Page 21 - НАЦІОНАЛЬНА АКАДЕМІЯ НАУК УКРАЇНИ

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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 de-
formed layer with the grain refined structure
It was shown that the surface pre-deformation intensifies the nitriding combined
with strengthening heat treatment of VT22 alloy, which was confirmed by a 50% higher
surface microhardness and a 30% deeper hardened layer compared to the nitriding with-
out pre-deformation. We can assume that accelerated nitriding kinetics is occurred for
two main reasons. Firstly, grain boundaries, deformation bands and twins act as diffusion
shortcuts by providing easy diffusion paths for nitrogen interstitial atoms. Moreover, the

higher density of non-equilibrium crystal defects increases the stored energy of the sur-
faces and their chemical reactivity especially providing additional preferential nucleation
sites for titanium nitrides.
The influence of deformation-diffusion treatment including a combination of the
previous cold surface plastic deformation (ball burnishing) and subsequent thermo-
chemical treatment (nitriding combined with strengthening heat treatment), on the tribo-
logical behavior of the VT22 two-phase titanium alloy in a pair with BrAZhN 10-4-4
bronze was studied. According to the results of the tribological test, SEM and EDX ana-

lyses of the wear surfaces, wear mechanisms (or processes) those of tribo-pairs in the
conditions of boundary lubrication were established. It is shown that deformation-
diffusion treatment of titanium alloy provided higher tribotechnical parameters (wear re-
sistance, friction coefficient and temperature near the friction zone) of investigated tribo-
pairs compared to ball burnishing or nitriding treatments. It was established that the de-
formation-diffusion treatment increases the wear resistance of VT22 titanium alloy in
tribo-pair with BrAZhN 10-4-4 bronze twice and reduces the friction coefficient by 30%
compared to nitriding without surface pre-deformation.
It is established that after deformation-diffusion treatment the fatigue strength ap-
proaches the values after strengthening heat treatment of VT22 titanium alloy. The frac-

tographic analysis of the fracture surfaces of the studied samples revealed a certain corre-
lation, namely: the lower the fatigue strength of the alloy samples after treatment, the
larger area of brittle intergranular fracture on the fracture surface.
It is shown that the strength of samples of VT22 titanium alloy after all hardening
treatments increases. The samples hardened by deformation-diffusion treatment were
somewhat inferior to the samples after cold surface plastic deformation and nitriding in
terms of the ultimate tensile strength. But, the plasticity (elongation) of VT22 alloy sam-
ples after deformation-diffusion treatment was the highest. The nature of the fractures

indicated the mechanisms of ductile rupture, namely, the formation of the so-colled di-
pole void structure was fixed on fracture surface. A correlation was observed between
the size and depth of the voids (holes) in the surface fracture zone and the strength of
VT22 alloys: the treatment that increases the strength the most has the smallest voids
(holes) on the fracture surface.
The technological recommendations for combined deformation-diffusion treatment
of VT22 titanium alloy were developed, which provide the regulated characteristics of
hardened layers while maintaining the volumetric characteristics of the alloy.

Keywords: VT22 titanium alloy, cold surface plastic deformation, intensification,
nitriding, deformation-diffusion treatment, surface hardening, wear resistance