MORPHOLOGICAL ALTERATIONS DURING TEXTURE-PRODUCING PLASTIC PLANE-STRAIN COMPRESSION OF HIGH-DENSITY POLYETHYLENE

Numerous specimens of a single linear polyethylene sample were deformed by plane strain compression in a channel die to very large plastic strains. The mechanisms of deformation were elucidated by density measurements, polarized light microscopy, transmission electron microscopy, and wide-angle and small-angle X-ray diffraction. A deconvolution procedure for separating overlapping X-ray diffraction peaks and the diffuse scattering from the amorphous material was also applied, and pole figures were reconstructed from the corrected data. At a compression ratio of 1.80 intense shear localization appeared at +/-45-degrees with respect to the flow direction. There is strong evidence for interlamellar sliding only at low compression ratios. Once this sliding is apparently exhausted, crystallographic slip on the (100)[001] chain slip system sets in. At a compression ratio above 1.80 and 3.13 respectively, (100)[010] transverse slip and (010)[001] chain slip processes are observed. The strong shear bands occurring at compression ratios of 2.5 and 3.13 originated mainly from (100)[001]and (100)[010] slip processes. The morphology at a compression ratio of 6.44 resembles a monocrystal texture. Further deformation occurs by (100)[001] slip and to a lesser extent by (010)[001] chain slip. The (110) twinning process shows limited activity, but only at very high compression ratios near 12. Associated TEM studies have shown that while the initial amorphous material layers attached to the lamellae rotate to lie normal to the loading direction, a new long period starts to evolve normal to the flow direction beginning at a compression ratio of 3.13; this new long period forms via an apparent widespread pinch-off of stretched lamellae and translation of interfaces above a compression ratio of 3.13 and becomes progressively better defined with continued deformation. It is suggested that the breakup of thinned lamellae is due to a deformation instability related to the increase of interface stretching resistance of the crystalline and amorphous layers and is initiated by thickness irregularities. Because of defects (tie links, etc.) that have good mobility along the chain in the newly evolved amorphous material, the fully textured quasi-crystal sample has a continuous background of crystals with layers of segregated chain defects making up the new long period. There is also some evidence for the formation of flaws and fissures in planes perpendicular to the load direction, which causes brittleness in tension associated with fracture along the (100) planes.