Effect of high-temperature vacuum annealing time on the structure and properties of titanium alloys
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In the process of vacuum annealing at 850° under a pressure of 2.5·10−3 Pa the following changes occur in the structure of titanium alloys VT1-0, PT-7M, and OT4-1:
Diffusion of oxygen into the metal and formation of a gas-saturated surface layer with high hardness, the thickness of which increases with the annealing time (through saturation occurs in alloy VT1-0); in this case the local concentration of oxygen in the bulk of individual grains may become so high that formation and growth of oxides are possible;
etching of grain boundaries and the development of sublimation microrelief on the surfaces of grains, an increase in the hardness of sublimated grains, and changes in the chemical and phase composition of the surface layer due to sublimation of alloying elements.
With increasing annealing time the processes of gas saturation and sublimation lead to progressive damage of the surface and an increase in the thickness and effectiveness of the surface layer with changes in composition, structure, and properties, which are accompanied by lower fatigue characteristics.
To obtain the optimal combinations of ductile, strength, and fatigue characteristics the vacuum annealing time at high temperature for parts of titanium alloys VT1-0, PT-7M, and OT4-1 should not exceed 1 h.
KeywordsFatigue Titanium Surface Layer Phase Composition Titanium Alloy
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