Subsequent Age-hardening Research Articles

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.

Near Net Shape Manufacturing of Sheets from Al-Cu-Li-Mg-Sc-Zr Alloy.

Thin twin-roll cast strips from a model Al-Cu-Mg-Li-Zr alloy with a small addition of Sc were prepared. A combination of a fast solidification rate and a favorable effect of Sc microalloying refines the grain size and the size of primary phase particles and reduces eutectic cell dimensions to 10-15 μm. Long-term homogenization annealings used in conventionally cast materials lasting several tens of hours followed by a necessary dimension reduction through rolling/extruding could be substituted by energy and material-saving procedure. It consists of two-step short annealings at 300 °C/30 min and 450 °C/30 min, followed by the refinement and hardening of the structure using constrained groove pressing. A dense dispersion of 10-20 nm spherical Al3(Sc,Zr) precipitates intensively forms during this treatment and effectively stabilizes the structure and inhibits the grain growth during subsequent solution treatment at 530 °C/30 min. Small (3%) pre-straining after quenching assures more uniform precipitation of strengthening Al2Cu (θ'), Al2CuMg (S'), and Al2CuLi (T1) particles during subsequent age-hardening annealing at 180 °C/14 h. The material does not contain a directional and anisotropic structure unavoidable in rolled or extruded sheets. The proposed procedure thus represents a model near net shape processing strategy for manufacturing lightweight high-strength sheets for cryogenic applications in aeronautics.

The effects of Ti content and quenching on phase transformations, microstructures, and mechanical properties in uranium-titanium alloys

• Increases in Ti content increase critical cooling rate. • Quenched microstructures progress from massive to acicular and banded martensites. • Acicular martensite forms directly from γ-phase: γ → α′ a . • Transition to banded martensite may occur when γ 0 intervenes: γ → γ 0 → α′ b . • Quenched alloys are ductile, supersaturated, and amenable to age hardening. The effect of Ti content on phase transformations, microstructures, and mechanical properties of U-Ti alloys are described for alloys containing 0.3 wt.% to 2.0 wt.%Ti. Rapid cooling is required to overcome diffusional decomposition of γ-phase and facilitate diffusionless transformation to supersaturated variants of α-phase. Critical cooling rate increases with increasing Ti content, opposite to the trend observed in U-Mo and U-Nb alloys. This difference occurs because the martensite transformation temperatures in these relatively dilute U-Ti alloys are above the knee of the C-curve for diffusional decomposition, unlike those in the more concentrated U-Mo and U-Nb alloys. In these U-Ti alloys critical cooling rate depends on the amount of undercooling required to reach M s , which increases with increasing Ti content, and the time for diffusional decomposition to occur just above M s , which decreases with increasing Ti content. The net result is that higher cooling rates are required as Ti content increases. Full quenching results in diffusionless transformation of γ-phase to supersaturated variants of α-phase. Very dilute alloys transform via a γ → β → α m sequence of massive transformations. Martensitic γ → α′ a transformation begins at ∼0.4%Ti, and 100% α′ a microstructures are obtained from ∼0.65% to ∼1.4% Ti. A transition to banded α′ b martensite occurs at ∼1.5%Ti. Evidence suggests that the α′ a to α′ b transition may occur when the cubic γ-phase first transforms to tetragonal γ°, which in turn transforms to orthorhombic α′ b via the sequence γ → γ° → α′ b . Fully quenched alloys exhibit moderate strengths and ductilities, and their supersaturation with Ti makes them amenable to subsequent age hardening. Subcritical quenching typically results in two-phase microstructures with lower ductilities and near-zero Ti-supersaturation, eliminating the possibility of subsequent age hardening.

Precipitation Hardening at Elevated Temperatures above 400 °C and Subsequent Natural Age Hardening of Commercial Al-Si-Cu Alloy.

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400 °C were systematically investigated in the commercially available AC2B alloy with a nominal composition of Al–6Si–3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480 °C. The hardness change is attributed to the precipitation of metastable phases associated with the α-Al15(Fe, Mn)3Si2 phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400 °C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable θ-Al2Cu and Q-Al4Cu2Mg8Si6 phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al–Si–Cu–Fe–Mn–Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400 °C to 500 °C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al–Si–Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400 °C.

Consequence of reinforced SiC particles and post process artificial ageing on microstructure and mechanical properties of friction stir processed AA7075

Friction stir processing and post process artificial ageing was successfully carried out on AA7075 with and without reinforcement of SiC particles producing defect free processed zone with uniform distribution of filler material. Effect of SiC particle reinforcement and artificial ageing times on the microstructural modifications was characterized using optical and electron microscopy, electron backscattered diffraction and X-Ray diffraction. Hardness, impact and wear tests were carried out to investigate mechanical behaviour before and after processing. Reinforcement of SiC particles during FSP and subsequent age hardening treatment brought about nearly twofold increase in hardness and impact toughness values by the combined effect of grain refinement, Zener pinning, dispersion strengthening and precipitation hardening. Significant improvement in wear resistance in terms of wear loss was also observed after processing compared to the reference material AA7075-T6. Fractured surface of post FSP age hardened AA7075 alloy exhibited features of ductile fracture during Charpy impact test.

Effect of solidification cooling rate on kinetics of continuous/discontinuous Al3(Sc,Zr) precipitation and the subsequent age-hardening response in cold-rolled AlMgSc(Zr) sheets

We explain the critical effect of casting processing route (i.e., the solidification cooling rate) on the kinetics of Sc–Zr precipitation upon solidification and, subsequently, on the age-hardening response of cold-rolled/aged Al–3Mg-0.36Sc-0.14Zr sheets. The evolution mechanisms of various types of Al3(Sc,Zr) precipitates are investigated in the as-cast specimens (fabricated via strip casting vs. mold casting techniques), as well as in the subsequently cold-rolled/aged sheet specimens. The Al3(Sc,Zr) precipitates appeared to have formed upon either discontinuous or continuous precipitation during cooling from solidification temperatures; the former leads to the formation of lamellar Al3(Sc,Zr) cells while the latter results in the formation of spherical precipitates, all of which possessing an L12 crystal structure and a coherent interface with the α-Al matrix. It is shown that slower cooling rates via mold casting lead to the occurrence of both continuous and discontinuous precipitation upon cooling from the solidification temperature. In contrast, higher cooling rates via strip casting results in the apparent prevention of all discontinuous precipitation reactions as well as in the suppression of continuous precipitation. The overall effect is the greater preservation of Sc supersaturation within the α-Al matrix of the as-cast specimens fabricated via strip casting which, subsequently, leads to higher precipitation kinetics and thus higher tensile properties upon age-hardening of the cold-rolled sheets.

Effect of filler material and post process ageing treatment on microstructure, mechanical properties and wear behaviour of friction stir processed AA 7075 surface composites

AA 7075 based surface composites reinforced with three different filler materials viz. Multiwall carbon nano tubes (MWCNT), Copper (Cu) and Silicon carbide (SiC) were successfully manufactured via Friction Stir Processing route with homogeneous dispersion of all the filler particles. The comparative effect of different filler materials and subsequent age hardening on the microstructural evolution of friction stir processed Aluminium matrix surface composites were characterized using optical and scanning electron microscopy (OM and SEM), electron backscattered diffraction (EBSD) and X-Ray diffraction (XRD). Hardness, impact toughness and wear behaviour was studied using microhardness test, Charpy impact test and pin on disc wear test respectively. Noteworthy enhancement in the microhardness (143 HV0.1 for as received AA 7075 T6 to 253 HV0.1 for FSPed with SiC filler followed by age hardening), impact toughness (15.8 kJ to 27.5 kJ) and wear resistance (wear loss dropped from 8.57 mg to 5.49 mg) were obtained through the addition filler particles during friction stir processing followed by age hardening. The enhancement in the mechanical and wear properties was primarily ascribed to grain refinement, Zener pinning effect (attributable to incorporation of filler material) and precipitate evolution during subsequent age hardening. SiC filler was found to be the best among the three fillers for the set of FSP and age hardening parameters used in the experiment. Nearly twofold increase in microhardness, impact toughness and wear resistance was achieved in age hardened SiC filler surface composite.

Fabrication of high-strength AZ80 alloys via multidirectional forging in air with no need of ageing treatment

A high-strength AZ80 alloy (with a tensile strength of 285 MPa, an ultimate strength of 388 MPa, and an elongation of 6.8%) has been fabricated by multidirectional forging (MDF) under air-cooling condition, at an initial temperature of 360 °C, with an accumulative strain of 1.8, with no need of ageing treatment. Extensive dynamic precipitation occurs in this sample, which deteriorates the subsequent age-hardening response but still contributes to improving the strength. In addition to the precipitation hardening caused by numerous nano-scale dynamic precipitates, the outstanding strength should also be attributed to the combined effects of other microstructural characteristics resulting from the increase of accumulative strain and decrease of forging temperature. They are 1) grain boundary strengthening by fine recrystallized grains, 2) strain hardening by the large dislocation density, and 3) texture strengthening by the low Schmid factor of basal slip. This paper provides a method of short process to manufacture high-performance large-scale AZ80 components for industrial production.

The Natural Aging Effect on Hardenability in Al-Mg-Si: A Complex Interaction between Composition and Heat Treatment Parameters

The technological relevance of Al-Mg-Si alloys has been rapidly growing over the last decade. Of particular interest to current and future applications is the problematic negative effect of prior natural aging on subsequent artificial age hardening. The influence of natural aging is dependent on both processing and compositional variables and has origins that are far from well-understood. This work examines the hardenability of 6000 series alloys under a wide range of conditions, paying particular attention to the natural aging effect. Experimental variables include alloy composition (Mg + Si, Mg/Si), cooling rate after solutionization, and duration of prior natural aging. Hardenability was evaluated with full hardness and conductivity aging curves for each condition, as well as select Transmission Electron Microscopy (TEM). Results are discussed based on the actions of naturally aged solute clusters during artificial aging. In particular, a complex interaction between vacancy concentration, cluster stability, and precipitation driving force is suggested.

Asymmetric Twin-Roll Casting of an Al-Mg-Si-Alloy

The industrial application of high-alloyed Al-Mg-Si alloys for the production of thin strips by means of twin-roll casting is limited due to the structural inhomogeneity and segregation formation. To reach the highest mechanical properties of the finished product, a direct influence on the strip formation conditions during the twin-roll casting can be applied. Analogous to the asymmetric rolling process, additional shear stresses were created in the strip forming zone by using different circumferential velocities and torques of the caster rolls. To provide the asymmetric process conditions, only one caster roll was left driven and the second one was left idling during the casting process. The microstructure and the mechanical properties of the strips in the as-cast state as well as after the homogenization and subsequent age-hardening were analyzed. A comparison of the test results showed a positive influence of the asymmetry conditions on the strips’ properties.

Process parameters for hot stamping of AA7075 and D-7xxx to achieve high performance aged products

This work examines the necessary process parameters for die quenching (DQ) during hot stamping and subsequent age hardening and paint bake cycle (PBC) response for two alloys: AA7075 and a developmental 7xxx alloy (referred to as AA7xxx), with a lower Chromium content, higher Zirconium content and higher Zinc-to-Magnesium ratio in comparison to AA7075. For both alloys, a minimum solutionizing time of 8 min was found to be required, along with a minimum quench rate of 56 °C/s and 27 °C/s for AA7075 and AA7xxx, respectively. Two-step aging treatments, leveraging a paint bake cycle (PBC) of 177 °C for 30 min as the second step, were considered after die quenching and were devised to achieve T6- or T76-level strengths. For AA7075, an aging treatment of 120 °C for 8 h, followed by the paint bake cycle (PBC) produced strength levels similar to a T6 temper. DSC experiments showed that the microstructure from this heat treatment was similar to a peak-aged T6 temper. For AA7xxx, a treatment of 100 °C for 4 h and followed by the paint bake produced a strength similar to a T76 temper, while 120 °C for 3 h followed by the paint bake yielded T6 strength levels. The properties of the custom aging treatments were validated through tensile tests. The resulting stress-strain curves show that it is possible to achieve T6 or T76 properties using a custom aging treatment incorporating the PBC that is 65–83% shorter than standard T6 or T76 treatments.

Mini-Thixoforming of Low-Carbon High-Alloy Steel

Semi-solid processing allows novel microstructures to be produced even in conventional materials. This is thanks to the peculiar conditions of the process and the rapid solidification. Despite that, semi-solid processing is not widely used in practice due to its technological complexity. Mini-thixoforming is an innovative method of processing metals in the region between their solidus and liquidus temperatures. With its small volume of the metal feedstock, it is a very precise and highly dynamic process. Consequently, it can be employed for materials with a very narrow freezing range which, until now, were impossible to thixoform conventionally. The present experiment focused on one of such materials: the low-carbon high-alloy age-hardenable X5CrNiCuNb16-4 steel. Owing to the low carbon level, the relevant temperature interval was 1380 - 1420 °C which, together with the need for strict control, posed a technological challenge. Once the semi-solid processing parameters were optimized, the die cavity was filled as required and the final products showed good surface quality. The resulting single-phase microstructure consisted of ferrite. Hence, given the 17% level of dissolved chromium, there is a potential for excellent corrosion resistance and, possibly, for subsequent age hardening of the material

Effects of Heat Treatment on the Microstructure and Mechanical Properties of Ductile AlSi9MgMn Die Castings

Conventional solution heat treatment is normally considered detrimental to high-pressure die-cast (HPDC) products. High-temperature solution treatment leads to expansion of pores of gas entrapped during the casting process inside the HPDC products causing surface blistering. Surface blisters act as crack initiators making the material less usable. To avoid surface blistering and improve the mechanical properties, a low-temperature solution heat treatment is suggested in this work. A heat treatment consisting of a low-temperature solution treatment and subsequent ageing is expected to give better properties in the HPDC AlSi9MgMn alloys owing to spheroidization of Si particles and subsequent age hardening. The heat-treated material had higher yield strength compared to the as-cast material, stored for 1 year (naturally aged), due to the precipitation of Mg2Si particles. However, it is observed that the as-cast and stored material work-hardened better as compared to the heat-treated material.

Quench sensitivity of Al–Mg–Si alloys: A model for linear cooling and strengthening

In this work quench-induced precipitation during continuous cooling of five Al-Mg-Si alloys is studied over a wide range of cooling rates of 0.05 K/min - 2x10^4 K/min using Differential Scanning Calorimetry (DSC), X-ray diffraction, optical microscopy (OM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and hardness testing. The DSC data shows that the cooling reactions are dominated by a high temperature reaction (typically 500 °C down to 380 °C) and a lower temperature reaction (380 °C down to 250 °C), and the microstructural analysis shows they are Mg2Si phase formation and B’ phase precipitation on dispersoids, respectively. A new, physically-based model is designed to model the precipitation during the quenching as well as the strength after cooling and after subsequent age hardening. After fitting of parameters, the highly efficient model allows to predict accurately the measured quench sensitivity, the volume fractions of quench induced precipitates, enthalpy changes in the quenched sample and hardness values

Ductility enhancement through texture control and strength restoration through subsequent age-hardening in Mg–Zn–Zr alloys

Hot plus warm rolling combined with an aging treatment was applied to Mg–Zn–Zr magnesium alloys for exploring the possibility of producing magnesium alloys capable of exhibiting good sheet formability at room temperature through texture control while becoming greatly strengthened after the sheet forming process using an aging treatment. Uniform ductility was significantly improved due to basal texture weakening. A considerable amount of strength loss due to the texture weakening could be restored by the subsequent aging treatment.

Fabrication of a High Performance Ti Alloy Implant for an Artificial Hip Joint

A new fabrication process of a stem for an artificial hip joint has been investigated to improve mechanical properties and to reduce both the fabrication cost and consumption of expensive Ti alloys by swaging and die-forging into near-net-shape at room temperature, followed by local heat treatment and precise machining. In this study metastable Ti-Nb-Sn alloys consisting of non-cytotoxic elements (biocompatible Ti alloys) were used for biomedical applications. It is found that swaging at both ends of a rod before die-forging saves material consumption by approximately 50% in comparison with turning, and enhances subsequent age-hardening. In addition the cold die-forging enables the fabrication costs to decrease. It is suggested that high strength of higher than 1100 MPa in the proximal part connected to a stem head and low Young’s modulus of less than 60 GPa in the distal part implanted in a femur can be obtained simultaneously in advanced Ti alloy stems.

Toughness after Interrupted Quench

We discuss data from a range of heat-treatable aluminum alloys, showing both yield strength and fracture toughness vs time at temperature of interrupted quench. Drop in toughness occurs at much shorter hold time than drop in strength. Concurrently the fracture becomes more intergranular. When later the yield strength falls, fracture becomes more transgranular, and toughness may rise. We attribute this pattern to two mechanisms: 1) Early quench precipitates nucleated on grain and/or subgrain boundaries grow to size sufficient to initiate fracture under tension, long before they withdraw significant solute from subsequent age-hardening. 2) Later quench precipitates nucleated on dispersoids and/or dislocations withdraw solute relatively uniformly, reducing matrix yield strength while increasing matrix ductility. We propose that quantitative modeling of change in strength and toughness with change in quench, requires multiple C-curves for multiple types of quench precipitates, and nonlinear relation of toughness to amount of boundary quench precipitate.

Grain growth of an aluminium [formula omitted] PMMC during solution heat treatment

Particulate reinforced metal-matrix composites (PMMC's), particularly those based on aluminum matrices, are candidate materials for both aerospace and automotive applications. Although processing routes have been developed and the mechanical properties of these materials examined, their microstructural behavior during deformation at both room and elevated temperatures and following subsequent annealing has not been widely studied. One aspect of importance is the effect of the reinforcement particles on grain growth following recrystallization. This is especially relevant in composites based upon age-hardenable aluminum alloys, where solution heat treatment following working is often desirable, so that a maximum amount of solute can be retained in solid solution prior to subsequent age hardening. Recrystallization may occur rapidly during such heat treatments and undesirable coarse grain structures may develop readily. It is clearly important to control of microstructure to maintain desirable mechanical properties. This paper presents data on the grain growth at 500 C of a PMMC consisting of a 2xxx series alloy matrix reinforced with alumina particles. This behavior is compared with the grain growth behavior of the unreinforced aluminium matrix. The size distribution of the matrix grains in the composite is compared with the particulate distribution. The grain growth data obtained are compared withmore » theoretical models of grain growth.« less

Non-linear stress-strain behaviour of unidirectional silicon carbide fibre reinforced aluminium alloy

Longitudinal tensile tests have been conducted on unidirectional SiC fibre reinforced 6061 aluminium matrix composites in the annealed and as-manufactured conditions. The results are presented in terms of stress-strain curves and tangent modulus-strain relations, which show considerable non-linearity. Corresponding micromechanical finite element modelling is performed including the effects of the manufacturing process on the matrix in-situ properties. The analysis shows that the non-linear behaviour of the composite is caused by the elastic-plastic deformation of the matrix alloy. The matrix fully yields during the cooldown from manufacturing. Residual stress relaxation plays an important role in the stress-strain characteristics of the annealed aluminium matrix composite by introducing some initial elastic deformation. The amount of elastic deformation for the as-manufactured condition is greater because of subsequent age hardening. However, more linear elastic deformation was observed than predicted in the as-manufactured specimens, which is believed to be due to higher precipitation hardening caused by metallurgical effects induced in the manufacturing process.

Strengthening mechanisms in Elgiloy

The effects of cold-working and age-hardening on the microstructure of Elgiloy (composition in wt %, 40 Co, 20 Cr, 15 Ni, 7 Mo, 2 Mn, 0.1 C, balance Fe) have been established. 36% and 72% cold reduction of the solution treated (ST) alloy (f c c) produced a network of thin f c c deformation twins and h c pe-martensite platelets. The subsequent age-hardening of the cold-worked strip noted at 500° C was attributed to the formation of additional e-phase (via theαf c c ⇌eh c p transformation), whereas the ST condition did not harden significantly at this temperature. In contrast, ageing the ST condition at 800° C for periods >1000 h caused an increase in hardness due to the formation of a coarse Mo-Co-Cr intermetallic compound together with a smaller amount of M23C6 carbide, while age softening of the cold-worked strip occurred at this temperature as a result of recovery and dissolution of the deformation inducede-phase.