Thermodynamics of hydrogen absorption and desorption in TC21 alloy
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Rare Met. DOI 10.1007/s12598-014-0318-z
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Thermodynamics of hydrogen absorption and desorption in TC21 alloy Xiao-Li Wang*, Yong-Qing Zhao
Received: 4 September 2013 / Revised: 29 October 2013 / Accepted: 16 May 2014 Ó The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2014
Abstract In this paper, it was addressed a hydrogen absorbing and desorbing thermodynamics in a?b type TC21 titanium alloy with high strength and toughness based on thermodynamic experiments and calculation. The relationship between concentration (C), temperature (T), and pressure (P) of TC21 alloy is shown by P–C–T curves during hydrogen absorption and desorption process, which were measured by multistep hydrogenation/dehydrogenation methods from 625 to 750 °C. The P–C–T isotherms at a given temperature were separated into three regions. The partial thermodynamic functions of hydrogen reaction were evaluated by a modified form of Sievert’s law and P–C– T relation of different regions was expressed by the modified Sievert’s law. The results show that the enthalpy of hydrogen reaction in the first and third region relies on hydrogen content. According to Vant’s Hoff law, enthalpy and entropy of hydrogenation platform in TC21 alloys are -53.58 kJ mol-1 and -127.41 JKmol-1, respectively. Compared with P–C–T curves of hydrogen absorption, that of hydrogen desorption exists hysteresis. Keywords Thermodynamic functions; Enthalpy; Entropy; Hydrogen absorption/desorption
X.-L. Wang* School of Material Science and Engineering, Xihua University, Chengdu 610039, China e-mail: [email protected] Y.-Q. Zhao Northwest Institute for Nonferrous Metal Research, Shaanxi 710016, China
1 Introduction In recent years, thermohydrogen treatment of titanium alloys has been investigated widely, which could lower the deforming temperature and resistance, improve cutting performance, and so on [1–6]. Zong et al. [7] investigated the high temperature deformation behavior of TC21 alloy with 0.7 wt% hydrogen. The results showed that the suitable hydrogen addition could significantly decrease the flow stress and improve the hot workability of TC21 titanium alloy. In order to formulate hydrogenated and dehydrogenated technology, thermodynamics of hydrogenation and dehydrogenation was investigated. The relationship between metal hydride concentration (C), temperature (T), and pressure (P) defines the solubility of hydrogen in titanium alloys, thermodynamic parameters (enthalpy, entropy, etc.) of hydrogenation reaction, and so on. Therefore, an accurate modeling of P–C–T correlation is the key factor in any metal hydride system [8]. Using experiential P–C–T curves, thermodynamics of hydrogenation reaction in titanium alloys was studied by Van’t Hoff equation in two-phase region and polynomial in single-phase region. Huang et al. [9] measured the hydrogenation P–C–T curves of titanium in 550–600 °C and calculated hydrogenated enthalpy of the first platform (-(82.5 ± 0.7) kJmol-1) and the second platform ((-120.9 ±
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