upon heating and cooling of steel have been reviewed. The Kampmann–Wagner model that describes the decomposition of supersaturated solid solution during the aging of aluminum alloys has also been considered. The main computational techniques to calculate microstructural evolution, such as the cellular automaton, Monte Carlo, and multiphase-field techniques have been considered. They exhibit high accuracy when calculating recrystallization processes and phase transformations. The systematization of the existing models that describe structural evolution has revealed the possibility to develop complex models for the comprehensive calculation of full cycles to process metal materials by heating and deformation. These models can be used for the optimization and development of new processing techniques.

Atomic scale understanding of the defects process in concurrent recrystallization and precipitation of Sm-Co-Fe-Cu-Zr alloys

Identifying the defects process in concurrent recrystallization and precipitation during aging a supersaturated solid solution is essential for understanding their interaction mechanisms and for manipulating the microstructure, but was rarely done at the atomic scale. Herein, the concurrent recrystallization and precipitation in the supersaturated hexagonal Sm(Co, Fe, Cu, Zr)7.5 alloys were studied through detailed transmission electron microscopy investigations, where the recrystallization, growth of recrystallized subgrains (cells) and precipitates stem from the gradual formation and dissociation of defects, including basal stacking faults (SFs), vacancies and excess interstitial atoms. The diffusion-controlled glides of <a>-type partial dislocations associated with the SFs not only transform the matrix from the mixture of hexagonal Sm(Co, Fe, Cu, Zr)7 (1:7H) and Sm2(Co, Fe, Cu, Zr)17 (2:17H) to Sm-depleted rhombohedral Sm2(Co, Fe, Cu, Zr)17 (2:17R) cells but also provide continuous diffusion channels to reduce the point defects to form the Sm-enriched hexagonal Sm(Co, Fe, Cu, Zr)5 (1:5H) cell boundary precipitates and Zr-enriched rhombohedral (Sm, Zr)(Co, Fe, Cu)3 (1:3R) platelets. It indicates a diffusion-controlled displacive phase transformation mechanism, characterized by the composition-dependent 2:17R’ intermediate phase due to incomplete basal slip and incomplete solute partitioning. The growth velocities of both recrystallized cells and precipitates are closely related to the defects density, being faster due to the high defects density at early stage, and being slower due to the reduced defects density at later stage. A basal slip model is proposed to explain the formation and dissociation of defects along with the stacking period change and the simultaneous formation of continuous atomic diffusion channels. These new findings may yield a deep understanding of the interaction between recrystallization and precipitation.

Nanoscale periodic gradients generated by laser powder bed fusion of an AlSi10Mg alloy

In this study, the formation of a periodic microstructural pattern designed by laser powder bed fusion (LPBF), commonly called selective laser melting (SLM), of an AlSi10Mg alloy is revealed at high resolution using scanning transmission electron microscopy and atom probe tomography. Special attention is paid to the description of non-equilibrium structures and compositional fields resulting from the ultrafast cooling of the LPBF process. Observations reveal the existence of a glass state in eutectic areas, wherein short-range ordering of the diamond-Si structure is observed. The apparent very fast solidification of eutectic regions is found to involve a local strain in adjacent Al cells, which extends up to 100 nm on average. In the supersaturated aluminium solid solution retained by the LPBF process, two populations of clusters are identified, for which the potential role of the selection of hardening phases is discussed. It is proposed that the microstructure of former melt pools outside heat-affected areas is described by the repetition of a periodic microstructural pattern consisting of eutectic regions /strain-hardened Al-crystals /strain-free Al-crystals with a high density of solute-rich clusters.

Surface modification of CoCrFeNi high entropy alloy by low-temperature gaseous carburization

Low-temperature (470 °C) gaseous carburization is successfully employed in surface hardening of a CoCrFeNi high entropy alloy. After low-temperature gaseous carburization, a carburized case of expanded face-centered cubic (FCC) structure, i.e., a carbide-free supersaturated interstitial solid solution of carbon (maximum of ~3.0 wt%) in FCC, forms in the surface region of the high entropy alloy. Due to solid solution strengthening by interstitially dissolved carbon atoms and strain hardening caused by plastic accommodation of the lattice expansion, the surface hardness of high entropy alloy is significantly enhanced. The surface hardness reaches 1221 HV0.1N, which is about 5 times the hardness of the substrate.

Designing ultrahard aluminum nanocomposites by severe mechanochemical processing

In this study, we deployed a severe mechanochemical approach to produce ultrahard Al–Nb nanocomposites. Two nanocomposites were fabricated by high-pressure torsion (HPT) at room temperature of commercially pure Al and 10 wt% Nb2O5 with distinct morphologies (extremely thin nanowires or nanorods). In addition to consolidation of powders, structural refinement and chemical mixing, HPT was applied here for inducing mechanochemical redox reactions between Al and nanostructured Nb2O5 (Nb2O5 + 10/3 Al → 2Nb + 5/3 Al2O3). These chemical reactions released metallic niobium in its atomic form, which contributed to the dynamics of mixing between Al and Nb at the atomic scale. This strategy demonstrated to be effective for promoting the formation of supersaturated (Al–Nb) solid solution, niobium clustering, and extensive segregation of Nb at lattice defects. The effect of different nanostructured Nb2O5 morphologies on the interface structures and hardening of the produced composites was discussed. Annealing at moderate temperature (373 K for 1 h) caused an exceptional increase in hardness of ~54% (~289 Hv) for the composite produced using Nb2O5 nanorods. This was attributed to the rearrangement of dislocations into a lower energy configuration (dislocation tangles) and an astounding segregation of Nb to this dislocation structure.

Structure and properties of rapidly solidifing foils Sn – 14 at.% In – 6.5 at.% Zn

The results of studies of the effect of ultra-high cooling rates of the melt equal to 10 5 K/s on the phase composition, microstructure, grain structure and mechanical properties of the Sn – 14 at.% In – 6.5 at.% Zn alloy are presented. To prepare the samples, the rapid quenching from the melt technique was used. A drop of melt was injected onto the inner surface of a rapidly rotating copper cylinder and solidifing in the form of a foil with a thickness of 30–90 μm. Investigations of the phase composition, carried out by the method of Х-ray diffraction analysis, made it possible to establish that the foil consists of a solid solution of zinc in the γ -phase (Sn4In) and zinc. Observations of the microstructure of the foil using scanning electron microscopy showed that the decomposition of a supersaturated solid solution proceeds at room temperature with the release of dispersed zinc particles. The character of the grain structure and texture of the foil is studied by the electron backscatter diffraction technique. A mechanism of the formation of elongated grains is proposed, which consists in the fact that at a high solidification rate, comparable to the rate of movement of the melt over the surface of the mold, grain growth can occur not only in the direction opposite to the direction of heat removal, but also in the direction of movement of the spreading. The formation of the preferred growth of grains, in which the most closely-packed plane (0001) is parallel to the foil surface, provides the maximum rate of decrease in the enthalpy of the alloy during crystallization. The features of the influence of the microstructure and grain structure on the mechanical properties of the foil are revealed. The microhardness of the rapidly solidifing Sn – 14 at.% In – 6.5 at.% Zn alloy is 105 MPa. The stress–strain curve of the Sn – 14 at.% In – 6.5 at.% Zn foil, obtained at room temperature, has a shape specific for the stress–strain curve of metals at high temperatures, which is due to the low melting point of the γ-phase.

Microstructure Evolution of Mg96.9Gd2.7Zn0.4 Alloy Containing Multiple Phases Prepared by Spark Plasma Sintering Method

The synergic strengthening of multiple phases is an essential way to achieve high-performance Mg alloys. Herein, Mg-Gd-Zn alloy containing four phases was prepared by rapid solidification (RS) ribbons and spark plasma sintering (SPS). The microstructure of the alloy consisted of α-Mg, nanosized β1 phase particles, lamellar long period stacking ordered (LPSO) phase, and β′ phase precipitates. The microstructural evolution was also investigated. The results show that the metastable β1 phase was formed in the as-cast solidification through rapid solidification, because both Zn atoms and the short holding-time at molten liquid facilitated the formation of the β1 phase. The β1 phase grew from 35.6 to 154 nm during the sintering process. Meanwhile, the fine lamellar LPSO phase was simultaneously formed after the Zn-Gd clusters were generated from the supersaturated solid solution, and the width of the LPSO phase was only in the range of 2–30 nm. The third strengthening phase, the metastable β′ phase, was obtained by aging treatment. The results of hardness testing implied that the hardness of the alloy containing the aforementioned three nanosized strengthening phases significantly improved about 47% to 126 HV compared with that of the as-cast ingot.

Microstructure evolution and mechanical properties of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy

In this work, the microstructures of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy plate were investigated in terms of the element segregation, grain structure and precipitation behavior by scanning electron microscopy, transmission electron microscopy and atom probe tomography observation as well as differential scanning calorimetry and electron probe micro analysis. The hardness distribution of the weld cross-section was also described using microhardness mapping. A model of strengthening mechanisms was proposed to analysis the microstructure-strength relationship of the welded joint. It is found that the occurrence of softening behavior in fusion zone after welding is closely related to the dissolution of Ω and θ′ strengthening phases. The precipitation of a large number of fine Ω, θ′ plates and some small dispersed precipitates (Ag-rich particles, GP zones and Mg–Ag phases) may be the main reason for the increase in the strength of the welded joint after post-welding heat treatment. It is because the supersaturated solid solution produced after welding due to the rapid cooling rate as well as the strong binding energy between Ag atoms and vacancies/Mg atoms promotes the precipitation of small dispersoids.

Dimensional Change of Selectively Laser-Melted and Gravity-Cast AlSi10Mg Alloy During Heat Treatment

AlSi10Mg alloy, processed by selective laser melting (SLM) in as-built state and gravity casting in solution-treated state, was heat-treated at several temperatures for different durations, and the resulting dimensional changes were investigated. For the cast AlSi10Mg alloy, the linear dimensional change heat-treated at 200 ˚C was approximately 0.10%, which agreed with that calculated by a theoretical model. The linear dimensional change of the SLM AlSi10Mg alloy heat-treated at 150 ˚C was 0.17%. This indicated that, in as-built state, the concentration of supersaturated solid solution Si in α-Al phase was 1.7 wt% on average, and the lattice constant of α-Al phase was 0.4047 nm. SLM AlSi10Mg alloy contained more than 3 cm3/100 g-Al hydrogen and showed a linear dimensional change of more than 0.6% after the heat treatment at 530 ˚C because of porosity expansion. The relative densities of the SLM AlSi10Mg alloy heat-treated at 150 ˚C for 1800 ks and 530 ˚C for 18 ks were 99.89% and 98.12%, respectively.

Powder Metallurgy Production of Ti-2 Wt Pct Si Alloy: Structural, Mechanical, and Electrochemical Characterization of the Sintered Material

Several commercial alloys use silicon (Si) to improve titanium (Ti) resistance to creep and oxidation at high temperatures and to improve Ti corrosion resistance in acid media. According to the Ti-Si phase diagram, reported stable solid phases in the Ti-rich region are β-Ti, α-Ti, Ti3Si, and Ti5Si3. Nevertheless, very few works in the literature discuss Ti3Si intermetallic production. As such, this work studied the possibility of obtaining an α-Ti-Ti3Si alloy by hot pressing α-Ti supersaturated solid solution powders obtained by mechanical alloying. The consolidation of milled powders was performed using uniaxial hot press equipment. Structural and morphological evolutions during the sintering process were investigated by X-ray diffraction and scanning electron microscopy. Electrochemical behaviors of sintered samples were evaluated by open circuit potential and linear sweep voltammetry. Results show a fine and uniform Ti3Si alloy distribution in the α-Ti matrix produced by the proposed powder metallurgy route. The sintered samples demonstrated high micro-hardness and resistance to sulfuric acid corrosion. Additionally, Ti3Si was shown to have a significant hardening effect on the α-Ti matrix. Electrochemical behavior further demonstrates that a fine and homogeneous Ti3Si distribution in the α-Ti matrix contributes to a more stable superficial oxide layer against sulfuric acid corrosion.

Isothermal Aging Behaviors of Copper–Titanium–Magnesium Supersaturated Solid-Solution Alloys

Herein, the isothermal aging behavior of copper–titanium–magnesium (Cu–Ti–Mg) supersaturated solid-solution alloys, with different compositions, under test conditions of 450°C for 100 h, has been thoroughly investigated in a comparative study using various electron microscopy and microanalytical techniques. The Vickers hardness and electrical conductivity of the ternary alloys were recorded at slightly elevated (during aging) and reduced levels than their binary counterparts without Mg doping. Hence, it is proposed that the hardness and conductivity values are approximated from the superposition effect of precipitation hardening stimulated by Ti solutes and solution hardening by both Ti and Mg solutes. Furthermore, the tensile tests for these ternary specimens have demonstrated that Mg doping has a substantial effect on the improvement of the tensile strength and fracture elongation properties of binary Cu–Ti alloys. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy imaging combined with atomic-resolution energy-dispersive X-ray spectroscopy mapping analysis confirmed that the same metastable precipitate phase is responsible for peak hardening in ternary and Cu–Ti binary alloys. In addition, a large part of the Mg solutes is homogeneously distributed over the matrix regions, while there is also a smaller part of those present in the precipitates. The potential effects of Mg doping on the microstructures of Cu–Ti alloys were elucidated and the structural environment, which may yield relatively high mechanical properties, was discussed using the aforementioned observations.

Effects of thermomechanical treatment on the microstructure, precipitation strengthening, internal friction, and thermal stability of Al–Er-Yb-Sc alloys with good electrical conductivity

Rare-earth elements improve the mechanical properties of aluminum owing to the formation of the L12-structured nanoprecipitates providing the precipitation strengthening effect. The precipitates type, size, and number density depend on the alloy chemical composition and a thermomechanical treatment regime. It is essential to develop the alloys and treatments, providing a combination of the enhanced mechanical strength and a high level of electrical conductivity. This study investigates the influence of thermomechanical treatments on the microstructure, precipitation strengthening, mechanical properties, and electrical conductivity of Al–Er-Yb-Sc alloys with differing Sc content. An as-cast structure of the alloys studied consists of a supersaturated aluminum solid solution and Al3(Er,Yb) phase particles of eutectic origin with the particle thickness of 50–200 nm. A significant strengthening during post-deformation annealing is achieved by precipitation of L12-Al3(Sc,Er,Yb) phase dispersoids of 4–8 nm in size. The mechanical spectroscopy method is successfully used to understand the precipitation kinetics of the studied alloys in comparison with analogous Al-Yb-Y-Sc alloys. This method is highly sensitive to lattice defects parameters, i.e., recrystallization and precipitation kinetics. High-scandium alloys demonstrate an increase of the hardness and tensile strength and insignificant changes in the internal friction background level during post-deformation annealing of cold-worked samples at 300 °C. The yield strength of the Al–Er-Yb-Sc alloys after post-deformation annealing is 142–231 MPa, elongation-to-failure is 3.6–13.5%, and electrical conductivity is 54.8–60.9% IACS, dependent on scandium content and annealing parameters. The studied alloys exhibit high thermal stability of the tensile properties, which remains unchanged during annealing at 300 °C for 100 h.

Hot Isostatic Pressing of Aluminum–Silicon Alloys Fabricated by Laser Powder-Bed Fusion

Hot isostatic pressing can be utilized to reduce the anisotropic mechanical properties of Al–Si–Mg alloys fabricated by laser powder-bed fusion (L-PBF). The implementation of post processing densification processes can open up new fields of application by meeting high quality requirements defined by aircraft and automotive industries. A gas pressure of 75 MPa during hot isostatic pressing lowers the critical cooling rate required to achieve a supersaturated solid solution. Direct aging uses this pressure related effect during heat treatment in modern hot isostatic presses, which offer advanced cooling capabilities, thereby avoiding the necessity of a separate solution annealing step for Al–Si–Mg cast alloys. Hot isostatic pressing, followed by rapid quenching, was applied to both sand cast as well as laser powder-bed fused Al–Si–Mg aluminum alloys. It was shown that the critical cooling rate required to achieve a supersaturated solid solution is significantly higher for additively manufactured, age-hardenable aluminum alloys than it is for comparable sand cast material. The application of hot isostatic pressing can be combined with heat treatment, consisting of solution annealing, quenching and direct aging, in order to achieve both a dense material with a small number of preferred locations for the initiation of fatigue cracks and a high material strength.

Influence of isothermal and non-isothermal aging treatments on microstructure and properties of Al-Zn-Mg alloy helical profile

Aging treatment was an important way to improve the mechanical properties of Al-Zn-Mg alloys. In this work, the Al-Zn-Mg (7 N01) alloy helical profile was extruded and treated by different kinds of isothermal and non-isothermal aging processes. It was found that the alloy had the peak strength at 39 h during the isothermal aging treatment. Its precipitation sequence was supersaturated solid solution (SSS) → GP zones → metastable η′ phase, where the stable η phase had not been formed. The peak strength of the alloy during the non-isothermal aging reached at 4-5 h, but the peak strength was lower than those with isothermal aging treatment. The maximum temperature Tmax of non-isothermal aging played a key role on the mechanical properties: the higher the maximum temperature was, the longer time was required, and the lower the strength and the hardness were. The grain boundary structure of the alloy with Tmax = 185 °C was similar to that with isothermal aging, and the grain boundary precipitate (GBP) was rarer than that with Tmax = 200 °C and Tmax = 215 °C, where there was no obvious precipitate free zone (PFZ) in the alloy, which was conducive to improving the corrosion resistance of the alloy.

Precipitation and aging phenomena in an ultrafine grained Al-Zn alloy by severe plastic deformation

The post-deformation aging response and underlying precipitation kinetics in ultrafine-grained, age-hardened alloys processed by severe plastic deformation are critical and heretofore poorly understood phenomena that ultimately influence mechanical behavior. In this study, a supersaturated solid solution 7075 Al alloy was processed by equal-channel-angular pressing (ECAP) route Bc at 250 °C for 6 passes to attain ultrafine grained (UFG) structures. Thermal and mechanical analyses revealed that there are three sequential phase transitions that govern post-deformation aging: first, dissolution reactions of Guinier-Preston (GP) zones, metastable ηp and η′ precipitates which correspond to slight decreases in strength and ductility; second, afresh precipitating of GP zones, metastable ηp and η′/stable η phases corresponding to an increase in strength with significant decrease in ductility; and third, subsequent η transformation of the metastable η′ and ηp as well as coarsening of stable η phases corresponding to a dramatic decreases in strength. Microstructural analysis suggests that the significant decrease in ductility may be attributed to grain boundary precipitation of the metastable ηp and η′/stable η phases.