Age-hardening Response Research Articles

Ageing response and microstructural evolution of biodegradable Zn-1.5Cu-1.5Ag alloy

In this study, the age-hardening response and microstructural evolution of an as-extruded biodegradable Zn-1.5Cu-1.5Ag (wt.%) alloy during ageing at 25 ℃, 100 ℃, 150 ℃ and 200 ℃ are studied. The age-hardening response is generally weak, and the largest hardness increment is observed after ageing at 150 ℃ for 24 h. Discontinuous precipitation (DP) and continuous precipitation (CP) occur competitively during ageing at 150 ℃ or 200 ℃, while only DP is observed during ageing at 25 ℃ or 100 ℃. All the precipitates formed through DP and CP are identified as ε-(Ag, Cu)Zn4 that has a hexagonal structure. Analysis of possible strengthening mechanisms shows that grain boundary strengthening and precipitation hardening contribute to the major part of yield strength in the as-extruded condition. Ageing treatments generate a limited increment in yield strength due to the small difference between the hardness of ε-(Ag, Cu)Zn4 and the Zn matrix and the reduced solid solution strengthening effect. Artificial ageing at 150 ℃ for 48 h effectively improves the stability of the mechanical properties of the as-extruded Zn-1.5Cu-1.5Ag alloy. This process fully depletes the excessive solutes in the supersaturated Zn matrix, ensuring that the alloy maintains consistent mechanical properties when stored at room temperature.

Severe brittleness in a peak-aged Mg-Gd-Sn alloy caused by segregated β precipitates along grain boundaries

This work elucidates the mechanism of brittle fracture in coarse-grained Mg-Gd-Sn alloy after aging. The results indicate that the Mg-14Gd-0.2Sn alloy extruded at 320 °C with a ratio of 28, exhibits a fully recrystallized microstructure with equiaxed grains of approximately 9.8 μm and a randomized texture. The elevated temperature extrusion process suppresses the formation of dynamic precipitates, resulting in a high elongation of 15.4 %. However, after aging, the alloy exhibits a significant age-hardening response arising from the high number density of β′ nano precipitates within matrix but much poor ductility, experiencing severe intergranular fracture due to the precipitated β phase along grain boundaries.

Effect of dispersoid formation on age hardening and precipitation behavior of an Al-Si-Mg(-Cu) alloy with Mn and Cr addition

In the present study, Mn and Cr elements were added to the alloy to induce the formation of the α-AlFeMnCrSi dispersoids during solution treatment. The effect of the dispersoids on subsequent age-hardening response and precipitation behavior was investigated. The results indicate that there is a partially coherent structure between the dispersoid and the α-Al matrix, but the coherence is weak. The precipitation of dispersoids resulted in accelerated age-hardening response and precipitation kinetics, but weakened peak-aged hardening effect. With the aging time increasing from 0 to 16 h, a new precipitation sequence was proposed in Mn+Cr modified Al-Si-Mg(-Cu) alloy: Supersaturated solid solution (SSSS) → GP zones (under-aged) → β'' + β' (peak-aged) → Si + β'' + β' + Mg2Si + Q + α (over-aged). Among them, the extra precipitates were heterogeneous nucleated on the dispersoids, which depended on the orientation relationship between the dispersoid and the precipitation, the interfacial energy of the dispersoid/matrix/precipitation, and the local solute element concentration. Heterogeneous precipitation of coarse particles on dispersoids resulted in the formation of precipitation-free zones (PFZ), which was mainly responsible for the weakened peak-age hardening effect. Moreover, compared to the base alloy, finer and denser precipitates were obtained in the matrix of the modified alloy, due to the increased density of dislocations induced by the dispersoids and the higher Mg supersaturation.

Microstructural evolution and precipitation behavior of multicomponent Al83Mg5Si5Cu5Li2 alloy

We investigated the microstructural evolution and precipitation behavior of a new multicomponent Al83Mg5Si5Cu5Li2 alloy using transmission electron microscopy and atom-probe tomography. As-cast alloy consists of θ-Al2Cu, Mg2Si, Q-Al5Cu2Mg8Si6, and AlLiSi phases. Solution treatment causes the formation of the Q-Al5Cu2Mg8Si6 phase, and slow furnace cooling causes the formation of θ-Al2Cu particles in the Al matrix, which depletes solute atoms. Nanoscale Cu/Si/Mg-rich solute clusters are formed during natural aging, contributing to an excellent age-hardening response.

A magnesium alloy with ultrahigh Gd content manufactured by laser directed energy deposition: Heat treatment and microstructure evolution

High RE content is able to improve the age-hardening response of Mg–RE alloys, thus enhancing mechanical properties. Here, we successfully produce a Mg–22Gd–2Zn–0.4Zr (wt%) alloy with ultrahigh Gd content using laser directed energy deposition (LDED) and investigate the microstructure evolution during heat treatment. Results show that for the studied alloy, the yield strength is 333 MPa after solution at 520 °C for 2 hours and aging at 200 °C for 64 hours, superior to most existing Mg–Gd alloys, due to the high number density of the strengthening β' phase resulting from high supersaturation. During solution treatment, increasing the temperature improves the solution efficiency by reducing the holding time, while preventing the overgrowth of grains. The peak hardness is around 139 Hv for both 175 °C and 200 °C aging, indicating that the peak hardness of Mg alloys with high Gd content is insensitive to aging temperature. This research can serve as guidance for the heat treatment and microstructure control of high-Gd-content magnesium alloys.

Effect of Pre-Deformation on Precipitation in Al–Zn–Mg–Cu Alloy

AbstractIn this work, strengthening effects and evolution of precipitates in a pre-deformed Al–Zn–Mg–Cu alloy during ageing were investigated using Vickers hardness measurements, tensile tests, and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). It was found that all cold rolled conditions had higher mechanical strength than the non-deformed condition for all ageing times and that this effect increases at higher deformation ratios. It was also found that the non-deformed condition has a higher age hardening response than that of the cold rolled conditions. A homogeneous precipitate distribution was observed in the non-deformed condition, while the cold rolled conditions contained non-uniformly distributed precipitates due to the introduced dislocations. This led to larger precipitate sizes and a reduction in the precipitate number densities in the pre-deformed conditions. HAADF-STEM analysis revealed differences in the fraction of different precipitate types between the non-deformed and the cold rolled conditions. η', η2, and disordered η phase were observed in the non-deformed condition, while η', η2 and the newly identified Y phase were observed in the cold rolled conditions. The disordered η phase contained structural units of the η1 phase and was associated with reducing the lattice misfit between this phase and the Al matrix. Formation of the Y phase was related to an accelerated nucleation rate in the regions of high dislocation density. Graphical abstract

Microstructure characterization and age hardening response of Mg-8Sn-2Al alloy with Bi, Ag addition

The effects of Bi and Ag addition on the microstructure and age hardening response of Mg-8Sn-2Al alloy were investigated in this study. The Mg2Sn phase distributed along grain boundaries was observed in as-cast Mg-8Sn-2Al alloys and disappeared after solution treatment due to the Sn dissolution into ɑ-Mg matrix. However, a majority of Mg2Sn phase disappeared and few Mg2Sn particles still existed in the solid solutioned Mg-8Sn-2Al alloys containing Bi element. Mg2Sn precipitates formed in all peak-aged alloys. And Al, Bi and Ag elements are enriched in the areas of lath-shaped and spherical Mg2Sn precipitates in peak-aged Mg-8Sn-2Al-1.5Bi-1Ag alloy. Bi and Ag elements can act as heterogeneous nucleation sites and restrict the growth of precipitates to improve the age hardening response of Mg-8Sn-2Al alloy. Meanwhile, the undissolved Mg3Bi2 phase or early precipitated Mg3Bi2 phase during ageing treatment probably change the morphology and elements distribution of Mg2Sn precipitates. In addition, Ag and Bi atoms play a role on grain refinement and solution strengthening in Mg-8Sn-2Al alloy. Therefore, the micro-hardness value of peak-aged Mg-8Sn-2Al-1.5Bi-1Ag alloy increased to 69.9 HV from 47.4 HV of solid solutioned Mg-8Sn-2Al alloy. The precipitation behaviour, refined second phase, grain refinement and solute atoms synergistically made contributions to the strength improvement. The roles of Bi and Ag elements on improving age hardening response and precipitates characteristics are discussed in this study, which provides an important reference for exploring aged Mg-Sn alloys.

Enhanced age hardening response of a Mg-Y alloy by Ca addition at an elevated temperature

The effect of Ca addition on the age hardening response and precipitation behavior of a Mg-7Y-0.5Zr (wt%) alloy during isothermal aging at 350 ℃ has been studied by hardness test and high-angle annular dark-field scanning transmission electron microscopy. The addition of 0.6 wt% Ca can effectively enhance the age hardening response of Mg-7Y-0.5Zr alloy at 350 ℃, and the enhanced hardness is mostly attributed to the uniform precipitation of a novel strengthening precipitate phase, designated βp. The βp phase forms as plates on {101̅0}α planes. It has a Cd45Sm11-type structure (space group F4̅3m, lattice parameter a = 2.24 nm) and a composition of Mg5Y0.8Ca0.2. Its orientation relationship with the α-Mg matrix phase is such that [1̅11]βp // [21̅1̅0]α and (110)βp // (0001)α. After prolonged aging at 350 ℃, the metastable phase βp can transform in-situ to the equilibrium phase β-Mg24Y5 with a cube-to-cube orientation relationship. The βp → β phase transformation is associated with the transition of constituent atomic clusters and the re-distribution of selected atoms within clusters.

Influence of Mg and Cu on precipitation behaviors and mechanical properties of Al–Si alloys

Al–8Si-0.5 Mg, Al–8Si–2Cu, and Al–8Si–2Cu-0.5 Mg alloys have been designed in this study, aiming to reveal the influence of Mg and/or Cu additions on microstructures and mechanical properties of Al–Si alloys. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Kampman-Wagner Numerical (KWN) model have been used to investigate age-hardening behaviors as well as their relation to mechanical properties. Microstructure analysis suggest that β'', θ′ and QP precipitates have formed in peak-aged Al–8Si-0.5 Mg, Al–8Si–2Cu and Al–8Si–2Cu-0.5 Mg alloys, respectively. Notably, the Al–8Si–2Cu-0.5 Mg alloy exhibits faster precipitation kinetics, larger age-hardening response and remarkable thermal stability compared to the other two alloys. Theoretical calculations indicate that the nucleation of QP precipitates is easier than β'' and θ′ precipitates, but the slower diffusivity of Cu limits the growth rate of QP precipitates during the growth stage. Furthermore, the contribution of β'', θ′ and QP precipitates to yield strength of peak-aged Al–Si alloys has been quantified, which shows a significant variation. β'' precipitates in the Al–8Si-0.5 Mg alloy exhibit the highest strengthening effect (∼186.4 MPa), followed by QP precipitates in Al–8Si–2Cu-0.5 Mg (∼160.2 MPa) and θ′ precipitates in the Al–8Si–2Cu alloy (∼13.4 MPa). This work provides valuable insights into the influence of Mg and/or Cu additions on the aging response and mechanical properties of Al–Si alloys, offering a reference for further enhancing the performance of Al–Si alloys.

Enhanced strength-ductility synergy in an Al–Cu alloy via Cd-induced hybrid θ″+θ′ precipitation

In this work, trace Cd addition (∼0.2 wt%) was exploited in Al–Cu alloys to improve the mechanical properties by introducing the hybrid θ″+θ′ precipitates, as governed by the solid-solution temperature T. In detail, if the solid solution temperature is low (T = 500 °C), Cd microalloying merely initiates undesirable dual precipitates of θ″ and θ′ with similar size and a homogenous distribution. In such case, the Cd-microalloyed Al–Cu alloy suffers from limited optimization on the strength-ductility synergy in comparison to the Cd-free Al–Cu alloy. However, elevating T to 530 °C results in the formation of dense Cd-rich nanoparticles, significantly promoting the hybrid θ″+θ′ precipitation with a bimodal distribution by offering numerous heterogeneous Cd-rich nucleation sites. Such hybrid θ″+θ′ precipitation imparts the architecture of microstructural heterogeneity, which was found to be responsible for the rapid age-hardening response and enhanced strength-ductility synergy in the Cd-microalloyed Al–Cu alloy.

Structural insights into T1 precipitation and age hardening by Zn alloying for T6 treated Al–Cu–Li based alloys with a high Cu/Li ratio

T1 (Al2CuLi) nano-phase precipitation has been a major focus in developing new multi-microalloyed Al–Cu–Li alloys. Nevertheless, more understanding is required. This work investigated the effect of Zn alloying on the microstructure and mechanical properties of multi-element Al–Cu–Li alloys with a high Cu/Li ratio using a combination of advanced electron microscopy and first-principles calculations. Zn alloying (i) induced a higher density of geometrically-necessary dislocations after solid-solution treatment, which increased the T1 nucleation rate during the early aging stage; (ii) promoted T1 growth by substituting Al or Cu atoms in the T1 structure; and (iii) profoundly increased the aging kinetics, the age hardening response, and the strength. In addition, the co-segregation of Zn, Mg and Cu at the T1/Al interface helped to stabilize the fine T1 nano-platelets. These structural insights into T1 precipitation and age hardening by Zn alloying can help to guide the optimal design of high-performance Zn micro-alloyed multi-element Al–Cu–Li alloys.

Enhanced corrosion performance by controlling grain boundary precipitates in a novel crossover Al-Cu-Zn-Mg alloy by optimizing Zn content

The effect of Zn contents on grain boundary precipitates (GBPs), intergranular corrosion (IGC), and stress corrosion cracking (SCC) of crossover Al–Cu–Zn–Mg alloys was investigated. The increase in Zn contents accelerates the age-hardening response, and both matrix precipitates (MPs) and GBPs are transformed from S phase to S and η phases, while the width of the precipitates free zones (PFZs) first increases and then decreases. The narrowed PFZ width, coarse and discontinuous GBPs were able to inhibit anodic dissolution resulting in enhanced IGC resistance. The η phase inhibits anodic dissolution leading to an enhanced IGC resistance. First-principles calculations demonstrate that the GBPs surface with Mg termination planes shows a lower work function than that of Al leading to preferential anodic dissolution. The evolution of GBPs and PFZ widths changes the mechanism of SCC susceptibility for different Zn contents alloys from anodic dissolution and hydrogen embrittlement to anodic dissolution.

Study on the precipitation behavior of the hot deformed 2195 Al–Li alloy after online quenching and its relation to the age-hardening response

The fine-grained structures following deformation would be retained by using online quenching treatment which would achieve the purpose of simplifying formation process and reducing energy consumption. This study examined the effects of hot deformations on grain morphology, dynamic precipitation behavior and age-hardening response of the alloy after online quenching. And the dynamic precipitates re-solution behavior after online assisted heating were analyzed. The results indicated that the age-hardening response of the alloy was primarily controlled by the deformation temperature, and the appropriate deformation temperature range for online quenching was determined as 475–500 °C. Most of the deformed grains exhibited <110>Al//CD orientation. The increase in deformation temperature induced the transformation of the deformed grains from <110>Al//CD to <100>Al//CD orientation. A number of coarse second phases would be observed when the alloy was deformed below the optimal temperature range (e.g. 450 °C). For this case, it was suggested that the deformed alloy was heated by online assisted heating for a short time with the remaining heat after hot deformation, promoting most of the dynamic precipitates to be re-dissolved before online quenching. As a result, the deformation temperature window suitable for online quenching in 2195 Al–Li alloy could be broadened to 450–500 °C.

Adjustment of a novel β′-type precipitate in a creep-aged Mg−15 wt%Gd alloy

A Mg−15 wt%Gd binary alloy was prepared by hot-extrusion and then creep-aged at 200 °C under a constant stress of 200 MPa in this work. Compared with the traditional aging, creep aging clearly accelerated the age-hardening response, as the aging time for the peak yield strength was shorten from 72h to ∼16h. In the peak-creep-aged sample, a novel precipitation structure, named as β′H herein, was observed as the dominant precipitate. Interestingly, these dense β′H precipitates preferred to distributing along a certain direction which generally deviated from the extrusion direction (ED) by 54–67°. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization revealed that this novel β′H precipitate is composed of distorted Gd-rich hexagonal units, with the chemical formula of Mg7Gd2 and having a monoclinic structure, a = 0.671 nm, b = 1.727 nm, c = 0.521 nm and β = 100.7°. Density functional theory (DFT) calculations suggest that the applied stress during creep aging could provide an effective tensile stress for the motion of the surrounding Mg atoms when forming the β′H structure.

Microstructure and Properties of 6 Series Aluminum Alloy Under Different Aging Treatment Systems

In the present study, an investigation is conducted into the effect of different aging treatments on the hardening properties of the 6151 aluminum alloy sheet. According to the research results, the artificial aging hardening response of the pre-aging sheet is significantly stronger compared to natural aging after the solution treatment. In the stage of artificial aging, 120 oC pre-aging treatment produces a more significant strengthening effect than 80 oC and 100 oC pre-aging treatment. The longer the artificial time, the higher the hardness value. When the artificial aging temperature reaches 200 oC, the time taken to achieve peak aging is the shortest, and the occurrence of softening is evident in the over-aged state. When the artificial aging temperature is 200 oC, the softening effect becomes more significant. Natural aging can inhibit the strengthening effect of artificial aging. With the extension of natural aging, the hardening effect of artificial aging diminishes gradually.

Effect of Sn Addition on the Microstructure and Age-Hardening Response of a Zn-4Cu Alloy

The aim of this research is to assess the influence of Sn inclusion on the microstructure evolution and age-hardening response of a Zn-4Cu alloy. This is the first study to correlate the age-hardening response to the microstructure of Zn-4Cu alloy reinforced with different Sn contents. A series of Zn-4Cu-Sn alloys were successfully fabricated with different Sn concentrations in the range of 0.0–4.0 wt.% using permanent mold casting. The microstructure of Zn-4Cu-Sn alloys was investigated by means of a scanning electron microscope (SEM) attached with an energy dispersive spectroscope (EDS) and X-ray diffraction (XRD) line profile analysis. At room temperature, the Vickers microhardness measurements were used to assess the age-hardening response of alloys. The results show that the microhardness of the Zn-4Cu (ZC) binary alloy increases a little bit from 76 to 80 HV as the aging time increases from 2 to 128 h, respectively. For aging times up to 16 h, the microhardness of all Sn-containing alloys decreases but then increases again. The lowest hardness belongs to the ZC-1.5Sn alloy, and the Sn-Zn-3.0Sn alloy has the highest; the other alloys fall somewhere in between. At high aging times (64 and 128 h), the microhardness of all Sn-containing samples increased continuously with an increasing Sn content from 0.0 to 3.0 wt.%. When the Sn-containing alloys (3.5 and 4.0 wt.% Sn) were aged for 64 and 128 h, the hardness declined by 7.94% and 8.90% compared to their peak aging hardness values, respectively. By considering the structural changes that occur in the Zn-4Cu-Sn alloys, the reasons for the observed variations in microhardness data with increasing Sn content and aging time were elucidated. X-ray diffraction (XRD) data was analyzed to determine the zinc matrix’s lattice parameters, c/a ratio, and unit cell volume variations.

Investigation on hot deformation behavior and quenching precipitation mechanism of 2195 Al-Li alloy

Al-Li alloys suffer from a narrow processing window. The hot deformation behavior of the ramp heating homogenized 2195 Al-Li alloy was investigated by hot compression test in the temperature range of 300–500 °C and strain rate range of 0.01–10 s−1, and the strain compensated Arrhenius constitutive relationship model and processing map of the alloy were established. The laws of Zener-Hollomon parameters on the age-hardening response and quenching sensitivity of the deformed alloy were analyzed. The quenching precipitation mechanism of the deformed alloy were revealed. The results showed that the hardness of the aged alloy tends to increase first and then decrease with increasing Z parameter, while the quenching sensitivity tends to decrease first and then increase. The quench-induced phases are precipitated under the induction of incoherent Al3Zr dispersoids within recrystallized grains. The boundary angle would affect the quenching precipitation behavior. It was also found that the types and amounts of second phases are significantly different between the homogenized and hot-deformed samples under slow cooling rate. The ramp heating homogenized 2195 Al-Li alloy after hot deformation at temperatures of 455–500 °C and strain rates below 1 s−1 possesses a lower flow stress, higher age-hardening response and smaller quenching sensitivity.

Long-term stress relaxation behaviors and mechanisms of 2219 Al–Cu alloy under various temperatures and initial stresses

Large 2219 Al–Cu alloy aerospace integral components suffer from long-term stress relaxation aging (SRA) due to complex temperature and stress loads during aging treatment/forming and service process, which makes it difficult to ensure their appropriate residual stress and excellent mechanical and service properties. However, the research is limited to a thorough understanding of macroscopic and microscopic features and underlying mechanisms of the long-term SRA under multivariable aging conditions. Therefore, this study investigated macroscopic and microscopic features of long-term SRA under different temperatures (120 °C to 190 °C), initial stress levels (100 MPa to 250 MPa) and durations (0 h to 50 h) through stress relaxation curves, metallographic traits, Vickers hardness, tensile performance, dislocations and phases of precipitation. On the basis of experimental outcomes, the comprehensive mechanisms beneath SRA were unraveled through dislocation theory, multiphase strengthening mechanisms and thermodynamics, where the interplays of stress relaxation behavior with age-hardening response were taken into consideration. The results showed elevations in the rate of stress reduction as the temperature and initial stress rose. At an initial stress greater than the yield stress of alloy, a marked increase in stress relaxation was found, and the mechanisms transform from the intragranular motion of dislocations and diffusion of grain boundaries to the intragranular and intergranular motion of dislocations and migration of grain boundaries. The stress reduction rate rose sharply when the temperature exceeded 175 °C, and the dislocation movement mechanisms transform from gliding to climbing of dislocations. Stress relaxation is in nature progressive transformation of strain from elastic into a permanently inelastic state via the motion of dislocations, leading to the decrease of movable dislocations and the increase of immovable dislocations with more stable configurations. The age hardening is mainly determined by precipitation strengthening, supplementarily by dislocation strengthening, and obvious stress orientation effect (SOE) of G.P. zones and θ" phases degenerates strengthening effect. The interplay between stress relaxation behavior and age-hardening response influences the thermal–mechanical coupling SRA of 2219 Al–Cu alloy, which depends fundamentally on the motion of dislocations and their interplay with precipitated phases. This is a thermal activation process concerning the interplay between internal (age-hardening resistance) stress and external (initial) stress. The initial energy of elastic strain offers Gibbs free energy as the SRA driver, and a steady state of stress relaxation is attained with the lowest energy of elastic strain. These findings provide valuable insights into exploring innovative aging treatment/forming for optimizing residual stress, mechanical performance and service property in a synergistic manner.

Effect of cold rolling on aging precipitation and mechanical properties of magnesium-aluminum alloy

Although magnesium-aluminum alloys, such as AZ80 and AZ91 have promising application potential in automotive, high-speed train and aerospace fields, their age-hardening response is generally not very appreciable. In this work, the aging-hardening response of AZ80 alloy was effectively enhanced by applying cold-rolling deformation before conducting conventional aging treatment at 200 °C. Compared to the directly aged sample, the yield strength of the pre-rolling and aged sample was increased by 35 MPa. Electron microscope examination confirmed that profuse {101¯1} and {101¯1}-{101¯2} twins, consisting of high density of dislocations and stacking faults, were generated by cold rolling. Blocky or ellipsoidal Mg17Al12 precipitates formed at the twin boundaries (TBs) during subsequent aging treatment. Crystallographic analysis indicated that the precipitates at {101¯1} TBs always held an identical Potter OR with both the matrix and twin, while the precipitates at {101¯1}-{101¯2} TBs exhibited three different ORs: Burgers OR, Potter OR and P-S OR with either the matrix or the twin. Moreover, recrystallized grains were found inside {101¯1}-{101¯2} double twins after peak-aging at 200 °C, implying that precipitation and recrystallization might occur concurrently along TBs at a relatively low temperature. It was speculated that the highly stored energy inside twins and the high elastic energy between the precipitates and twins were driving factors for the occurrence of recrystallization.

Effect of Room-Temperature Pre-rolling and Pre-cryorolling on Natural Aging and Bake Hardening Response of an Al–Mg–Si Alloy

Effect of Room-Temperature Pre-rolling and Pre-cryorolling on Natural Aging and Bake Hardening Response of an Al–Mg–Si Alloy