Under-aged State Research Articles

A novel under-aging design to improve the creep rupture life of a precipitation-strengthened Fe–Ni-based superalloy

The under-aging, peak-aging, and over-aging microstructures of a precipitation-strengthened Fe–Ni-based alloy were designed by applying different heat treatment procedures. Tensile creep tests were conducted at 700 °C and 250 MPa on samples exhibiting different strengthening γ′ precipitate volume fractions and diameters. The creep life of the alloy in the under-aged state was increased by more than 10% compared to the peak-aged condition, which provided a new approach to designing the heat treatment regime that improve the creep rupture life and plasticity of precipitation-strengthened alloys. The improvement in creep properties of the under-aged state was attributed to the transition from shearing to Orowan looping of γ′ particles by matrix dislocations during the dynamic precipitation and coarsening of γ′ phase, increasing intragranular deformation resistance. The under-aged microstructure design allowed reducing the grain boundary sliding contribution to overall strain during creep. As a result, the steady-state creep stage of the alloy in the under-aged state was extended. Additionally, relationships between applied stress and nucleation, growth, and coarsening of the γ′ phase have been evidenced.

The precipitates and properties evolution behaviors of AlZnMgCu alloy during the retrogression process with slow heating

In order to develop the retrogression and re-aging (RRA) effect on the performance of AlZnMgCu alloy products with large scale, the hardness, electrical conductivity, tensile properties, and exfoliation corrosion resistance of the Al7.7Zn2.1Mg2.0Cu alloy were investigated at two pre-aging temperatures and a 3 °C/min retrogression heating rate. Transmission electron microscopy (TEM) was used to investigate the evolutions of precipitates at different RRA routes. Results revealed that the standard compliant performances could also achieved by setting the pre-aging as the under-aged state when the slow retrogression heating rate existed. Continuous precipitation occurred at the early stage of heating retrogression. The dissolution behavior of the GP zone with the radius lower than the critical value had finished in heating retrogression. The η′ phase re-precipitated firstly in the isothermal retrogression, then grew and coarsened with the retrogression time increase. During the whole retrogression including heating and isothermal stages, the hardness undergone an “up-down-up-down” transversion. After RRA-treated at (105 °C, 24 h + (heating to 190 °C at 3 °C/min)190 °C, 50 min +120 °C, 24 h), excellent corrosion resistance achieved with only a 5.5% sacrifice of tensile strength when compared to peak aged state. Due to the slow temperature-rising process in retrogression for the large section products, the optimized RRA schedule is more practical for industrial applications.

Research on hydrogen embrittlement behavior of L-PBF 18Ni(300) maraging steel by experiments and numerical simulations

Affected by complex microstructural evolution, dominant factors in hydrogen embrittlement (HE) of maraging steels in different aging states were not clear. This fundamental issue was clarified systematically by experiments and numerical simulations in this work. In under-aged and peak-aged states, diffuse coherent ω/matrix interfaces and sharp semi-coherent η-Ni3Ti/matrix interfaces acted as reversible traps, respectively. Their different interfacial properties determined the differences in trap density in the two states. The lowest trap density led to the fastest hydrogen diffusion and the highest hydrogen content in lattice, causing the worst HE in under-aged state. In peak-aged state, the situation was relieved by the highest trap density. In over-aged state, the newly formed Laves-Fe2Mo/matrix interfaces acted as slightly deeper traps compared with the η-Ni3Ti/matrix interfaces, but the reduction in interfacial area decreased trap density and accelerated hydrogen diffusion instead. Even so, the improvement in ductility by reverted austenite further relieved HE susceptibility in this state.

Effect of Aging on Impact Toughness and Fracture Behavior of Three Wrought Aluminum Alloys

Herein, the effect of the aging state on the impact toughness of three typical wrought Al alloys (7075, 2024, and 6A01) is investigated. At room temperature, the fracture mode of the three aluminum alloys at different aging states is a ductile transgranular fracture. At low temperatures, high‐strength aluminum alloys exhibit a mixture fracture of transgranular and intergranular, while medium‐strength aluminum alloys are still dominated by transgranular fractures. The effect of the aging state on the impact toughness of aluminum alloys was analyzed based on the deformation mechanism and morphology near grain boundaries. The underaged state with planar slip dislocations exhibits good plasticity and work‐hardening ability, which reduces the strain localization caused by dislocation plugging and thus improves the impact toughness of the alloy. Furthermore, the fine grain boundary precipitates and the narrow precipitation‐free zone of the underaged increase the uniform deformation capacity near the grain boundaries and inhibit the initiation and propagation of cracks. Compared with high‐strength aluminum alloys, medium‐strength aluminum alloys exhibit higher impact toughness due to the formation of a larger plastic zone and shear lip area during impact.

Effects of aging state on fatigue properties of 6A01 aluminum alloy

AbstractThe high‐cycle fatigue (HCF) experiments were carried out for different aging states to study the effects of the aging state and yield strength on the fatigue properties of a 6A01 Al alloy. The results show that the 6A01 Al alloy with the highest yield strength at a peak‐aging (PA) state can exhibit the highest fatigue strength in comparison with the overaged (OA) state and the underaged (UA) state. The main reason is that the increased strength of the 6A01 Al alloy at the PA state can improve the plastic deformation resistance and inhibit the fatigue crack initiation. Additionally, the UA has a similar yield strength but a higher fatigue strength compared with the OA, which shows that the aging state also has a significant influence on the fatigue properties. Finally, combined with the previous studies on the 2XXX and 7XXX Al alloys, the effects of the aging state and yield strength on the fatigue properties of Al alloys are summarized.

Precipitation behavior of an Al–Cu–Li–X alloy and competing relationships among precipitates at different aging temperatures

Abstract The precipitation behavior of strengthening phases has a significant effect on the properties of Al–Cu–Li–X (X = Mg, Ag, Zr) alloys during aging treatment. In this work, the precipitation behavior and competing relationships among precipitates were studied, and the relationship between the microstructure and mechanical properties was analyzed in detail at four aging temperatures for an Al–Cu–Li–X alloy (AA2195). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) were used to systematically explore the precipitation characteristics of the alloy. Then, the precipitation sequence and the interrelationship among the precipitates were determined in the under-aged and peak-aged states at various temperatures. The results show that the alloy exhibits different precipitation characteristics in various aging conditions, particularly for the under-aged state. Furthermore, the T1 phase (subject to aging temperature) preferentially multiplies to dominate the microstructure, governs the formation of other precipitates, and eventually gives rise to the multi-precipitate coexistence of the peak-aged state. Therefore, the influence of temperature on the precipitation behavior is realized by its effect on nucleation of the T1 phase. Moreover, the increase in the strength of the alloy arises from the comprehensive strengthening effect of all precipitates, in which the T1 phase plays a decisive role.

Microstructure, mechanical properties and fracture behavior of a new WE43 alloy

The microstructures, mechanical properties and fracture behaviors of a new WE43 alloy (Mg–4Y–1.6Nd–2Sm–0.5Zr) were investigated. The microstructure of the as-cast alloy includes a Mg matrix, Mg41Sm5, Mg41Nd5 and Mg24Y5 eutectic phases, as well as β-phase. After an optimal solution treatment, the eutectic phases are almost completely dissolved; only a few spots of blocky Mg–Y compounds remain at grain boundaries. After an aging treatment, a large amount of β′-phases are dispersed and precipitated at the grain interior, which provides good comprehensive mechanical properties of the alloy, particularly in the under-aged state. The tensile strength is up to 290 MPa, the yield strength reaches 209 MPa, and the elongation is slightly improved. Furthermore, the fracture behaviors of the studied alloy in different states significantly differ. In addition, a comparison of mechanical properties of the new WE43, traditional WE43 and other modified WE43 alloys is presented.

Effect of Trace Silver on Precipitation Behavior of Strengthening Phases and Mechanical Properties of Aluminum-Copper Alloys

The effects of trace silver (Ag) on the precipitation behavior of strengthening phases and the mechanical properties of Al-Cu alloys have been investigated by performance testing, scanning electron microscopy, and transmission electron microscopy. Ag retarded the aging response of the Al-Cu alloys. The density of the θ′′ and θ′ phases decreased with increasing Ag content in under- and peak-aged alloys, and there was almost no θ′ phase in the 0.12 wt.% Ag alloy in the under-aged state. The θ′ phase morphology was narrower and longer after addition of Ag. Fatigue striations became more obvious and their spacing reduced at the fracture surface of experimental alloys with increasing Ag content, reflecting an improvement in the fatigue life of the alloys. On addition of 0.06 wt.% or 0.12 wt.% Ag, the fatigue life of the Al-Cu alloys was increased by 72.7% and 155.5%, respectively.

To improve strength and bio-corrosion resistance of Mg-4Zn alloy via high strain rate rolling combined with double aging

High strain rate rolling (HSRR) and artificial aging are conducted to improve mechanical properties and bio-corrosion resistance of Mg-4Zn alloy by modifying grain size and precipitates. The under-aged state is the most corrosion resistant among four states in single and double aging. Bio-corrosion resistance of the as-rolled alloy can be improved by double aging of 70 °C × 10 h + 160 °C × 2 h, resulting from stress relief and uniformly distributed nano-scale G.P. zones and β1′ precipitates.

T6 Treatment and Its Effects on Corrosion Properties of an Mg⁻4Sn⁻4Zn⁻2Al Alloy.

The effects of T6 treatment (solid-solution and artificially aging at 200 °C) on the microstructure and corrosion properties of an Mg–4Sn–4Zn–2Al (TZA442) alloy were systematically investigated. The alloy exhibits a double-peak age-hardening behavior, i.e., one is 78 HV after 10 h of aging, and the other is 83 HV after 50 h of aging. The strengthening effect is mainly attributed to the simultaneously and mutually independent precipitation of the dispersively distributed MgZn2 and Mg2Sn precipitates. Solid-solution treatment can significantly decrease the corrosion rate of the TZA442 alloy. The following aging treatment can initially further decrease the corrosion rate in the under-aged state, but can afterward slightly increase it after 50 h of aging. The relationship between the microstructure and corrosion properties is also discussed.

Dislocation dynamics modeling of precipitation strengthening in Fe–Ni–Al–Cr ferritic superalloys

Abstract

Precipitation behavior of the high-Li-content in-situ TiB2/Al-Li-Cu composite

The microstructural evolution and mechanical properties of the cast in-situ TiB2/Al-3.3Li-1.2Cu composite during heat treatment were systematically investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and Vickers hardness (HV). The nano TiB2 particles mainly agglomerated along the grain boundaries, showed a clean and clear interface between matrix. Much fine δ′ (Al3Li) particles were distributed homogeneously after quenching, and its size increased with the increase of aging time. Primary θ′ and T1 precipitates were visible in the under-aged (UA) state, which contributes to a rapid increase in hardness. And the plate-like T1 precipitates of very high aspect ratio were precipitated with prolonging aging time. It was worth mentioning that the thickness of this plate-like T1 precipitates was stable as the aging proceeded. The studied composite, which was largely strengthened by the δ′ and T1 phases in the peak-aged condition, had a strong age-hardening response. After aging at 175°C for 214h, a peak-hardness, which was nearly double than that of the as-quenched, was obtained.

An investigation of the strain dependence of dynamic precipitation in an Al-Zn-Mg-Cu alloy

The use of high strength Al-Zn-Mg-Cu alloys in automotive components is restricted by their low formability. Warm forming in the under-aged state improves formability, but induces a strain-dependent microstructure evolution due to dynamic precipitation. We present a new methodology to quantitatively evaluate the dynamic precipitation strain dependence which is applicable to strain rates relevant to forming. The plastic strain is varied using a tapered tensile sample and the precipitation state is measured in terms of size and volume fraction using spatially resolved small-angle X-ray scattering, showing its evolution during straining is dependent on the imposed strain level and rate.

Hydrogen Embrittlement Susceptibility and Hydrogen-Induced Additive Stress of 7050 Aluminum Alloy Under Various Aging States

Hydrogen embrittlement susceptibility of 7050 aluminum alloy under various aging states has been investigated by means of cathodic hydrogen permeation, slow strain rate test, hydrogen determinator, x-ray diffraction, and scanning electron microscope, and effect of hydrogen on atomic binding force of charged alloy has been calculated by free electron theory in this paper. Simultaneously, hydrogen-induced additive stress (σad) of 7050 aluminum alloy hydrogen charged with different current densities under various aging states have been investigated by flowing stress differential method. The results showed that hydrogen concentration of examined alloy increased with increasing charging time or current density under the same aging state. Hydrogen segregation occurred at grain boundaries which enlarged the crystal lattice constant, meanwhile, it reduced the average bonding energy and interatomic bonding force of the grain boundary atoms, thus resulting in hydrogen embrittlement; moreover, σad of 7050 aluminum alloy increased linearly with increasing hydrogen concentration under the same aging state, i.e., under aged: σad = −1.61 + 9.93 × 105 C H, peak aged: σad = −1.55 + 9.67 × 105 C H, over aged: σad = −0.16 + 9.35 × 105 C H, correspondingly, σad increased the susceptibility to hydrogen embrittlement (I HE) further. Under the same charging condition, aging states had a great influence on σad and I HE, the under-aged state alloy was of the highest, the over-aged state alloy was of the lowest, and peak-aged was in the middle.

Quantitative measurements of dynamic precipitation during fatigue of an Al–Zn–Mg–(Cu) alloy using small-angle X-ray scattering

The room temperature cyclic deformation behaviour of an Al–Zn–Mg–(Cu) alloy was examined in the under-aged and peak-aged states. The tests were performed under constant plastic strain amplitude loading in tension–compression mode, and the effect of cumulative strain (number of cycles), strain rate and plastic strain amplitude was examined. Pronounced cyclic hardening was observed in the under-aged states, and characterization of the microstructure using small-angle X-ray scattering illustrated that this was due to profuse dynamic precipitation at room temperature during cycling. Regardless of deformation conditions, the radius of the dynamically formed GP zones remained constant at 4–5Å, and only the volume fraction of the zones increased during cycling. The strain, strain rate and plastic strain amplitude dependence of the evolution of the volume fraction of zones forming dynamically was interpreted in terms of a nucleation-dominated reaction, and it is shown that the behaviour can be satisfactorily rationalized when the role of strain-induced vacancies is considered.

Fundamental mechanisms of surface damage associated to the localization of the plastic deformation in fatigue

Nowadays fatigue damage is becoming a very important issue for it may lead to a service failure at any moment. In low cycle fatigue, the crack initiations influence strongly the fatigue life of a material. The study done here is carried out on Waspaloy™at its under-aged state, a superalloy based Nickel characterized by the hardening precipitates, γ′ in matrix Nickel, γ. Fatigue resistance depends on the material microstructure. Damage mechanisms due to the localization of plastic deformation by fatigue are studied on the precipitate and grain from nano to meso scale. This work aims to have a better understanding on the crack initiation mechanisms related to the metallurgical parameters: the influence of the grain and γ′ size. Complete investigation is performed by using multiple analysis instruments: SEM and AFM.

The effect of Cu addition on the precipitation and redissolution in Al–Zn–Mg alloy by the differential dilatometry

With the aim to determine the effect of the addition of micro-constituents (copper and magnesium, essentially the copper) on the phenomena of phase transformation and kinetic of precipitation in an Al–Zn–Mg alloy we proceeded to study the effect of addition of Cu and Mg by dilatometry and measurements of microhardness. The results show that the addition of copper modifies appreciably the number and perhaps the nature of some phases that precipitate in this alloy system. We showed that the number of the types of the equilibrium η phase increases and that for the underaged states this equilibrium η phase seems to precipitate from the η′ intermediate phase.

Effect of Heat Treatment on Grain Boundary Nanostructure and Corrosion of Low Copper AlMgSi Alloy

AlMgSi (6000-series) aluminium alloys are generally known to be re-sistant to intergranular corrosion (IGC). However, unfavourable alloy-ing and thermomechanical history may introduce susceptibility to IGC. A model alloy in the 6000-series, with 0.2 wt% Cu, was studied using an accelerated corrosion test. In the naturally aged T4 temper, the ma-terial did not exhibit localized corrosion, while in the underaged state (below peak strength), the alloy became highly susceptible to IGC. Samples aged to peak strength T6 temper or overaged were nearly re-sistant to IGC, exhibiting only shallow, localized IGC and pitting. TEM investigation of the IGC-susceptible material revealed nanosize AlMgSiCu particles along the grain boundaries, along with Cu-enriched nanofilm connecting the particles, indicating the cause of sharp IGC propagation. Further ageing caused coarsening of the grain boundary film thereby destroying its continuity and giving increased resistance against IGC.

Effects of heat treatments on Microstructure and mechanical properties of Mg–4Y–4Sm–0.5Zr alloy

Microstructure and mechanical properties of Mg–4Y–4Sm–0.5Zr alloy during heat treatments were investigated. The eutectic phase dissolved into the matrix and there was no evident grain growth after solutionized at 798 K for 8 h. The alloy showed very fine scale precipitates in the grains and along the grain boundaries after ageing at 473 K for 16 h. And the ultimate tensile strength of 348 MPa, yield strength of 217 MPa and elongation of 6.9% were attained in this under-aged state. The elongation of alloy greatly decreased, and the ultimate tensile strength decreased respectively with increasing the ageing time, for increasing amount and coalescence of the precipitates along the grain boundaries. The mechanical properties of the alloy could hold up to 473 K, and decreased steeply at temperatures higher than 523 K, for the softening of the precipitates and activation of non-basal slip systems.

Significance of Low Copper Content on Grain Boundary Nanostructure and Intergranular Corrosion of AlMgSi(Cu) Model Alloys

Intergranular corrosion (IGC) of model alloys in the 6000-series, with and without 0.2 wt% Cu, was studied using an accelerated corrosion test (BS ISO 11846 B), FE-SEM and FE-TEM. Low Cu alloys (0.02wt%) did not exhibit IGC even though they contained excess Si. The high-Cu, naturally aged material (T4) was susceptible to severe superficial etching. In the underaged state (below peak strength), the Cu-containing material was highly susceptible to IGC. Materials aged to peak strength (T6) or overaged were only slightly susceptible to IGC, with localized, shallow attacks. FE-TEM investigation of the underaged material revealed scattered, small AlMgSiCu-type precipitates, as well as a Cu-enriched film along the grain boundaries. The overaged material showed more extensive, coarse grain boundary precipitation. However, the Cu-enriched film was still present at localized sites. The reduced susceptibility to IGC upon artificial ageing was attributed to breaking of the continuity of the grain boundary film. The possible role of matrix precipitation is also discussed.