Creep transitions in an Al-Zn alloy

Abstract

The creep behavior of an Al-10 at. pct Zn alloy was investigated at temperatures in the range 573 to 800 K and in a normalized shear stress range, τ/G, extending from 10^•6 to 10^•3, where τ is the applied shear stress andG is the shear modulus. The results reveal the presence of four deformation regions: region I (very low stresses), region II (low stresses), region III (intermediate stresses), and region IV (high stresses). In regions II and III, the creep characteristics including the stress exponent, the shape of the creep curve, and the nature of creep transients after stress reductions are consistent with dislocation climb and viscous glide, respectively. The experimental transition stresses between region II (climb) and region III (glide) were compared with those predicted from the deformation criterion for solid-solution alloys, and it is demonstrated that the correlation between experiment and prediction becomes satisfactory when corrections concerning the appropriate diffusion coefficients for describing glide-controlled creep and climb-controlled creep, the stacking fault energy of the alloy, and the contributions of Suzuki interaction and Fisher interaction to solute atom drag forces are incorporated in the analysis. Experimental data in region IV are not sufficient to unambiguously identify the rate process, but calculations show that the values of the transition stress between region III and region IV agree reasonably well with those of the critical stress for dislocation breakaway from a solute-atom atmosphere. The characteristics of deformation in region I are still under investigation.