Effect Of Strain Path On The Morphology Evolution Of Twin-structured And Properties Of Magnesium Alloys

Twin dominates the plastic deformation at extruded magnesium alloys with a strong basal texture under compression deformation. Due to the pre-compression causes the crystal orientation turn to nearly 86°, the detwinning dominates plastic deformation under subsequent reverse tensile deformation. The change of strain path can lead to the change of morphology twin structure, twin incidence and properties of alloys. However, how the strain path affects the morphology of twin structure and properties is still worth further research. It is good for the further understanding of the mechanism of twinning plastic deformation to determine the influence of the law.In this work, the compression and compression–tensile deformation tests were conducted under extension direction with the material of the extruded and annealed Mg-0.087%Mn and Mg-0.037%Zr alloys. The twin characteristics(include the aspect ratio and incidence of twin) were be count after deformation tests. And the effect of stain path on the twin–structured morphology and yield stress were analyzed systematically. Base on the compression–tensile deformation, in order to study the effect of annealing treatment on twin-structured and mechanical response of the twined magnesium alloys, the selected samples were been compressed after annealing treatment at 200?×0.5h compare to the without annealing treatment samples before compression.After compression, compression-tensile test of the extruded Mg-0.087%Mn and Mg-0.037%Zr alloys. The {10(1|-)2}twin dominates the plastic deformation, with the compression plastic strain increased from 1% to 6%, the fraction of twin aspect ratio which is relatively small(less than 0.1) gradually decreased, and the twin incidence increased gradually. Two kinds of alloys showed significantly the detwinning phenomenon under reverse tensile deformation after compressing to strain value with 2%, 4%, 6%. The larger the tension strain, the larger the amount of the detwinning. At the different strain paths and same strain value, when the accumulate plastic strain value is 1%, 2%, 4% and 0% respectively, with the increase of the plastic deformation, the finer the twin lamellae and the higher the twin incidence. The strain paths with the detwining process are benefit to get the finer twin-structured magnesium alloy. The analysis shows that this is the mutual coordination between the twinning(compression deformation) and the detwinning(reverse tension deformation), which leads to the different morphology of the twin-structured. The twins are easy to be opened under only compression deformation at same strain value, when the compress to the larger plastic strain and then tensile to the same accumulate plastic strain value, these twins experienced detwinning process, course twins had gradually eliminated and produced more small residual twins.The compressive yield strength of Mg-0.087%Mn alloy changes from 56 MPa to 68 MPa, while the yield strength of reverse tensile increase from 37MPa(compression plastic strain is 2%) to 67MPa(compressive plastic strain is 6%). The compressive yield strength of Mg-0.037%Zr alloy changes from 81 MPa to 85 MPa, while the yield strength of reverse tensile increase from 43MPa(compression plasticstrain is 2%) to 65MPa(compressive plastic strain is 6%), furthermore, the compression–tensile asymmetric ratios are improved with the compression strain increase, nearly to 1. Under the reverse tensile deformation after compress to low strain value, it is benefit for twinning again and detwinning occurred, the detwinning needn't to renucleation and has lower yield stress. When the compression plastic deformation is large, the contribution of twinning to plastic deformation is almost complete, and more slips are involved in the plastic deformation at the later deformation stage, resulting in many defects such as dislocation, these dislocation tangles in the vicinity of the twin boundary, hindering the twin boundary migration, leading to greater force to make the twin boundary motion under reverse tensile deformation, therefore, detwinning is not easy and tensile yield strength is higher.The annealing treatments on the effects of twin-structured magnesium alloy were studied. The results show that: the compression–tensile deformation samples which were reverse tensile to the accumulate plastic strain with 1% after compressed to plastic strain with 2%, 4%, 6% respectively and tensile to the accumulate plastic strain with 2% after compressed to plastic strain with 6% were as the initial sample of re-compression test. The experimental results show that the annealing treatment before compression is helpful for getting the small twin, and the twin incidences were lower or equal than that of the samples without annealing treatment. There are more twins in the four samples, and the chance that alloys element segregate to the twin boundary is more, thus the alloy element pined the twin boundary at the annealing treatment process. The twin boundary is not easy to expand to the matrix under recompression, therefore, there are more finer twin lameallce and higher twin incidence at these samples which had annealing treatment before compression deformation. The yield strength of the sample after annealing treatment is higher than that without annealing treatment before compression deformation. The annealing treatment of twin magnesium alloy is helpful for strengthening the alloy.