Oriented polymer blends whose major component is high-density polyethylene (HDPE) are strained until failure. Two-dimensional (2D) small-angle X-ray scattering (SAXS) patterns monitor the nanostructure evolution, which is related to the macroscopic mechanical evolution. Data evaluation methods for high-precision determination of macroscopic and nanoscopic parameters are presented. The hardest materials exhibit a very inhomogeneous nanodomain structure. During straining, their domains appear to be wedged and inhibit transverse contraction on the nanometer scale. Further components of the blends are polyamide 6 (PA6) or polyamide 12 (PA12) (20-30%) and Yparex(R) 8102 (YP) as compatibilizer (0-10%). Some HDPE/PA6 blends are additionally loaded with commercial nanoclays (Nanomer(R) or Cloisite(R)), the respective amounts being 7.5% and 5% with respect to PA. Blending of HDPE with PA12 causes no synergistic effect. In the absence of nanoclay, PA6 and HDPE form a heterogeneous nanostructure with high macroscopic Young's modulus. After addition of YP a rather homogeneous scaffold structure is observed in which some of the PA6 microfibrils and HDPE crystallites appear to be rigidly connected, but the modulus has decreased. Both kinds of nanoclay induce a transition in the HDPE/PA6 blends from a structure without transverse correlation among the microfibrils into a macrolattice with 3D correlations among the HDPE domains from neighboring microfibrils. In the range of extensions between 0.7 and 3.5%, the scattering entities with 3D correlation show transverse elongation instead of transverse contraction. The process is interpreted as overcoming a correlation barrier executed by the crystallites in an evasion-upon-approaching mechanism. During continued straining, the 3D correlation is reduced or completely removed. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 237-250, 2010