Successive Translucent and Opaque Shear Bands Accompanied by a Pronounced Periodic Waves Observed in a Polypropylene (PP) Processed by Single ECAE Pass

Abstract

The equal channel angular extrusion (ECAE) is an ingenious severe plastic deformation process used to modify texture and microstructure without reducing sample cross-section. The ECAE of polypropylene (PP) was conducted under various extrusion velocities and back-pressure levels using a 90° die. The application of single ECAE pass to PP was meticulously investigated at room temperature. The ECAE-induced deformation behaviour was examined in relation to the load versus ram displacement curves. Depending on extrusion conditions, PP displayed various types of plastic flow. For ram velocities beyond 4.5 mm/min, severe shear bands consisting of successive translucent and opaque bands were observed, accompanied on the top surface by more or less pronounced periodic waves. Although the application of a back-pressure significantly reduced the wave and shear-banding phenomena, slightly inhomogeneous shear deformation was still observed. Shear bands were only suppressed by decreasing extrusion velocity. The strain-induced crystalline microstructure was investigated by X-ray scattering. Shear-banded samples exhibited a strong texturing of the (hk0) planes along the shear direction in the translucent bands whereas perfect crystalline isotropy appeared in the opaque bands. Application of backpressure and/or reducing ram velocity resulted in uniform texturing along the extruded sample. Yet, texturing changed from single shear to twin-like shear orientation about the shear direction. Mechanical properties changes of the extruded specimens due to back-pressure and extrusion velocity effects were analyzed via uniaxial tensile tests. The tensile samples displayed multiple strain localizations in shear-banded materials whereas quite homogeneous deformation appeared for non-banded ones. These effects were connected with the crystalline texturing. The results also revealed significant increase in the strain hardening after ECAE. Full-field strain was measured under tensile loading using an optical strain measuring technique based upon Digital image correlation technique, suitable for large deformation, which confirms these effects.