Effect of cooling aging on microstructure and mechanical properties of an Al–Zn–Mg–Cu alloy

In the present work, Al–Zn–Mg–Cu alloy was aged by non-isothermal cooling aging treatment (CAT). At high initial aging temperature (IAT), the hardness was decreased with the decreased cooling rate. However, when IAT was lower than 180 °C, the hardness was increased with the decreased cooling rate. Conductivity was increased with the decreased cooling rate regardless of IAT. The tensile strength, yield strength and conductivity of Al alloy after (200–100 °C, 80 °C/h) CAT were increased 2.9%, 8.1% and 8.3% than that after T6 treatment, respectively. With an increase of IAT and decrease of cooling rate, the fine GP zone and η′ phase were transformed to be larger η′ and η precipitates. Moreover, continuous η phase at grain boundary was also grown to be individual large precipitates. Cooling aging time was decreased about 90% than that for T6 treatment, indicating cooling aging could improve the mechanical properties, corrosion resistance and production efficiency with less energy consumption.     

Heating aging behavior of Al–8.35Zn–2.5Mg–2.25Cu alloy

In the present work, the influence of heating aging treatment (HAT) on the microstructure and mechanical properties of Al–Zn–Mg–Cu alloy was investigated. When the final aging temperature (FAT) was lower than 180 °C, the hardness increased with the decreasing of heating rate, however, in the case of the FAT was higher than 180 °C, the variation of hardness was opposite. The electrical conductivity of Al–Zn–Mg–Cu alloy increased with the decrease of heating rate regardless of FAT. The tensile strength, yield strength and conductivity of the Al alloy after (100–180 °C, 20 °C/h) HAT increased by 1.6%, 4.5% and 14.1% than that after T6 treatment, respectively. The precipitates sequence of HAT was coincident with that of isothermal aging, which is SSS → GP zone → η′ → η. With the increase of FAT and the decrease of heating rate, the fine precipitates became larger and the continuous η phase at grain boundary grew to be individual large precipitates. The HAT time was decreased about 80% than that for T6 treatment, indicating HAT could improve the mechanical properties, corrosion resistance and production efficiency with less energy consumption.     

A new 1.9 GPa maraging stainless steel strengthened by multiple precipitating species

A new ultra-high strength maraging stainless steel with composition of 13Cr–13Co–4.5Ni–3.5Mo–0.5Ti (at.%) has been developed. It was demonstrated that the ultimate tensile strength of the steel could reach 1.9 GPa with reasonable ductility. This breakthrough was achieved by a combined strengthening of three different species of precipitates. The evolution of precipitates with respect of size, morphology and chemical composition during aging at 500 °C was characterized using atom probe tomography (APT) and transmission electron microscopy (TEM). The precipitates were identified to be η-phase Ni₃(Ti, Al) phase, Mo-rich R′ phase and Cr-rich α′ phase, developing out of the precursor clusters, Ni–Ti–Al-rich cluster, Mo-rich cluster and Cr-rich cluster, separately. The segregation of Mo and Cr atoms at the precipitate/matrix interfaces was detected and is considered to impede the coarsening of η-phase. Based on the characterizations, the precipitation process of these phases and their effect on mechanical properties were analyzed.     

Age hardening behaviors,mechanical and corrosion properties of deformed Mg–Mn–Sn sheets by pre-rolled treatment

The age behaviors, mechanical and corrosion properties of Mg–1.5Mn–xSn (x = 1 and 5 wt.%) alloys under three aging conditions have been investigated. The results reveal that both age behaviors and mechanical properties are improved with the increment of Sn. Meanwhile, the mechanical properties of EA (extrusion + artificial aging) state Mg–1.5Mn–5Sn alloy are higher than those of SA (solid solution + artificial aging) state sample, which are mostly attributed to fine grain and high density secondary precipitate. In addition, an accompanying improvement in age hardening response and strength is achieved in ERA (extruded + rolled + aging) state alloy compared with EA state one. The main reasons are related to the formations of a large number of dislocations and deformation twins, which provide effective nucleation sites to form fine β-Mg₂Sn strengthening precipitates during the following aging process. In addition, compared with EA state sample, a lower corrosion rate of ERA alloy is confirmed by Tafel curves and electrochemical impedance spectroscopy results, which is mainly related to the formation of a thick anodic passivation film on the surface.     

Effects of heat treatment on the microstructure and mechanical properties of AA2618 DC cast alloy

Direct chill (DC) cast ingot plates of AA2618 alloy have been increasingly used for large-mold applications in the plastics and automotive industries. The effects of different heat treatments on the microstructure and mechanical properties of AA2618 DC cast alloy were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and hardness and tensile testing. The as-cast microstructure contained a considerable amount of coarse intermetallic phases, including Al₂CuMg, Al₂Cu, Al₇Cu₄Ni, Al₇Cu₂(Fe,Ni) and Al₉FeNi, resulting in poor mechanical properties. Solution treatment at 530 °C for 5 h dissolved the first three phases into the solid solution and consequently improved the mechanical properties of the alloy. By utilizing the appropriate aging temperature and time, different combinations of strength and ductility could be obtained to fulfill the design requirements of large-mold applications. The strengthening of AA2618 DC cast alloy under the aging conditions studied was caused by GPB zones and S′ precipitates. The evolution of both precipitates in terms of their size and density was observed to have a significant effect on the mechanical properties of the alloy.     

Microstructure and residual stress distributions in friction stir welding of dissimilar aluminium alloys

The aim of this investigation was to study the effect of welding heat input and postweld natural aging on residual stress, microstructure, and precipitation distribution in different zones of dissimilar friction stir welding of 8 mm thick plates of AA6082-T6 and AA7075-T6. It was found that atomic diffusion occurs at the interface of the materials in the stir zone of the joints. Transmission electron microscopic investigations showed that reprecipitation of fine Guinier–Preston zone, β′, and η′ precipitates resulted in increased micro-hardness in the SZ after natural aging. An increase in welding heat input resulted in decreased maximum tensile residual stress and increased size of the tensile residual stress region. Natural aging within the SZ and thermo-mechanical affected zone resulted in 15–20 MPa reduction of the residual stress in these zones.     

Effect of T4 heat treatment on microstructure and hardness of A356 alloy refined by Ga + In + Sn mixed alloy

A356 alloy was solidified with cooling rate enhanced by phase transition cooling (PTC) of Ga + In + Sn mixed alloy, and then solution treated at 540 °C, 500 °C and 450 °C for different time, followed by quenching and immediately natural aging. Evolution of the corresponding microstructure and hardness with the solution temperature and time were studied. The as-cast sample was hardened due to the refined microstructures, the large solubility of Si- and Mg-clusters in Al matrix and the nano-scale Si precipitates. The modification of eutectic Si was verified to depend on the initially solidified morphology of Si, the solution temperature and time, for example, the Si particles subjected to high cooling rate and high solution temperature become more readily spheroidized. The age hardening, from the hardness tests and differential scanning calorimetry (DSC) results, was ascribed to be associated with the Guinier–Preston (GP) zones. It can be concluded that an optimal combination of solidification with heat treatment may achieve the better mechanical property but with the less product cost.     

A study on preparation of Mo–0.6Ti–0.2Zr–0.02C alloy by mechanical alloying and hot isostatic pressing,and its characterization

Mo–0.6Ti–0.2Zr–0.1C alloy was prepared by mechanical alloying (MA) and subsequently consolidated by powder processing techniques. The pellets prepared from the fine size MA powder showed a high rate of densification during sintering in the temperature range of 1300–1500 °C. Close to theoretical density was attained by hot isostatic pressing (HIP) at 1250 °C and TEM studies revealed the uniform distribution of complex carbide precipitates (<100 nm) in the fine grain microstructure of the consolidated alloy. The alloy consolidated by HIP showed a high hardness of the order of 500 HK due to the presence of the carbides in the fine grain microstructure.     

Effects of stacking fault energy on the creep behaviors of Ni-base superalloy

Cobalt in a 23 wt.% Co containing Ni-base superalloys was systematically substituted by Ni in order to study the effects of stacking fault energy (SFE) on the creep mechanisms. The deformation microstructures of the alloys during different creep stages at 725 °C and 630 MPa were investigated by transmission electron microscopy (TEM). The results showed that the creep life increased as the SFE decreased corresponding to the increase of Co content in the alloys. At primary creep stage, the dislocation was difficult to dissociate independent of SFE. In contrast, at secondary and tertiary creep stages the dislocations dissociated at γ/γ′ interface and the partial dislocation started to shear γ′ precipitates, leaving isolated faults (IFs) in high SFE alloy, while the dislocations dissociated in the matrix and the partials swept out the matrix and γ′ precipitates creating extended stacking faults (ESFs) or deformation microtwins which were involved in diffusion-mediated reordering in low SFE alloy. It is suggested that the deformation microtwinning process should be favorable with the decrease of SFE, which could enhance the creep resistance and improve the creep properties of the alloys.     

Effect of thermo mechanical treatments and aging parameters on mechanical properties of Al–Mg–Si alloy containing 3 wt.% Li

In the present work, microstructure and mechanical properties of 3 wt.% Li addition in a Al–Mg–Si alloy of target composition 0.5 wt.% Mg and 0.2 wt.% Si in W (solution heat treated), T6 (solution heat treated and artificially aged) and T8 (solution heat treated, cold worked and artificially aged) conditions was studied. The age-hardening response of the alloy was determined after systematic cold reductions from 10%, 20%, 30%, 40%, 50% and 60% in quenched condition followed by aging at 175 °C (448 K) for 2, 4, 6, 8, 10 and 12 h (T8 condition). The results were compared with samples aged in the same conditions with 0% cold reduction (T6 condition). The alloy displayed a strong artificial aging response and maximum hardness value achieved was after 60% cold work and 10 h of aging time. Furthermore, the yield strength and the ultimate tensile strength were increased from 123 MPa to 224 MPa and 356 MPa to 540 MPa respectively with a slight decrease in ductility. Scanning electron microscopy (SEM) based fractography showed a uniform network of bigger and deeper dimples with round morphology in T6 condition while a ductile tearing with few discernable cleavage planes was observed in T8 condition. The interplay of various precipitation hardening mechanisms and relevant phases was established by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was concluded that the enhancement in mechanical properties, with the degree of cold work, was attributed due to a possible refinement of δ′ (Al₃Li) precipitates resulted after aging.     

One-step mechanical processing to prepare LSM/ScSZ composite particles for SOFC cathode

Novel one-step mechanical processing was proposed to prepare LSM/ScSZ composite particles from the starting raw powders of LSM (strontium doped lanthanum manganite) and ScSZ (scandium stabilized zirconia) fine powder. In this paper, the properties of composite particles made by this method and the properties of the resultant cells were evaluated. As a result, LSM/ScSZ composite particles were obtained by only 10 min mechanical processing using three kinds of raw powder materials of LSM and ScSZ fine particles without extra heat. The maximum power density of the cell made by the composite particles at 800 °C reached 320 mW/cm². It was higher than that made by commercially available LSM nanosized powder. Besides, the polarization resistance of the cathode made by the composite particles at 800 °C was 0.5 Ω cm² which was lower than that made by using commercially available LSM powder. It suggests that the proposed method is very promising for producing high quality composite particles used for SOFC cathodes by more simple and energy-saving way.     

Precipitates,zones and transitions during aging of Al-Zn-Mg-Zr 7000 series alloy

Abstract

Age hardening of an industrial 7000 series alloy in the temperature range 70-150° C has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate η′ and the stable phase η-MgZn₂, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450°C is emphasized.     

The influence of chemical composition and thermo-mechanical treatment on Ti–Nb–Ta–Zr alloys

Ti–Nb–Ta–Zr quaternary alloying system is very promising for biomedical alloys. It is due to good mechanical properties and corrosion resistance of titanium alloys. Moreover no potentially harmful elements are contained in this system.Mechanical properties were influenced by changing the chemical composition and by various heat-treatment operations. The alloys were prepared by arc melting and then they were hot forged (900–1000 °C). After solution treatment 850 °C/0.5 h/water quenched, cold swaging was carried out with section reduction about 85%. As final heat treatment aging at 450 °C/8 h/furnace cooling was used.Mechanical properties were measured from tensile tests results at cold swaged and aged specimens. The microstructure was observed by using light microscopy and transmission electron microscopy (TEM)-thin foils method. X-ray diffraction analysis reveals the phase composition. By using these techniques the changes in microstructure caused by precipitation during aging treatment were clarified. After aging, the presence of ω or α phases may occur. Influence of changing Zr and Ta contents on mechanical properties and also on precipitation of secondary phases during aging treatment was observed.     

Research into mechanical properties of reaction-bonded SiC composites at cryogenic temperatures

The mechanical properties of reaction-bonded silicon carbide (RBSC) composites at cryogenic temperatures have been reported for the first time. The results show that the flexural strength and fracture toughness increase from 277.93 ± 23.21 MPa to 396.74 ± 52.74 MPa and from 3.69 ± 0.45 MPa·m^1/2 to 4.98 ± 0.53 MPa·m^1/2 as the temperature decreases from 293 K to 77 K, respectively. The XRD analysis of the phase composition reveals that there is no phase transformation in the composites at cryogenic temperatures, indicating cryogenic mechanical properties are independent of phase composition. The enhancement of mechanical properties at 77 K over room temperature could be explained by the transition of fracture mode from predominant transgranular fracture to intergranular fracture and stronger resistance to crack propagation resulting from higher residual stress at 77 K. The above results demonstrate that such composites do not undergo similar deteriorations in the fracture toughness as other materials (some kinds of metals and polymers), so it is believed that such composites could be a potential material applied in cryogenic field.     

Microstructure and mechanical properties of spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy after two-step aging treatment at 110 and 150 °C

The microstructure and mechanical properties of a spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy were investigated after a two-step aging treatment at 110 and 150 °C. The results indicate that GP zones and η′ are major precipitates for the alloy under the two-step aged condition. Discontinuous grain boundary precipitates are favorable for SCC resistance in the over-aged condition, which reduces its strength about 7% compared to the peak-aged condition.     

Effect of welding parameters on microstructure and mechanical properties of friction stir welded joints of a super high strength Al–Zn–Mg–Cu aluminum alloy

Samples made of a super high strength aluminum alloy with high Zn content were friction stir welded with rotation rates of 350–950 rpm and welding speeds of 50–150 mm/min. The effect of welding parameters on the microstructure and mechanical properties was investigated. It was observed that the grain size of the nugget zones decreased with the increasing welding speed or the decreasing tool rotation rate. Most of the strengthening precipitates in the nugget zone were dissolved back and the intragranular and grain boundary precipitates in the heat affected zone coarsened significantly. The greatest ultimate tensile strength of 484 MPa and largest elongation of 9.4 were obtained at 350 rpm−100 mm/min and 350 rpm−50 mm/min, respectively. The ultimate tensile strength and elongation deteriorated drastically when rotation rate increased from 350 to 950 rpm at a constant welding speed of 100 mm/min.     

Investigation of mechanical and corrosion properties of an Al–Zn–Mg–Cu alloy under various ageing conditions and interface analysis of η′ precipitate

The mechanical and corrosion properties under various ageing treatment conditions were investigated in an Al–6.0Zn–2.3Mg–1.8Cu–0.1Zr (wt.%) alloy. The results showed that the retrogression and re-ageing (RRA) were capable of providing higher strength and improved corrosion resistance in comparison with the conventional T6 and T74 ageing. The optimised ageing process had been found to be 120 °C/24 h + 180 °C/60 min + 120 °C/24 h for the experimental alloy. The results obtained from the high resolution transmission electron microscopy (HRTEM) interface analysis revealed that a semi-coherent stress field between the η′ precipitate and the Al matrix was critical in controlling the strength of the Al–Zn–Mg–Cu alloy heat-treated under different conditions. Furthermore, Transition Matrix calculation showed that the η′ phases had only two zone axes: [1̅21̅3]_η′ and [108̅2̅3]_η′, which were parallel to the [112]_Al zone axis, when being precipitated from the Al matrix. Therefore, the orientation relationships between the η′ precipitates and the Al matrix under the [112]_Al zone axis could be described as: [1̅21̅3]_η′//[112]_Al;(12̅12)_η′//(11̅)_Al and [108̅2̅3]_η′//[112]_Al;(12̅12)_η′//(111̅)_Al. Consequently, a new diffraction pattern model from η′ precipitates in two variants under the [112]_Al zone axis had been established, which was in a good agreement with the experimental data.     

High-temperature low cycle fatigue behavior of a gray cast iron

The strain controlled low cycle fatigue properties of the studied gray cast iron for engine cylinder blocks were investigated. At the same total strain amplitude, the low cycle fatigue life of the studied material at 523 K was higher than that at 423 K. The fatigue behavior of the studied material was characterized as cyclic softening at any given total strain amplitude (0.12%–0.24%), which was attributed to fatigue crack initiation and propagation. Moreover, this material exhibited asymmetric hysteresis loops due to the presence of the graphite lamellas. Transmission electron microscopy analysis suggested that cyclic softening was also caused by the interactions of dislocations at 423 K, such as cell structure in ferrite, whereas cyclic softening was related to subgrain boundaries and dislocation climbing at 523 K. Micro-analysis of specimen fracture appearance was conducted in order to obtain the fracture characteristics and crack paths for different strain amplitudes. It showed that the higher the temperature, the rougher the crack face of the examined gray cast iron at the same total strain amplitude. Additionally, the microcracks were readily blunted during growth inside the pearlite matrix at 423 K, whereas the microcracks could easily pass through pearlite matrix along with deflection at 523 K. The results of fatigue experiments consistently showed that fatigue damage for the studied material at 423 K was lower than that at 523 K under any given total strain amplitude.     

Effect of heat treatment and Fe content on the microstructure and mechanical properties of die-cast Al–Si–Cu alloys

The effect of solution and ageing heat treatment on the microstructure and mechanical properties of the die-cast Al–9 wt.%Si–3.5 wt.%Cu alloys containing 0.1–1.0 wt.% Fe was investigated. The results showed that the dendritic primary α-Al phase was varied from 20 to 100 μm in size and the globular α-Al grains were smaller than 10 μm in size. The Fe-rich intermetallics exhibited coarse compact or star-like shapes with the sizes from 10 to 20 μm and the fine compact particles at an average size of 0.75 μm. The solution treatment of the alloys could be achieved in a short period of time, typically 30 min at 510 °C, which dissolved the Cu-rich intermetallics into the primary α-Al phase and spheroidised the eutectic Si phase. During the subsequent ageing treatment, numerous fine precipitates of θ′ and Q′ phases were formed to provide effective strengthening to the α-Al phase, significantly improving the mechanical properties. Therefore, Fe content in the die-cast Al–Si–Cu alloys needs to be controlled at a low level in order to obtain the improved ductility and strength under solution and aged condition.     

Effect of short nylon-6 fibres on natural rubber-toughened polystyrene

Composites based on polystyrene and natural rubber at a ratio of 85/15 were prepared by melt mixing with nylon-6 fibres using an internal mixer. The loading of short nylon-6 fibre, untreated and resorcinol formaldehyde latex (RFL)-treated, was varied from 0 to 3 wt.%. Tensile and flexural test samples were punched out from sheets and tested to study the variation of mechanical and dynamic mechanical properties. The tensile behaviour of the composite has been determined at three different strain rates (4.1 × 10⁻⁴ s⁻¹, 2 × 10⁻³ s⁻¹ and 2 × 10⁻² s⁻¹). Both the tensile strength and Young’s modulus of the composite increased with strain rate. The tensile strength, tensile modulus, flexural strength and flexural modulus increased with the increase in fibre content up to 1 wt.%, above which there was a significant deterioration in the properties. The RFL-treated fibre composites showed improved mechanical properties compared to the untreated one. Dynamic mechanical analysis (DMA) showed that the storage modulus of the composite with RFL-treated fibre was better compared to the untreated one. The fibre–matrix morphology of the tensile fractured specimens was studied by scanning electron microscopy (SEM). The results suggested that the RFL treatment of nylon fibre promoted adhesion to the natural rubber phase of the blend, thereby improving the mechanical properties of the composite.