Low cycle deformation behaviour of Zircaloy-4 at elevated temperatures

The LC deformation behaviour of Zry-4 at 400°C and 600°C was examined by means of tension/compression experiments conducted in load and in strain control respectively. The main results were compared to those obtained at comparable conditions on the stainless steel type AISI 304. For both the materials the influence of the stress ratio R = σ_min/σ_max (where within one test σ_max > 0 was kept constant) upon the lifetime T_f at low and high homologeous temperature T_h was examined. Whereas at the lower T_h for R < 0 the lifetime decreased with decreasing R, the opposite was true at the higher T_h. The explanation of the influence of R upon t_f suggests that at high temperatures the fatigue damage rate Å_f drops below the rate for creep damage Å_c Two cases are considered. If the above damage mechanisms are sequentially independent the resulting damage rate Å ≈? Å_c and hence Å_c is the failure (rate) determining mechanism. In the case that the mechanisms are sequentially dependent then Å ≈? Å_f. TEM investigations conducted on Zry-4 cycled at 600° C have shown that the typical dislocation pattern revealed is a band structure consisting of dense dislocation walls separating denuded zones. The habit and the crystallographic characteristics of the band structure resemble the structure associated with PSBs observed in fee metals. The comparison of the values of the saturation stress τ_s and the wall spacing d for different fee and hep metals shows that there is a proportionality between τ_s and 1/d which is independent of stress and temperature.