A high-entropy alloy syntactic foam with exceptional cryogenic and dynamic properties

Metal foams are in great demand in extreme service environments with cryogenic temperatures and high strain rates due to their unique properties. However, the reduced ductility and impact toughness under such conditions limit their applications. In this work, we tested the quasi-static and dynamic compression properties of CoCrFeMnNi high entropy alloy syntactic foam at liquid nitrogen temperature (-196 degrees C). We found that it exhibits ultra-high strengths and energy absorption capacity, especially a superior resistance to embrittlement at cryogenic temperature. Microstructural characterizations reveal that the foam matrix has a high twinning activity and shear localization propensity in cryogenic environments. Dense deformation twins and multiple shear bands intersected, forming a weave-like microstructure that can disperse the deformation and benefits energy absorption. Deformation twins can also strengthen the matrix and delay the crack nucleation and growth. While under dynamic loading, an FCC to HCP phase transformation was activated, forming a nano-laminated dualphase (NLDP) FCC/HCP structure in the matrix, leading to further strengthening and toughening. Deformation twinning and HCP phase transformation act as additional plastic deformation mechanisms along with stacking faults and shear bands to offset the shortcomings caused by limited dislocation movements at low temperatures and high strain rate dynamic loading, enabling the high strength and energy absorption of the syntactic foam.

Automated summary

This study investigated the compression properties of CoCrFeMnNi high entropy alloy syntactic foam at liquid nitrogen temperature. The foam exhibited ultra-high strength and energy absorption capacity, especially at cryogenic conditions. The microstructural analysis revealed a high twinning activity and shear localization propensity in the foam matrix, which dispersed deformation and benefited energy absorption. Dynamic loading activated a phase transformation, further enhancing the strength and toughness of the foam. Deformation twinning and phase transformation acted as additional plastic deformation mechanisms, compensating for the limitations of low temperature and high strain rate loading.