PSI - Issue 59

Lyudmyla Bodrova et al. / Procedia Structural Integrity 59 (2024) 731–738 L. Bodrova et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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conducted at macro- and microlevels using an electron microscope REM – 106, magnifications ranging from 12 to 800 times. 4. Results and discussions The results of the investigation of alloys with different contents of WC/nano WC are presented in Fig. 1. The alloying of alloys with an equal amount of metallic binder with both fine and nano WC in quantities ranging from 5 to 15 wt.% results in the increase in thermal shock resistance in 1.1…1.3 times (Fig . 1). At ΔT = 600 and 700 °C, with 5 and 10 wt.% nano WC content, the number of cycles that the alloys withstand before the cracks initiation as compared with fine grained WC increases in 1.8…1.9 times. The ΔT = 800 °C and 15 wt.% nano WC content is increased in 3.9…4.4 times.

Fig. 1. Thermal shock resistance of TiC – 5VC – 5NbC – xWC – 18NiCr (a), TiC – 5VC – х nano WC – 18NiCr (b) alloys, where х = 5, 10, 15 wt.% WC/nano WC.

The maximum number of cyclic loading (N = 74) undergoes an alloy containing 15 wt.% nano WC at ΔT=600°C (Fig. 1b). It is likely to be caused by the fact, that the amount of nano WC (Koval et al. (2014)), being increased, the strength of the alloy fracture toughness increase, as well as the formation of a fine grained structure is observed, which results in a reduction in thermal stresses when subjected to sharp temperature gradients. The research results of alloys with different Ni/nano Ni content in the metallic binder are presented in Fig. 2. The change in the thermal shock resistance of alloys with different nickel content in the metallic binder is of a different nature. In the alloys with fine-grained nickel, this dependence is of the extreme appearance (Fig. 2a), on the contrary to the alloys with nano Ni, in which the thermal shock resistance increases with an increase in the binder and nano Ni content (Fig. 2b). When the metallic binder content increases to 24 wt. % containing 18 wt. % of fine grained Ni, the thermal shock resistance of the alloys decreases due to a decrease in strength (Koval et al. (2022)), whereas in alloys with the same amount of nano Ni, thermal shock resistance increases. At all temperature gradients, the thermal shock resistance of alloys with nano-Ni is in 1.5 to 4.2 times higher than that of fine grained Ni. The alloy with 18 wt.% nano-Ni withstands the maximum number of cyclic loading (N = 83) at ΔT=600°C (Fig. 2b). To assess the operational properties more thoroughly, the relationship between thermal shock resistance and the mechanism of alloy fracture has been investigated. The effect of nano WC on the macrostructure of alloy fractures was analyzed for both the lowest (5 wt. %) content of fine grained and nano WC, as these alloys have the lowest thermal shock resistance. Macrofractographic analysis of the alloys testified signs of fatigue failure on all fractures - crack initiation, stable dynamic propagation, and fractured area (Fig. 3). The surface of the alloy with fine-grained WC fracture is composed of flat aggregates that appear as smooth, featureless areas with low waviness (Fig. 3a, b). Surfaces with smooth delamination are most often observed under conditions where the number of microvoids in the material is small, its deformation before failure is significant, and the failure occurs mainly due to shearing. In the alloy with nano WC, the relief of the surface was significantly increased at the macrolevel, and the transition between areas with different relief was sharper as compared with the previous alloy (Fig. 3c, d). For both alloys, the intergranular fracture was observed