  Precipitation on deformation defects is essential for enhancing mechanical
properties of age-hardenable alloys. {10-12} twinning is common in deformed
alloys with hexagonal-close-packed structure. In this work, we revealed how
{10-12} twin boundary (TB) influences beta-Mg17Al12 precipitation in Mg-9Al-1Zn
alloy. The precipitates on the TB are rod-like, while others are lath-shape.
The precipitates hold the Burgers orientation relationship (OR) only with twin
or matrix, contrary to traditional wisdom in others alloys that precipitates on
TB symmetrically keep an OR with both twin and matrix. Moreover, certain
precipitate variants are absent, and a new rule for variant selection on TB is
proposed.
