Al Eutectoid Alloy Research Articles

Effect of Al−10Nb−3Ce−2.5B inoculant on microstructures, damping and tensile mechanical properties of Zn−Al eutectoid alloy

A novel Al−10Nb−3Ce−2.5B inoculant was prepared, and its effect on microstructures, damping and tensile mechanical properties of Zn−Al eutectoid (ZA22) alloy was systematically studied. It was found that the inoculant had an effective refining effect on the α-phase in the ZA22 alloy (α-phase could be refined to 16 μm), and the morphology of the α-phase could also change from coarse dendrite to small petal. Orientation relationships (ORs) between the α-phase and the CeB6 and NbB2 particles in the inoculant were established by the edge−edge matching model (E2EM), and based on the E2EM, the refining mechanism of the inoculant on the ZA22 alloy was revealed. Compared with the uninoculated ZA22 alloy, the high temperature damping of the inoculated ZA22 alloy, especially the damping peak arising from the reverse eutectoid transformation, was significantly improved. Moreover, the room temperature tensile mechanical properties of the inoculated ZA22 alloy were also significantly improved. The tensile strength and elongation of the ZA22 alloy with the best refining effect were 18.56% and 119.04% higher than those of the unrefined ZA22 alloy, respectively. Correlated mechanisms were discussed.

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.

Micro-Mechanical Responses of Ultrafine-Grained Materials Processed through High-Pressure Torsion

The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk metal solids. These ultrafine grains in the bulk metals usually show superior mechanical and physical properties. Especially, the development of micro-mechanical behavior is observed after significant changes in microstructure through processing and it is of great importance for obtaining practical future applications of these ultrafine-grained metals. Accordingly, this presentation demonstrates the evolution of small-scale deformation behavior through nanoindentation experiments after HPT on various metallic alloys including a ZK60 magnesium alloy, a Zn-22% Al eutectoid alloy and a high entropy alloy. Special emphasis is placed on demonstrating the essential microstructural changes of these materials with increased straining by HPT and the evolution of the micro-mechanical responses in these materials by measuring the strain rate sensitivity.

Effect of Sn Addition on Superplastic Properties of Zn-22mass%Al Eutectoid Alloy

Superplastic behavior of a Zn 22 mass % Al eutectoid alloy (SPZ) with small addition of Sn (SPZSn) was investigated. Granular grain size of about 0.3 μm was obtained by water quench after annealing SPZ and SPZ05Sn (addition of 0.05 mass % Sn into the SPZ) at 653 K for 2 h. The fundamental microstructure of the SPZ05Sn was similar to that of the SPZ, but, microstructure observation by STEM showed additive Sn was present at the α’ grain boundary in the SPZ05Sn. Excellent high strain rate superplasticity was achieved in the SPZ05Sn, with elongation of more than 1300 % at 523 K at strain rate of 10-1 S-1. Furthermore, large elongation of about 1100 % was recorded at 473 K at strain rate of 10-1 S-1. The large elongation and high strain rate sensitivity value of the SPZ05Sn tend to shift to higher strain rate region as compared to those of the SPZ. It was considered that the small addition of Sn into the SPZ effectively suppressed the grain growth of α and β phase during the superplastic deformation, because granular grains less than 2 μm is maintained after superplastic deformation at 523 K.

Significance of Si impurities on exceptional room-temperature superplasticity in a high-purity Zn-22%Al alloy

Recent numerous studies demonstrated the advantages of producing bulk metals with submicrometer grain sizes which provide the opportunity to demonstrate improved mechanical characteristics including superplastic properties. Besides the effort, although the impurity may cause low ductility due to grain boundary segregation, there are limited studies to date on the influence of general impurities upon flow behavior of conventional superplastic materials. Accordingly, the present report demonstrates the significance of Si impurity on superplastic properties in an ultrafine-grained high-purity Zn-22%Al eutectoid alloy at room temperature. The alloy was prepared to include different levels of Si contents up to 1500ppm in the high-purity alloy and the consistent fine grain sizes of ~0.60µm were introduced through a series of solutionizing followed by cold rolling. Tensile testing showed an occurrence of excellent room-temperature superplasticity and the maximum elongation of 500% was recorded at an optimal superplastic strain rate of 1.0×10−3s−1 in the alloy with less Si. Increasing Si contents reduced ductility without changing the strain rate sensitivity, thereby implying the consistency in the deformation mechanism for superplastic flow but the difference in the fracture mode. The present analysis estimates a threshold stress and demonstrate the validity of applying the conventional superplastic relationship for depicting the room-temperature superplastic flow in the high-purity Zn-22%Al alloy. Moreover, the separate fracture modes are proposed for the alloy with increasing Si impurity contents by taking fractographs after superplastic elongations.

Incorporating dislocation variables into Mohamed's and Kawasaki–Langdon's deformation mechanism maps containing superplasticity mechanism regimes

Deformation mechanism map (DMM) is a useful tool to depict pictorially the deformation mechanisms and to elucidate the nature of material flow in creep and superplasticity. In this paper, the number of dislocations and the dislocation density inside the grain and at the grain boundary were incorporated into Mohamed's and Kawasaki–Langdon's DMMs in an attempt to give their distribution. It is found that in superplasticity or Region II regime, whether or not the dislocations exist inside the grain depends on the Burgers vector compensated grain size and the shear modulus compensated stress. In the DMMs, the number of dislocations at the grain boundary is about one fifth as much as that inside the grain. The dislocation forecasts in Coble creep and dislocation creep regimes are consistent with the thoughts of Coble and Weertman mechanisms. A new DMM containing the dislocation density was proposed with Zn–22wt% Al eutectoid alloy as the subject and its prediction is in good agreement with some of available experimental evidences.

Fabrication of miniature hollow helical gear by powder extrusion of gas-atomized Zn-22wt%Al powder

A powder extrusion pr°Cess based on the superplastic behavior of Zn-22wt%Al eutectoid alloy is proposed as a means to achieve fine microstructures in the manufacture of miniature hollow helical gears by reducing the forming load. In this paper, the Zn-22wt%Al powder compromising fine spherical particles is first produced by gas atomization, compacted, and then consolidated by solution heat treatment. This is found to produce billets with a very fine-grained microstructure, which were subsequently utilized in extruding helical gears at a temperature of 250°C and a strain rate of 2.36×10−3s−1. The specifications of these helical gears are: module, 0.2; number of teeth, 12; helix angle, 20°; pitch diameter, 2.55 mm; and internal diameter, 1.5 mm. An investigation of the mechanical properties of the extruded gears through Vickers hardness test and extrusion load measurement, and SEM observation, confirms the effectiveness of superplastic Zn-22wt%Al eutectoid alloy powders for the production of miniature hollow helical gears.

Achieving room-temperature superplasticity in an ultrafine-grained Zn-22% Al alloy

The mechanisms of creep and superplasticity occurring in conventional coarse-grained materials are now understood well. However, recent study advances in the production of bulk metals with submicrometer grain sizes which provide the opportunity to demonstrate improved mechanical properties. Thermo-mechanical processing is used in conventional industrial practice to achieve substantial grain refinement in bulk metals whereas the smallest grain sizes achieved in this way are of the order of a few micrometers and generally it is not possible to achieve grain sizes within the submicrometer or nanometer range. In this report, synthesis of an ultrafine-grained Zn-22 % Al eutectoid alloy was demonstrated through solutionizing followed by thermo-mechanical processing. Microstructural investigations revealed there are stable equiaxed ultrafine grain sizes of ~0.63 μm with homogeneous distributions of Zn and Al grains. Tensile testing demonstrated the occurrence of excellent room-temperature superplasticity with a maximum elongation of 400 % at a strain rate of 1.0×10-3 s-1 where the elongation is one of the highest room-temperature superplastic elongation recorded to date in Zn-22 % Al alloy. However, the strain rate sensitivity of superplastic flow was measured as ~0.24 which is lower than the theoretical value of ~0.5 for conventional superplasticity. The present study estimates a threshold stress as one of possible reasons for lowering the strain rate sensitivity of room-temperature superplastic flow in the ultrafine-grained Zn-22 % Al alloy.

High-cycle fatigue behavior of Zn–22% Al alloy processed by high-pressure torsion

A Zn–22% Al eutectoid alloy was processed by high-pressure torsion (HPT) and its high-cycle fatigue behavior was explored using novel small-scale bending fatigue experiments. Testing of the finest grain region in each HPT disk showed that the fatigue life decreases continuously with increasing numbers of torsional revolutions. The results are discussed in terms of the HPT-induced hardness change and the underlying fatigue failure mechanism.

The significance of self-annealing in two-phase alloys processed by high-pressure torsion

The Zn-22% Al eutectoid alloy and the Pb-62% Sn eutectic alloy were processed by high-pressure torsion (HPT) over a range of experimental conditions. Both alloys exhibit similar characteristics with significant grain refinement after processing by HPT but with a reduction in the hardness values by comparison with the initial unprocessed conditions. After storage at room temperature for a period of time, it is shown that the microhardness of both alloys gradually recovers to close to the initial unprocessed values. Electron backscatter diffraction (EBSD) measurements on the Pb-Sn alloy suggest that the self-recovery behaviour is correlated with the fraction of high-angle grain boundaries (HAGBs) after HPT processing. Thus, high fractions of HAGBs occur immediately after processing and this favours grain boundary migration and sliding which is important in the self-annealing and recovery process. Conversely, the relatively lower fractions of HAGBs occurring after annealing at room temperature are not so conducive to easy migration and sliding.

Evolution of hardness, microstructure, and strain rate sensitivity in a Zn-22% Al eutectoid alloy processed by high-pressure torsion

Severe plastic deformation (SPD) is an attractive processing method for refining microstructures of metallic materials to give ultrafine grain sizes within the submicrometer to even the nanometer levels. Experiments were conducted to discuss the evolution of hardness, microstructure and strain rate sensitivity, m, in a Zn-22% Al eutectoid alloy processed by high- pressure torsion (HPT). The data from microhardness and nanoindentation hardness measurements revealed that there is a significant weakening in the Zn-Al alloy during HPT despite extensive grain refinement. Excellent room-temperature (RT) plasticity was observed in the alloy after HPT from nanoindentation creep in terms of an increased value of m. The microstructural changes with increasing numbers of HPT turns show a strong correlation with the change in the m value. Moerover, the excellent RT plasticity in the alloy is discussed in terms of the enhanced level of grain boundary sliding and the evolution of microsturucture.

Evolution of hardness in ultrafine-grained metals processed by high-pressure torsion

The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk metals. Numerous reports are now available demonstrating the application of HPT to a range of pure metals and simple alloys. In practice, excellent grain refinement is achieved using this processing technique with the average grain size often reduced to the true nano-scale range. Contrary to the significant grain refinement achieved in metals during HPT, the models of the hardness evolution are very different depending upon the material properties. For a better understanding of the material characteristics after conventional HPT processing, this report demonstrates the hardness evolutions in simple metals including high-purity Al, commercial purity aluminum Al-1050, ZK60A magnesium alloy and Zn-22% Al eutectoid alloy after processing by HPT. Separate models of hardness evolution are described with increasing equivalent strain by HPT. Moreover, a new approach for the use of HPT is demonstrated by synthesizing an Al–Mg metal system by processing two separate commercial metals of Al-1050 and ZK60A through conventional HPT processing at room temperature.

Microstructure Development and Superplasticity in a Zn-22% Al Eutectoid Alloy Processed by Severe Plastic Deformation

Severe plastic deformation (SPD) is an attractive processing method for refining microstructures to have ultrafine grain sizes within the submicrometer or even the nanometer levels. In SPD, the most promising techniques are equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). A conventional superplastic Zn-22% Al eutectoid alloy was processed by ECAP and HPT. Experiments were conducted to demonstrate the evolution of hardness and microstructure and the enhancement of superplastic properties of the Zn-Al alloy after processing by the SPD techniques. In addition, flow mechanisms of the Zn-22% Al alloy are discussed by utilizing a deformation mechanism map.

Interpretation of hardness evolution in metals processed by high-pressure torsion

The processing of metals through the applica- tion of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk disks. Numerous reports are now available describing the appli- cation of HPT to a range of pure metals and simple alloys. Excellent grain refinement was achieved using this pro- cessing technique with the average grain size often reduced to the nanoscale range. By contrast, the development of microstructure and local hardness is different depending upon the material properties. In order to make HPT pro- cessing more practical, it is indispensable to investigate the nature of the sample characteristics immediately after conventional HPT processing. Accordingly, this report demonstrates the different models of hardness evolution using representative materials of AZ31 magnesium alloy, high-purity aluminum, and Zn-22 % Al eutectoid alloy processed by HPT. Separate models are described for the evolution of hardness with equivalent strain, and the cor- relation between these models is suggested by the homol- ogous temperature of HPT processing. A special emphasis is placed on examining the numerical expression of the level of strain hardening or softening of these metals with increasing equivalent strain.

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.

Evolution of plasticity, strain-rate sensitivity and the underlying deformation mechanism in Zn–22% Al during high-pressure torsion

This study explores the evolution of plasticity, strain-rate sensitivity and the underlying deformation mechanism of a Zn–22% Al eutectoid alloy during high-pressure torsion processing. The experiments reveal an optimal torsional straining condition for achieving the largest plasticity; beyond this condition the strain-rate sensitivity decreases and activation volume increases. The results are discussed in terms of changes in the microstructure and the underlying deformation mechanism.

The significance of grain boundary sliding in the superplastic Zn–22 % Al alloy processed by ECAP

A Zn–22 % Al eutectoid alloy was processed by equal-channel angular pressing (ECAP) to reduce the grain size to ~0.8 μm. Tensile testing at 473 K showed superplastic characteristics with a maximum elongation of ~2230 % at a strain rate of 1.0 × 10−2 s−1. The significance of grain boundary sliding (GBS) was evaluated by measuring sliding offsets at adjacent grains from the displacements of surface marker lines in samples pulled to elongations of 30 % at a series of different strain rates. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic ductility. There were relatively large offsets at the Zn–Zn and Zn–Al interfaces, but smaller offsets at the Al–Al interfaces. Analysis shows the results are affected by the presence of agglomerates of similar grains which are present after ECAP processing and specifically by the increased fraction of Al–Al boundaries. The experimental results are in excellent agreement with the predictions of a deformation mechanism map depicting the flow behavior in the Zn–22 % Al alloy, and the results confirm the importance of GBS as the dominant mechanism of flow in superplasticity after processing by ECAP.

Microstructural evolution in two-phase alloys processed by high-pressure torsion

The Zn-22 % Al eutectoid alloy and the Pb-62 % Sn eutectic alloy were processed by high-pressure torsion (HPT) over a range of experimental conditions. Both alloys exhibit similar characteristics with significant grain refinement after processing by HPT but with a reduction in the hardness values by comparison with the initial unprocessed conditions. It is shown that there are generally smaller grains at the edges of the disks by comparison with the disk centers. The hardness results in these two alloys are mutually consistent, but they are different from those generally reported for conventional single-phase materials. The significance of this difference is examined.

Fabrication of miniature helical gears by powder extrusion using gas atomized Zn–22%Al powder

Powder extrusion, which is based on the superplastic behavior of Zn–22%Al eutectoid alloy, was proposed to reduce the forming load and promises to provide fine microstructures in the manufacture of miniature helical gears. The specifications of the helical gears were as follows: module, 0.3; number of teeth, 12; and helix angle, 15°. Compacted powders were consolidated by sintering and solution heat treatment. The consolidated billets consisted of lamellar and fine-grained microstructures. Extrusion experiments were carried out under the following conditions: temperature, 250 °C; strain rates, 2.36×10−3 s−1 and 1.18×10−1 s−1. The mechanical properties of the extruded helical gears were investigated by measurement of the Vickers hardness and extrusion load, and by scanning electron microscopy.