Internal Friction Transient Effects of Titanium Hydrogen Solid Solution

The internal friction of pure Ti between 500K and700K is constant at 10-4 when measured during a continuous increase of temperature. In opposite when the temperature is increased stepby step, each increase of 10K induces immediatly a raising up of 4 10-4 internal friction. During the following stage at the new temperature the internal friction falls slowly down to its initial value. This behaviour can be attributed to either the sudden modification of the thermodynamic equilibrium of pure titanium or the presence of a small amount of hydrogen trapped by the specimen during the experiment. This paper compares the internal friction of pure titanium and titanium containing 7 w.ppm of hydrogen. The presence of hydrogen has three consequences : i the internal friction background is increased from 10up to 30 ii the transient maximum of internal friction due to thetemperature steps is 10 times as high as the corresponding value obtained with pure titanium. iii at temperatures higher than 700K large instabilities both of damping and modulus are recorded and it is not possible to reach an equilibrium value. It can be concluded that the transient maximums of internal friction are specific of the metal, the presence of hydrogen increases the amplitude of the effects. Introduction.The spectrum of damping of titanium at 1 Hz is determined by increasing the temperature, step by step, from 200°C to 47S°C(1). Each step gives rise to a sudden increase of the damping, and once the temperature is stabilized, the internal friction decreases slowly to reach its equilibrium value for this temperature. The maximum value of the transient internal friction, is twice the equilibrium value for a change of temperature of 10°C. This transient effect is observed for an increase or a decrease of the temperature greater than 5OC. This transient effect has been observed on high purity titanium, hence it is difficult to attribute it to chemical instabilities; nevertheless its very low activation energy (0.2 eV) could indicate the contribution of hydrogen to this process. Hydrogen could come from a superficial chemical reaction between titanium and residual oil vapour in vacuum during the heat treatment which follows the mounting of the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981527 C5-176 JOURNAL DE PHYSIQUE specimen of pure titanium on the pendulum. In order to verify this hypothesis the transient effects on pure titanium and Ti containing 0.15 at. % H are compared. Experimental procedure.The specimens are platelets (80x8~1 mm) of high purlty titanium with major impurities (at.ppm): 0, 20 ; N, 20 ; C, 6 ; Fe, 2 ; Mn ; Mg ; Si < 1. The high degree of purity is confirmed by the very low elastic limit : 40 MPa. Hydrogen is added to titanium by electrolysis in H2 SO4 (4N) with a current density of 2OmA/cm 2 for 1 mm. The internal friction is measured in an inverted torsion pendulum by the free decay method at a frequency of about 1 Hz. The maximum amplitude is 5x10-~ and the accuracy of measurement of damping AQ-' is The temperature is maintained constant within 0.25OC as long as necessary. After mounting on the pendulum, the specimens of pure titanium are annealed at 800°C for 1 hour in vacuum of 5x10-~ torr. The specimens of Ti-0.15 at % H do not receive this heat treatment to avoid the loss of hydrogen in vacuum (2) as three specimens heat treated at 800°C in ultra-vacuum behave like pure titanium. Transient effects of pure titanium.The transient of damping is shown on figure 1. After several hours at 35g°C the specimen exhibits a nearly stable damping : Q-l = Z X ~ O ~ . The temperature is then brought to 370°C, this change of temperature takes one hour. In the figure 1 it can be checked that the period of the pendulum increases simultaneously with the temperature indicated by the thermocouple. As soon as the temperature of the specimen increases its internal friction rises . The maximum value Q ; ' = 4.2 x is reached 20 mn after the start of the heating and then the damping decreases exponentialy. The equilibrium value Qzl = 2xl0-~ is reached after 6 hours following the relationship (fig. 2) : 1 Q = pzl + IQ~;' Q_ exp 7 111 T The transient effect is defined by 2 parameters : its magnitude ( Q ~ Qz1) and its decreasing rate related to the characteristic time