Age-hardenable Alloys Research Articles

A Study on the Nucleation Mechanism of Nano-Scale Precipitates of 7055 Aluminum Alloy

7055 aluminum alloy is a typical age-hardening alloy whose properties change with microstructural evolution as a result of heat treatment. In this paper, the microstructural evolution of the alloy after solid solution treatment and manual aging is examined using SEM, TEM and HADDF. The precipitation, nucleation and growth mechanisms of [Formula: see text] and [Formula: see text] phases are also analyzed. Results show that, as aging proceeds, the Al–Cu phase gradually enlarges and thickens the following precipitation sequence of supersaturated solid solution (SSSS) [Formula: see text] GPI zone [Formula: see text] GPII zone [Formula: see text] phase [Formula: see text] phase; some [Formula: see text] phases formed during aging, which have two different nucleation mechanisms: homogeneous nucleation and heterogeneous nucleation, the [Formula: see text] phases of the latter being marginally larger in size than those of the former. There is a transition in the transformation of (Al3Cu) in GPII zone into [Formula: see text]-phase (Al2Cu). The orientation relation between [Formula: see text] phase and Al matrix is (100)Al//(100)[Formula: see text], (010)Al//(010)[Formula: see text], (001)Al//(001)[Formula: see text]. According to this relationship, [Formula: see text] phase has three variants in the matrix: Variants 1, 2, and 3, which represent the [Formula: see text] phase along {100}Al, {010}Al and {001}Al. As [Formula: see text] phase has a special orientation relation with Al matrix (100)Al//(100)[Formula: see text], (010)Al//(010)[Formula: see text], (001)Al//(001)[Formula: see text], [Formula: see text] phase is distributed in disc form on {001}Al. Significantly larger [Formula: see text] phase can be observed. The orientation relation between S phase and Al matrix is [100]Al//[100]S, [021]Al//[010]S, [012]Al//[001]S. [Formula: see text] phase grows in step form for a period equaling to 1.75-fold unit cell of [Formula: see text]. As [Formula: see text] phase grows, the Cu atoms segregated on the edge of [Formula: see text] phase are gradually consumed.

Strengthening of A2024 alloy by high-pressure torsion and subsequent aging

An age-hardenable A2024 alloy is processed by high-pressure torsion (HPT) for grain refinement and further aged for fine precipitation. The HPT is conducted under an applied pressure of 6GPa for 0.75, 1 and 5 turns with a rotation speed of 1rpm at room temperature and this results in a significant grain size reduction to a grain size of ~ 240 ± 80nm. The hardness sharply increases with imposing strain at an early stage but level off after 5 turns. Further aging at temperatures of 373K and 423K leads to extra hardening above the elevated hardness of the HPT-processed condition. Components contributing to the strengthening were evaluated in terms of grain refinement and fine precipitation including the contributions from dislocation accumulation and solid solution. In this study, a conclusion is reached such that simultaneous strengthening due to grain refinement and fine precipitation is achieved by application of HPT processing and subsequent aging.

Carbon Nanotube-Reinforced Aluminum Matrix Composites Produced by High-Energy Ball Milling

Although multiwall carbon nanotubes (MWCNT) are promising materials to strengthen lightweight aluminum matrix composites, their dispersion into the metallic matrix is challenge. In the present work, MWCNT were dispersed into age-hardenable AA6061 aluminum alloy by high-energy ball milling and the blend was subsequently hot-extruded. The composite bars obtained were heat-treated by solution heat treatment at 520 °C and artificially aged at 177 °C for 8 h, in order to reach the T6 temper. Special attention was given to the integrity of the MWCNT along the entire composite production. The microstructure of the obtained bars was evaluated by optical and scanning electron microscopy, and the mechanical properties were evaluated by Vickers microhardness tests. Raman spectroscopy, x-ray diffraction and transmission electron microscopy were employed to evaluate the structural integrity of MWCNT. It was found that milling time is critical to reach a proper dispersion of the reinforcing phase. The composite hardness increased up to 67% with the dispersion of 2% in weight of MWCNT, when comparing with un-reinforced bars produced by similar route. However, age hardening was not observed in composite bars after heat treatment. It was also found that MWCNT continuously degraded along the process, being partially converted into Al4C3 in the final composite.

Concurrent strengthening of ultrafine-grained age-hardenable Al-Mg alloy by means of high-pressure torsion and spinodal decomposition

In this study, the age-hardening behavior and precipitate microstructures of severely-deformed and then artificially-aged Al-13.4 wt%Mg alloy has been investigated by Vickers hardness test, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and atom probe tomography (APT). The combined processing of high-pressure torsion (HPT) and aging treatment at a temperature below spinodal lines results in a higher attained hardness of ∼HV296 with an age-hardenability (i.e ΔHV31 ± 2) comparable to that of the undeformed specimen without HPT (i.e. ΔHV33 ± 2). The corresponding TEM microstructures consist of modulated structures associated with spinodal decomposition, and quantitative estimation of the amplitude, as well as the wavelength, of Mg fluctuations was successfully conducted by APT for the first time for this alloy system. The linear relationship between the increment of Vickers hardness and the estimated amplitude of the undeformed specimen supposed that Kato's spinodal-hardening mechanism works even in the HPTed specimen with a high number density of grain boundaries. Therefore, our proposed strategy; i.e. taking advantage of spinodal decomposition, is regarded as a convincing approach to achieving concurrent strengthening by ultrafine-grained and precipitation hardenings for the alloys that decompose via spinodal decomposition.

Design of novel age-hardenable aluminium alloy using evolutionary computation

Abstract This work considers the experimental data of tensile properties as a function of composition and processing of the three series of age-hardenable aluminium alloys, i.e. 2XXX, 6XXX and 7XXX, for designing new age hardenable alloy. Computational approach of designing better alloys with desired properties is employed with a target of breaking the barrier of class or series of age hardenable Al alloys. The alloy designed with the help of two evolutionary computation tools, viz. genetic programming and multi-objective genetic algorithm, having better combination of properties, i.e. strength and ductility, is experimentally developed. The designed Al-Zn-Cu-Mg alloy with complex aging characteristics and encouraging tensile properties seem to have the potential for further study.

Atomistic analysis of the driving force behind the formation of G.P. zones in age-hardening alloys

Atomistic analysis of the driving force behind the formation of G.P. zones in age-hardening alloys

Performance evaluation on high speed machining of custom 465 steel

Custom 465 steel is a martensitic, age-hardenable alloy having a superior combination of strength and corrosion cracking resistance, which are commonly used in aerospace industries. When machining Custom 465 steel, tool-work interface temperature and tool wear are the major parameters which influences the machinability factors at a greater extend. Even though, researchers have performed many operations based on this subject, no significant data’s were available regarding the Custom 465 steel.A dry machining operation known for its ecological desirable and cost effective process was implemented to experimentally investigate the tool wear and tool-work interface temperature when machining Custom 465 steel using uncoated, TiAlN and TiCN coated carbide cutting tool inserts with a cutting speed of 100 m/min and 200 m/min and also the effects of cutting speed on tool wear and tool-work interface temperature were also determined. The highest tool life of 6.15 minutes was recorded for TiAlN coated insert at a cutting speed of 2000 rotations per minute (rpm), feed rate of 200 mm/min and depth of cut of 0.5 mm. The maximum tool wear recorded for TiAlN coated insert was 0.25 mm and the maximum temperature on the tool-work impact was 142.43 degrees. In all cutting speeds, TiAlN coated insert outperformed uncoated and TiCN coated insert in term of tool life and temperature. For a cutting speed of 2000 rpm and above uncoated inserts are not suitable to be used for machining Custom 465 steel due to the rapid wear of cutting edge.

Presence of silver in the strengthening particles of an Al-Cu-Mg-Si-Zr-Ti-Ag alloy during severe overaging and creep

The results of a study aimed at investigating the presence of silver in the strengthening phases of age-hardenable AlCuMgSi alloys are presented. HR STEM observations were performed on Al-4Cu-0.55Mg-0.5Si--0.15Zr-0.1Ti-0.5Ag (wt%) alloy samples aged or crept at 200 °C to monitor the evolution of secondary phases over long times. Results showed that in the peak aged condition silver mostly segregated at the surface of some strengthening precipitates types. This occurred specifically at the particle/matrix interface of θ' phase while particles, matching the wide compositional range of the quaternary Q' phase were free from Ag. A third population of strengthening particles having the shape of a core-shell rod was also observed. They were characterized by a Mg- and Si-rich core and a Cu- and Ag-rich shell. The exposure at 200 °C for up to 570 h, caused an evident coarsening of Q' and θ' plate-like particles, and the Ag surface enrichment of the latter went progressively lost. On the contrary, silver concentrated inhomogeneously on the surface of the core-shell particles for which a relatively low coarsening rate was observed.

Effect of grain size, texture and density of precipitates on the hardness and tensile yield stress of Mg-14Gd-0.5Zr alloys

In this study, an age-hardenable Mg–14Gd–0.5Zr (wt.%) alloy was used as a model material to investigate the effects of grain size, texture and density of precipitates on the hardness and 0.2% proof tensile yield stress. Six kinds of samples with different grain size, texture and density of precipitates were prepared by varying the extrusion process and aging treatment. The microstructure and texture of the samples were examined by electron backscatter diffraction and transmission electron microscope. The examination results indicate that 0.2% proof yield stress of the Mg-Gd alloys were largely affected by the grain size and texture, while the hardness of the alloys was predominately controlled by the density of precipitates. This study reveals that the grain size, texture and density of precipitate can have much different influences on the hardness and 0.2% proof yield stress for the Mg-Gd alloys. It raises an important issue about the mechanical-testing standards and product-design guidelines for the age-hardenable magnesium alloys.

Effect of Natural Ageing on Strength and Anisotropy in Aluminium Alloy AA 6005C

During processing of age-hardenable AA 6xxx series alloys for automotive applications the sheets may experience significant time spans between solution heat treatment at the aluminium supplier and age hardening upon the final paint bake cycle at the carmaker. Natural ageing during these pause times is known to greatly affect materials properties of autobody sheet. In the present study we explore the impact of natural ageing on the tensile properties and the in-plane anisotropy of alloy AA 6005C. Materials properties at various degrees of natural ageing are modelled with the help of a nanoscale material model NaMo, which consists of a precipitation model simulating the formation of clusters and phases upon natural ageing as input to a mechanical model simulating the evolution of yield strength and work hardening. Plastic anisotropy is modelled from the materials crystallographic texture by a visco-plastic self-consistent polycrystal-plasticity code VPSC.

Controlling the high temperature mechanical behavior of Al alloys by precipitation and severe straining

The aim of this work was to investigate the influence of the precipitate distribution on the microstructure and on the room and high temperature mechanical properties of an age-hardenable aluminium AA6082 alloy following severe straining by high-pressure torsion (HPT). With this goal, specimens in the as-cast and T6 peak-aged conditions were processed by HPT using 0.5, 1 and 5 turns at room temperature. At high strain levels (γ>100), similar saturation grain sizes (~250nm), high-angle boundary fractions (~80%) and hardness values (Hv~160) were obtained for both initial conditions. Grain refinement led to significant strengthening and to good ductility values at room temperature. Analysis by TEM and EDS elemental mapping revealed that HPT processing of the as-cast condition led to fracture of the stable β-phase into many small precipitates located preferentially along grain boundaries and triple junctions. By contrast, HPT processing of the T6 peak-aged specimens revealed a partial dissolution of the needle-shaped nanoprecipitates. The different evolutions of the precipitate distributions following straining in the as-cast and peak-aged conditions gave rise to dramatic differences in the mechanical properties and the operative deformation mechanisms at warm temperatures. These results provide evidence that the high temperature mechanical behavior of age-hardenable Al alloys may be conveniently controlled by altering the precipitate distribution followed by severe straining.

Improving the ductility of nanostructured Al alloy using strongly textured nano-laminated structure combined with nano-precipitates

A ductile nanostructured Al alloy with desired microstructure is tailored via the precipitation optimization and texture design. The microstructure consists of two-dimensional nano-laminated structure with strong texture and one-dimensional elongated sub-micron structure with intra-precipitations at nanoscale and sub-nanoscale level. The precipitation optimization is realized by the appropriate introduction of its embryo in the NS matrix before aging. The embryo is the solute-rich structures (solute clusters) segregated to dislocations due to strain-induced dissolution of initial precipitations into the matrix. These nano-precipitations not only impede dislocation motion but also accommodate plastic deformation during loading. The textured laminate activates the partial dislocation mechanism in high stacking-fault energy alloys and the extrinsic toughening mechanism during loading. In addition, the laminated structure exhibits significant texture strengthening. Therefore, both nano-precipitations and textured laminated structure are responsible for the simultaneous increase in strength and ductility. Additionally, the initial deformable precipitations play a positive role in microstructural refinement and the refined structure shows significant cluster strengthening, ~20% relative contribution to total strength. This paper provides a new understanding of designing nanostructured materials for achieving high ductility by several toughening strategies, which is expected to be applicable to other age-hardenable alloys and steels as well.

Temporal evolution of coherent precipitates in an aluminum alloy W319: A correlative anisotropic small angle X-ray scattering, transmission electron microscopy and atom-probe tomography study

Small-angle X-ray scattering (SAXS), transmission electron-microscopy (TEM), and atom-probe tomography (APT) are utilized correlatively to study the evolution of nanometer length-scale coherent precipitates in a commercial age-hardenable aluminum alloy, W319. SAXS, with its high-field-of-view and resultant large statistics, perfectly complements TEM and APT at the nanometer length-scale. The combined results provide a comprehensive description of the evolution of the fine precipitates. W319 exhibits two types of fine coherent precipitates on aging namely, θ′ and Q, whose temporal evolution has been studied using time-resolved in situ SAXS analyses at three different aging temperatures: 438, 463, and 533 K. Complementary information about the morphology, dimensions and number density of the precipitates were determined from ex situ APT and TEM analyses. Coherent precipitates from a single grain of the α-Al-matrix of W319 give rise to an anisotropic SAXS signal, which is subsequently simulated, taking into account the relative orientation of the sample and the crystallographic orientation relationships of the different precipitates. The simulated anisotropic SAXS intensity, based on the results from APT and TEM, is validated utilizing the experimentally obtained 2D SAXS patterns. This permits us to determine the dimensions and number densities of the precipitates from anisotropic SAXS signals, and a complete quantitative analysis of the in situ temporal evolution of the coherent precipitates is performed. Additionally, the present study demonstrates an alternative technique for extracting structural parameters from anisotropic SAXS analyses. The methodology presented herein will be useful for age-hardening analyses of commercial age-hardenable Al-based alloys and Ni-based super alloys.

Effect of carbon content and annealing on structure and hardness of the CoCrFeNiMn-based high entropy alloys

A set of CoCrFeNiMn-based high entropy alloys, containing 0, 2.0, 3.4 and 4.8 at.% of carbon (CoCrFeNiMnCx (x = 0; 0.1; 0.175; 0.25) alloys), was prepared by vacuum arc melting. The structure of the alloys was examined in as-solidified conditions and after annealing at 600 °C, 800 °C, 1000 °C and 1200 °C for 14 h. In the as-solidified condition, the CoCrFeNiMn alloy is composed of single fcc (face-centered cubic) solid solution phase, the CoCrFeNiMnC0.1 alloy contains an insignificant fraction of nanoscale M7C3 carbides, and in the CoCrFeNiMnC0.175 and CoCrFeNiMnC0.25 alloys, coarse M7C3 carbides with the volume fraction of 5% and 9.5% respectively are found. The microhardness increases with the increase of carbon content in the alloys from 160 HV of the CoCrFeNiMn alloy to 275 HV of the CoCrFeNiMnC0.25. After annealing at 800 °C nanoscale M7C3 carbides precipitate in the CoCrFeNiMnC0.1, CoCrFeNiMnC0.175 and CoCrFeNiMnC0.25 alloys. Precipitation of carbides is accompanied by a strong hardening of the alloys. For example, the microhardness of the CoCrFeNiMnC0.1 alloy increases from 205 HV in as-solidified condition to 240 HV. Further increase of annealing temperature results in a decrease of hardness. Carbon concentration in the fcc solid solution is estimated and compared with equilibrium thermodynamic predictions. Thermodynamic modeling also demonstrates that increase of the number of components in fcc equiatomic alloys of the Co-Cr-Fe-Ni-Mn system decreases possible carbon solubility level in the fcc solid solution phase, and in the studied alloys Cr is the main element which determines carbon solubility. Hardening mechanisms of the carbon-containing alloys are analyzed quantitatively, and prospects of development of age-hardenable high entropy alloys of Co-Cr-Fe-Ni-Mn-C system are discussed.

Modeling of the mutual effect of dynamic precipitation and dislocation density in age hardenable aluminum alloys

A model has been proposed to capture the complex strain rate effect on dynamic precipitation of GP zones in an age-hardenable aluminum alloy. The contributions of vacancies and dislocations to dynamically formed GP zones have been specified in the model. It has been demonstrated that the proposed model is capable for predicting the contribution of each dynamic precipitation mechanisms, accurately, which are acting during deformation. Furthermore, the vacancy and dislocation evolutions during deformation have been considered in this modeling. The effect of strain rate by considering different mechanisms of dynamic precipitation of GP zones has been studied and confirmed by experimental data reported in the work of other researchers. Both experimental results and model outputs demonstrate that at high strain rate regime, strain rate dependency of dynamic precipitation is increased and in comparison with vacancy-assisted nucleation, dislocation-assisted nucleation of GP zones is encouraged. In this study, the mutual effects of dislocation density and dynamic precipitation have been considered. Outputs of model demonstrate that there is a weak negative strain rate sensitivity in the age-hardenable supersaturated aluminum alloy.

OUTOKUMPU TYPE 630

Abstract Outokumpu Type 630 is a martensitic age hardenable alloy of composition 17Cr-4Ni. The alloy has high strength and corrosion resistance similar to that of Type 304 stainless steel. This datasheet provides information on composition, physical properties, hardness, and tensile properties as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, and joining. Filing Code: SS-1238. Producer or source: Outokumpu High Performance Stainless.

Strengthening of age-hardenable WE43 magnesium alloy processed by high pressure torsion

An age-hardenable Mg-Y-Nd-Zr alloy (WE43) was severely deformed using high-pressure torsion (HPT) at room temperature, 200°C or 300°C to refine the microstructure and improve its mechanical properties. Thermal stability of microstructure of HPT processed alloy was studied by monitoring its microhardness and age behavior. The deformation by HPT in the temperature range of 20–300°C was accompanied by twinning with the twin size of 0.4–8.1µm. A very fine grain structure with the average grain size of 30–100nm was formed, predominantly within the twins, during and after HPT. Severe deformation by HPT resulted in substantial strengthening of the WE43 alloy, especially after processing at room temperature. The strength of the HPT processed WE43 alloy was additionally improved by subsequent aging. It was found that thermal stability of strengthening induced by HPT did not depend on the deformation temperature and sustained to 250°C.

Precipitation Reactions in Age-Hardenable Alloys During Laser Additive Manufacturing

We describe and study the thermal profiles experienced by various age-hardenable alloys during laser additive manufacturing (LAM), employing two different manufacturing techniques: selective laser melting and laser metal deposition. Using scanning electron microscopy and atom probe tomography, we reveal at which stages during the manufacturing process desired and undesired precipitation reactions can occur in age-hardenable alloys. Using examples from a maraging steel, a nickel-base superalloy and a scandium-containing aluminium alloy, we demonstrate that precipitation can already occur during the production of the powders used as starting material, during the deposition of material (i.e. during solidification and subsequent cooling), during the intrinsic heat treatment effected by LAM (i.e. in the heat affected zones) and, naturally, during an ageing post-heat treatment. These examples demonstrate the importance of understanding and controlling the thermal profile during the entire additive manufacturing cycle of age-hardenable materials including powder synthesis.

時効硬化したダイカスト用アルミニウム合金の析出相の透過電子顕微鏡観察

時効硬化したダイカスト用アルミニウム合金の析出相の透過電子顕微鏡観察

Severe Plastic Deformation under High Pressure for Production of Superplastic Materials

Grain refinement is an important prerequisite for advent of superplasticity. In particular, as the grain size is smaller, the superplasticity appears at higher strain rates and lower temperatures. Severe plastic deformation (SPD) is a useful process for achieving significant grain refinement. This presentation shows that applicability of the SPD process is enhanced when it is operated under high pressure through high-pressure torsion (HPT) and high-pressure sliding (HPS). It is demonstrated that commercially available conventional alloys but less ductile alloys such as Mg alloys, age-hardenable high-strength Al alloys (A2024, A7075) and Ti alloys become superplastic after processing by HPT or HPS.