Ductile damage development in two-phase metallic materials applied at cryogenic temperatures

FCC metals and alloys are frequently used in cryogenic applications, nearly down to the temperature of absolute zero, because of their excellent physical and mechanical properties including ductility. These materials, often characterized by the low stacking fault energy (LSFE), undergo at low temperatures three distinct phenomena: dynamic strain ageing (DSA), plastic strain induced transformation from the parent phase (γ)(γ) to the secondary phase (α′)(α′) and evolution of micro-damage. Especially the third phenomenon leads to irreversible degradation of lattice and can accelerate the process of material failure therefore a suitable constitutive description appears to be fundamental for the correct analysis of structures applied at very low temperatures. The constitutive model presented in the paper takes into account the plastic strain induced phase transformation and the evolution of ductile damage. The FCC–BCC phase transformation results from metastability of LSFE metals and alloys at very low temperatures. The phase transformation process leads to creation of two-phase continuum where the parent phase coexists with the inclusions of secondary phase. Such heterogeneous material structure induces strong strain hardening related to two distinct mechanisms: interaction of dislocations with the inclusions and increase of tangent stiffness as a result of mixture of two phases, each characterized by different parameters. For the micro-damage evolution a generalization of the classical isotropic ductile damage concept (Chaboche, 1988a and Chaboche, 1988b) to anisotropic model has been introduced. The kinetics of damage evolution is based on the accumulated plastic strain as driving force of ductile damage. The damage rate tensor depends on the strain energy density release rate (conjugate force) and on the tensor of material properties, that reflects the damage anisotropy.