Maximum shear stress-controlled uniaxial tensile deformation and fracture mechanisms and constitutive relations of Sn-Pb eutectic alloy at cryogenic temperatures

Sn-Pb eutectic alloy is widely applied to the deep-space electronics that work under cryogenic environments, but little is known about its responses to the mechanical loading at cryogenic temperatures. In this work, a comprehensive investigation about the deformation and fracture behaviors as well as constitutive relations of the eutectic alloy at cryogenic temperatures was conducted through uniaxial tensile experiments over a full temperature range from 293 K to 77 K, in-situ cryogenic tensile experiments and fractography. With the declining temperature, the tensile strength and quasi-static toughness increase substantially while the elongation is maintained at 25%-30%; Sn-Pb eutectic alloy achieves the optimal combinations of strength, ductility and toughness at 123 K, which is due to a number of deformation twins activated in the Sn matrix together with the compatible deformation between the Pb-rich phases and the Sn matrix. Multiple 45 degrees shear bands induced by the maximum shear stress contribute to reconcile the deformation difference between the two phases, which reaches its minimum at around 123 K. A ductile shear fracture on the 45 degrees shear planes with respect to the tensile axis occurs under the maximum shear stress at temperatures ranging from 293 K to 123 K, while the maximum normal stress leads to a brittle fracture on the 90 degrees planes at 77 K. Moreover, the Anand model fails in fitting the constitutive relations of Sn-Pb eutectic alloy when the temperature declines to 233 K or lower. Instead, the Hollomon equation has been successfully applied to fit the constitutive relations at these invalid temperatures.

Automated summary

This study comprehensively investigated the deformation and fracture behaviors, as well as the constitutive relations of Sn-Pb eutectic alloy at cryogenic temperatures through a series of experiments and analyses. The optimal combination of strength, ductility, and toughness was found at 123K for the alloy. Different fracture modes were observed under shear and normal stresses at various temperatures.