Effect of creep substructure on the stress exponent of Al following stress reductions

Abstract

The mechanical and substructural behaviour of aluminium was investigated in the temperature range 573–600 K using both uninterrupted and stress-change tests. The present results, like those most recently reported for Al in the temperature range 610–923 K, show that the subgrain size has the ability to coarsen after a stress reduction. However, unlike the data on Al in the range 610–923 K, the present data at high stresses (τ > 1·8 MPa, where τ is the shear stress) indicate that the stress exponent obtained from stress reduction teats is slightly higher than that obtained from uninterrupted tests; at low stresses (τ < 1·8 MPa), there is no difference between the two exponents. Careful and detailed electron microscopic examination of the interiors of subgrains developed in Al in the whole temperature range of 923–573 K reveals the presence of cellular substructure at temperatures lower than 650 K and high stresses. Based on a correlation between the characteristics of the cellular substructure and creep data, it is suggested that the small difference between the stress exponent of stress reduction tests and that of uninterrupted tests at T < 600 K and high stresses may be the result of a strengthening effect associated with the relative stability of the cellular substructure after stress reductions in this temperature range.