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

733

3

under complex temperature conditions. Since thermal shock resistance is directly caused by structural characteristics, it is important to investigate macro- and microstructural changes in alloys during repeated cyclic heating and cooling. The objective of this paper is to determine the influence of nano and fine grained powders of tungsten carbide and nickel on the thermal shock resistance of TiC-VC/NbC/WC-Ni-Cr alloys under thermal cyclic loading at different temperatures and their fracture mechanism. 3. Materials and methods The alloys were produced by the powder metallurgy method. Carbides and metal powders, in quantities corresponding to the specified composition of the alloy, were ground in ethyl alcohol to obtain a homogeneous mixture, which took 72 hours. The compacting of samples was carried out with a specific pressure of 120-200 MPa. The green compact density was 65% of the theoretical density. Sintering was performed in a high-temperature furnace under a vacuum of 1.33×10 -2 Pa at a temperature of 1350°C and a hold time of 40 minutes. The chemical composition of the investigated alloys is presented in Table 1.

Table 1. Chemical composition of the investigated alloys. No Chemical composition, wt. % TiC VC NbC WC Nano WC* Ni

Nano Ni** Cr

1 2 3 4 5 6 7 8 9

67 62 57 72 67 62 75 61 72 80 66

5 5 5 5 5 5 5 5

5 5 5

5

-

13.5 -

4.5 4.5 4.5 4.5 4.5 4.5 2.5

10 15

13.5 13.5 13.5 13.5 13.5

- - -

- - -

5

10 15

5 5 5 5 5

5 5 5 5 5

- - - - -

7.5

- -

18

6

- - -

- - -

13.5

4.5 2.5

10 11

7.5

18

6

* Grain size 150-200 nm; **Grain size 70-90 nm

The investigated polycarbide based alloys exhibit a relatively high combination of mechanical and thermal properties: transverse rupture strength 850… 1030 MPa, fracture toughness 7.5…8.3 MPa∙m 1/2 , thermal conductivity 12.2 W/m·°C, and a thermal expansion coefficient 8.92 × 10 -6 °C -1 . The thermal shock resistance of cemented carbides and cermets is assessed using two methods: the value of the temperature difference required for crack initiation under a specified number of heating-cooling cycles or the number of cycles required for crack initiation in specimens with a given temperature difference between heating and cooling. Since the temperature difference values for the investigated alloys are close and within the margin of error, the thermal shock resistance of the alloys was assessed on the number of heating-cooling cycles required for the first crack to appear in a polished specimen being rapidly cooled in water from a specified temperature to 20°C. For the investigation specimens of 8 mm diameter and 12 mm height were prepared, 5 for each measurement. The specimens were simultaneously heated in a muffle furnace to the specified temperature and then rapidly quenched in water. To compare the thermal shock resistance of alloys with different chemical compositions, dependencies of the number of cycles N required for crack initiation caused by the temperature difference between heating and cooling ΔT were constructed based on the obtained results. The data processing was performed using the Statistika 10 software. To determine the mechanism of alloy fracture, fractographic examinations were