Zn-Al Eutectoid Alloy Research Articles

Phenomenological Model for the Dynamic Superplastic Deformation Mechanism in a Zn-Al Eutectoid Alloy Modified with 2 wt% Cu

Phenomenological Model for the Dynamic Superplastic Deformation Mechanism in a Zn-Al Eutectoid Alloy Modified with 2 wt% Cu

Microstructural role of TiB addition in modifying ZnAl alloy

The microstructural role of TiB addition in modifying a Zn-Al eutectoid (ZA27) alloy was investigated using scanning electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and energy X-ray spectroscopy. A series of Zn-27Al-2Cu-0.02Mg-x(Ti+B) (x=0, 0.01, 0.02, 0.05, 0.10, 0.20, wt.%) alloys were cast. The results show that the hardness and strength of the ZA27 alloy are improved with increasing (Ti+B) content, and peak at a (Ti+B) content of 0.05 wt.%. The TiB modifier refines the primary α(Al) phase grains and changes the microstructure of coralloid (α(Al)+η-Zn) eutectoid, which is responsible for the increased mechanical properties of the ZA27 alloy. The modified eutectoid demonstrates featured basket-like and strip-like eutectoid microstructures. The refinement of the primary α(Al) phase grains is very likely related to the constitutional supercooling caused by the segregation of B at the solid-liquid interface. Participation of B and/or Ti in the eutectic and eutectoid reactions modifies the morphology of the eutectoid microstructure.

Effects of combined use of inoculation and modification heat treatment on microstructure, damping and mechanical properties of Zn–Al eutectoid alloy

Inoculation and modification heat treatments were conducted on the Zn–Al eutectoid alloy (ZA22) with the aim of grain refinement. Evolution law of microstructure, and changes of damping and mechanical properties of resultant specimens were also studied. It was found that inoculation could refine the primary α phase and effectively improved the damping arising from the reverse eutectoid transformation, as well as the tensile strength and elongation. After modification heat treatments, all original phases and structures disappeared completely. The newly formed ZA22 alloy only consisted of sub-micron granular eutectoid structure. The significantly increased density of interfaces and the change in α/η interface microstructure resulted in remarkable improvement of damping over the whole temperature range. The damping of all ZA22 alloys showed obvious strain amplitude dependence in two strain regions, which has been ascribed to the depinning of sliding interfaces and the dynamic recrystallization due to microplasticity at the surface of the specimens, respectively. Sub-micron grains and uniform composition of granular eutectoid structure could effectively promote the sliding of grain boundaries, which leaded to the appearance of room temperature superplasticity.

Microstructural changes in a Zn–Al eutectoid alloy modified with 2 wt.% Cu after superplastic deformation

This work focuses on the characterization of microstructural changes in Zn–Al eutectoid alloy modified with 2wt.% Cu when it is superplastically deformed varying the strain rate and temperature. Microstructural characterization was carried out by SEM and AFM techniques and it was shown that increasing the superplastic strain leads to a change of an initial fine equiaxed grain microstructure (0.85μm), formed by a Zn rich (η) and Al rich (α) phases, toward a microstructure consisting on flow bands. These flow bands formed mainly by elongated grains of the η phase, tend to be aligned along the tensile direction. The flow bands formation is promoted by an increase in temperature or a decrease in strain rate during deformation process. The morphologies and dimensions of flow bands were found to strongly depend on the testing conditions. The largest and thinnest flow bands were observed at 10−3s−1 and 513K. Experimental results evidenced that the origin of these microstructural changes, where deformation capability and growth of Zn-rich phase is significant, was not associated with a micro-superplastic phenomenon. Rather, it is associated to a non-homogeneous plastic flow and possible changes in the controlling deformation mechanism of this alloy, where recrystallization assisted by a redistribution of elements by diffusion is significant.

Calculation of the Activation Energy for Super Plastic Flow in Zn-22Al Alloy

The application of the unified physics is the way to understand the phenomenology and mechanics of super plastic flow (SPF). In this scenery, the main proposed in this work is to establish the effect of grain size and thermomechanical conditions on the activation energy for super plastic flow (Qspf) in Zn-22Al eutectoid alloy by applying the quantum mechanics and relativistic model (QM-RM) proposed by Muñoz-Andrade. Analyses on the experimental results reported before by some authors, it is shown for grain size of 0.35 μm that the calculated Qspf by using QM-RM for grain boundary sliding is 55.669 kJ/mol at 303 K and strain rate of 1 s−1. These results are in closed agreement with the value of Qa = 54 kJ/mol reported previously by using the theoretical and conventional methodology set up by Mohamed and Langdon. However, for grain size of 0.8μm, the calculated Qspf is 67.864 kJ/mol at 473 K and strain rate of 1×10−2 s−1. Furthermore, in order to understand the phenomenology and mechanics of SPF in Zn-22Al eutectoid alloy, the variation of the activation energy with the temperature; stress and strain rate is analyzed in association with coupled mechanisms during SPF, such as grain boundary sliding, cooperative grain boundary sliding and self-accommodation process related to the microstructure.

Dendritic Segregation of Zn-Al Eutectoid Alloys

The structural investigations performed and described in this paper are focused on both the segregation phenomenon at the dendrite level, and on the external structure in the equilibrium state. Qualitative and quantitative investigations of the Zn-Al eutectoid alloy (22% Al) structures in equilibrium and non-equilibrium states were considered and studied, revealing that, in this composition, dendritic segregation determines the eutectic transformation. Solidification in non-equilibrium conditions controls both the eutectic transformation and the significant variation of the chemical composition within the dendritic unit, as well as in other constituents. The most important effect of dendritic segregation is on the ratio of the different phases and on their geometry and distribution within the metallic matrix, which determines the material topology. The phenomena involved in such conditions are governed by the structural transformations, which take place during solidification and cooling in a solid state.

Analysis of Plastic Flow Instability During Superplastic Deformation of the Zn-Al Eutectoid Alloy Modified with 2 wt.% Cu

The aim of this work is to analyze the plastic flow instability in Zn-21Al-2Cu alloy deformed under 10−3 s−1 and 513 K, which are optimum conditions for inducing superplastic behavior in this alloy. An evaluation using the Hart and Wilkinson–Caceres criteria showed that the limited stability of plastic flow observed in this alloy is related to low values of the strain-rate sensitivity index (m) and the strain-hardening coefficient (γ), combined with the tendency of these parameters to decrease depending on true strain (e). The reduction in m and γ values could be associated with the early onset of plastic instability and with microstructural changes observed as function of the strain. Grain growth induced by deformation seems to be important during the first stage of deformation of this alloy. However, when e > 0.4 this growth is accompanied by other microstructural rearrangements. These results suggest that in this alloy, a grain boundary sliding mechanism acts to allow a steady superplastic flow only for e 0.7 as another mechanism is thought to take over.

Study on Preparation of Foamed Zn-Al Eutectoid Alloy by Air Pressure Infiltration Process

This work reports the technology of preparation about foamed Zn-Al eutectoid alloy, which were prepared by an air pressure infiltration process using NaCl crystals as place holders. The method consists of five sequential processes: particles preprocessing, preparing preform, preheating, infiltration casting, machining and cleaning. The porosity of foamed Zn-Al eutectoid alloy is 61%. The mean diameter of macroscopic pore is on the order of a millimeter (1.0 mm) in the foamed Zn-Al eutectoid alloy with open-pore structure.

Effect of combined addition of Al-Ti-B ribbon and Zr element on the microstructure, mechanical and damping properties of ZA22 alloy

A novel Al-5Ti-1B ribbon was prepared and used as inoculant to refine the grains of Zn-Al eutectoid (ZA22) alloy with the aim to significantly enhance the mechanical properties while remaining the high damping capacity. It was found that compared with the commercial Al-5Ti-1B rod the present ribbon had better refining effect on the ZA22 alloy due to its finer and more uniformly distributed TiB2 and L12-Al3Ti particles formed during rapid cooling process. It has been confirmed that both of the two kinds of particles could act as effective heterogeneous nucleation sites of α-Al phase, and then led to the refinement of α-Al grains. The combined addition of Al-5Ti-1B ribbon and Zr element could achieve the best refining effect because Zr could further promote the heterogeneous nucleation and hinder the growth of α-Al grains. Properties tests illustrated that with the decrease of average grain size, the tensile strength, elongation and hardness of the ZA22 alloy increased obviously because of the fine-grain strengthening and dispersion strengthening effects, while the damping showed different variation patterns within different temperature regions, which has been ascribed to the synthetic effect of two opposite influencing factors.

Microstructural evolution and mechanical properties in a Zn–Al eutectoid alloy processed by high-pressure torsion

Experiments were conducted to examine the evolution of microstructure and mechanical properties in a Zn–22% Al eutectoid alloy processed by high-pressure torsion (HPT). Measurements of the Vickers microhardness revealed significant weakening in the alloy after HPT, and microstructural analysis showed that the initial duplex structure, consisting of equiaxed grains and lamellae, was retained at the disk centers after HPT, whereas equiaxed fine grains were observed at the disk edges. Direct evidence is presented for a transformation of the lamellae into equiaxed very fine grains and subsequent dynamic recrystallization in the early stages of HPT. The reduction of the lamellar structure and the loss of Zn precipitates account for the weakening in the alloy in HPT processing. Excellent high strain rate superplasticity was recorded after HPT, with elongations up to ∼1800% at 473K at a strain rate of 10−1s−1. The experiments show that the maximum elongations are displaced to faster strain rates with increasing numbers of HPT turns.

溶湯発泡法で作製されたポーラス Zn-22Al 超塑性合金のセル形態に及ぼす発泡条件の影響

Closed-cell superplastic Zn-22Al eutectoid alloy foams were manufactured through the melt foaming process using titanium hydride powder as a foaming agent. Foaming tests were carried out under different foaming temperatures, times and additive amounts of foaming agent. Maximum porosity of 75% and small pore diameter were obtained during the temperature range of semi-solid condition from 738-753 K. This is because of increased melt viscosity. Though the small amount of the foaming agent is effective to decrease the pore diameter, the porosity becomes low. Cell morphology of the present Zn-22Al alloy foam is identical to conventional aluminum foams. Semi-solid condition of Zn-22Al foam has an advantage over increasing melt viscosity on manufacturing of metal foam.

Effects of Macroscopic Pores on the Damping Behavior of Zn-Al Eutectoid Alloy

The objective of present work is to investigate the effect of macroscopic pores on the damping behavior of Zn-Al eutectoid alloy. The damping behavior of the foamed Zn-Al (FZA) eutectoid alloy prepared by air pressure infiltration process with macroscopic pores is characterized by internal friction (IF). Macroscopic pores size are on the order of a millimetre (0.51.0mm). The IF was measured on a multifunction internal friction apparatus (MFIFA) at frequencies of 0.5, 1.0 and 3.0 Hz over the temperature range of 25 to 400°C, while continuously heating temperature. The damping capacity of the FZA eutectoid alloy, with two different porosity and sizes of macroscopic pores, were compared with that of bulk Zn-Al eutectoid alloy specimen. The damping capacity of the materials is shown to increase with introducing macroscopic pores. Finally, the operative damping mechanisms in the FZA eutectoid alloy were discussed in light of IF measurements.

Superplastic behavior of Zn–Al eutectoid alloy with 2 % Cu

The effects of deformation temperature and strain rate on the superplastic behavior of the Zn–21Al–2Cu alloy (Zinalco alloy) were investigated by uniaxial tensile tests. Results were compared with those of the Zn–22Al eutectoid alloy without Cu. It was observed that additions of 2 % Cu leads to a decrease of the maximum strain attainable from 2600 % to 1000 %. The maximum strain in Zinalco alloy is obtained at lower strain rates. The presence of Cu increases the values of flow stress up to 600 % compared with those reported in the Zn-22Al alloy. Grain size sensitivity (p), true activation energy (Qt), and constant A of the constitutive equation were not affected by presence of Cu unlike the stress exponent (n) which increased from 2.5 to 3.9. The main effect of Cu was to decrease the plastic flow stability of the Zn–22Al alloy. The results indicate that presence of Cu in the Zinalco alloy causes a hardening effect at low strain rates leading to a decrease in the strain rate sensitivity which promotes the formation and growth of sharp necks. Microstructural characterization suggests that the large deformations at necking could possibly be due to the substantial elongation capability of the Zn-rich phase (η).

Microstructural Control of a Zn-Al Eutectoid Alloy by Hot-Rolling

The Zn-Al eutectoid alloy has been well known as a typical superplastic metallic material with small grain sizes. In the present study, controlling conditions in hot rolling such as temperature, total number of passes, reduction in a pass, etc. have been investigated in a ZnAl eutectoid alloy to obtain finegrained microstructures. Homogenized ingot specimens were hotrolled by a total rolling reduction of 90% with different pass schedule. Microstructural observation on the final sheet showed that nearly equiaxed finegrained microstructure with grain size of about 2µm was formed when the ingot was waterquenched after homogenization.

Effects of Macroscopic Pores on the Damping Behaviors of Metal Materials

A large number of macroscopic pores were introduced into commercially pure aluminum (Al) and Zn-Al eutectoid alloy by air pressure infiltration process to comparatively study the influence of macroscopic pores on the damping behaviors of the materials. Macroscopic pores size are on the order of a millimetre (0.5~1.4mm) and in large proportions, typically high 76vol.%. The damping behavior of the materials is characterized by internal friction (IF). The IF was measured on a multifunction internal friction apparatus (MFIFA) at frequencies of 0.5, 1.0 and 3.0 Hz over the temperature range of 25 to 400 °C, while continuously changing temperature. The damping capacity of the metal materials is shown to increase with introducing macroscopic pores. Finally, the operative damping mechanisms in the metal materials with macroscopic pores were discussed in light of IF measurements.

Developing superplasticity and a deformation mechanism map for the Zn–Al eutectoid alloy processed by high-pressure torsion

A Zn–22% Al eutectoid alloy was processed by high-pressure torsion (HPT) for 1, 3 and 5 turns at room temperature to produce an ultrafine grain size of ∼350 nm. Tensile testing at a temperature of 473 K gave excellent superplastic properties with elongations to failure up to a maximum of 1800% at an imposed strain rate of 1.0 × 10 −1 s −1: this is within the range of high strain rate superplasticity and represents the highest elongation recorded to date for a specimen processed by HPT. It is shown that the experimental data are in excellent agreement with a deformation mechanism map constructed for a temperature of 473 K.

Phase transformations in the Zn–Al eutectoid alloy after quenching from the high temperature triclinic beta phase

Ribbons of the Zn–Al eutectoid alloy obtained by melt-spinning, were heat treated at 350 °C during 30 min in a free atmosphere furnace, and then quenched in liquid nitrogen. The temperature correspond to β phase zone, which has a triclinic crystalline structure [1, 2]. Some evidence, obtained by X-ray diffraction, show that the structures present in the just quenched material are both close-packed hexagonal (η-phase) and rhombohedral (R-phase). X-ray diffractograms taken in the same ribbons after annealed 500 h at room temperature, show that the R phase its transform to α and η phases.

413 Zn-Al超塑性制振合金の微細組織制御(OS4-(4)オーガナイズドセッション《材料と組織》)

As a typical superplastic metallic material, Zn-Al eutectoid alloy has been well known, where fine-grained microstructure required for the occurrence of superplasticity is obtained by solution treatment, water-quenching and subsequent aging at an elevated temperature such as 200℃. Recently thermo-mechanically controlled process (TMCP), where hot rolling is conducted under temperature control, has also been reported to provide a fine-grained microstructure. As the critical cooling rate in this technique is slower than in the above-mentioned traditional process, it can be applied to thick-gage components and product alloy plates have been employed as seismic damper in an actual high-rise building for protection from giant earthquakes. However, condition to obtain fine-grained microstructure and its mechanism has not elucidated yet. Hence in the present study, control conditions in hot rolling such as temperature, total number of passes, reduction in a pass, etc. have been investigated in a Zn-Al eutectoid alloy to obtain fine-grained microstructures. Homogenized ingot specimens were hot-rolled using conventional mill without a heating device from 20 to 2mm in thickness with 9 passes. The specimens were held at 260℃ for 30min before each pass, and water-quenched after the pass. Microstructural observation on the final sheet with 9 passes and that of 3mm thickness with 8 passes showed that equi-axed fine-grained microstructure with grain sizes of about 2μm was observed in the two sheets.

Influence of macroscopic graphite particulates on the damping properties of Zn-Al eutectoid alloy

The paper presents in detail the effects of macroscopic graphite (Gr) particulates on the damping behavior of Zn-Al eutectoid alloy (Zn-Al). Macroscopic defects are graphite particulates with sizes of the order of a millimeter (0.5 mm and 1.0 mm). Macroscopic graphite particulate-reinforced Zn-Al eutectoid alloys were prepared by the air pressure infiltration process. The damping characterization was conducted on a multifunction internal friction apparatus (MFIFA). The internal friction (IF), as well as the relative dynamic modulus, was measured at different frequencies over the temperature range of 20 to 400°C. The damping capacity of the Zn-Al/Gr, with two different volume fractions of macroscopic graphite particulates, was compared with that of bulk Zn-Al eutectoid alloy. The damping capacity of the materials is shown to increase with increasing volume fraction of macroscopic graphite particulates. Two IF peaks were found in the IF-temperature curves. The first is a grain boundary peak, which is associated with the diffusive flux on a boundary between like phases, Al/Al. Its activation energy has been calculated to be 1.13±0.03 eV and the pre-exponential factor is 10−14 s in IF measurements. The second is a phase transition peak, which results from the transformation of Zn-Al eutectoid. In light of internal friction measurements and differential scanning calorimetry (DSC) experiments, its activation energy has been calculated to be 2.36±0.08 eV.

Corrosion Resistances of As-Cast and Quenched Samples of a Zn-22Al Eutectoid Alloy

To address the increasing demand for high performance and high quality die castings, Zn-Al casting alloys have been developed, in particular for applications in the automotive industry. This family of alloys has good physical, mechanical and tribological properties and is commonly used as foundry alloys in a variety of applications. Particularly, the Zn-22 wt% Al eutectoid alloy is a classic commercial material and a fine-grained structure can be obtained by using a suitable heat treatment which can induce a superplastic behavior. On the other hand, this heat treatment can affect the corrosion behavior of such alloy. The present work focuses on the influence of as-cast and quenched microstructures of the Zn-22wt%Al eutectoid alloy on its electrochemical corrosion behavior. The typical microstructural patterns were examined by using optical microscopy techniques. In order to evaluate the corrosion behavior of such alloy, corrosion tests were performed in a 0.5 M NaCl solution at 25°C by using an electrochemical impedance spectroscopy (EIS) technique and potentiodynamic polarization curves. An equivalent circuit by using the ZView software was also used to provide quantitative support for the discussions and understanding of the electrochemical corrosion behavior. It was found that the heat-treated samples are more susceptible to the corrosion action than the as-cast alloy sample.