Mechanical properties of spinodally decomposed Fe-30 wt% Cr alloys: Yield strength and aging embrittlement

The age-hardening due to spinodal decomposition in Fe-30 wt% Cr alloys was studied. The yield stress was measured using monotonic tensile tests over a temperature range from 77 to 473 K. The observed incremental yield stress was found to be essentially test temperature independent, increasing substantially with increasing aging time as long as deformation occurs by a slip dominant mode. TEM observation of these test samples showed many dislocation pile-ups for aged materials while as-quenched materials exhibited a homogeneous dislocation distribution. The composition profile and consequent internal stress field due to isotropic decomposition were simulated using Cahn's multi-wave method. The results of this model were then used to estimate the yield stress for such an isotropic decomposition. Theory and experiment were compared using estimates of the extent of decomposition derived from detailed analyses of small angle neutron scattering (SANS). The results indicated that the misfit effect due to the coherent internal stress field is the dominant mechanism responsible for the observed age hardening. Experimental observations are consistent with a model for aging embrittlement which attributes the origin of twinning deformation to a Cr-rich second phase with Cr concentration greater than some critical value.