| Superplastic Zn-Al alloys have attracted wide attention as potential new anti-seismic materials to replace the current steel/lead-base anti-seismic materials,due to their low flow stress,little work hardening and high plasticity.Severe plastic deformation process such as equal-channel angular pressing(ECAP),conventional rolling and extrusion have been introduced to inhibit the formation of lamellar microstructure and form equiaxed grains of Zn-Al binary alloys.Zn-Al alloys with the equiaxed grains exhibit superplasticity at room temperature with a certain strain rate(below 100 s-1).However,little work has been focused on the mechanical properties of Zn-Al alloys with high strain rates(over 101 s-1)as anti-seismic materials.In addition,on one hand,although the influences of the shape and size of phase on the room temperature superplasticity of deformed Zn-Al binary alloys have been demonstrated,there are few studies conducted on the effects of the type and volume fraction of grain boundaries on their superplasticity.On the other hand,the applications of Zn alloys in industry are mainly concentrated in the Zn-Al-Cu-Mg and Zn-Cu-Ti alloy systems,owing to their good wear resistance and corrosion resistance.It is meaningful to expand their applications by developing new Zn-based alloys.The addition of Mn element into Zn alloys can not only realize high temperature superplasticity,but also exhibit excellent comprehensive mechanical properties.Nevertheless,the effect of Mn addition on the room temperature mechanical properties and microstructures of Zn-Mn binary alloys has not been reported.In the present work,the microstructure and room temperature mechanical properties of Zn-Al and Zn-Mn binary alloys were studied by using scanning electron microscope(SEM),X-ray diffraction(XRD),electron backscattering diffraction(EBSD),transimission electron microscopy(TEM),room temperature tensile and compression,and especially the deformation behaviors of Zn-Al binary alloys under high strain rates.The present work conducted tried to understand the relationship among composition-preparation technology-microstructure-mechanical properties of Zn-Al(Mn)alloys.And,the microstructures close to the fracture site in the as-extruded Zn-Al binary alloys after room temperature tensile tests with strain different rates and the microstructures of Zn-Mn alloys at different uniaxial tensile/compressive strains were investigated.The main results are as follows:(1)When the extrusion ratio is fixed,with the decreasing of extrusion temperature from 350? to 200 ? the tensile ultimate strength of Zn-15A1 alloy decreases gradually 250 MPa to 106 MPa,while the elongation of Zn-15A1 alloy increases gradually from 71%to 168%.Although the lamellar microstructure of Zn-15AI alloy is not totally suppressed with decreasing the extrusion temperatures,the grain sizes of ? phase and 1 phase decrease from 0.44?m and 0.38 ?m down to 0.39 ?m and 0.33 ?m,respectively.Meanwhile,the volume fraction of ?/? grain boundaries and ?/? phase boundaries increases from 89%to 98%,which are the easy slip boundaries and result in the room temperature superplasticity of Zn-15Al alloy extruded at low temperatures.For the Zn-15A1 alloy extruded at 150 ? the extrusion ratio slightly influences the final elongation to failure(160-180%).All these results indicate that the superplasticity of Zn-Al alloy is mainly dependent on the extrusion temperature other than the extrusion ratio.(2)For the Zn-15A1 alloy extruded either above or below the eutectoid temperature followed by heat-treating at 320 ? for 1 h and then cooling at different cooling rates,the Zn-15 A1 alloy quenched in water is composed of coarse ? phase and granular a phase.In contrast,the Zn-15A1 alloy cooled in air contains both lamellar eutectoid and ? phase.Quenching in water results in the elongation of Zn-15A1 alloy extruded at either 200 ?or 350 ? decreasing from 170%to 78%or from 70%to 58%,respectively.The tensile ultimate strength of Zn-15A1 alloy extruded at 200 ? increases from 110 MPa to 132 MPa after quenching in water,whereas the tensile ultimate strength of Zn-15A1 alloy extruded at 350 ? decreases from 250 MPa to 160 MPa.However,after cooling in air,the elongation of Zn-15A1 alloy extruded at 200 ? and 350 ? decreases from 170%to 38%and from 70%to 28%,respectively.The tensile ultimate strength of Zn-15A1 alloy extruded at 200 ? and 350 ? increases from 110 MPa to 220 MPa and from 250 MPa to 280 MPa,respectively.It is suggested that the cooling rate of Zn-Al alloy be one of the important parameters to control the microstructure and mechanical properties of the alloys during high temperature plastic forming and heat treatments.(3)All the Zn-xAl(x=6,15,22)alloys extruded at either 200 ? or 350 ?demonstrate large elongation more than 150%under a tension rate of 10-3 s-1 for the alloy extruded at 200?,which indicates good room temperature superplasticity.The grain size,fraction of hard sliding a/a boundaries and lamella microstructure are the main influence factors on its superplasticity.(4)The elongations of the Zn-xAl(x=6,15,22)alloys extruded at 200 ?and 350 ?are about 30?40%under high strain rates.In addition,the strain rate slightly influences the elongation for these alloys,showing that the plasticity of extruded Zn-Al alloys is not sensitive to the microstructures and strain rates in the case of high strain rates.(5)Under the low strain rate condition,the grains of ?l and ? phase in the Zn-xAl(x=6,22)alloys extruded at either 200 ? or 350 ? increase along the extrusion direction.Meanwhile,the grains of ? phase usually rotate to accommodate the deformation process.At the high strain rate,the recrystallization occurrs in both the ?1 and ? phase,which results finally in the equiaxed grains.The crystallographic orientation of recrystallized ? phase is mainly dependent on the strain rate.(6)With increasing the Mn concentration,the mean grain size of ? phase in Zn-xMn(x=0.2,0.4,0.6)alloys extruded at 200 ? decreases from 4.21 ?m to 2.11?m.Meanwhile,the volume fraction of twining decreases from 14.2 vol.%to 2.3 vol.%.What's more,the tensile ultimate strength decreases from 220 MPa to 180 MPa,while the elongation increases from 44%to 71%.It indicates that the effect of the addition of Mn into Zn alloys is different from that of either Mg or A1 addition.(7)The grain size of ? phase of Zn-0.6Mn alloys during tensile/compression process is refined.At the same time,the orientation of ? phase grain in the deformation process changes with different strains.This is mainly due to the dynamic recystallization in the deformation process. |
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