| The high-temperature flow behavior,processing map and and microstructure evolution of heat-resistant Al-0.2Sc-0.04Zr aluminium alloy were investigated by isothermal compression tests at the temperatures from 440? to 600? and strain rates from 10-3s-1 to 5s-1 on Gleeble-1500D machine.The effects of extrusion ratio parameters from ?=5:1 to 100:1 on the microstructure and mechanical and electrical properties of Al-0.2Sc-0.04Zr alloy were studied by hot extrusion tests on the THP61.630 Type Oil Hydraulic Press.The main research is as follows:the effects of hot deformation parameters(deformation temperature and strain rate)on flow stress(?)and activation energy(Q)were analyzed.An Arrhenius-type constitutive equation suitable for Al-0.2Sc-0.04Zr alloy was established,and the correlation coefficient(R)and average absolute relative error(AARE)were also evaluated.Based on the dynamic material model(DMM)theory,the Kumar-Prasad(K-P)and Murty-Rao(M-R)stability/instability criteria were employed to establish their instability map and processing map respectively,which was verified by microstructure observation.Microstructure evolution and heat resistance of Al-0.2Sc-0.04Zr alloy were analyzed by ways of Electron Backscatter Diffraction(EBSD),Scanning Electron Microscopy(SEM)and Energy Dispersive Spectroscopy(EDS).According to the M-R processing map and microstructure evolution,a deformation mechanism map was established to discriminate the optimal processing region and unstable region.The main results show that,(1)During hot compression deformation,the flow stress-strain curve of Al-0.2Sc-0.04Zr alloy is the dynamic recrystallization type and the dynamic recrystallization(DRX)is mainly softening mechanism.The peak stress decreases with the increase of the deformation temperature and the decrease of the strain rate.(2)The average value of the activation energy(Q)of Al-0.2Sc-0.04Zr alloy was calculated as 642.575kJ/mol,which is greatly higher than that of commercial pure aluminum(192.92kJ/mol).The Q value is actually a variable,it increases with the increase of deformation temperature and strain rate.The obtained Arrhenius-style equation of Al-0.2Sc-0.04Zr alloy can be expressed as,? = 8.11 × 1038[sinh(0.0463?)]13,8105 exp[-642574.5/(RT)](3)For experimental and calculated peak stress values,their correlation coefficient(R)and average absolute relative error(AARE)were calculated as 0.9708 and 7.428%respectively,which shows that the relationship between the flow stress and the temperature and the strain rate can be accurately described by the Arrhenius-style constitutive model during the hot compression deformation of Al-0.2Sc-0.04Zr alloy.(4)According to microstructure verification and the comparison between K-P and M-R criterion theories,the M-R criterion is more suitable for the establishments of processing map for Al-0.2Sc-0.04Zr alloy.A deformation mechanism map was developed on the basis of EBSD microstructure changes and the M-R processing map;the M-R processing map indicates that the region ranges of T=520-560? and ? ?10-3s-1 and ?=0.1-5s-1 are the optimal processing regions for obtaining a relatively completely reconstituted structure useful for hot deformation,and that the region ranges of ? =0.015-0.135s-1 and T=470-520? and T=550-600? are the main unstable regions for the deformation localization.(5)During compression deformation,the deformation temperature had a significant impact on the extent of DRX,which increases significantly with increasing temperature.520? is the critical temperature for the transition of the deformation mechanism.The structure after lower temperature(T?520?)deformation exhibits mainly the dynamic recover A dominantly or completely dynamic recrystallized structure is obtained under higher temperature(T>520?),and obvious growth of the grains at 600?.(6)The excellent heat resistance and high dynamic recrystallization temperature of Al-0.2Sc-0.04Zr are related to the dispersed phase particles Al3(Sc,Zr)with the dimension of 1-100 nm within the matrix.During plastic deformation,these particles effectively block sub-grain boundary movement,pin dislocations,resist recrystallization,increase the resistance of activated atoms and activated dislocations across the necessary energy barriers and increase additional power dissipation,leading to the great increase of the thermal stability and activation energy(Q value)and the general decrease of the efficiency parameter(? value)in the processing map.(7)After hot extrusion,a fiber structure and a preferred orientation of the<111>Direction paralleled to the extrusion direction are obtained.In the extrusion ratio ranges from 5:1 to 20:1(?=5?20),the fiber structure is continuously refined and the extent of DRX increases with the increase of the extrusion ratio parameter.When ??20,the effect of refining on structures is weakened.When ?=100,the plastic deformation is very uneven,and the structure on the edge region of as-extruded specimen is drastically refined and the center structure is greatly coarsened.(8)Although hot extrusion can not significantly improve the strength of Al-0.2Sc-0.04Zr alloy,it significantly improves the plasticity and high-temperature electrical conductivity of alloy.When ?=20,the electrical conductivity of alloy at room temperature reaches 62.94%IACS,and the average resistance temperature coefficient(??3.46×10-3?-1,T=25-300?)is the lowest,far less than the coefficient(??z4.06×10-3?-1,T=25-300?)of industrial pure aluminum,exhibiting the optimal high-temperature electrical conductivity(T=150-300?).The tensile fracture mechanism of as-extruded Al-0.2Sc-0.04Zr alloy at room temperature is transgranular ductile fracture.(9)A comprehensive consideration for microstructure,mechanical properties and high-temperature electrical properties of as-extruded Al-0.2Sc-0.04Zr alloy,?=20:1 is the best-optimized extrusion ratio parameter. |
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