Microstructure and mechanical properties of multidirectionally forged Al alloy 1570С subjected to cryogenic rolling

Abstract

The effect of cryogenic rolling at −196 °C up to 90 % reduction on the microstructure and static strength of a 1570C alloy (Al–5.0Mg–0.18Mn–0.2Sc–0.08Zr–0.01Fe–0.01Si wt%), pre-processed by multidirectional isothermal forging (MIF) at 325 °C to a total strain of about 12, is investigated. MIF of the homogenized ingot allowed to obtain a near-homogeneous (ultra)fine grained (UFG) structure with a grain size of 2.3 μm, leveled Mg in solid solution and a uniform distribution of second phases, which ensured subsequent rolling without cracking. Rolling transformed the forged structure into a pancaked (flattened) UFG high work-hardened structure with a halved fraction of high-angle boundaries, characterized by a dislocation density of 10¹⁵ m⁻² and a partially developed nano-sized crystallite structure with a coherent domain size of 15 nm. These structural changes resulted in significant improvements in room temperature tensile strength compared to the initial homogenized/forged conditions: yield strength increased from 245/235 to 590 MPa and ultimate tensile strength increased from 355/365 to 620 MPa, while elongation decreased from 26/37 to 9 %. The increase in strength with reduced ductility is attributed to the superposition of grain boundary and dislocation strengthening mechanisms amid dispersion strengthening. Therewith, cryogenic rolling promotes uniform dislocation structures and a homogeneous distribution of Al₃(Sc,Zr) precipitates, which interact with the dislocations to maintain relatively high ductility even in a highly deformed state.