Experimental Study On The Relationship Between Twin Deformation And Polycrystalline Microstructure Of Extruded AZ31 Magnesium Alloy

In order to reveal the correlation between the grain size and microstructures such as the number of twin nucleation,twin thickness and twin-twin junctions,the relevant deformation tests were carried out,The compression tests were conducted on the extruded magnesium alloy AZ31 with compression strain variables of 1%,3%,5%,7% and 10%,respectively.The strain amplitude was 0.3%,0.8%,1.2%,1.5% and 2.0%.Loading test and microstructure statistical analysis of extruded AZ31 magnesium alloy with pure torsional cycle variables of 0.5%,1.0% and multi-angle combined torsional cycle with amplitude of 0.5%(15°,45°,75°)and 1.0%(15°,45°,75°).The results of mechanical tests show that the compression stress-strain curve has an obvious s-shaped character due to the twin deformation,while the tension and compression cyclic hysteresis curve has an asymmetric flow and plasticity and a strong bauschinger effect,as well as a strictly symmetric mechanical response curve in the torsional deformation.The twin deformation was studied from the micro point of view and the evolution law of magnesium alloy in the micro dimension was analyzed by experiments.This study can provide experimental basis for more accurate description of twin deformation mechanism,and these results can be used to verify the plasticity model of magnesium alloy and develop a more widely applicable plastic constitutive relationship of magnesium alloy,which has important guiding significance for the manufacture and application of magnesium alloy products.The main contents and conclusions of this paper are as follows:(1)AZ31 magnesium alloy materials suitable for testing were selected and processed into mechanical test samples.First,the sample materials were heat treated,followed by mechanical test,and then the samples were wire-cut for metallographic test.Metallographic test results showed that there were statistical differences in microstructure parameters of the materials.(2)The results of the statistical analysis of the plastic microstructure in the compression test of magnesium alloys show that the distribution of microstructure such as grain size and twin thickness basically conforms to the gaussian probability distribution characteristics.The effect of grain size on twin growth was small.The number of twin cores increases with the increase of strain,and under different strain variables,the number of twin cores also increases with the increase of grain size,but there is no absolute correspondence.However,the grain size per unit area showed a decreasing relationship with the increase of the grain size.Twin-twin junctions is relatively easy to occur in large grains,and the appearance of twin-twin junctions hinders the growth of twins to some extent.(3)The results of uniaxial cyclic test of magnesium alloy show that the cyclic hysteresis curves show significant asymmetry under different strain amplitudes,and the accumulation of residual twins plays an important role in cyclic deformation.Under different cyclic strain amplitudes,the statistical distribution of grain size of magnesium alloy under all strain amplitudes is approximately in accordance with Weibull probability function,while the statistical distribution of twin thickness is closer to Gaussian probability function.The number of twin nucleation increases with the increase of grain size,but there is no obvious function relationship between twin thickness and grain size.The twinning fraction increases with the increase of grain size under different strain amplitudes,and it is mainly realized by the increase of twinning nucleation number.(4)The results of statistical analysis of microstructure in torsion test show that the grain area and twin thickness of magnesium alloy under various loading conditions are in accordance with the Gaussian probability function distribution,and the grain area does not change significantly due to the change of loading conditions;under different strain amplitudes,the number of twin nuclei in the grain shows obvious size effect,that is,the number of twin nuclei will change with the grain size In the pure torsion cycle,the higher the strain amplitude,the greater the thickness of twin,but the less the difference in the number of nucleation.In the tension torsion cycle,when the angle of tension and torsion is larger,that is to say,the closer to the meridional direction,the higher the strain amplitude,the more the number of twin nucleation and the thickness of twin increase,while in the low strain amplitude,the difference is not significant Remarkable.(5)Considering the twin distribution under various strain conditions,the volume fraction of twin(no matter the number of twin nuclei or the thickness of twin)is much larger than the torsional deformation under radial load when the loading direction is axial compression or tension compression cycle.