Strengthening mechanisms and Hall-Petch stress of ultrafine grained Al-0.3%Cu

Abstract

An ultrafine grained Al-0.3 wt %Cu has been produced by cold rolling to a thickness reduction of 98% (εvM = 4.5). The deformed structure is a typical lamellar structure with a boundary spacing of 200 nm as characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Coarsening of the deformed structure to recrystallization is achieved by heat treatment in the range of 100–300 °C. Good thermal stability has been observed up to 175 °C with some segregation of Cu to the boundaries as observed by 3D atom probe characterization. Tensile tests have shown a flow stress (0.2% offset) of 198 MPa with continuous flow with no yield drop and Lüders elongation. To quantify the contribution of boundary strengthening to the flow stress, dislocation strengthening and solid solution hardening have been calculated and subtracted from the flow stress. It has been found that boundary strengthening can be expressed by a Hall-Petch relationship and that the constants in this equation are in very good agreement with previous observation of recrystallized pure polycrystalline aluminium with a grain size in the tens of micrometer range. Thereby the Hall-Petch relationship of aluminium can be extended an order of magnitude from the micrometer to the sub-micrometer range, which is of both scientific and technical importance.