Experimental And Theoretical Investigations On The PLC Effect In Pressed AZ Magnesium Alloy

Portevin-Le Chatelier（PLC） effect, a plastic instability in metals and alloysduring plastic deformation under at certain range of temperatures, affectedsignificantly the mechanical properties of materials. To investigate the PLC effectoccurred in AZ magnesium alloy can help not only to understand better themacro-process and micro-mechanism of plasticity, but also to utilize better to extendcapacity of magnesium alloy.A series of tensile tests of AZ pressed magnesium alloys, which have beenwidely used in the aerospace and industry, were systematically carried out underdifferent temperature and strain rate ranges, by an Instron1185testing machine. Somemicroscopic analysis methods were used to survey microstructure evolution duringdeformation and to investigate micro-mechanism when PLC effect occurred. Thevisco-plastic constitutive relation was modeled to describe PLC effect inmathematical simulation and parametric recognition based on universal model wasadvanced.The PLC effects in three AZ magnesium alloys, AZ31, AZ61and AZ91, weresystematically investigated under different temperature and strain rate ranges. Theresults show that no PLC effects are found in AZ31magnesium alloy, but someserrations characteristics appear in AZ61magnesium alloy under testing conditions.However, PLC effects occur in AZ91magnesium alloy treated by solid solutiontreatment, in which are subjected to plastic deformation at a set range of strain rates(ε=1.11×10^-4s^-11.67×10^-3s^-1)and a certain range of temperatures（T=248⁴23K）.Here, the critical plastic strain εc, which is the strain for the onset of serrations,decreases with the increasing of deformation temperatures, while it increases with theincreasing of strain rate. Especially, the related serrated types are defined as A andA+B type, which is correlated to different strain rates. Moreover, the five typicalcharacteristics of DSA are found, including the serrated flow（PLC effect）, a yielding stress platform independent of temperature, negative strain-rate sensitivity and theabnormal variation of work-hardening rate, and “Blue Brittleness”, and so on.The interaction betweem the diffusion solute atoms and the mobile dislocationshas been widely accepted as micro-mechanism of PLC effect. The diffusing activationenergy of solute atoms during serrations occurring was calculated according to therelationships of the critical plastic strain εc, strain rate ε and temperature T byMcCormick theory. The result shows that the diffusing activation energy of soluteatoms during the PLC effect occurring in AZ91magnesium alloy is140.7634kJ/mol,which is highly correspondence with the diffusing activation energy of Al soluteatoms in Mg matrix. Therefore, the micro-mechanism of PLC effect in AZ91magnesium alloy is believed that the Al atoms in solid solution gather arounddislocations to form Cottrell solute atmospheres by vacant diffusion and then pin themoving dislocations, so it needs to increase applied forces to make pinningdislocation moving forward and finally results in PLC effect when repeated thisprocess.Furthermore, the effects of the content of precipitates on the PLC effect of theAZ91magnesium alloy were systematically investigated, by means of differentheat-treatments such as annealing, solid solution and aging to get different precipitatesand microstructure of the AZ91magnesium alloy. According to the quantitative andqualitative analysis, the result reveals that the mass or concentration of Al soluteatoms dissolved into magnesium matrix are the major factor for the DSA occurring inthe AZ91magnesium alloy. It has been proved that there is a great effect on serratedyielding characteristic after different heat treatments such as temperature and timesabout annealing, solid solution treatment and aging, etc. Especially, the effects ofsecond-phase precipitation（β-Mg17Al12）on PLC effect of theAZ91magnesium alloyrepresent that the tensile curves do not appear serrated yielding when the content ofprecipitates is more than1.0wt.%. Also, it is not only to make the strain rate ranges ofPLC narrowing, but also to reduce the temperature ranges when the second-phaseprecipitation occurs. Moreover, compared with its solid solution state, the serratedyielding characteristic is not obvious and the stress drop is smaller in the tensilecurves. According to the present study and other viewpoint in the literatures, a newmicro-mechanism of controlling PLC effect has been developed, which demonstratesthe influences of second-phase precipitation on PLC through the interaction betweenthe precipitates and the mobile dislocations. What is worth mentioning, it is indispensable to theoretically enrich the mechanism of serrated yielding while takinginto account the interaction between the precipitates and the mobile dislocations. Also,different serrated yielding characteristics appear to vary with the grain size.Investigations on PLC effect of AZ91magnesium alloys with different grain sizeswhich have been obtained by heat-treatments or by equal channel angularpressing（ECAP）, show that the critical plastic strain εcdecreases with the reducing oftheir sizes while the stress drop increases in the tensile curves. It seems very obviouschange when the grain size is in range from micro-scale to nano-scale.Finally, a new visco-plastic constitutive relation and model for the PLC effect ofAZ magnesium alloy, quantitatively considering the interaction between the soluteatoms and the mobile dislocations, is constructed and developed by modifyingYaguchi model to express accurately the micro-mechanism of PLC effect in numericalmethods. Based on the experimental investigations in the present, the parameteridentification of the constitutive model is further developed in multi-object strategywith the multi-island genetic algorithm（MIGA） and the series quadratic programming（SQP）. Also, the results of numerical simulation are well matched with the ones ofexperiments.