Research On Deformation Behavior Of HCP Metals Under Biaxial Tension

In this thesis,AZ31 magnesium alloy in the forms of hot rolled plate and CP-Ti with a strong texture were choosen as the starting materials.The mechanical response,microstructure,deformation mechanism and texture evolution in the process of plastic deformation were investigated systematically by biaxial tension and crystal plasticity approach.First of all,the ND-TD biaxial tensile test was used to study the twinning behavior under biaxial stress and verify the applicability of Schmid law under biaxial loading.On this basis,the biaxial tensile tests along 45° and 45°(TD-ND)and EVPSC-TDT model were used to study the competitive behavior of slip and twinning and the influence of the competitive behavior on flow stress curve.Furthemore,the competitive behavior between slip was systematically investgated using biaxial tensile tests along RD and 45°(TD-ND).Then,the slip and twinning behaviors of CP-Ti under RD-TD biaxial tension were also studied.The results in this thesis show as follows:? The Schmid law for the {10(?)2} twinning is more efficient under biaxial tension than under uniaxial tension,and a higher efficiency is observed with a higher stress ratio along the TD.The higher efficiency of Schmid's law for biaxial tension than that for uniaxial tension mainly stems from the fact that the biaxial tension increases the difference among the Schmid factors(SFs)for the six twin variants,and a higher stress ratio along the TD further generates a larger SF difference.The selection of {10(?)2} twin variants is highly dependent on the stress ratio?ND:?TD.The variants(0(?)12)[01(?)1]and(01(?)2)[0(?)11]are more favorable under biaxial tension and their summed fraction rises with higher stress ratio along the TD.This is mainly because a higher stress ratio along the TD slightly varies the SFs for the variants(0(?)12)[01(?)1]and(01(?)2)[0(?)11]but reduces those for other variants to a large extent.Unlike uniaxial tension,the volume fraction of the {10(?)2} twins under biaxial tension will be impacted by detwinning,and the impact of detwinning is related to ?ND:?TD.With a low stress ratio along the TD,a high fraction of twins with(0002)poles around the TD will be activated,and the tensile stress along the TD will trigger the detwinning of those twins and reduce the twin volume fraction.In contrast,when the stress ratio along the TD is high,the twin variants with(0002)poles close to the TD are difficult to activate.Thus,detwinning hardly occurs and will not impact the twin volume fraction.? The dominant deformation mechanism under biaxial loading along 45° and 45°is basal slip and {10(?)2} twinning.With the decreases of biaxial stress ratio,the proportion of basal slip gradually increases and the activity of twinning gradually decreases,which could be explained successfully by the SF law.The difference of deformation mechanism has a significant impact on the flow stress curve.Under?45:?45=1:1 and 1:1.5,the strain along x-axis and y-axis is positive strain.For?45:?45=1:3,the x-axis strain is negative and the y-axis strain is positive.For ?45:?45=1:2,with the increase of the strain,the x-axis strain changes from negative to positive.Different from slip,twinning has polarity.As the stress ratio along x-axis gradually increases,the contribution of twinning to plastic deformation gradually increases,which induces the strain changing with the change of the stress ratio.? The dominant deformation mechanism under biaxial loading along 45° and RD is basal slip and prismatic slip.And the grain favoring basal slip increase with the decrease of the biaxial stress ratio,while the prismatic is the opposite.The competition obeys the Schmid law.The contribution of twinning to the deformation was so limited.And the activation of {10(?)2} twinning decreases with the decrease of the biaxial stress ratio.This is because the resistance effect of RD stress for twinning would be weakened with the increase of RD-stress.These differences in twinning and slip characteristics introduced certainly different flow stress,and this could be explained successfully from the viewpoints of activities of twinning and slips in the deformation.When the stress ratio along RD is low,the basal slip is the dominant mechanism,resulting in a positive strain along the 45° direction and a negative strain along RD.When the RD stress was large enough to start the prismatic slip,and the positive strain was induced in two directions simultaneously.? The twinning and slip behavior of CP-Ti under biaxial tension along TD and RD was totally different with that under the uniaxial tension.The activity of pyramidal slip increase and the prismatic was inhibited because of the decrease of the SF.The multiple slip was enhanced under biaxial tension namely,the sum of multiple slip is approximately 43%for equivalent biaxial tension,but approximately 15%and 25%for RD and TD uniaxial tension,which can be ascribed to the specific external strain.For twinning,there is significant difference between the uniaxial tension and biaxial tension.For example,under equivalent biaxial tension,although the tension twin has a higher SF,compression twin with a small or negative SF was also selected to accommodation the tension strain along RD in the blue,green or purple grain,in which the title angle is large between its basal pole and ND.This is because that the compression twin is better for the external strain demands.This suggests that accommodation of the externally applied strain is more important during biaxial tension than during uniaxial deformation.