Investigation of the Compressive Behavior of a Freeze‐Cast Cu Foam Using Acoustic Emission Measurement

Csilla Kádár,Péter Jenei,Jenő Gubicza,Heeman Choe,Hyeji Park,Myung-Yeon Kim,Ábel Szabó

doi:10.1002/adem.202100378

Abstract

Herein, the compressive behavior of Cu foams with lamellar structures processed by the freeze‐casting method is interrogated in directions parallel and perpendicular to the direction of freezing. Deformation is monitored using simultaneous acoustic emission (AE) measurements and video recordings of changes at the sample surface. Significant differences are observed between the stress−strain curves acquired during compression in the two directions. For parallel loading, relatively high peak stress is detected at a strain of about 3%, followed by a plateau, and, at high enough loadings, the Cu foam hardens. The AE results suggest that the decrease in stress from the peak value to the plateau value arises from fractures in the thick lamellae lying nearly parallel to the freezing direction. For perpendicular loading, the energy of the AE events is lower because, in this case, the thinner struts that connect the thick lamellae bend and break more easily. For this case of perpendicular loading, the peak stress is missing from the stress–strain curve. Further deformation yields a gradual increase in the energy of the AE signals, suggesting that the thicker lamellae also break during densification.

Highlights

The X-ray diffraction measurement was acquired on the sample surface that was parallel to the freeze-casting direction
This study investigated the compression behavior of an anisotropic Cu foam manufactured by the freeze-casting method
The failure mechanisms were revealed using acoustic emission (AE) techniques and cinematography for foams subjected to compression both parallel and normal to the direction of freezing up to an engineering strain of about 0.5

Summary

Microstructural Characterization

The X-ray diffraction pattern in Figure 1 reveals that the foam struts are single-phase Cu materials without significant oxide content. The X-ray diffraction measurement was acquired on the sample surface that was parallel to the freeze-casting direction. The microstructure of the freeze-cast Cu foam is composed of lamellar Cu walls with elongated pores. These aligned lamella-shaped colonies of Cu walls are parallel to the freezing direction in accordance with the temperature gradient (Figure 2b). The foam surface was etched by Ga ion milling to produce a surface of sufficiently good quality for EBSD analyses. The details of this surface treatment are provided in the Experimental Section. The samples are texture-free based on both the X-ray diffraction pattern and the EBSD analysis

Deformation of Cu Foams

Conclusion

Experimental Section

Data Availability Statement