1. Introduction

An increasing of fatigue strength and endurance of titanium alloys of various structural classes can be achieved by the formation of surface layers with regulated structure and phase state during surface hardening processes of chemico-thermal treatment (CTT). CTT in the present time the thermodiffusion hardening of surface layers by interstitial impurities is not used to increase the fatigue properties of workpieces made of titanium alloys. However, works performed at PMI NASU have shown that the range of the parameter K exists where the endurance of alloys with gas-saturated layers is higher than fatigue endurance in the initial state. An optimal level of surface hardening depends on the phase-structural state of metal and relative depth of gas-saturated zone. The problem arises in the controlling of intensity of the physical and chemical processes in a "titanium alloy/gas medium" system to obtain the optimal phase-structural state of surface layers and increase the

operating characteristics of metal. Therefore the aim of investigations on the second stage of the project was to determine (a) regularities of solid solution hardening of surface layers of titanium alloys (α, near-α, α + β) depending on the conditions of thermodiffusion saturation in rarified gas medium containing oxygen and (b) general regularities of influence of methods and regimes of surface deformation on phase, structural, and substructural state of various titanium alloys. It is expected that obtained results will allow to forecast the influence of the regime of CTT on the phase-structural state of surface layers of metal and level of hardening and determine the parameters of thermal treatments for achieving the regulated level of hardening.

Increasing of fatigue strength and durability of titanium alloy workpieces remains an actual modern problem. It is known that fatigue properties of titanium alloys can be increased sufficiently by means of optimization of phase-structural state of surface layer. The aim of the investigations of paper was to determine the correlations between parameters of surface-hardened layers (surface hardness, depth of hardened zone, microstructure) and fatigue properties of titanium alloys VT1-0, VT5, ОТ4-1, VT16, and VT22 under various methods of surface hardening: thermodiffusion saturation in gas medium containing oxygen (CTT). Determination of such correlations allow to define the parameters of surface hardening of titanium alloys necessary for increasing of fatigue properties and approximate these methods of surface hardening to the practical application.

The influence of temperature-time and gas-dynamical parameters (T = 650,

Changes of free energy of formation of oxygen solid solutions in alpha-titanium and titanium monoxide as a

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

thermodiffusion saturation in controlled gas medium on the regularities of interaction of titanium alloys VT1-0, VT5, OT4-1, and VT16 is investigated by means of gravimetric analysis. The kinetic parameters of interaction of investigated titanium alloys determined by means of gravimetric analysis are shown in Tables 1–4.

According to the obtained results for α-titanium alloys (VT1-0, VT5), the process of gas saturation is intensified with the increasing of interaction temperature and pressure of gas medium (partial pressure of oxygen). The α-alloy VT1-0 (tech-

The mass loss caused by intensification of sublimation of alloying element Mn is possible for near-α-alloy OT4-1 (2% β-phase) under definite conditions of interaction with rarefied gas medium. The ratio of parameters T and P exists for this alloy when the rates of the gas saturation and sublimation processes become comparable. For the predomination of gas saturation processes, it is necessary to increase the

Rate of gas saturation in rarefied gas medium decreased substantially with the increasing of quantity of β-phase in alloys (VT16 ! VT22). This is caused by the decreasing of maximal solubility of oxygen in β-phase of titanium (6 at.%) in comparison with α-phase (33 at.%). With the increasing of interaction temperature, this difference appeared most appreciably. Therefore, it was concluded that alloys with large quantity of β-phase are the ones less sensitive to the changing of the

) at residual pressure of gas medium

<sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup> Pa <sup>P</sup> = 1.33 � <sup>10</sup>�<sup>2</sup> Pa <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>2</sup> Pa 1h 3h 5h 1h 3h 5h 1h 3h 5h

650 3.46 9.905 15.98 4.58 12.9 20.61 7.95 21.34 33.14 700 8.109 22.96 36.82 10.67 29.68 47.07 18.25 48.11 73.9 750 17.416 48.83 77.85 22.82 62.68 98.64 38.45 99.49 151.23

The specific mass gain of specimens of titanium alloy VT1-0 as a result of interaction with rarefied gas medium

nical titanium) has the largest rate of interaction with rarefied gas medium containing oxygen under the all conditions. Alloying of titanium by 5% Al-alloy VT5-slows down slightly the rate of mass gain under the same conditions of inter-

partial pressure of oxygen or decrease the temperature of interaction.

conditions of interaction with rarefied gas medium containing oxygen.

, 1.33 � <sup>10</sup>�<sup>2</sup>

, 6.6 � <sup>10</sup>�<sup>2</sup> Pa) of

700, 750°С, <sup>τ</sup> = 1, 3, 5 h, <sup>P</sup> = 6.6 � <sup>10</sup>�<sup>3</sup>

T, °C ΔM/S (μg/cm2

function of (a) oxygen pressure and (b) temperature [1].

DOI: http://dx.doi.org/10.5772/intechopen.82545

action.

Table 1.

79

containing oxygen.

Figure 1.
