Unidirectional solidification of a Zn-rich Zn–2.17 wt%Cu hypo-peritectic alloy

Unidirectional solidification of a Zn-rich Zn-2.17 wt%Cu hypo-peritectic alloy has been carried out to investigate the microstructure evolution over the growth velocity range 0.02–4.82 mm/s ata temperature gradient of 15 K/mm by means of the Bridgman technique. Regular and plate–like two-phase cellular structures were observed in samples grown at growth velocities V above 0.48 and 2.64 mm/s,respectively. The dominant microstructure in samples grown below 0.22 mm/s was dendrites of primary sin a matrix of secondary η. Intercellular spacing ι decreased ε with increasing growth velocity V such that ιV¹² is a constant of 316 ± 55 µm^3/2/s^1/2. Secondary dendrite arm spacing λ₂ of primary decreased with increasing V such that λ₂V^1/3 isa constant of 14.9 ± 0.9 µm^4/3/s^1/3. The observed transition from regular cells to plate-like cells of η is discussed on the basis of competitive growth and crystallographic effect.     

Improvement of magnetic properties of an Fe–6.5 wt.% Si alloy by directional solidification

The magnetic properties of an Fe–6.5 wt.% Si alloy can be improved through texture and microstructure control during directional solidification process. With the increasing of directional solidification rate, the main texture of the Fe–6.5 wt.% Si alloy along specimen withdrawing direction evolved in the way of < 130> → < 100> → < 142>, and the coercivity initially decreased and then increased. For the directional solidification rate of 1 mm/min, a homogeneous microstructure of the Fe–6.5 wt.% Si alloy specimen with low energy boundaries between columnar grains was obtained. The main texture of the specimen was < 100>, and the coercivity of the alloy was reduced by 44% compared with that of the alloy consisting of equiaxed grains.     

Anaxial columnar dendrites in directional solidification of an Al–15 wt.% Cu alloy

A new dendrite morphology, anaxial columnar dendrites, was found in directional solidification using the real-time X-ray imaging technique. The dendrites are composed of a pair of stems, which are divided by a narrow liquid zone located in their center. The formation process of a free dendrite in the melt indicates that it grows along < 110> directions. According to the basic morphology of the dendrites, a tip model was developed to explain the growth preference of the secondary arms. The formation of anaxial columnar dendrites indicates that the dendrite morphology is closely related to the selection of the growth direction of the dendrites.     

Role of graphene oxide (GO) for enhancing the solidification rate and mechanical properties of Sn–6.5Zn–0.4 wt% Cu Pb-free solder alloy

Journal of Materials Science: Materials in Electronics - The role of incorporation of the 0.5 and 1.0&nbsp;wt% GO with the Sn–6.5Zn–0.4&nbsp;wt% Cu has been researched. The...     

Microstructure and mechanical properties of a sand-cast Mg–Nd–Zn alloy

The microstructures and mechanical properties of a sand-cast Mg–Nd–Zn alloy in the as-cast, solution-treated and peak-aged conditions were investigated. The as-cast alloy was comprised of α magnesium matrix and Mg₁₂Nd eutectic compounds. The eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors, due to the solution treatment. After the solution treatment, two kinds of cooling manner, either cooling in air or quenching in water, were employed. It was worth noting that some basal precipitates formed in the matrix during the in-air cooling process after solution treatment, which led to the succedent weak ageing hardening response and low strength in peak-aged condition. The hardness, yield strength, ultimate tensile strength and elongation at room temperature, of the samples in the T61 condition, were HV81, 191 MPa, 258 MPa and 4.2%, respectively. When tensile tested at high temperature, they exhibited serrated flow. Moreover, the casting surface of the tensile testing bar also had a great influence on its mechanical properties.     

Solidification microstructures of the undercooled Co–24 at%Sn eutectic alloy containing 0.5 at%Mn

Solidification samples of undercooled Co–24 at%Sn eutectic alloy containing a small amount of Mn (<1.0 at%) were prepared by the glass fluxing technique. The surface and internal solidification microstructures of the samples were observed by a scanning electron microscope (SEM) and an optical microscope (OM), respectively. The experiment results revealed that the addition of 0.5 at%Mn remarkably changed the solidification behaviors of the undercooled Co–24 at%Sn eutectic alloy. The addition of 0.5 at%Mn influenced the morphological selection of eutectic growth interface by increasing the interface energy anisotropy during the solidification of the undercooled Co–24 at%Sn eutectic melt. As undercooling increases, the coupled eutectic growth interface morphology successively experienced dendritic pattern, factual seaweed pattern and compact seaweed pattern. Besides, the addition of 0.5 at%Mn decreased the critical undercooling for the formation of anomalous eutectic by introducing a new formation mechanism of anomalous eutectic, i.e. divorce eutectic mechanism.     

Precipitation hardening of Zr-modified Mg–Ca–Zn alloy

The microstructure and mechanical properties of Mg–Ca–Zn alloys with 1 wt.% Zr were investigated in as-cast and heat-treated conditions. A substantial decrease in grain size (from 65 μm for the Mg–Ca–Zn base alloy to 22 μm) was observed. The alloy was solution treated at 410 °C for up to 96 h followed by aging at 175 °C for up to 24 h. Conventional techniques, X-ray diffraction, EM + EDS, and TEM were used to characterize the microstructure of the alloy. The microstructure obtained after heat treatment had equiaxed grains with evenly distributed binary phase Zn₂Zr. The binary Mg₂Ca and ternary Mg₂Ca₆Zn₃ phases were identified in the matrix and at grain boundaries surrounded by precipitate-depleted zones (PDZs). The thermal stability of the Zr-modified alloys was examined by microhardness measurements conducted after prolonged exposures of the alloys to elevated temperatures. It was found that Zr is a structure-stabilizing factor. Its influence was associated with the formation of Zn₂Zr phase that does not undergo coarsening at the elevated temperatures used (due to the low diffusivity of Zr). The nanoscale mechanical properties of grain boundary PDZs were analyzed using combined nanoindentation and atomic force microscopy. These mechanical properties were then correlated to the composition and precipitate distribution in PDZs. An increase in the solution treatment duration from 10 to 96 h at 410 °C resulted in expansion of PDZs from ~0.75 to ~3 μm, while the following aging at 175 °C for up to 24 h did not lead to a detectable change in PDZs. The analysis indicates that the lowest hardness was found in the region where Zn₂Zr precipitates density was low, regardless of the solute concentration.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Development of high-strength,low-cost wrought Mg–2.5 mass% Zn alloy through micro-alloying with Ca and La

A high-strength low-cost Mg–2.5Zn–0.3Ca–0.4La (mass%) alloy was fabricated by hot extrusion following direct-chill casting. Yield strength (YS), ultimate tensile strength (UTS) and elongation to failure of the alloy are 325 MPa, 341 MPa and 15%, respectively. The high strength of the extruded Mg–2.5Zn–0.3Ca–0.4La alloy is mainly due to grain refinement, dense precipitation and high density of dislocations. The extruded alloy exhibits a bimodal microstructure containing fine dynamic recrystallized (DRXed) grains and deformed regions. High density of dislocations is stored in the deformed regions while dense precipitates are homogeneously distributed in both the DRXed grains and the deformed regions. However, precipitates in the DRXed regions show in spherical shape only, while they are in rod-like shape and spherical shape in the deformed regions.     

Investigation of mechanical properties of advanced Al–Zn–Mg–Cu alloy

Abstract

The mechanical properties of the rapidly solidified 7000 series powder alloy CW 67 were investigated for various extrusion and heat treatment conditions. The principal aim of the work was to ascertain the optimum processing route for peak aged (T6) material. The highest proof stress in the T6 condition was found to be 572 MN m^?2 for material extruded at 325°C and aged for 13·5 h at 120°C after solutionising. The ductility of this material was found to be 13·5%. The fracture toughness was measured in two orientations and found to be approximately 21 MN m^?3/2 in the short transverse direction and 44 MN m^?3/2 in the longitudinal direction. Degassing and hot compaction was found to improve the fracture toughness of the material substantially.

MST/1504     

Microstructure and mechanical properties of directionally solidified Mg–Zn alloy as a biomaterial

Directionally solidified Mg-4wt-% Zn alloy was prepared and the effect of growth rate on its microstructure evolution and mechanical properties was investigated. A typical cellular structure was observed when the growth rate was lower than 60?µm?s^?1. The microstructure evolved from cell to columnar dendrite as the growth rate increased. The ultimate tensile strength of the directionally solidified alloy was found to be higher than that of the alloy ingot with the same cooling rate. The ultimate tensile strength of the directionally solidified alloy increased with increasing growth rate but it decreased during the cell–dendrite transition. The results indicate that the mechanical properties of the directionally solidified alloy with fine cellular and columnar dendritic structures meet the requirements of biomaterials.     

Effects of pre-stretching on corrosion resistance of a creep-aged Al–5.87Zn–2.07Mg–2.42Cu alloy

In this experiment, the corrosion resistance of an Al–5.87Zn–2.07 Mg–2.42Cu alloy with different pre-stretchings (0, 2, 4, 6%) after creep aging (CA) was investigated. It is found that the corrosion performance reaches the optimal value at 4% pre-stretched sample, which owns the largest stress corrosion sensitivity factor r_tf (93.8%) and the shallowest intergranular corrosion depth (24.7 µm). When the degree of pre-stretching before CA rises, coarser and sparser intragranular precipitates (IGPs) are observed after CA. The average size of IGPs rises from about 2.2 to 5.8 nm while the density declines from about 5.4?×?10¹⁶ to 3.5?×?10¹⁶ cm^?3, and thus diminish the electron scattering effect and release the strain field. Moreover, the grain boundary precipitates (GBPs) become larger and more discontinuous and the precipitate-free zones (PFZs) broaden. The average size of GBPs rises from about 9.3 to 23.9 nm, and the average distance between GBPs increases from about 11.8 to 39.4 nm. Additionally, the segregation degree of Mg reduces and the content of Cu of GBPs rises with the increase of pre-stretching. These influence factors reduce the possibility of anodic dissolution and hydrogen-induced cracking (HIC). However, the expanding PFZ increases the potential difference between matrix and PFZ, promoting the anodic dissolution. Therefore, the corrosion resistance of an Al–5.87Zn–2.07 Mg–2.42Cu alloy can be improved by applying 4% pre-stretching before CA.

     

Effects of pre-treatments on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy

The effects of pre-treatments (solution and retrogression) on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy are investigated by means of transmission electron microscope (TEM), scanning electron microscope (SEM) and cyclic potentiodynamic polarization experiments. It is found that the aging precipitates and corrosion resistance are greatly affected by the pre-treatments. For the creep-aged alloy after solution pre-treatment, fine aging precipitates with high density are formed within grains. Meanwhile, large and continuously-distributed aging precipitates appear along grain boundaries. Also, this creep-aged alloy is strongly sensitive to the electrochemical corrosion, and the corrosion pits are easily induced in the 3.5 wt.% NaCl solution. For the creep-aged alloy after retrogression pre-treatment, when the retrogression pre-treatment time is increased, the density of intragranular aging precipitates first increases and then decreases, while the size of grain boundary precipitate and the width of precipitate free zone continuously increase. Compared with the creep-aged alloy after solution pre-treatment, the corrosion resistance of the creep-aged alloy after retrogression pre-treatment is greatly improved.     

Hot deformation behavior of spray-deposited Al–Zn–Mg–Cu alloy

The flow behavior of spray-deposited Al–10.21Zn–2.76Mg–1.45Cu–0.16Zr (wt.%) alloy has been systematically investigated by thermal compression tests with temperature and strain rate ranging from 613 K to 733 K and 0.001–1 s⁻¹, respectively. Microstructural observations revealed that the average grain size of spray-deposited alloy was below 25 μm due to the high cooling rate. Both relatively high temperature and low strain rate could promote the formation of dynamic recrystallization (DRX). The stress level of the alloy decreased with increasing deformation temperature and decreasing strain rate, which could be characterized by a Zener–Hollomon parameter in the hyperbolic-sine equation. Furthermore, the strain-dependent constitutive equation could lead to a good agreement between the calculated and measured flow stresses in the elevated temperature range for spray-deposited alloy. The deformation activation energy for spray-deposited alloy was relatively lower than that of the as-cast alloy owing to ultrafine grains and high supersaturated solid solubility.     

Non-isothermal aging of a high-Zn-containing Al–Zn–Mg–Cu alloy: microstructure and mechanical properties

The non-isothermal aging behaviour of a newly developed Al–Zn–Mg–Cu alloy containing 17?wt-% Zn was investigated. Hardness and shear punch tests demonstrated that during non-isothermal aging, the mechanical properties of the alloy first increased and then decreased. The best properties were obtained in a sample which was non-isothermally aged upto 250°C with heating rate of 20°C?min^?1, due to the presence of η′/η (MgZn₂) phases. This was confirmed by differential scanning calorimetery. After homogenisation, residual eutectic phases remained at triple junctions or in a spherical form. During aging, these phases transformed into rodlike S (Al₂CuMg)-phase at 400°C, with sizes ranging from 50 to 250?nm. The precipitation sequence in this high-Zn alloy was similar to that for conventional Al–Zn–Mg–Cu alloys.     

Tensile deformation of a Mg–2.54Nd–0.26Zn–0.32Zr alloy at elevated temperature

The engineering stress versus engineering strain curves for a Mg–2.54Nd–0.26Zn–0.32Zr cast alloy were measured by Gleeble-1500D thermo-simulation machine in the temperature range of room temperature to 400 °C at initial strain rates of 10⁻⁴–10⁻² s⁻¹. The effects of strain rate on stress, elongation to facture, and section shrinkage were analyzed. The fractograph morphologies were investigated by using SEM. It was found that strain rate has little effect on engineering stress for the Mg–2.54Nd–0.26Zn–0.32Zr alloy when tested at below 250 °C. When tested at above 250 °C, low strain rate resulted in decreased engineering stress, increased elongation to fracture, and section shrinkage. The fracture mode is cleavage fracture with elongated dimple below 250 °C and changes to typical ductile failure when tested above 250 °C.     

High-pressure homogenization treatment of Al–Zn–Mg–Cu aluminum alloy

Absract The microstructures and aging hardening response of Al–12Zn–3.5Mg–3.0Cu–0.14Zr aluminum alloy after a high-pressure homogenization treatment at 750 °C for 45 min under 5 GPa were investigated. The results showed that the constituent phases dissolved completely and formed α-Al single-phase solid solution comparable to that formed after ambient-pressure homogenization at 450 °C/96 h + 460 °C/128 h. The complete dissolution of the constituent phases increased the solubility of the alloying elements, as well with the over-burning temperature and aging hardness.     

Investigation of high-strength and superplastic Mg–Y–Gd–Zn alloy

The Mg–7Y–4Gd–1Zn (wt.%) alloy was prepared by hot extrusion technology, and the microstructure, tensile properties and superplastic behavior have been investigated. The extruded alloy possesses high tensile strength both at room temperature and 250 °C, and especially the yield strength can remain above 300 MPa at 250 °C. The outstanding microstructure, i.e. bent 18R long period stacking ordered (LPSO) strips and dynamic recrystallization (DRX) Mg grains containing fine lamellae with 14H LPSO or stacking fault structures, is responsible for the excellent mechanical properties, and it is considered that the integrated performance can be further improved by controlling the size of LPSO phase. The alloy shows the maximum elongation of 700% at 470 °C and 1.7 × 10⁻⁴ s⁻¹. The predominant superplastic mechanism is considered to be grain boundary sliding assisted by lattice diffusion. The fracture of superplastic deformation is related to the microstructure evolution, i.e. the disappearance of LPSO phase and the formation of cubic phase. Both high temperature and stress contribute to the phase transformation.