Page 8 - Thesis_Lavrys

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kinetic constants for a wide temperature range (750…950 °C) of the two-phase

VT22 titanium alloy were specified.

2. The effect of the initial structural state of VT22 alloy on the physico-

mechanical characteristics of the surface hardened layer and structural changes in

the matrix after nitriding combined with strengthening heat treatment was

investigated for the first time. It was shown that for the alloys with initial equiaxed

(α+β)-globular and β-structures, the level of surface hardening is higher than for

alloy with initial (α+β)-lamellar structure. Instead, the most intense structural

changes (α-phase growth) were observed in the alloys with the initial β- and (α+β)-

lamellar structures.

3. It was found for the first time that the most effective cold surface plastic

deformation was realized in VT22 alloy with initial two-phase lamellar or globular

structures. For the alloy with the initial β-structure, the cold surface plastic

deformation requires higher loads.

4. For the first time, on the basis of microstructural, durometric and X-ray
structural analyzes, the thermal stability of deformation hardened surface layer of

VT22 alloy under conditions of saturation by nitrogen up to 820 °C was confirmed.
It was shown that during heating in nitrogen, the processes of recrystallization of

the deformation hardened layer were reduced due to the processes of solid-soluble
hardening, which is confirmed by the preservation of the deformed layer with the

grain refined structure.
The practical significance of the obtained results

1. A new solution of the scientific and technical problem of intensification of

gas nitriding of VT22 titanium alloy by pre-modification of its surface by the cold

surface plastic deformation was proposed: the surface microhardness increases by

50%, and the depth of the hardened layer thickens by 30% compared to nitriding

without surface pre-deformation.

2. The scheme and regime of deformation-diffusion treatment were

recommended, which with regulated volumetric strength of VT22 alloy provide

increasing of its wear resistance in tribo-pair with BrAZnN 10-4-4 bronze by twice