Effect of Zn addition on the precipitation behaviors of Al–Mg–Si–Cu alloys for automotive applications

Effect of Zn addition on the precipitation kinetics and age-hardening response of Al–Mg–Si–Cu alloys was investigated by differential scanning calorimetry (DSC), hardness measurements, tensile tests and microstructural characterization. The results show that, compared with the Zn-free alloy, both the starting and peak temperatures in the DSC curve, and activation energy of β″ precipitation of Zn-added Al–Mg–Si–Cu alloy decrease significantly, corresponding to the greatly improved precipitation kinetics and age-hardening response, i.e., a hardness increment of 70HV after aging at 185 °C for 20 min. Moreover, the peak hardness and tensile properties can also be greatly enhanced after adding 3.0 wt% Zn even exhibiting a ductile fracture feature in the peak-aged state. No precipitates of the Al–Zn–Mg alloy system appear in the Zn-added Al–Mg–Si–Cu alloys after aging at 185 °C, and pre-β″, β″, and L precipitates are still the main precipitates in the two alloys after peak aging treatment. Finally, based on the microstructural evolution, a schematic diagram of precipitation in the Al–Mg–Si–Cu–Zn alloy is put forward, and the relationship between mechanical properties and microstructure is also established.     

Effect of thermal cycling on martensitic transformation temperatures in Ti–Ni–Cu shape memory alloys

Abstract

Changes in martensitic transformation temperatures during thermal cycling in Ti–Ni–Cu shape memory alloys have been investigated by means of electrical resistivity measurements, thermal cycling tests under constant load and transmission electron microscopy. During thermal cycling without applied stress, the B2→B19′ transformation temperature M _s decreased, while the B2→B19 transformation start temperature M _s′ kept almost constant. During thermal cycling with applied stress, in solution treated Ti–45Ni–5Cu alloy, changes in M _s depended on the amount of applied stress. That is, M _s decreased when the applied stress was 39.2 MPa, while its value kept almost constant when a stress of 117.2 MPa was applied. It was also found that M _s′ increased during thermal cycling in the solution treated Ti–35Ni–15Cu and Ti–30Ni–20Cu alloys, irrespective of the amount of applied stress. All changes in M _s and M _s′ during thermal cycling with applied stress in Ti–Ni–Cu alloys were explained well by a combination of the thermal cycling effect and the structural refinement effect.     

Effect of multi-directional forging and ageing on fracture toughness of Al–Zn–Mg–Cu alloys

Strength, ductility and fracture toughness are the most important mechanical properties of engineering materials. In this work, an Al–Zn–Mg–Cu alloy was subjected to multi-directional forging (MF) and ageing treatment. Microstructural evolution was studied by optical and electron microscopy and strength, ductility and fracture toughness were researched. After MF, the dislocation density was increased and the microstructure was refined. The strength and fracture toughness were increased, while the ductility was decreased sharply. Without compromising the strength, the ductility was improved significantly after ageing. The fracture toughness was increased further. The coarse and discontinuously distributed grain boundary precipitates were found to be responsible for higher fracture toughness of the fine-grained structure Al–Zn–Mg–Cu alloy.     

Effect of prior cold working on microstructural evolution of precipitation in Zn–22Al–2Cu alloy during aging

Abstract

The microstructural evolution of precipitation was characterised by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and microhardness measurements during two different types of isothermal aging in a Zn–22Al–2Cu alloy. Solution treated samples were quenched and rolled at room temperature and then aged at 250°C for different times. The X-ray results showed that the decomposition of the β _sss phase and the formation τ′ phase, during the aging treatment occurred by the following reactions β _sss→α + η + ? and α + ?→η + τ′ respectively. Similar phase transformations were observed in the sample solution treated, quenched and then cold worked before aging condition, with a faster kinetics. Besides, the microstructure of the latter samples consists of equiaxial grains of the τ′, α and η phases. In contrast, the former sample showed a lamellar structure of the α and η phases. This difference of microstructure caused a slightly higher hardness in the samples without cold working. Furthermore, the aging process caused a decrease in hardness with aging time, which seems to be related to the coarsening process of the equilibrium phases for both heat treatments.     

Development of rolling textures in Cu–Ni alloys

Abstract

The development of texture during the cold rolling of Cu–12·5Ni and Cu–27Ni (wt-%) alloys has been studied using X-ray analysis and transmission electron microscopy (TEM). Pole figures and diffractometer intensity measurements from rolling sections confirm that the texture is of the ‘copper’ type, although the preferred orientation develops more slowly and is consequently less sharp than in the pure metal at equivalent strains. The microstructures were consistent with deformation by slip, no evidence of mechanical twinning being found despite the greater hardness of the alloys compared with copper. However, the presence of nickel in solid solution was found to alter the deformation sequence observed by TEM. Beyond 80% reduction (ε=2·0), the cell structure characteristic of deformed copper, both at low and high strains, was almost entirely replaced by an assembly of small, slightly elongated crystallites whose boundaries often lay at ~±35° to the rolling direction. Long microbands, associated with fine scale rippling in the optical microstructure, appeared after only ~90% reduction (ε=2·5), there being a much reduced tendency for such lamellae to group into transition bands than in copper. Compared with the pure metal, the macroscopic deformation of cupronickels thus proceeds more homogeneously, although larger orientation differences, e.g. of ~10;°, as measured by a precision convergent beam technique, existed between adjacent crystallites, adjacent microbands, and across crystallite/microband boundaries. Possible causes of these differences of behaviour in the alloys are discussed and related to the higher hardness and work hardening rates of Cu–Ni alloys.

MST/499     

Effect of heat treatment on morphology of Fe-rich intermetallics in Al–Cu alloys

The iron-rich intermetallics are considered to be stable in aluminium alloys and usually hard to dissolve into the matrix during solution heat treatment (SHT). Nevertheless, solid-state transformations between Fe-rich intermetallics may happen and will play a great role in breaking-up of the large Fe-rich intermetallics particle during hot deformation process. In this study, the solid-state phase transformation of the iron-rich intermetallics from Al_mFe to β-Fe (Al₇Cu₂Fe) has been studied using SEM, XRD and deep-etched technique in a 2xxx Al–Cu cast alloy during SHT. The mechanism of solid-state transformation for the Fe-rich intermetallics was discussed, including the solid-state reaction, nucleation and growth of iron-rich intermetallics and Al phases.     

Effect of Cu/Zn on microstructure and mechanical properties of extruded Mg–Sn alloys

In this paper, the effect of Cu and Zn addition on mechanical properties of indirectly extruded Mg–2Sn alloy was investigated. Mg–2Sn–0.5Cu alloy exhibits a moderate yield strength (YS) of 225?MPa and an ultimate strength of 260?MPa, which are much higher than those of the binary Mg–2Sn alloy, and the elongation (EL) evolves as ～15.5%. Mechanical properties of the Mg–2Sn–0.5Cu alloy are deteriorated with more 3 wt-% Zn addition, and YS and EL are reduced as 160?MPa and ～10%. The detailed mechanism is discussed according to the work-hardening rate and strengthening effect related to the grain sizes, second phases and macro-textures. Grain refinement and proper texture are believed to play a critical role in both strength and ductility optimisation.     

Effect of high-density electric current pulses on precipitation and mechanical properties of a Cu–Zn alloy

ABSTRACT

By applying high-density electric current pulses (ECP) on the Cu–Zn alloy, the high-temperature precipitated phase is explored and the related mechanical properties are investigated. Results show that the nucleation rate of the high-temperature β precipitation can be greatly accelerated due to the ECP treatment. Especially, an obvious orientation relationship can be detected between the matrix α phase and the product β precipitation by increasing the electric current density. In addition, for the samples with a dense distribution of refined β precipitation, a slight decrease in the tensile strength and a great increase in the elongation-to-failure can be observed.

This paper is part of a themed issue on Materials in External Fields.     

Microstructural change and precipitation hardening in melt-spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified by melt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400°C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200°C in the Mg–Al–Si–Ca alloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200°C for 1 h.     

Microstructural change and precipitation hardening in melt–spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified bymelt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400%C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200%C in the Mg–Al–Si–Caalloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200%C for 1h.     

Effect of ultrasonic treatment on the mechanical behaviour of Al–Ni alloys

Applications of Al–Ni alloys are limited because their matrix is weaker than other binary aluminium alloys. Ultrasonic treatment (UST) is an effective tool for grain refinement that can strengthen the matrix phase. It not only reduces the grain size and porosity but also refines and uniformly distributes the secondary phase, which can influence the mechanical properties of Al–Ni alloys. Varying the amount of nickel (1, 2, 3, and 5?wt-%) in molten aluminium along with ultrasonication of the melt is investigated through grain-structure, mechanical properties, and fractography. Mechanical properties of the alloys subjected to UST are superior to respective as-cast alloys. UST also altered the fracture behaviour from dominant ductile fracture in as-cast alloys to dominant mixed mode fracture.     

Effect of silver addition on formation of secondary phases in squeeze cast Al–Cu–Mg alloys

Abstract

The effect of silver addition on the formation of secondary phases in squeeze cast Al–4.0Cu–1.5Mg and Al–4.0Cu–1.5Mg–0.7Ag (all wt-%) alloys has been investigated using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometry, and transmission electron microscopy. The as cast microstructure of both alloys consists of primary dendritic α-Al and various types of secondary solidification phase, e.g. Al₂Cu, Al₂CuMg, Al(Cu,Ag)Mg, and icosahedral (I) and decagonal (D) quasicrystalline phases. However, the solidification path in the interdendritic region during squeeze casting is different for each alloy, i.e. L→ternary α-Al–Al₂Cu–Al₂CuMg eutectic in Al–4.0Cu–1.5Mg and L→L′+Al₂Cu→α-Al–Al₂Cu–Al(Cu_0.75Ag_0.25)Mg eutectic in Al–4.0Cu–1.5Mg–0.7Ag. This indicates that silver acts as an alloying element stabilising the formation of Al(Cu,Ag)Mg Laves phase. The remaining copper and iron rich liquid in the interdendritic region at the final stage of solidification solidifies into a mixed structure of α-Al, Al₂Cu, and AlCuFe I (or D) phases. The composition of the I and D phases, measured by energy dispersive X-ray spectroscopy, is in the range Al–(27～28)Cu–(9～10)Fe and Al–(26～27)Cu–(7～9)Fe (all at.-%) respectively.     

Effect of Sm on hydrogen storage performance of La–Sm–Mg–Ni alloys

In this study, Sm was adopted in order to completely replace the expensive Pr/Nd elements in the A2B7 type alloy. The results indicate that Sm is a favourable element for forming Ce2Ni7 type and Ce5Co19 type phases. With the increasing amount of Sm, the discharge capacity of the alloy retains a value of 283·3 mAh g^?1 at the current density of 1200 mA g^?1. The maximum discharge capacity of the alloys increases with the increasing Sm content when Mg content is relatively low. By optimising the composition and processing technology, the cycle life the alloy enhances from 74 cycles to more than 540 cycles, and the maximum discharge capacity also increases from 300 to 355 mAh g^?1.     

Effect of Gd on physics properties of the dual-phase Ni–Mn–Ga-based alloys

The microstructure, martensitic transformation (MT) and shape memory effect (SME) of the dual-phase Ni₅₈Mn₂₅Ga_17?xGd_x (x?=?0, 0.1, 0.2, 0.5, 1) alloys have been investigated. The results show that the refined the grain size and adjust the distribution of γ phase by added rare earth Gd in the Ni₅₈Mn₂₅Ga_17?xGd_x alloys. With increasing Gd content, the MT temperatures of Ni₅₈Mn₂₅Ga_17?xGd_x alloys first gradually decrease and then increase with the increasing content of Gd, reaching their minimum values when the content of Gd is 0.5?at.-%. In addition, the Ni₅₈Mn₂₅Ga_16.9Gd_0.1 has a SME of 5.1% owing to the favourable γ phase distribution, which is mainly attributable to the γ phase grain refined and weaken resistance of reverse MT.     

Correlation of microstructure and magnetic properties in Cu–Co–Ni alloys

The present study concerns correlation of microstructure and magnetic properties of nanocrystalline binary 50Cu–50Co and ternary 50Cu–25Co–25Ni (wt%) alloys prepared by ball milling and subsequent isothermal annealing of the ball milled alloys. High resolution transmission electron microscopic (HR-TEM) investigation has shown deformation-induced microstructural features. Field emission scanning electron microscopy (FE-SEM) has revealed a distinct change in morphology of as-milled CuCoNi alloys after annealing. Differential scanning calorimetric (DSC) and X-ray diffraction (XRD) analysis have revealed that annealing of the CuCoNi alloy above 350 °C results into precipitation of nanocrystalline Co (fcc) in the CuNi matrix by spinodal decomposition. It is also demonstrated that isothermal annealing of the ball milled alloys in the temperature range between 350 and 650 °C significantly influence the magnetic properties, e.g. coercivity (H_c), remanence (M_r) and magnetic saturation (M_s) due to annihilation of defects such as stacking and twin fault along with dissolution and/or precipitation of magnetic phases in the Cu-rich matrix.     

Effect of Zn addition on hot tearing behaviour of Mg–0.5Ca–xZn alloys

The influence of Zn addition (0, 0.5, 1.5, 4.0 and 6.0 wt.%) on hot tearing behaviour of Mg–0.5 wt.% Ca alloy was investigated using a constrained rod casting (CRC) apparatus. The effects of mould temperature and grain refinement on the hot tearing susceptibility (HTS) were studied. Hot tears were observed with 3D X-ray tomography and the tear volumes were quantified. Results show that the Zn addition increases the HTS of Mg–0.5Ca alloys. At a mould temperature of 250 °C, all alloys investigated except Mg–0.5Ca–6Zn alloy show severe HTS. An increase in the mould temperature from 250 °C to 450 °C did not reduce the HTS in Mg–0.5Ca–1.5Zn and Mg–0.5Ca–4Zn alloys. Among all the investigated alloys, Mg–0.5Ca–4Zn alloy exhibits severe HTS as it completely broke away from the sprue–rod junction. The HTS of alloys was well correlated with the susceptible temperature range (ΔT_s). An increase in ΔT_s increased the HTS. The hot tears propagated along the grain boundaries through liquid film rupture. Grain refinement by Zr addition improved the hot tearing resistance of Mg–0.5Ca–4Zn alloy as the fine grain structure facilitated the easy feeding of liquid into the last area of solidification and accommodated the developed strain more effectively.     

Extrusion of AI–Zn–Mg–Cu–Zr alloys

Abstract

Four aluminium alloys of different zinc/magnesium ratio have been studied under various extrusion conditions. The alloys were cast in steel book moulds and subjected to initial thermomechanical treatments. Studies were made of hot extrusions and cold hydrostatic extrusions and in each case the changes in the extrusion parameters were analysed. An attempt has been made to explain some of the extrusion defects which appeared in various extruded sections. The extrusion speed was found to be crucial, since sections developed surface cracks at higher speeds. The extrusion speed was also found to vary inversely with the extrusion ratio, with higher speeds at low ratios. A well defined solute–depleted weld zone was observed on each of the four faces of a square tube extruded using a porthole die. Thermal treatment was not found to improve this weak weld zone. Tubes extruded using a floating-mandrel die withstood pressure testing up to 550 MPa.

MST/43     

Hexagonal martensite decomposition and phase precipitation in Ti–Cu alloys

The mechanical behavior of Ti–Cu alloys can be improved by controlling Ti₂Cu precipitation. In eutectoid alloys, such precipitation can be achieved by the decomposition of martensite in response to aging heat treatment. The purpose of this work is to discuss the evolution of precipitates during the decomposition of hexagonal martensite in Ti–Cu alloys. First, samples with near-eutectoid compositions were prepared in an arc furnace equipped with a non-consumable tungsten electrode and water-cooled copper hearth under a high purity argon atmosphere. After chemical homogenization at a temperature in the beta field, the samples were water-quenched and examined by differential scanning calorimetry and high-temperature X-ray diffraction. The results indicate that rapidly quenched near-eutectoid Ti–Cu alloys present Ti₂Cu precipitates. Regardless of the cooling rate applied, such precipitation is unavoidable. No evidence of beta phase stabilization was found in the rapidly quenched samples. Precipitation temperatures of coherent and incoherent phases of 415 °C and 550 °C, respectively, were determined from the differential scanning calorimetry measurements. Ti₂Cu precipitation was examined in situ by high temperature X-ray diffraction experiments. The total decay of martensite was found to occur above 575 °C. Vickers hardness testing of aged samples revealed a correlation between phase precipitation and hardening.     

Effect of small alloying additions on behaviour of rapidly solidified Cu–Cr alloys

Abstract

The mechanical properties of as well as microstructural changes in rapidly solidified ternary Cu–Cr based alloys were studied for various ternary additions. The flow stresses of the binary and various ternary alloys are explained in terms of Orowan strengthening mechanisms. Zirconium, magnesium, and, to a lesser extent, silicon affected the age hardenability of the alloys, refining the chromium dispersion by modifying the precipitation sequence described by Tang. The coarsening kinetics was insensitive to the presence of a third alloying element, showing that these additions did not affect chromium transport within the copper matrix. Nevertheless, these additions influenced the morphology of the chromium particles during coarsening; zirconium tended to keep the particles spherical, while titanium additions increased their aspect ratio. Titanium reduced dramatically the age hardenability of the alloy by provoking heterogeneous nucleation of bcc chromium on small Ti0₂ particles present in the as cast structure, resulting in a coarser particle size distribution at peak strength.

MST/1171     

Effect of Homogenization Process on Microstructure of Al–Zn–Mg–Cu Aluminum Alloys

Herein, the best homogenization process of 466.5 °C × 36 h + 490 °C × (14–26.4 h) that can completely eliminate the coarse phases σ[Mg(Zn, Al, Cu)₂] and S(Al₂CuMg) in the Al–Zn–Mg–Cu aluminum alloy is developed. The homogenization process is determined by the method of calculation phase diagram, and the experimental verification. It is shown in the results that, first, in the microstructure of the as-cast alloys, the crystal structure of the σ[Mg(Zn, Al, Cu)₂], Al₇Cu₂Fe, and Mg₂Si phases is determined. Second, during the homogenization process, the σ[Mg(Zn, Al, Cu)₂] phase dissolves and also transforms into the S(Al₂CuMg) phase. Most importantly, the dissolution temperature range of the σ[Mg(Zn, Al, Cu)₂], S(Al₂CuMg), and Al₇Cu₂Fe phases is determined from 472.56 to 476.36 °C, from 484.09 to 485.39 °C, and from 540.18 to 547.23 °C, respectively. At best homogenization process, the residual Al₇Cu₂Fe phase area fraction ranges from 1.28 ± 0.16% to 1.60 ± 0.18%. In addition, dispersed η(MgZn₂) phase precipitates in supersaturated Al-matrix during differential scanning calorimeter heating. And, the concentration differences between the grain center and the eutectic of structure of Zn, Mg and Cu regression equations are established, which can provide some reference for the design of experimental parameters, thus reducing the experimental workload.