Snoek-type relaxation caused by interstitial atoms in sintered β-type Ti-Nb alloy

Abstract

<sec> The <i>β</i>-type Ti-Nb alloys are potential shape memory and superelasticity materials. The interstitial atoms in the alloys have important effect on their physical and mechanical properties. For the interstitial atoms, the internal friction technique can be used to detect their distributions and status in the alloys. The influences of chemical compositions and heat treatments on the microstructures of the containing-oxygen Ti-Nb alloys are given, and a clear understanding and the relaxational mechanism of the internal friction peak correlated with oxygen are also clearly discussed by investigating the internal friction behavior of the alloys and the detecting their microstructures. </sec><sec> The Ti-Nb alloys with different Nb content values are prepared by powder metallurgy. The internal friction behaviors of Ti-Nb alloys with different Nb content values and heat treatments are investigated by using dynamic mechanical analysis (dynamic mechanical analyzer, DMA) Q800 from TA Instruments in single cantilever mode under different testing parameters and conditions from room temperature to 350 ℃. The X-ray diffraction experiments are also carried out in order to detect the differences among the microstructures of the specimens with different heat treatments for the Ti-35.4Nb alloy. It is shown that relaxational internal friction peaks are found on the internal friction temperature dependent curves of the sintered and water-quenched alloys. The internal friction peak is correlated with Nb content. The peak does not appear in the sintered Ti-Nb alloys with low Nb content. The maximum of the internal friction peak appears in the quenched alloy with about 35% Nb. The internal friction peak height increases monotonically with Nb content increasing in the present testing composition range for the sintered alloys. The relaxation parameters are the activation energy <i>H_wq</i> = (1.67 ± 0.1) eV and the preexponential factor <i>τ</i>_o<i>wq</i> = 1.1 × 10^{－17 ± 1} s for the quenched Ti-35.4Nb alloy . In addition, the peak height also depends on heat treatment. The water-quenched Ti-35.4Nb alloy has much higher internal friction peak than the as-sintered alloy with identical compositions. The internal friction peak height is also correlated with the quenching temperature. It is found that the peak is linked to the <i>β</i> phase of Ti-Nb alloys and that the peak height is determined by the stability and amount of the <i>β</i> phase from their microstructures. When the stability of the <i>β</i> phase decreases, the peak height increases, and the increase in the amount of <i>β</i> phase results in the increase of the peak height. The <i>β</i> phase in the quenched Ti-35.4Nb specimen is metastable <i>β</i> phase (<i>β</i>_M), which can be transformed into the stable <i>α</i> and <i>β</i>_S by ageing. The <i>β</i> phase in as-sintered specimen is the stable <i>β</i> phase (<i>β</i>_S). The modifications of microstructures of the specimens with different heat treatments result in the difference in peak height between the water-quenched and as-sintered Ti-35.4Nb specimens. That the peak height presents a maximum in the vicinity of 35 wt.% Nb for the quenched alloys results from the variation of the stability and amount of <i>β</i>_M with Nb content. That the height of the peak increases monotonically with Nb content increasing in as-sintered alloys is attributed to the increase of the amount of <i>β</i>_S. It is suggested that the internal friction peak is related to oxygen jump in lattice or the interaction between the oxygen-substitute atoms in <i>β</i>_M phase for the water-quenched alloys and those in <i>β</i>_S phase for the as-sintered alloys.</sec>