The phenomenon of transition from elastic deformation to plastic deformation that occurs when the stress on polycrystalline metals exceeds a certain value, resulting in yielding of poIycrystal.
The yielding of polycrystals can occur within or between grains, and the basic methods of deformation are slip or twinning, as well as the movement and rotation of grains. The movement and rotation of grains can promote the improvement of grain deformation conditions, but it is also the root cause of the destruction of intergranular connections and the occurrence of internal microcracks.
When polycrystalline yielding, the deformation of one grain must be coordinated with the deformation of adjacent grains to overcome the hindrance of grain boundaries to slip and the mutual interference of dislocations when multiple slip systems are activated simultaneously. The yield stress of polycrystals is higher than that of single crystals, and the degree of difference mainly depends on the type of crystal lattice and grain size.