The poor oxidation resistance of niobium is caused by the oxidation process and the oxides formed by it. Property determined. X - ray diffraction analysis showed that the main oxides of niobium NbO, NbO2, and Nb2O5, the light and dark outer layers are made of high-priced oxides. Nb2O5 is composed, and the different color is caused by inner layer hypoxia. The inner grain has selectivity. The optimal orientation can be closely attached to the matrix, while the outer grains are disordered and loose Fall. This is due to the high volume ratio between Nb2O5 and the matrix (~ 2.69), which is generated in the surface layer.
High bidirectional internal stress. The greater the oxide thickness is, the greater the internal stress will be when the inner layer grows to at a certain critical thickness, cracks are produced and the outer layer becomes loose. Nb2O5 has two species isomers -Nb2O5 and -Nb2O5. Alpha -Nb2O5 is a metastable phase as long as if you meet the thermodynamic conditions, you go to a stable phase, but very slowly at low temperatures above 800 ℃ -Nb2O5 is rapidly generated, so 800 ℃ is generally considered as an phase the transition temperature to phase.
The oxidation process of niobium can be described as follows :(1) oxygen dissolves into metal
Niobium, the formation of interstitial solid solution, niobium lattice expansion. The composition is uneven at first homogenize while approaching saturation. The -NB2O5 phase is generated, with each grain size gradually grew up. NB2O5 gradually forms preferentially oriented compactness during grain growth dense layer, with the increase of temperature, gradually changed into loose oxide layer with disordered orientation.
According to the oxidation mechanism of niobium, the methods of alloying can be considered from several anti - oxidation properties of niobium were improved.
(1) Add the alloy in Nb2O5 to form a solid solution, the valence number of solute element ions.
And ionic radius obviously affect the crystal defect structure and volume of Nb2O5.
The addition of ions can be expected to change the diffusion characteristics of the shell and reduce the expansion of oxygen ions speed.
(2) The alloying elements added exceed the solid solubility in Nb2O5 to generate other oxygen
The oxide phase, or the low - cost oxide, or the amorphous surface layer, gives the oxide layer a variety of properties
Produce favorable changes.
(3) To improve the strength and creep property of the oxide layer, release the internal stress slowly, and reduce the crack is generated.
(4) Densify the surface layer, reduce the voidness, and block the oxygen to the interior of the matrix to spread.
