Fracture borides

To increase the wear resistance of surfaces, silicon and metals are often coated with a hard nitride, carbide, boride, or oxide film. Nanoindentation and fracture simulations have been used extensively to elucidate failure mechanisms of these typically more brittle surfaces, which include crack propagation and film delamination. Considerable attention has also focused on nanocomposite materials, which possess nanocrystalline inclusions in an otherwise amorphous matrix. The nanocrystalline component is sufficiently small to preclude the formation of stable dislocations, and thus provide a higher hardness. 

It is necessary to note that both borides and silicides in the investigated systems exhibit a good cohesion with titanium matrix. In particular, it is practically never revealed on fracture surfaces of these alloys any lamination of particles of the strengthening phase from matrix. 

Key words Titanium in-situ composites, discontinuous reinforcement, silicide reinforcement, boride reinforcement, mixed silicide-boride reinforcement, ductile reinforcement, microstructure, a -phase, (3-phase, mechanical properties, fracture mechanisms. 

Ti-6.6Al-3.5Zr-1.3Si-l.lB. In as-cast state, as-dendritic structure is practically invisible. Lamellar a -structure of matrix is recognized well. Close to uniformly distributed borides have rod-like morphology and fogged like brushwood. Thickness of rods is between 0.1-1.5 pm. Their length is up to 0.2 mm. Fine (0.1-0.3 pm) silicides are uniformly distributed along matrix. Fracture feature is intensive cleavage microcracking of RT and... 

A further example of production of boride-based fully-dense materials is recently reported by Olevsky et al. [99]. The product, a TiB2 TiN-Ni cermet, a material with a unique combination of high temperature hardness and stability and with a reasonable ductility/fracture toughness. 

The ability of steel to withstand moderate stress without plastic deformation and fracture at high temperature is its high-temperature strength. In steam turbines, hardened chromium steels with 9-12% Cr are used. If they are alloyed with some tenths of a percent of boron, small boride precipitations are formed in the iron matrix. These act as obstacles to dislocation movements (slips in the crystal grains) and by that means improve the high-temperature strength. 

Lancasta microindentation tester, 50 Lanthanide dicarbides pendulum hardness, 295 Vickers hardness, 304 Lanthanum boride, LaB , 297 Lanthanum oxide (1 03) effect on silica hardness, 238 as network modifier, 238 Lanthanum silicate (La2 i207) hardness, 242 precipitate in silica, 238 Lateral vent crack, 55, 126, 148, 154, 158-161, 235 analysis of, 159-161 circular contours of, 159 critical flaw size, 154 length as a function of load, 161 and surface distortion, 159 at thin film interfaces, 205 Lattice energy and fracture energy, 198 and hardness, 22, 24 of silicon, 24... 

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