Influence of Deformation Twins on Mechanical Properties and β Precipitation of Large-Cross-Section (290 mm × 230 mm) AZ80 Bar Fabricated by “Multi-direction Forging + Extrusion + Cold Compression” Technique

An AZ80 bar with the cross-section of 290 mm×230 mm was fabricated by “multi-direction forging (MDF) + extrusion + on-line water quenching”. The bar was cold compressed along 230 mm direction (ND) with a reduction of about 20 mm, which brought about numerous twins in the center (C) of the cross-section, while the outer (O) had few twins. The influences of deformation twinning on mechanical properties and precipitation reactions were studied. Results show that room-temperature (RT) tensile properties of C and O along extrusion direction (ED), 290 mm direction (TD) and ND exhibit remarkable anisotropy and heterogeneity. ED in O (ED-O) has the highest strength, i.e. RP0.2, Rm and A are 295 MPa, 401 MPa and 10%, respectively, while ND-C has the lowest strength, i.e. RP0.2, Rm and A are 164 MPa, 273 MPa and 7%, respectively. Such remarkable anisotropy and heterogeneity are mainly determined by texture, and deformation twinning has a major influence on texture. Before cold compression, two orientations exist in C, i.e. [0001]Mg//TD and [0001]Mg//ND; after cold compression, the texture of O is almost unchanged, however, that of C is largely transformed by deformation twins, that is, [0001]Mg//TD orientation is immensely weakened, while [0001]Mg ∼//ND is accordingly enhanced. After 155 °C/7 h aging, β particles continuously precipitate in deformation twins, and these twin-related β particles are much finer than discontinuous β lamellae.