Zn-Al Eutectoid Alloy Research Articles

Milling characteristics of extruded eutectoid Zn–Al alloy

Mechanical milling-induced phase transformations, microstructural changes and preferred crystal orientation of the extruded eutectoid Zn–Al-based alloy are reported. Decomposition of a metastable η′ E phase and a four-phase transformation: α+ ε→ T′+ η, were greatly accelerated by mechanical milling. The mechanism of the milling-induced microstructural change from a lamellar structure to a fine grain structure was found to be different from that which occurred under tensile and creep deformations in the extruded eutectoid Zn–Al-based alloy. A preferred crystal orientation was observed as well, in the alloy during milling, which was supported by a rolling-induced preferred orientation in the same alloy.

Microstructures and dimensional stability of extruded eutectoid ZnAl alloy

Two microstructures were observed in extruded eutectoid ZnAl alloy. A new metastable phase η E ′ was detected in both of the microstructures, possessing an h.c.p. crystal microstructure which decomposed during isothermal holding. The decomposition of the newly-observed η E ′ phase and the four-phase transformation, α+ε→ T′+η, were found to be responsible for dimensional changes occurring during the isothermal holding.

Tensile Creep Deformation and Microstructural Change in Cast Zn–Al Alloy

Phase transformation and microstructural change of an as-cast eutectoid Zn–Al alloy specimen Zn76Al22Cu2 (mass%) were investigated during tensile creep testing by using scanning electron microscopy (SEM) and X-ray diffraction techniques. Creep induced decomposition of a metastable η′T phase and a four-phase transformation, α+ε→T′+η, occurred during the creep testing. Also the detailed microstructural change from decomposed dendritic structure into spheroidized structure was observed in the neck zone and the rupture part of the specimen. The rupture was initiated within the spheroidized structure and developed along the grain boundaries of the fine lamellar structure and the decomposed η′T phase.

Creep-induced phase transformations in furnace cooled Zn-Al alloy

Phase transformation and microstructural change of a furnace-cooled eutectoid Zn-Al alloy specimen (76 wt% Zn-22 wt% Al-2 wt% Cu) were investigated during creep testing by using scanning electron microscopy and X-ray diffraction techniques. Creep-induced decomposition of a metastable η′T phase and a four-phase transformation, α+e→T′+η, occurred during the creep testing. Also microstructural change was observed from a lammellar structure partially into a spheroidized structure in the rupture part of the furnace-cooled alloy. The mechanism of the creep rupture of the furnace-cooled eutectoid Zn-Al alloy was also discussed.

Microstructural changes in welded Zn–Al alloy

Extruded eutectoid Zn–Al alloy was welded by a melt of the same eutectoid alloy. Two different microstructures were observed in the joint part and in the bulk of the welded alloy. Typical dendritic structure of as-cast Zn–Al alloy was observed in the joint part of the welded alloy. The bulk of the welded Zn–Al alloy appeared as fine grain structure. Two different metastable phases, η′T decomposed from η′s of the chilled as-cast state and η′E of the extruded state, were found to be unstable during the early stages of aging. A four-phase transformation occurred after the decompositions of these two metastable phases of η′T. Microstructures of both the joint part and bulk of the welded alloy were investigated simultaneously during the aging processes.

Microstructures of thermomechanically treated eutectoid Zn–Al alloy (II)

The evolution of microstructure in a cast, extruded, and aged eutectoid Zn-Al-Cu alloy is described based on x-ray diffraction an micrographic observations. Whereas the three phases α′, β′, and η′ were found in the as-cast state, the three phases α, T′, and η′E were observed in this alloy after extrusion at 250 °C. The Al-rich fcc α phase appeared as isolated particles with clear boundaries. The Zn-rich η′E phase decomposed upon aging into α, T′, and η phases. It was found possible to control the phase makeup of the alloy by controlling extrusion temperature. At extrusion temperatures greater than 268 °C, α, E, and η′E were detected, whereas temperatures below this, the phases α, T′, and η′E were found. After prolonged aging, the phase makeup of the alloy was found to be the same regardless of prior treatment routes. Early shrinkage of the extruded alloy occurred after aging at 91 and 150 °C and was related to precipitation of aluminum from the η′E phase during its decomposition to α, T′, and η products.

Influence of the Structure and Temperature on the Extrusion Pressure of an Zn-20Al-2Cu Alloy

Abstract The feasibility of developing a method to successfully extrude the eutectoid Zn-Al alloy modified with 2 mass% Cu was studied. The best conditions found for extrusion are determined by the dendrite size of the extrusion ingot, the extrusion rate, and the extrusion temperature. Alloys extruded below the eutectoid temperature show high strength (380 MPa) and good ductility (25-35%). The microstructure of this alloy is composed of fine grains of aluminum solid solution a and zinc solid solution η and e phases. When the alloy is extruded between 280°C and 320°C, the resultant alloy is super plastic with microstructure formed by fine grains (0.5 μm to 2 μm). Higher extrusion temperatures, cause embrittlement of the material, likely caused by the presence of a low melting point Zn rich eutectic phase at the grain boundaries formed during the solidification.

Complex microstructural changes in as-cast eutectoid Zn-Al alloy

Phase transformations and microstructural changes of an as-cast eutectoid Zn-Al alloy (ZnAl22Cu2) were investigated during isothermal holding. The typical dendritic structures consisted of α′s phase as a core with the edge of decomposed β′s phase and decomposed η′s in the interdendritic regions. A series of complex phase transformations was observed. Both decompositions of β′s and η′s were determined at an early stage of ageing and a four-phase transformation, αf+ɛ→T′+η, was observed at the boundaries of αf phase and the ɛ phase, instead of clearly observed at the boundaries of ɛ phase, in a solution-treated Zn-Al alloy during prolonged ageing.

Effect of Copper Modification on Impact Strength of Zinc-Aluminum Alloys

The effects of the microstructure and copper content on the notched impact strength of the eutectoid Zn-Al alloy modibed with 0.58 mass%, 2.34 mass% and 4.95 mass%Cu were studied as a function of temperature from room temperature to 350 o C. The fracture mode changes from transgranular to completely intergranular above 350 o C. It is suggested that the change in fracture behavior to be associated with the formation at the grain boundaries, of a low melting point compound with the eutectic composition

Microstructural Changes of Welded Zn-AI Alloy

AbstractExtruded eutectoid Zn-Al alloy was welded by a melt of the same eutectoid alloy. Two different microstructures were observed in the joint part and the bulk of the welded alloy. Typical dendritic structure of as cast Zn-Al alloy was observed in the joint part of the welded alloy. The bulk ofthe welded Zn-Al alloy appeared as fine grain structure. Two different metastable phases η'T decomposed from η's of chilled as cast state and η'E of extruded state were found to be unstable during early stage of ageing. A four phase transformation occurred after the decompositions of these two metastable phases of η'T. Microstructures of both joint part and bulk of the welded alloy were investigated parallely during ageing processes.

Microstructural evolution and threshold stress for superplastic flow in the Zn-Al eutectoid

The microstructural evolution and the stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy were investigated by prestraining specimens at two strain rates corresponding to Regions I and II. Even though the scale of microstructure was similar, the stress-strain rate curves of differently prestrained specimens were distinctly different in the lower strain-rate regime. While Region I of low rate sensitivity was more prominent when prestrained at a lower strain rate of Region I, it was less distinct because of prestrain in Region II. The threshold stress for superplastic flow, as assessed by an extrapolation procedure, varied with the nature of prestrain. The interphase boundaries were more rounded (higher mean curvature) on prestraining on Region II, compared to Region I. The correlation between the changes in the mean curvature of phase boundaries and the threshold stress arising from the nature of prestrain was consistent with the boundary-migration controlled sliding mechanism to interpret the threshold stress for superplastic flow.

Threshold Stress for Superplastic Flow in the Zn–Al Eutectoid Alloy

The high temperature deformation of a commercial Zn-Al eutectoid alloy was investigated over the grain size range of 0.8 to 2.6 μm and the temperature range of 423 to 473 K, with a view to assessing the interpretation of region I in terms of a threshold stress for superplastic flow. The threshold stress evaluated by an extrapolation procedure is observed to decrease with increasing temperature and grain size. The activation energy and grain size exponent for the threshold process are 34 kJ/mol and 0.85, respectively. Boundary sliding controlled by boundary migration is the likely origin of the threshold stress

超塑性Ｚｎ‐Ａｌ合金表面へのセラミック粒子の圧入過程の解析

In order to obtain a high frictional and good wear resistance surface, the superplastically embedding process of SiC powder into Zn-Al eutectoid alloy plates was developed. The embedding mechanism was investigated using the experimental simulation. The upper surface of a Zn-21.5%Al-0.5%Cu-0.01%Mg alloy (SPZ) specimen of the size 13×13×3mm was covered with SiC powder classified into +150, +250 and +355μm in size, with the density of 0.08-0.15mg•mm-2, and then forged superplastically at the punch speed of 1.7×10-3 to 1.7×10-1mm•s-1 at the temperature range of 473 to 523K. The embedding mechanism of SiC powder into SPZ specimen was analyzed using a glass-ceramics sphere of 6.0mm in diameter. The projection height and embedding efficiency depended on the applied stress ratio of SiC powder to the yield stress of SPZ.

The stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy

The stress-strain rate behaviour of a superplastic Zn-22% Al alloys was investigated by the differential strain rate and constant load cycling tests on an Instron machine. A region with a rate sensitivity approaching unity was observed at low strain rates. On increasing the strain rate, a transition to the superplastic region occurred through an intermediate region of lower m (∼0.35). These observations are interpreted in terms of a transition from diffusional creep to superplastic flow with a threshold stress for superplastic deformation.

Constitutive equation of plastic deformation of a zn-al alloy under tension-tension strain controlled cyclic loading

A constitutive equation of plastic deformation under tension-tension, strain controlled cyclic loading condition was derived from the transition state theory of rate processes. It was considered that the rate of plastic flow during the (tension-tension) cyclic deformation is controlled by a system of two consecutive energy barriers and that the material structural characteristics remain constant during cyclic deformation. The study revealed that within the stress, time, and temperature range, where the backward activations over the energy barriers are negligibly small, tension-tension, strain controlled cyclic deformation is essentially a stress relaxation process. The theory described well the cyclic deformation behavior of a near eutectoid ZnAl alloy. The constitutive parameters determined from the analysis of stress relaxation and tension-tension, strain controlled cyclic loading experimental results were identical. Consequently, it was recommended that stress relaxation can be used to determine the material structural characteristics which can then be used to predict the tension-tension, strain controlled cyclic deformation behavior of the alloy, using the constitutive equation derived in this report.

Internal friction and thermal losses in Zn-Al eutectoid alloy

A Kê pendulum was used to determine the internal friction, Q -1, of Zn-Al eutectoid alloy (22 wt% Al) in relation to its microstructure. The internal friction for the as-quenched and slowly cooled samples from above the eutectoid temperature, showed an exponential rise with temperature, characteristic to existing phases and microstructure of the samples. The internal-friction value at any particular temperature decreased as the quenched, metastable state α'-phase transformed to (α + β) phases in thermodynamic equilibrium below the eutectoid temperature. The phase transformation was made by giving the prescribed heat-pulses to the as quenched samples. The energy activating the process responsible for the rise of Q -1 with temperature decreased from 0.544 × 10 -19 J for the α'-phase to 0.448 × 10 -19 J for the (α + β) phases at temperatures below the eutectoid temperature. This decrease was attributed to the role of the different phases in the relaxation mechanism responsible for this effect. The internal friction was used to assess the degree of transformation from the high-temperature phase to the low-temperature phase. Results were discussed in terms of Ham's theory of diffusion-limited precipitation of supersaturated solid solutions.

Production of whisker crystal by straining ZnAl eutectoid alloy at elevated temperatures

Production of whisker crystal by straining ZnAl eutectoid alloy at elevated temperatures

Microstructural observations on the frequency effect in the fatigue behaviour of superplastic Zn-Al eutectoid

Fatigue tests carried out over the frequency range 8–100 Hz and temperature range 293–373 K on a commercial, superplastic Zn-Al eutectoid alloy showed a marked frequency effect even at 293 K, which is almost certainly the result of residual superplasticity, for when the superplastic microstructure is destroyed, the effect disappeared. Following this previously reported mechanical study, microstructural features of the fatigue process have been investigated in an attempt to identify the metallurgical factors which control the fatigue behaviour. Microscopic studies revealed two significant features: β-phase segregation and large (∼50 μm) inclusions from which fatigue cracks were multi-initiated after a few hundred cycles. Stage I occupies ∼95% of the fatigue life and failure occurs soon after the change to Stage II, for now the crack travels rapidly along the interphase boundaries. It is postulated that the frequency effect arises from the frequency dependence of Stage II, for the transition to Stage II is a function of flow stress, which is in turn a function of strain rate, which is, in turn, related to frequency.

A Transmission Electron Microscopy Study of Decomposition in the Zn-Al Eutectoid Alloy

There has been considerable interest in the Zn-Al eutectoid alloy in recent years mainly because it exhibits superplastic deformation under certain conditions. A feature of particular importance is that the microstructural requirements for superplasticity can be achieved.in this alloy by heat treatment alone. However in spite of this, the mechanisms of phase transformation have not been widely studied; some aspects of these are described in this paper. The eutectoid reaction takes place at a composition of 78 wt% Zn and a temperature of 276°C by the decomposition of the high temperature γ phase (F.C.C.) to the Zn-rich β phase (C.P.H.) and the Al-rich α phase (F.C.C.) terminal solid solutions. Experiments have mainly involved quenching directly from the γ - phase field and isothermally transforming at various temperatures below 276°C.

Crystallographic slip during superplastic deformation of the ZnAl eutectoid alloy

Crystallographic slip during superplastic deformation of the ZnAl eutectoid alloy