Microstructural analysis of the thermal fatigue tested Be/CuCrZr joint with artificial defects between Ti/Cu interlayers

The ITER enhanced-heat-flux (EHF) FW fingers undertook by China are made by hot isostatic pressing (HIP) method to bond beryllium armor tiles to CuCrZr/316L(N) heat sinks with 10 μm-thick Ti coating on beryllium tiles and 0.5 mm-thick oxygen free copper sheet as interlayer. To study its thermal fatigue behavior, small scale mock-up was manufactured with artificial defects (AD) on the beryllium tile surface with Ti coating. Thermal cycling of the mockup was carried out at 4.7 MW/m2 high heat flux (HHF) for 5000 cycles. The interface between Be/Ti/Cu and the fracture surface on beryllium tiles were observed by stereomicroscopy and scanning electron microscopy with energy spectrometer. The Be/Ti/Cu interfacial phase composition and the behaviors of defects after high heat flux test were analyzed. In addition, the generation and propagation mechanism of thermal fatigue defects were preliminarily studied in this paper. The results show that uniform bonding between Be and Cu was achieved and the interfacial phases are Be base/ Be-Ti intermediate/ α-Ti/ Ti2Cu/ TiCu /Cu base in the sequence. The formation of complex brittle phases reported in previous study, such as Ti3Cu4 or α-TiCu4, can be avoided by using Ti single interlayer. When the original defect dimension is only 3 × 3 mm2, the defect will not propagate after 5000 cycles 4.7 MW/m2 HHF testing (HHFT). Besides, decreasing beryllium tile size can reduce the possibility of thermal fatigue failure. In addition, from the research on fracture surface on beryllium tiles, the final fracture made for beryllium detachment mainly happened in Ti layer near the beryllium side. While thermal fatigue cracks preferentially originated at Be/Ti interface and the oxidation of beryllium surface under thermal cycling test would accelerate the generation and propagation of thermal fatigue defects.