Non-uniform Deformation And Microstructure Evolution Of TWIP Steel Under Tensile Loading

Twinning Induced Plasticity(TWIP)steel has excellent mechanical properties.TWIP steel absorbs a lot of energy in car impact and is widely used in cars and high-speed trains.Non-uniform deformation has a strong influence on the damage and fracture of the material.Therefore,studying the non-uniform deformation is of great significance to the engineering application.This topic studies the characteristics of non-uniform deformation during TWIP steel deformation,its control factors and microstructure evolution.The in-situ tensile test of TWIP steel is conducted by self-made micro material testing system(MMTS)in scanning electron microscopy(SEM),and image of sample surface at different tensile strain is perform the digital image correlation process to obtain the strain field.During the deformation,the microstructure is characterized by the electron backscatter diffraction(EBSD).During the deformation,slip bands can pass through the grain boundary with misorientation of 10°-30°,and activate the slip band at adjacent grain without strain concentration at the grain boundaries(GBs).When the slip band is partially blocked by the GBs with misorientation of 30°-50°,causing a relative smaller strain concentration at GBs;The slip bands are completely blocked by GBs with misorientation angle above 50°,leading a large strain concentration.For inhomogenous deformation,the slip length dominates the non-uniform deformation,since the number of piled-up dislocation is larger for the grain with larger slip length.The softer grain orientation facilitates the dislocation slip,resulting that the strain concentration is larger for the grain of softer orientation.The strain concentration also prefer to occur at high angle GBs.At larger strain,the inhomogenous deformation is also existed due to the occurrence of macrodeformation twin.EBSD results show that the rotation angle and rotation direction of different grains are different.The grain rotation basically follows Taylor model: grains with orientations at<001>-<111> boundary rotate toward <001>-<101> at 0-12% strain;other grains rotate towards <001>-<111> boundary.At larger strain,many grains have a bunch of twins,andsome of them also activate multiple twinning.Then the twins can intersect with each other,leading to strain concentration.