Page 6 - Thesis_Lavrys

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β- structures, the level of surface hardening was higher than for the alloy with the

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.

The thermal stability of the deformation hardened layer under conditions of

nitrogen saturation up to 820°C due to the dominance of solid-soluble nitrogen

hardening processes over recrystallization processes was confirmed. On the basis

of the microstructural, durometric and X-ray structural analyzes 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

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 without 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 surfaces 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 tribological 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 analyses 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