Oxygen Solid Solution Hardening in Titanium by Heat Treatment

WATARI Fumio, NISHIMURA Fumio

doi:10.18939/jjdm.10.2_266

Heat treatment of titanium from 300 to 1, 200℃ under air, in vacuo and under oxygen partial pressure was done for solution hardening by oxygen.The surface oxide film was stable up to 800〜900℃.At a higher temperature oxidation showed the different phenomena.The surface oxide layer was formed by the single step heat treatment even under vacuum at a lower temperature(<800℃).By the double step heat treatment, where the oxide film was dissolved and absorbed into metal by the temporary heating to a higher temperature(>900℃), and oxidation process was followed at a lower temperature, more oxygen was dissolved into titanium.By the further homogenizing anneal after oxidation, the surface oxide film disappeared through the dissolution into metal and the metallic brilliance reappeared.By measurement of the micro-Vickers hardness along the depth from surface in the specimen cross-section, the diffusion range of oxygen could be confirmed as the increase of the hardening layer thickness.The experimentally obtained value of hardening layer thickness was quantitatively in good accordance with that calculated from the activation energy of ox gen diffusion in titanium.Whether oxygen contributes to the formation of oxide layer or it becomes solute atoms in metal depends on the balance between the reactivity of the gas oxygen to surface oxide and the dissolution rate of oxygen from layer to metal.When there exists a stable oxide layer at the surface, the solution and diffusion of oxygen into metal is blocked.These models could explain the phenomena well.

Oxygen Solid Solution Hardening in Titanium by Heat Treatment

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