Effect of grain boundaries on the preferential orientation distribution of θ′ precipitates in stress-aged Al–2Cu alloy bicrystals

Abstract

Grain boundaries in metals would introduce extra nucleation energy to promote the growth of θ′ precipitate during ageing process. The influence of grain boundaries on the distribution behavior of two θ′ precipitates variants (010) and (001) were studied in Al–2Cu alloys bicrystals. Ageing conditions without external stress and with 40 MPa and 60 MPa stresses were conducted to study the effect of grain boundaries on the oriented distribution of two θ′ precipitates variants. The hardness differences between the near grain boundaries area and the single crystal of the bicrystal samples were tested to understand the relationship between microstructures and mechanical properties. Results of transmission electron microscopy (TEM) images showed that the stress-orienting effect on precipitates was inhibited by the grain boundary. TEM images and quantitatively measurement of two θ′ precipitates variants indicated that grain boundaries inhibited the orienting effect of external stresses on the distribution of precipitates. Especially in 60 MPa stress-aged sample, θ′ precipitates near the grain boundary distribute homogeneously and have the same density as in single crystal areas. Hardness difference in stress-aged sample was smaller compared to the stress-free-aged sample. External stresses during ageing also narrowed the PFZ width near grain boundaries. The anisotropy of precipitation strengthening contribution of two θ′ variants were reduced due to the extra energy provided from grain boundaries. The compressive yield strength of Al–2Cu alloys bicrystal increased with the applied stresses during ageing. This work provides new insight into solving the anisotropy problem during stress ageing based on the effect of grain boundaries.