PSI - Issue 23

W. Teraud et al. / Procedia Structural Integrity 23 (2019) 390–395 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

391

2

p

creep deformation

percentage of the hydrogen

C H

1. Introduction

It is known that titanium interacts very actively with hydrogen, this is due to the fact that hydrogen has a high diffusion mobility, and this causes the phenomenon of hydrogen embrittlement. Therefore, hydrogen dissolved in titanium alloys, on the one hand, is widely used in technological processes, reducing the required force for deformation, and on the other hand, hydrogen dissolved in titanium alloys can provoke destruction of structures made of these alloys. Therefore, since most of the destruction is preceded by the stage of localization of deformations, it is of interest to study the effect of excess hydrogen content on the localization of deformations. Off-road samples, in addition to embrittlement under the action of excess hydrogen in the crystal lattice, may also show an increase in strength and time to failure Grabovetskaya et al (2017) and Lokoshchenko et al (2008). The effect of hydrogen on the creep of a titanium alloy is also known at room temperature as Gao and Dexter (1987). The study of the moment and conditions for the formation of strain localization was carried out by many scientists, however, without the influence of hydrogen. Among these works, it is possible to distinguish works using the force approach to describe the time of the appearance of the neck, examined by P. Hora et al (1996) and Michel et al (2007); deformation criterion considered Malygin (2011); the time criterion is presented by Srinivas et al (2009); in addition, other approaches are used, for example, based on learning algorithms Volk and Suh (2012). In the presented works, hydrogen embrittlement and its effect on plasticity and long-term strength are investigated, on the one hand, and on the other, the localization of deformations in the material without regard to the hydrogen content is investigated, but the authors do not know the effect of hydrogen on the localization of deformations. This paper proposes such study. 2. Setup and performance of real experiments In tests, samples of a two-phase titanium alloy ( α + β ) Ti – 6Al – 4V (grade VT6) were used. Preparation and testing was carried out at the Research Institute of Mechanics of Lomonosov Moscow State University on the installation IMEX-5. A total of 15 experiments were conducted with a test duration ranging from several hours to three weeks, but in this paper only eight experiments are presented, combined with a study of the hydrogen effect. For these experiments, the tensile stress at the initial moment of time was 352 MPa or 552 MPa in various tests. At each value of the initial tensile stress, the experiment was repeated several times. The test temperature was 450 ° C. The initial working length of the samples is l 0 = 25 mm, width w 0 = 5 mm, thickness  0 = 1 mm. In tests, samples of two types were used: initial and samples with a high content of hydrogen. Six samples of VT6 titanium alloy were prepared, which prior to experiments were pre-hydrogenated in the Moscow Aviation Institute laboratory by thermal diffusion method in Sievers equipment up to C H content of 0.1 %, 0.28 % and 0.6 % by weight of the sample. Samples with a hydrogen content of C H = 0.6 % were so fragile that they could not be tested. The fixed sample, heated to the operating temperature, was quickly loaded to a given initial voltage level  0 with the help of the weight P 0 , which later remained constant until the end of the experiment. The camera was turned on, which was configured to shoot at a specified interval of sample elongation. Further deformation was performed under creep under constant tensile strength. The experiments used a contactless system for measuring the geometric parameters of the sample, developed by the author of the work Teraud (2018). The geometry of the sample was measured without contact from the photographs of the sample obtained automatically in the course of the experiment. With help a camera installed at a distance in which the effect of temperature is not significant, the sample dimensions were recorded during high-temperature deformation at a specified sample deformation interval, the test applied a value of a one frame for every 0,1 mm of sample elongation. In each experiment, the sample elongation values  l ( t ), deformations p ( t ), the maximum contraction w max ( t ), the true stress distribution  ( t , x ) along the working part, the sample width distribution w ( t , x ), were obtained, parameters of neck formation, values of the moments of formation of localization of deformations  , etc. Under x is meant the longitudinal coordinate of the centerline of the sample.