IMPROVING THE PERFORMANCE CHARACTERISTICS OF GROUP IV METALS (Ti, Zr, Hf) AND THEIR ALLOYS BY METHODS OF CHEMICAL-THERMAL TREATMENT

Abstract

The influence of oxygen diffusion saturation parameters on the characteristics of Group IV metals (Ti, Zr, Hf) and their alloys, including surface microhardness and the thickness of the hardened layer size, was studied. An increase in the performance of hardened thin-walled products made from these metals under cyclic and static loads was experimentally demonstrated. The effective diffusion coefficients of interstitial elements in Group IV metals (Ti, Zr, Hf) were calculated as a function of saturation temperature. Oxygen concentration profiles in Group IV metals and their alloys were analytically calculated depending on the oxygen saturation regime (temperature, duration, vacuum). The hardness values of surface microhardness and size of the hardened layer that enhance the durability of thin-walled products made of Ti, Zr, Hf, and their alloys under cyclic tension and pure bending were determined. Fractographic fracture characteristics of the hardened samples are presented. The effect of vacuum in an oxygen-containing environment (in the range of 133 · 10⁻³ to 133 · 10⁻¹ Pa) during surface modification of thin-sheet hafnium specimens of the GFE-1 grade at a temperature of 850 °C for 2 hours on fatigue strength under pure bending was determined.

It was found that diffuse saturation of the surface layers of the studied alloys with oxygen increases their durability under cyclic and static loads, which is important for increasing the service life of thin-walled products made of titanium, zirconium, and hafnium-based alloys through oxygen modification. Based on the experimental data, nomograms were constructed to select parameters (surface microhardness — δHV^surf, hardened layer size — l) for the studied α-titanium VT1-0, with oxygen-modified surface layers that would meet regulatory requirements for their rigidity and surface hardening depth. Based on these data, the authors substantiated the corresponding σ₋₁ levels for each processing option, below which the products will retain their functionality under operating conditions.

Two possible algorithms for monitoring the effectiveness of surface layer modification in α- and pseudo-α-titanium alloys are proposed: the first is based on one of the proposed indicators δHV^surf or l, and the second is based on the metal's fatigue endurance limit σ₋₁.