SIMS analysis of low content hydrogen in commercially pure titanium

Abstract

Due to their excellent specific strength and environmental resistance, titanium and its alloys have been widely used as structural components. However, titanium is very active, so that not only oxygen and nitrogen, but also hydrogen may be readily introduced into the material during manufacturing. Titanium usually contains a specific amount of hydrogen, and it exists in the titanium as a hydride or solid solution dependent on the hydrogen content. The hydrogen solid solubility limit of pure titanium (a-phase titanium) seems to be higher than 10 ppm [1], although some values are reported. That is to say, if the hydrogen content of pure titanium is less than 10 ppm, all the hydrogen is thought to be a solid solution in the titanium. On the other hand, when the hydrogen content is more than the solid solubility limit, hydrides would appear. As the hydride is brittle, it weakens the strength of the titanium. Therefore, the hydrogen content in commercially pure titanium (CP-Ti) is within 150 ppm [2]. The present authors carried out a fatigue strength evaluation of titanium samples using the hydrogen removed sample (hydrogen content is 2.7 ppm) and normal sample (34 ppm) [3, 4]. On the surface of the 2.7 ppm one, the hydride did not exist, while on the 34 ppm one, the hydride was present. It was found that the fatigue crack nucleation life of the 34 ppm one is greater than that of the 2.7 ppm one, and the fatigue crack propagation life of the 34 ppm one is the same as the 2.7 ppm one. This specific observation of the crack propagation indicated that the material behavior near the grain boundary is important for the crack propagation [3, 4]. It is then important to understand the actual state of hydrogen near the grain boundary for the CP-Ti applications. There are some methods to detect hydrogen in materials, and each method has some merits and demerits. The hydrogen thermal desorption analysis method (TDS and TDA) can be used for the average hydrogen content and energy state analysis [5]. As already mentioned, hydrogen in the CP-Ti exists as a solid solution or hydride, and the hydrogen localized around the grain boundary should be evaluated. The localized hydrogen can then be visualized by the following method.