First observation of spalling in tantalum at high temperatures induced by high energy proton beam impacts

Spalling in tantalum has been observed in an experiment aiming at testing an antiproton production target prototype at CERN’s HiRadMat Facility. The experiment consisted in impacting 47 intense and high-energy proton beams onto a target equipped with ten cylindrical cores made of tantalum of Ø8 mm by 16 mm. Each of these proton beam impacts induced a sudden rise of temperature in the bulk of the Ta cores around 1800 ◦C in 0.9 μs leading to the excitation of a vibration mode which exposed their material to compressive-to-tensile pressures ranging from 2 GPa to 9 GPa, with pressure rates up to 20 GPa/μs. Post-irradiation analyses such as neutron tomography and metallographic examination of the cores, revealed the creation of voids in the bulk of the tantalum cores ranging from to 2 μm to 1 mm in diameter. These voids present a non-uniform size and density distribution within the cores, with limited growth and coalescence in areas subjected to higher temperatures and tensile pressures. Grain-growth due to a fast, thermally-induced, recrystallization has been also observed in some zones. In this work, we present a detailed characterization of the unique thermal and mechanical load that has induced this spall process by means of Finite Element and hydrocode simulations, together with post-experiment microscope and EBSD observations of the Ta rods. This analysis suggests that spall-induced void growth and coalescence is enhanced in the temperature and pressure window of 1300-1800 ◦C and 3-6.5 GPa, whereas is restrained at temperatures and tensile pressures above 2000 ◦C and 6.5 GPa. In addition, the analysis suggests that full thermal-recrystallization in tantalum can take place when exposed to temperatures above 2000 ◦C for less than 2 s. Four different hypotheses to explain the observed void size distribution trends are presented.