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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At the temperature gradient 600 °C for an alloy containing 10% Ni -Cr (7.5 wt.% nano Ni), besides the main crack, a significant number of subcracks located normal to the main crack (Fig. 4 a, b) are specified. As the nano Ni content increases to 18 wt.%, their quantity significantly decreases, but larger macrocracks of intergranular cracks appear. It is likely to be caused by the difference in the properties of thin films compared to the properties of the same material in significant amount. In the latter, the limit load that the material can withstand is determined by the appearance of plastic flow. When a metal as a thin film is located between the rigid surfaces of carbide grains, they resist the plastic flow of the metal, resulting in high strength. This phenomenon is similar to the strength curve of alloys with a maximum dependence on the binder content. Besides, residual stresses, which occur in carbides during cooling due to differences in thermal conductivity and coefficients of thermal expansion of metals and carbides, also demonstrate plastic flow.

Fig. 4. Macrofractographs of TiC – NbC – WC based alloy with 7.5 (a.b), 18 (c,d), wt. % nano Ni (d, e). ΔT = 600 °C.

Comparison of the macrostructure of alloys with different nano Ni content in the binder at temperature gradient 800°C testified, that in alloys with 10 wt.% binder (7.5% nano Ni) and with 18 wt.% binder (13.5 wt. % nano Ni) in particular, elements of intergranular fracture prevail, involving the tearing apart of not only individual grains but also their conglomerates. Increasing of the binder content to 24 wt.% (18 wt.% nano nickel) results in a reduction in facet size on the fracture and an increase in elements of ductile fracture. This testifies the influence of dispersion strengthening of the binder due to the dissolution of carbides in it. The microfractographs of alloys with different WC content, magnified up to 800 times, demonstrate a distinctive feature: different orientations of secondary submicrocracks. In alloys with 5 wt.% WC, the grid of secondary submicrocracks is randomly oriented (Fig. 5a). In alloys containing 10 and 15 wt.% WC, the secondary submicrocracks, with lengths up to 20- 25 µm, are predominantly oriented along the propagation front of the main crack, i.e., normal to the direction of the main crack propagation (Fig. 5c, e). In the microfractographs of alloys with nano WC, more dispersed microstructure with fewer sharp grain boundaries (Fig. 5d, e, f) and lower signs of brittle fracture were observed totally. Mostly it is observed in the alloy