FCC–BCC phase transformation in rectangular beams subjected to plastic straining at cryogenic temperatures

Abstract

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 ) to the secondary phase ( α ′ ) and evolution of microdamage. 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. The strain hardening model is based on micromechanics considerations (first mechanism) and on the Hill concept (1965) including the Mori–Tanaka (1973) homogenization scheme (second mechanism). Identification of parameters of the constitutive model is based on the available experimental data. The model is used to describe phase transformation in rectangular beams subjected to elastic–plastic bending at cryogenic temperatures. Several examples of rectangular beams with FCC–BCC phase transformation induced functionally graded (FGM) microstructure are presented.