Microstructure and Mechanical Properties of Severely Deformed Polypropylene in ECAE (Equal Channel Angular Extrusion) via Routes A and C

Abstract

Equal channel angular extrusion (ECAE) is a solid-state extrusion process for modifying microstructures via severe plastic deformation without modifying the specimen cross section. In this study, changes in the microstructure and mechanical properties of polypropylene resulting from extrusion orientation route A (no rotation between extrusions) and extrusion orientation route C (a rotation of 180° between extrusions) are investigated using a 90° die-angle tooling outfitted with back pressure. Important differences are reported for the ECAE-induced deformation behavior between the two processing routes. A focus is made on the occurrence of heterogeneous plastic deformations (periodic shear banding and warping) for both routes and the control and inhibition of the plastic instabilities via regulated back pressure and ram velocity. Wide-angle X-ray scattering is carried out to characterize the structural evolution as a function of the processing conditions including route, extrusion velocity and BP application. The mechanical properties of the specimens machined from the ECAE pieces are examined under different loading paths including uniaxial tension/compression and simple shear. Full-field displacements converted to volumetric strains revealed the profound impacts of the processing route on the deformation mechanisms during tensile deformation.

Equal Channel Angular Extrusion of Polymers: Structural Changes and Their Effects on Properties

“…30) The possibility of forming biaxially oriented structures in the strain range e = 6.78.5, consisting of larger and more perfect crystallites connected by a large number of stretched tie chains, the possibility of activating phase transformations (occurrence of crystal polymorphism and the formation of crystalline phases with higher plasticity or thermal stability), makes it possible to obtain a unique combination of multiple increased values of stiffness, strength (with low anisotropy of the latter) and wear resistance, while maintaining plasticity at a level close to that of the original samples. 27,30,46) Compared to the latter, the density and melting point increase. Figure 3 shows an example of the dependence of tensile strength and strain at break on accumulated strain for high density polyethylene subjected to ECMAE.…”

“…Textural changes also occurred in this region of accumulated strain, but the crystalline textures differed from each other depending on their localization in the sample volume, as there was a deformation gradient in the transverse direction and the local plastic strain decreased from the top to the bottom of the sample. 20,27) At the same time, there was a steady flow region where the plastic strain was quite uniform along the longitudinal direction except for the edges. It was observed that routes A and C reached their saturation levels quickly (after about four passes, e = 3.3), compared to routes B A and B C , which require a high number of passes to reach their steady state.…”

“…For example, in Ref. 27) it was found that the homogeneous simple shear that the ECAE-deformed piece with e = 0.82.4 should experience was a heterogeneous deformation manifested by periodic shear bands and warps. The results obtained with Route A showed significant warping.…”