1.School of Mechanical and Electronic Engineering, Suzhou Vocational University, Suzhou 215104, China 2.Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
Abstract: The β-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. 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 Hwq = (1.67 ± 0.1) eV and the preexponential factor τowq = 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 β phase of Ti-Nb alloys and that the peak height is determined by the stability and amount of the β phase from their microstructures. When the stability of the β phase decreases, the peak height increases, and the increase in the amount of β phase results in the increase of the peak height. The β phase in the quenched Ti-35.4Nb specimen is metastable β phase (βM), which can be transformed into the stable α and βS by ageing. The β phase in as-sintered specimen is the stable β phase (βS). 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 βM 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 βS. It is suggested that the internal friction peak is related to oxygen jump in lattice or the interaction between the oxygen-substitute atoms in βM phase for the water-quenched alloys and those in βS phase for the as-sintered alloys. Keywords:internal friction/ Ti-Nb alloy/ interstitial atoms/ Nb concentration
图1显示了不同振动频率下加热过程中水淬的Ti-35.4Nb合金的内耗(tanδ)和存储模量(storage modulus)随温度的变化. 可以看出, 一个内耗峰出现在250 ℃左右, 存储模量在内耗峰的位置有一个快速的下降, 这一现象符合Kronig-Kramers关系[5, 6]. 当振动频率增加时, 峰移向温度较高的位置, 说明这个内耗峰是由热激活的弛豫过程引起的. 从图2还可以看出, 这个峰的高度与应变振幅没有关系, 当振幅增加时, 峰的高度和峰温不变, 这种现象的出现说明是线性滞弹性弛豫. 图 1 水淬的Ti-35.4 Nb合金tanδ和存储模量随温度和频率的变化关系(ε = 3 × 10–4) Figure1. tanδ and storage modulus as a function of temperature for the water-quenched Ti-35.4 Nb alloy at different vibration frequencies (ε = 3 × 10–4)
图 2 水淬的Ti-35.4Nb合金在不同应变振幅条件下tanδ 随温度的变化(f = 1 Hz) Figure2. tanδ as a function of temperature for the water-quenched Ti-35.4Nb alloy for different amplitude (f = 1 Hz)
图4显示了烧结的Ti-35.4Nb合金在不同振动频率下的内耗-温度曲线. 从图4可以看出, 尽管内耗峰高度与水淬的Ti-35.4Nb合金相比要低很多, 但内耗峰仍然存在, 说明热处理对内耗峰的高度有影响. 从峰温对频率的依赖性看, 在烧结合金中出现的内耗峰也有弛豫特征, 激活能是Hs = (2.13 ± 0.2) eV, 指数前因子是τos = 2.7 × 10–23 ± 2 s. 然而, 烧结合金的内耗峰高度随Nb含量的变化趋势与水淬的合金不同, 从图5可以看出, 峰高单调地随Nb含量的增加而增加. 图 4 烧结态Ti-35.4 Nb合金Tanδ和存储模量随温度和频率的变化关系(ε = 3.7 × 10–4) Figure4. Tanδ and storage modulus as a function of temperature for the as-sintered Ti-35.4 Nb alloy at different vibration frequencies (ε = 3.7 × 10–4)
图 5 烧结态的Ti-Nb合金样品内耗峰高度随Nb含量的变化关系(ε = (2.5—4.0) × 10–4, f = 1 Hz) Figure5. The dependence of the peak height on Nb content for the as-sintered Ti-Nb alloys (ε = (2.5?4.0) × 10–4, f = 1 Hz)